potters council studio reference guide to successful ... · by deanna ranlett 6 the potter’s...

how to select, process, and test materials

for great results

Guide to SucceSSful GlazinG and firinG

Potters Council Studio Reference

ceramicartsdaily.org/potters-council/ | copyright © 2014, ceramic Publications company | Guide to Successful Glazing and firing | 2

3 Building a Glaze Pantry by Deanna Ranlett

6 The Potter’s Pallette Studio Reference: Colorant by Robin Hopper

8 Feldspar: The Potter’s Pet Rock by Mimi Obstler

10 In the Bucket: The Key to Consistent Glazes by Richard A. Eppler with Mimi Obstler

Guide to Successful Glazing and FiringHow to select, process, and test materials for great results

16 Kiln Firing Chart

17 Heatwork by Dave Finkelnburg

18 The Many Layers of Kiln Wash by John Britt

22 Glossary of Common Ceramic Raw Material by Vince Pitelka

23 Primary Function of Common Ceramic Raw Materials

all content excerpted from ceramic arts daily, and Potters council members receive a 20% discount on all publications.

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Much like cooking, there are staples to any great glaze pantry. Having a stockpile of materials on hand opens your options to hundreds of recipes you’ll be able to mix in minutes.

A Good FoundationIt’s important to start your glaze pantry with a good foundation of commonly used glaze ingredients. There are specific lists for each range of firing temperatures: low fire (cone 08–02), mid fire (cone 4–6), and high fire (cone 8–10). Many ingredients are shared between the ranges, but some, like frits, you will use in a higher per-centage at lower temperature ranges. Others, like stains, need to be tested at mid- and high-fire temperatures be-fore stocking up on them as potential colorants.

While it’s hard to recommend material quantities with-out knowing what batch sizes you’ll be mixing, I usually give the following guidelines:

Test Mixing: If you’re mixing test batches, start with 5–10 pounds of the basics (feldspars, frits, kaolin, silica), ¼-pound bags of colorants, opacifiers, and stains, and maybe 1–2 pounds of rare ingredients. Planning ahead and buying in bulk may save you money in the long run.

Large Batching: If you’re mixing larger batches (5-gal-lon buckets), start with 50-pound bags of the basics, 5 pounds of Zircopax, ½–1 pound of oxides and common stains, and 5–10 pounds of less common ingredients. I’d recommend using a spreadsheet (see page 13) or glaze cal-culation software to keep track of ingredients, amounts, and pricing.

Essential EquipmentA great glaze pantry also needs the proper equipment:

■ Scales: Having a good scale is critical to proper mixing and not wasting ingredients. I recommend buying two scales; one that is accurate to the 1 gram and has high-capacity (5000 grams) capabilities and one for small test batches that is accurate to one-hundreth of 1 gram so that you can measure expensive colorants accurately. The highly accurate scales generally have a lower-weight ca-pacity (figure 1).

■ Sieves: Uniform and consistent glazes depend on the sieving of mixed glazes. I recommend a 40-mesh sieve for raku glazes and for granular ingredients; a 60-mesh sieve for the first run of a glaze; an 80-mesh sieve for glazes with fine colorants (i.e. oxides, stains, etc.); and if you are mixing high-firing glazes such as celadons, you may want a 100-mesh sieve (figures 1–2).

■ Storage containers: Chemicals should be stored in plastic bags or plastic containers. Paper bags can dry rot and the adhesive can dry out allowing chemicals to spill and cost you money. Paper bags also allow mois-ture to get through the bag and chemicals can harden. Always label your bags and containers with the chemi-cal name. It’s a good idea to keep a record of where and when you purchased your materials in case you have questions later (figures 3–4).

■ Safety equipment: A dust mask or a professionally fitted respirator; gloves; wet clean up equipment such

Building a

by Deanna Ranlett

Glaze Pantry

Scales, whisk, and assorted sieves needed for mixing both test batches and large quantities of glaze.

A Talisman Rotary Sieve for screening large batches into 5-gallon buckets. This is an optional piece of equipment.

1 2


MAtEriAl/QuAntity low-FirE: ConE 08–02 Mid-FirE: ConE 4–6 HiGH-FirE: ConE 8–10

Soda Feldspar: nC4/Minspar* occasional common common

Potash Feldspar: Custer* occasional common common

Potash Feldspar: G200HP* occasional common common

nepheline Syenite* occasional common common

Spodumene rare occasional comon

talc common occasional rare

Barium Carbonate occasional rare rare

Strontium Carbonate rare occasional common

lithium Carbonate occasional occasional occasional

Ferro Frit 3110 common rare rare

Ferro Frit 3124 common occasional rare

Ferro Frit 3134 common common rare

Ferro Frit 3195 common occasional rare

wollastonite* occasional common common

whiting* (calcium carbonate) occasional common common

Gerstley Borate* common common occasional

Borax common rare rare

dolomite rare occasional common

Magnesium Carbonate rare rare occasional

Kaolin* (EPK) common common common

oM4 Ball Clay* common common common

red Art occasional rare rare

6 tile Clay rare common common

tin oxide common common common

Zircopax common common common

titanium dioxide common common common

Cobalt Carbonate common common common

Cobalt oxide common common common

Copper Carbonate common common common

Black Copper oxide common common common

Chromium oxide common common common

red iron oxide common common common

rutile Powder (light or dark) occasional common common

Manganese dioxide common common common

Commercial Stains (¼ lb bags) common common common

Silica* common common common

Color-coded shopping list: feldspars = blue; fluxes = purple (note: although separated here, feldpars are a category within fluxes); clays = orange; opacifiers = yellow;

colorants = green, glass former = pink. * = materials recommended for bulk purchase.


Put smaller quantities of raw glaze materials into air-tight containers. Clearly label each one on the container itself rather than the lid, as the lids can be lost or switched with other lids.

A more elaborate materials storage and glazing area including rolling carts for large buckets and a wall designed to display fired text tiles.

Put bulk materials in storage bins and clearly mark them. Try to keep bins off the floor if possible to make the floor easier to clean of unwanted dust. Either use sturdy racks or rolling carts.

A simple work bench set up for mixing small batches of test glazes. Within convenient reach: scale, gloves, scoops, containers, sponge, reference notebook, and glaze materials

3 4

5 6

as a sponge and a mop; a well-ventilated area to work, whether that is near an open door or window or by a vented space.

■ Miscellaneous equipment: Scoops, buckets with lids, whisks or stirring paddles, labeling tools; MSDS safety sheets.

Shopping listUse the shopping list (on the previous page) to help you get started at your local clay supplier. The list includes the essential materials you’ll want to start a glaze pan-try and labels them as “common.” Those that are less needed but occasionally called for in recipes are noted as well followed by “occasional” or “rare”. Note: I have placed an asterisk (*) next to ingredients to buy in bulk

if you’ll be mixing multiple batches or large 5-gallon buckets—although most ingredients can be purchased in various small quantities, 50-pound bags can save a sig-nificant amount of money. If it’s your first time mixing and you’re only mixing a few tests, a 5-pound bag would suffice.

Once you have the foundations of your glaze lab set up, you may want to consider adding a few additional items: a drill mixer for mixing large batches of glaze in 5-gal-lon buckets; an immersion blender for mixing small test batches; a rack or rolling cart to keep bulk ingredients off the floor; a sturdy table for mixing; glaze software; and a vent equipped with a dust filter (figures 5–6).

Have fun experimenting and building your glaze pantry!Deanna Ranlett owns Atlanta Clay (www.atlantaclay.com) and MudFire Clayworks and Gallery (www.mudfire.com). She has been a working ceramic artist for 13 years.

This article was excerpted from Pottery Making Illutrated, by Deanna Ranlett and published by The American Ceramic Society.


The Potter’s Palletteby Robin Hopper

Studio RefeRence: coloRant

red to orangeThe potter’s palette can be just as broad as the painter’s. Different techniques can be closely equated to working in any of the two-dimensional media, such as pencil, pen and ink, pastel, watercolor, oils, encaustics or acrylics. We also have an advantage in that the fired clay object is permanent, unless disposed of with a blunt in-strument! Our works may live for thousands of years—a sobering thought.

Because a number of colors can only be achieved at low tempera-tures, you need a series of layering techniques in order to have the fired strength of stoneware or porcelain and the full palette range of the painter. To accomplish this, low-temperature glazes or overglazes are made to adhere to a higher-fired glazed surface, and can be super-imposed over already existing decoration. To gain the full measure of color, one has to fire progressively down the temperature range so as not to burn out heat-sensitive colors that can’t be achieved any other way. Usually the lowest and last firing is for precious metals: platinum, palladium, and gold.

For the hot side of the spectrum—red, orange, and yellow—there are many commercial body and glaze stains, in addition to the usual mineral colorants. Ceramists looking for difficult-to-achieve colors might want to consider prepared stains, particularly in the yellow, vio-let, and purple ranges. These colors are often quite a problem with standard minerals, be they in the form of oxides, carbonates, nitrates, sulfates, chlorides or even the basic metal itself.

Minerals that give reds, oranges, and yellows are copper, iron, nick-el, chromium, uranium, cadmium-selenium, rutile, antimony, vana-dium, and praseodymium. Variations in glaze makeup, temperature and atmosphere profoundly affect this particular color range. The only materials which produce red at high temperature are copper, iron, and nickel . The results with nickel are usually muted. Reds in the scarlet to vermilion range can only be achieved at low temperatures.

The chart should help pinpoint mineral choices for desired colors (note that the color bars are for guidance only and not representative of the actual colors —Ed.). Colors are listed with the minerals needed to obtain them, approximate temperatures, atmosphere, saturation percentage needed, and comments on enhancing/inhibiting factors. Because of the widely variable nature of ceramic color, there are many generalities here. Where the word “vary” occurs in the column under Cone, it signifies that the intended results could be expected most of the time at various points up to cone 10.

COLORANT CONE ATMOS. % COMMENTS

Dark Red

Copper Vary Red. 0.5%-5% Best in glazes containing less than 10% clay content, and a high alkaline content. Needs good reduction. In low temperatures it can be reduced during cool-ing. Good reds as low as cone 018.

Iron Vary Both 5%-10% Good in many glaze bases at all temperatures. Can be improved with the addition of 2%-5% tin oxide.

Nickel 4-10 Ox. 5%-8% Use in barium-saturated glazes.

Burgundy

Iron See Dark Red, Iron.

Copper See Dark Red, Copper. Owing to the unstable nature of copper, this colorant can produce a wide range of results. Very controlled reduction firing and cooling are important.

Maroon

Chrome-Tin Stains Vary Ox. 1%-5% Use in glazes with calcium. There should be no zinc in the glaze.

Copper Vary Red. 0.5%-5% Best in high alkaline glazes.

Crimson

Copper + Titanium 8-10 Red. 1%-5% Try various blends of copper (1%-5%) and titanium (2%-5%).

Calcium- 010-05 Ox. 0.5-5% Best with special frits.

Indian Red

Iron Vary Both 5%-10% Best in high calcium glazes; small amount of bone ash helps. Tin addition up to 5% also helps. Also works well in ash glazes.

Brick Red

Iron Vary Both 5%-10% Similar to Indian Red. Tin to 2% helps.

Orange-Brown

Iron + Rutile Vary Both 1%-10% Various mixtures (up to 8% iron and 2% rutile) in most glaze bases.

Iron + Tin Vary Both 1%-5% Various mixtures (up to 4% iron and 1% tin) in most glaze bases. Creamier than iron with rutile.

Orange-Red

Cadmium- 012-05 Ox. 1%-4% Best with special frits such as Ferro 3548 or 3278 or both. Helps to opacify with zirconium.

Orange

Iron Vary Both 1%-5% Use in tin or titanium opacified glazes.

Rutile Vary Both 5%-15% Many glaze types, particularly alkaline. More suc-cessful in oxidation.

Copper 8-10 Both 1%-3% Use in high alumina or magnesia glazes. Addition of up to 5% rutile sometimes helps.

Orange-Yellow

Iron Vary Both 2%-5% With tin or titanium opacified glazes.

Rutile Vary Ox. 1%-10% Best with alkaline glazes.

Yellow Ocher

Iron Vary Both 1%-10% Use in high barium, strontium or zinc glazes.

Iron + Tin Vary Ox. 1%-5% Various mixtures (up to 3.5% iron and 1.5% tin) in many glaze bases.

Iron + Rutile Vary Both 1%-5% Various mixtures (up to 2.5% iron and 2.5% rutile) in many glaze bases.

Vanadium- Vary Ox. 5%-10%Various mixtures in many Zirconium stain glaze bases.

Lemon Yellow

Praseodymium Stains Vary Both 1%-10% Good in most glazes. Best in oxidation.

Pale/Cream Yellow

Iron + Tin Vary Both 2%-5% Various mixtures (up to 3.5% iron and 1.5% tin) in high barium, strontium or zinc glazes. Titanium opacification helps.

Vanadium Vary Both 2%-5% Use in tin-opacified glazes.

Rutile + Tin Vary Ox. 2%-5% Various mixtures (up to 2.5% iron and 2% tin) in variety of glaze bases. Titanium opacification helps.

Selenium Stains

Zirconian Stains

Selenium Stains

note: colors bars are for visual reference only, and do not represent actual colors.


yellow-Green to navy BlueThe cool side of the glaze spectrum (from yellow-green to navy blue) is considerably easier, both to produce and work with, than the warm. In the main, colorants that control this range create far few-er problems than almost any of the red, orange, and yellow range. Some are temperature and atmosphere sensitive, but that’s nothing compared to the idiosyncrasies possible with warm colors.

Yellow Green

Copper + Rutile Vary Both 2%-10% Various mixtures in a wide variety of glazes, particu-larly those high in alkaline materials. Almost any yel-low glaze to which copper is added will produce yellow green.

Chromium Vary Both 0.5%-3% In yellow glazes without tin or zinc.

Chromium 4-8 Ox. 0.25%-1% In saturated barium glazes.

Chromium 018-015 Ox. 0-2% In high alkaline glazes with no tin.

Cobalt Vary Both 0-1% In any yellow glazes.

Light Green

Copper Vary Ox. 0-2.5% In various glazes except those high in barium or mag-nesium. Best in glazes opacified with tin or titanium.

Cobalt Vary Both 0-2% In glazes opacified with titanium, or containing rutile.

Apple Green

Chromium Vary Both 0-2% In various glazes without zinc or tin. Good in al-kaline glazes with zirconium opacifiers. Also use potassium dichromate.

Copper 1%-2% See Light Green; use in non-opacified glazes.

Celadon Green

Iron Vary Red 0.5%-2% Best with high sodium, calcium or potassium glazes. Do not use with zinc glazes.

Copper Vary Ox. 0.5%-2% Good in a wide range of glazes.

Grass Green

Copper 010-2 Ox. 1%-5% In high lead glazes; sometimes with boron.

Chromium 018-04 Ox. 1%-2% In high alkaline glazes.

Olive Green

Nickel Vary Both 1%-5% In high magnesia glazes; matt to shiny olive green.

Iron Vary Red. 3%-5% In high calcium and alkalines, usually clear glazes.

Hooker’s Green

Copper + Cobalt Vary Ox. 2%-5% In a wide variety of glaze bases.

Cobalt + Vary Both 2%-5% In a wide variety of glaze Chromium bases: no zinc or tin. Good opacified with zirconium or titanium.

Chrome Green

Chromium 06-12 Both 2%-5% In most glazes; no zinc or tin.

Dark Green

Copper Vary Ox. 5%-10% Many glaze bases, particularly high barium, strontium, zinc or alkaline with a minimum of 10% kaolin.

Cobalt + Chromium Vary Both 5%-10% Blends of these colorants will give a wide range of dark greens.

Cobalt + Rutile Vary Both 5%-10% Dark greens with blue overtones.

Teal Blue

Cobalt + Rutile Vary Both 1%-5% In a wide variety of glazes.

Cobalt + Chromium Vary Both 1%-5% In most glazes without tin or zinc.


Turquoise

Copper Vary Ox. 1%-10% In high alkaline and barium glazes. Bluish with no clay content; tends toward greenish tint with added clay.

Copper + Rutile Vary Both 1%-5% In high alkaline and barium glazes.

Copper + Tin Vary Ox. 1%-10% In high alkaline and barium glazes; usually opaque.

Light Blue

Nickel Vary Ox. 1%-2% In high zinc or barium glazes.

Rutile Vary Red. 1%-5% In a wide range of glazes; best with low (10% or less) clay content.

Cobalt Vary Both 0.25%-1% Use in most glazes, particularly those opacified with tin. Also use mixed with small amounts of iron.

Celadon Blue

Iron 6-10 Red. 0.25%-1% In high alkaline or calcium clear glazes. Black iron is generally preferable to red iron.

Wedgewood Blue

Cobalt + Iron Vary Both 0.5%-2% In most glazes; small amounts of cobalt with iron, manganese or nickel yield soft blues. Added tin gives pastel blue.

Cobalt + Manganese Vary Both 0.5%-2%

Cobalt + Nickel Vary Both 0.5%-2%

Cobalt 4-10 Both 0.5%-3% In high zinc glazes.

Nickel 4-10 Ox. 1%-3% In high barium/zinc glazes; likely to be crystalline.

Blue Gray

Nickel Vary Ox. 0.5%-5% In high barium/zinc glazes.

Rutile Vary Red. 2%-5% In a wide variety of glazes, particularly high alumina or magnesia recipes.

Cobalt + Manganese Vary Both 0.5%-2% In most opaque glazes.

Cobalt Vary Ox. 0.5%-5% In high zinc glazes.

Ultramarine

Cobalt Vary Both 0.5%-5% In high barium, colemanite, and calcium glazes; no zinc, magnesium or opacification.

Cerulean Blue

Cobalt Vary Both 0.5%-5% In glazes containing cryolite of fluorspar.

Cobalt + Chromium Vary Both 2%-5% In most glazes except those containing zinc or tin.

Prussian Blue

Nickel 6-10 Ox. 5%-10% In high barium/zinc glazes.

Cobalt + Manganese Vary Both 5%-10% In most glaze bases.

Cobalt + Manganese Vary Both 5%-10% In most glazes; for example, cobalt 2%, chromium 2% and manganese 2%.

Navy Blue

Cobalt Vary Both 5%-10% In most glazes except those high in zinc, barium or magnesium.


The colorants known for creating cool hues are copper, chro-mium, nickel, cobalt, iron, and sometimes molybdenum. For variations, some are modified by titanium, rutile, manganese or black stains. The usual three variables of glaze makeup, tempera-ture, and atmosphere still control the outcome, though it is less obvious in this range.

This article was excerpted from Ceramic Spectrum by Robin Hopper and published by The American Ceramic Society.


Feldspar: The Potter’s Pet Rockby Mimi Obstler

Feldspars are minerals of varying composition commonly used by potters. Feldspars form a glassy, white surface when fired high enough. They have a very long range, they begin melting at cone 4 and continue fusing beyond cone 10. They also tend to stiffen a glaze due to their high alumina content.

In ceramics there are two basic categories of feldspars: potash feld-spars, in which the primary melting oxide is potassium, and soda feld-spars in which the primary melter oxide is sodium. Soda and potash have the highest thermal expansion and contraction rate of all the ceramic melter oxides, they promote color brilliance and luster at most firing temperatures, and they encourage specific color results.

Soda feldspars melt at a higher temperature than the potash feld-spars; however, the actual flow of the soda feldspar, once it begins, is more fluid and less viscous than its potash feldspar counterpart. Hence, after a firing, the soda feldspar displays a shinier and more melted surface. Both potash and soda melters create a glaze with low surface tension, which means they flow freely over the surface of the clay form. A glaze batch of high surface tension crawls or beads up on itself, leaving bare patches of exposed clay body. Potash has a slightly lower surface tension than soda and has the lowest surface tension of all materials used in ceramics.

Potash FeldsparsThe presence of potash feldspar in a glaze or clay body has a more re-fractory effect on the ceramic surface compared to equivalent amounts of soda feldspar. Although potash feldspar actually begins its melt at a lower temperature than soda feldspar, once the melt begins, the forma-tion of leucite crystals causes a slower and more viscous flow.

Soda FeldsparsKona F-4 and NC-4 feldspars contain a fair amount of potassium oxide, and their total sodium content is not as high as the total con-tent of potassium in potash feldspars. These feldspars are hybrids that incorporate some qualities of both potash and soda feldspars. This is especially evident when they’re compared to stronger sodium materials, such as nepheline syenite. Hence, it’s often possible to substitute some soda feldspars for potash feldspars without causing a dramatic surface change.

nepheline SyeniteNepheline syenite is a low-silica, high-soda, high-alumina mineral re-ferred to as a feldspathic rock. It is available in various particle sizes ranging from coarse to very fine. The fluxing power and shrinkage rate of nepheline syenite depends on the grade number. The finest grades (A400 and 700) have the greatest melting power and shrink-age rate and are used in electrical porcelain and by manufacturers of ceramic wares. Grade A270 has a medium melting and shrink-age rate and is the most commonly used form of nepheline syenite in ceramic studios and schools. Grades A40–A200 (used by glass manufacturers) are the coarsest grades and produce the lowest melt-ing and shrinkage rates.

Cornwall StoneCornwall stone, also a feldspathic rock, contains more silica and less melter oxides than do the feldspars. Since silica has a high melt-ing point, Cornwall stone has a higher melting temperature than the feldspars and appears stiffer and less melted when fired alone

Feldspar rocks and test potsBarbara Beck mixed a glaze consisting of 90% feldspar, 10% whiting, 0.5% red iron oxide and varied the feldspar in each batch to show variation in surface and color. The stoneware pots shown were fired to cone 9–10 in reduction. From left to right the glaze used contains potash feldspar, Cornwall stone, and soda feldspar, with nepheline syenite in the rear. The rocks shown left to right are soda felspar, potash feldspar, nepheline syenite, and Cornwall stone.


to stoneware temperatures. This is especially apparent at the lower stoneware temperatures. Even the potash feldspars show more fu-sion at the cone 5-6 oxidation firing temperatures than does Corn-wall stone, so this would not be a first choice as a glaze core at these firing temperatures unless a stiffer surface is desired.

SubstitutingWhen recipes call for an uncommon or extinct feldspar, substitu-tions are possible, but you may need to make adjustments to other components in the glaze, such as clay, silica, and/or one of the fluxes. In many tests, substituting a soda feldspar for the potash feldspar caused little change in the glaze surface. Of all the glaze cores, potash and soda feldspars produced the least difference when substituted for each other; and can often be interchanged without causing drastic changes in the glaze surface.

Potash feldspars can usually substitute for each other in most glaze formulas without producing major changes in surface, provided the silica and alumina content are not too different. The difference in the feldspars’ silica content can be compensated for by adding or removing silica from the glaze formula. In any case, before making large-scale substitutions, compare the oxide structure of both feld-spars and recompute the percentage oxide analysis of the glaze with the substituted feldspar.

Substituting nepheline syenite for a feldspar or Cornwall stone appreciably lowers the firing temperature of the glaze or clay body.

A comparison of the quantities of potash feldspar and nepheline syenite necessary to produce an equivalent melt shows that ap-proximately 25% more potash feldspar is required to produce the same fluxing activity obtained with nepheline syenite. Despite its increased melting action, the lower silica and higher alumina content of nepheline syenite can cause a gloss surface to become more matt.

When using Cornwall stone as a substitute for a soda or potash feldspar in a glaze formula, it may drastically alter the glaze surface. It brings an increased amount of silica and less melter oxides to the glaze combination. This substitution can raise the firing tempera-tures of the glaze, with the result that a formerly shiny and glassy surface may appear more opaque and more matt.

Although substituting Cornwall stone for a feldspar lowers the total melter content of the glaze formula, the large amount of auxil-iary melters already present in the glaze may suffice to melt the addi-tional silica brought in by the Cornwall stone. The glaze’s color may change if Cornwall stone is substituted for a feldspar; it contains more iron oxide and other impurities than do the feldspars. These impurities, together with the increased silica content, can produce distinctive color differences, especially in high iron glazes.

Potash and soda feldspars, nepheline syenite, and Cornwall stone provide the basis for the bulk of stoneware and porcelain glaze surfaces.

This article was excerpted from Out of the Earth, Into the Fire, by Mimi Obstler and published by The American Ceramic Society.


In the Bucket: The Key to Consistent Glazesby Richard A. Eppler with Mimi Obstler

After chemical formulation of a glaze, the three most impor-tant factors for success are application, application, and applica-tion. The flow characteristics (how it sprays, brushes, dips, etc.) are what govern application, but the flow of material cannot be understood through a simple measurement of viscosity, because there are far too many variables at work. This excerpt is meant to be an overview of those variables that are most important in studio ceramics.

It is rare for any slip or glaze—that is any mixture of ground frit and raw materials suspended in water—to be usable as is, particularly with any degree of reproducibility. The rheological properties (viscosity, thixotropy, etc.) of the mixture are influ-enced by the particle sizes and shapes of the various compo-nents, and these flow properties can change with time.

Control is needed over the thickness of coating and evenness of application of a glaze. Hence, additions of rheology modifiers

are required to control sedimentation (settling out over time), improve wetting properties on and bonding to the ware body, control drying time, prevent drying cracks, and improve green strength. Command of these properties provides the means to control the application process and, therefore, final results.

Most additives often influence more than one of the rheologi-cal properties. These properties are also somewhat dependent on details of the water and other raw materials, the person, the processing equipment (mixers, mills, spray guns, etc.), and the methods used in preparation and application. Thus, several trials are usually required to find a suitable combination of additives for a given glaze and application procedure at a particular loca-tion. However, once you have a suitable combination of addi-tives that works for you, that combination of additives will be usable in almost all the glazes you prepare, apply, and fire by the same procedure.


ViscosityThe property of most immediate concern is the viscosity (resis-tance to flow) of the slip. The viscosity of simple liquids, such as water, used in making ceramic slips is said to be Newtonian. If we push a fluid down a pipe or a channel, a stress develops between the moving fluid and the stationary container through which it is flowing. When we say that a liquid is Newtonian, we mean that the stress is proportional to how fast the fluid is moving. The constant of proportionality is the viscosity. Thus, viscosity indicates the re-sistance to flow due to friction between the molecules of the fluid and its stationary container.

Viscosity measurement of ceramic slips can be performed sev-eral ways. The most widely used instrument in industry for ceram-ic suspensions is the variable-speed rotating cylinder viscometer. However, an indication of viscosity can be obtained from a simple flow test. Apply a level teaspoon of glaze slip to a non-absorbing substrate (e.g., a glass panel). For comparative purposes the appli-cations should be arranged in a row along one side of the panel. The glass panel is then raised to an incline of 45 degrees. Once the glaze flows have stopped moving, the lengths of flow are mea-sured. The greater the length, the lower the viscosity.

Initially, at low concentration of solid particles, the effect of add-ing solid particles to a liquid is merely to gradually increase the vis-cosity, as the liquid media has to flow around the solid particles. At a concentration of solids above 5–10 percent by volume, the solid particles begin to interfere with each other. They become entangled in each other as the flow rate (stirring) is increased. Thus, the viscos-ity increases as the flow rate increases. This is called dilatant flow, and it basically means that the faster you stir, the more viscous a flu-id becomes. Dilatant flow is characteristic of large particles. Either polymer additives or agglomerates of ceramic particles, especially clays, can produce dilatant flow. This is an additional reason (beyond avoiding crawling defects) for limiting clay additions to the amount needed for suspension of the solid particles, as dilatant flow is often undesirable in a glaze.

Binders Though some glaze compositions high in clay content can be easily handled in the green state, most can easily be damaged in preparing the ware for firing. The binder acts as a temporary ce-ment that holds the glaze particles on the surface until firing. The binder must be strong enough to permit handling of the ware in the dried-but-not-fired state, but soft and pliable enough to ac-commodate the drying shrinkage without cracking off the ware. Thus, lower-viscosity grades are preferred for glaze hardening. It is almost never possible to use the same grade of binder for glazes as is used for binding a ceramic body, where the high-viscosity grades are more effective.

The amount of binder added can range up to 3 percent, but 0.5 percent is typical. Excessive amounts make the coating brittle and introduce shrinkage upon firing. Therefore, one should add the least amount of binder that will permit handling without difficulty. The ideal binder burns away freely below 400°C (752°F) without ash, and doesn’t cause shrinkage or disruption of the coating.

defining the termsrheology: The science of how a liquid responds to force. For our purposes this typically means whether a glaze sticks on a surface, drips off or runs off in sheets, and whether it smooths out or shows brush strokes. Flocculants and deflocculants modify glaze rheology.

Specific Surface Area: The sum of the surface areas of all the particles in a given weight of a powder, typically given in square meters per gram. Smaller particles have more surface area per unit weight than larger particles.

Viscosity: A material’s resistance to flow. The thickness of a liquid due to internal friction.

thixotropy: The property of a material that enables it to thicken to a gel in a relatively short time upon standing, but upon agitation or manipulation to become softer or more fluid. Many high-solid glazes exhibit thixotropy. Generally considered detrimental, but can assist in application, permitting the glaze to flow over the ware and smooth out, then quickly set up and remain in place during drying.

Specific Gravity: Ceramic slurries contain solid particles suspended in fluid. The density of a slurry (concentration of solid particles) is often expressed in terms of specific gravity, which is a comparison between the density of water and the density of the slurry. To determine specific gravity, weigh a given volume of water (be sure to subtract the weight of the vessel) in grams. Then weigh the same volume of slurry. Divide the slurry weight by the water weight to get specific gravity. For the majority of ceramic slurries, the number will be between 1 and 2 (1 represents the density of water, and 2 would be twice the density of water).

Set: The ability of a suspension to adhere to a vertical surface and not run off.

Binder: Natural gum or synthetic polymer added to glaze mixtures to increase the durability of the raw, dry glaze coating in order to protect it during the production process.

deflocculant: An electrolyte added to a slurry of water and clay or glaze solids to establish equal dispersion of the particles in the mixture, resulting in a lower amount of water needed to suspend the same amount of solids.

Flocculant: Compound added to a slurry of water and clay or glaze solids to encourage formation of loosely bonded aggregates of particles (called flocs). Often used to counteract inadvertent deflocculation caused by soluble materials in a mixture. This is important in controlling application thickness and consistency.


Both natural gums and synthetic polymers (plastics) are used as binders, sometimes mixed, but the most commonly used bind-ers are cellulose ethers (water-soluble derivatives of cellulose; carboxymethyl cellulose—trade name CMC). They are chosen as coating hardeners because their properties are more consistent than natural gums and starches. In a glaze of stable flocculation (i.e., a stable arrangement of the suspended particles in the slip), there is an improvement in the stability of the viscosity. Hence, coatings of consistent and controllable thickness can be applied by various techniques. Drying shrinkage is also predictable.

The physical properties of the binder are affected by tem-perature, pH, and the presence of electrolytes and preservatives. Vigorous stirring permits solution of the binder in cold water. Usually a 10 percent solution is made up, from which additions up to 1 percent are made to the coating slip, or to the water being used to make the slip. Mechanical stress and heat can degrade the binders, but small quantities (up to 5–10 gallons of slip) can be milled or mixed without damaging the binders.

There are side effects to using binders that cannot be ignored. CMC, although a preferred binder, also acts as a deflocculant in most glazes (deflocculants are discussed on page 11). Because they are organic, cellulose solutions, if kept more than a couple days, glazes containing organic binders require protection against bio-logical and mold attack.

Natural gums are carbohydrate polymers of high molecular weight. Gum tragacanth is a natural hydrophilic (water loving) gum found on a bush in much of Asia. It is only partially soluble in wa-ter, where it swells to form first a gel (a viscous jelly-like product), and then a sol (a liquid colloidal dispersion). These sols have low surface tension and are useful as coating stabilizers.

Starch has wide application in industry as a thickener, extender (i.e., a filler), and adhesive. However, the large amount of ash re-maining after firing is a major limitation to its use in ceramics.

There is a range of polyvinyl alcohol compounds (PVA) that are efficient binders. Low-molecular-weight versions disperse more readily and are necessary for glaze applications. Additions of up to 1 percent to the glaze produce tough, coherent layers. Wetting agents (soaps) improve the use of polyvinyl alcohol. Other possi-bilities include alginates (seaweed), water-soluble acrylics, and resin emulsions (a stable mixture of resins).

Surface AreaThe most important factor in glaze suspension is the amount of surface area of the glaze solids. Frit, silica, feldspar, and whiting are relatively coarse glaze ingredients, on the order of 45 microns (45 millionths of a meter) in diameter. These coarse materials have about one square meter of surface area per gram of material. Kaolins and ball clays, in contrast, are much smaller and finer, roughly 1 to 10 microns in diameter, with a surface area of 15 to 30 square meters per gram. They also tend to be plate-shaped, which gives them a lot of surface area for their weight. Bentonite and Veegum® are less than one micron in diameter, and may have 30 to 100 square meters of surface area per gram of material. Just like dust in air, fine clay particles tend to settle extremely slowly in water if they ever settle at all. While chemically inert, these powerful glaze suspenders prevent coarser glaze materials from settling out. A good starting point for testing glaze recipes is to use a minimum of 10% kaolin or ball clay or 1% bentonite. Use care in dispersing and hydrating fine clays to prevent them from clumping. Don’t use too much clay; it can make a glaze suspend well, but dry too slowly.

A flocculated glaze will settle out in a layer of particles that is less dense than a flocculated glaze, making it easier to remix. It will also result in a glaze layer on ware that is less dense, possibly requiring multiple applications in order to achieve the desired results.


Glaze AdditivesADDiTiVES THAT WORK ON THE pHySiCAl pROpERTiES OF THE GlAzE SlURRy

Material Function % used

Kaolin Suspender. increases bisque strength. 0–10.00

Ball clay Strong suspender. increases bisque strength. 0–10.00

Bentonite, Hectorite Strong suspender. five times stronger than other clays.

causes thixotropy. 0–2.00

colloidal Silica aids suspension. improves gloss and acid resistance. 0–2.00

ADDiTiVES THAT WORK ON THE SURFACE CHEMiSTRy OF THE GlAzE SlURRy pARTiClES

Material Function % used

Water-Soluble cellulose (cellulose ether) Binder. Hardens bisque bodies. Reduces handling damage. 0–1.00

Gum tragacanth Binder. Hardens bisque bodies. Reduces handling damage. 0–0.25

Polyvinyl alcohols Strong binder. Greatly hardens bisque bodies.

Prevents handling damage. 0–0.25

tetrasodium Pyrophosphate Strong deflocculant. Rapidly decreases set.

use with care. 0–0.25

Sodium tripolyphosphate deflocculant. decreases set. 0–0.25

Sodium Metaphosphate deflocculant. decreases set. 0–0.25

Sodium nitrite Widely used deflocculant. increases set. 0–0.50

Borax Slurry stabilizer. increases set. 0–0.50

Sodium aluminate Strongly increases set. 0–0.50

ammonium Hydroxide Weak deflocculant. for alkali-free systems. 0–1.00

Sodium or Potassium carbonate increases set. aids other deflocculants. 0–0.50

Sodium Silicate (n brand) Strong deflocculant. often used with sodium carbonate.

also has binding characteristics. 0–0.20

Potassium chloride increases set. Brightens whites.

difficult to avoid defects. 0–0.50

urea Reduces tearing. add just before use. 0–0.25

calcium chloride long-time flocculant. 0–0.25

Magnesium Sulfate (epsom Salts) long-time flocculant. 0–0.25

calcium Sulfate longest-time flocculant. 0–0.25

alum Short-time flocculant. 0–0.25

ammonium chloride Short-time flocculant. Suitable for alkali-free systems. 0–0.25

deflocculantsIn a glaze mixture, the solid particles can either be individually dis-persed or agglomerated into flocs (loosely bonded aggregates of particles). Stokes Law shows that heavier particles, or agglomer-ates, settle out much faster than small particles. Therefore, control of the dispersion of the particles in a mixture is critical.

This control is achieved by adding materials that are called de-flocculating electrolytes, or deflocculants. Their action in the sus-pension may be compared to magnets, having a north and south pole, or a positive and negative charge. For example, when dis-solved in water, sodium nitrite has a positive charge on the sodium and a negative charge on the nitrite. Clay in suspension carries a negative charge. As a result, the positively charged sodium will

adhere to the clay particle surface. This charged clay, with sodium ion, will in turn attract water, forming a three-part sphere known as a clay micelle. Instead of a small clay particle moving about freely in water, there is now a much more bulky shape of lower density, which cannot move with the same freedom as the origi-nal clay particle. Hence, the slurry becomes able to suspend larger quantities of the heavier frit and other solid particles.

Though all deflocculants work the same way, they vary in ef-fectiveness and in the balance between improving suspension of the solids in the liquid and altering viscosity (resistance to flow), or set (the ability of a suspension to adhere to a vertical surface, and not run off). Thus, some of the milder agents may increase set, whereas the overall effect of some strong agents is to produce a free-flowing suspension of lower viscosity.


Deflocculants and Glaze FluidityThe image at the upper left shows a glaze batch with just enough water to bind it together. in this case, it is 35% water by weight. This result is a very soft ball that will gradually slump when placed on a flat surface. The addition of several drops of deflocculant (sodium silicate) made it possible for the mixture to become fluid to the point that the surface of the mixture became flat immediately after stirring. The amount of deflocculant and water necessary will vary by recipe, and will depend largely on the specific materials used in the glaze batch.

Water was then added to a second batch of the same glaze, at 5% increments (by weight). The glaze was thoroughly mixed with each addition, until the same level of fluidity was reached in the bucket. For this recipe, 60% water by weight was necessary to achieve a flat surface in the bucket immediately after mixing.

This does not mean, however, that the batches are the same—far from it. Because the deflocculated glaze has far less water, it can coat a vitreous surface with a thick layer of glaze, while the non-deflocculated glaze will only build up a thin coat and will not stay put. in addition to requiring far less water to create a slurry, sodium silicate acts as a binder and improves the ability of the glaze to adhere to itself.

Most of the time, we are not glazing vitreous surfaces in studio practice, so what would be the point of deflocculating a glaze? The answer is application, especially if you are looking for a thicker layer of glaze. increasing the fluidity of a glaze while decreasing the water content allows a thicker coating of solids to be deposited on the ware surface (whether vitreous or bisque). While a glaze slurry can be mixed thicker using less water than is typical (like the one with 50% water by weight on the left) in order to adhere to a vitreous surface or produce a thicker layer, it will still contain considerably more water than the deflocculated glaze and is therefore more likely to shrink and crack off the surface or cause crawling problems.

Some glaze recipes will deflocculate over time as the batch sits in the bucket, because of soluble alkalis in the recipe. This can cause the glaze to settle to a hard cement-like layer on the bottom of the bucket (see illustration on page 48). it can also result in the glaze particles not remaining in suspension and an application that is too thin to be practical, which will cause a glaze to look like the one on the left even when applied to bisque. you will want to counteract this with a flocculant (see the chart on page 10). So even if you are not deflocculating a glaze on purpose, being aware of why it happens and what it looks like will help you correct it.

35% water

50% water

60% water

35% water plus deflocculant


The amount of deflocculant needed depends upon the viscosity of the slip needed for the application process you have chosen (dip-ping, spraying, brushing), plus the desire to maximize the amount of solids applied to the ware. The higher the density of application, the less the amount of water to be removed in drying, and the less the glaze will shrink upon firing. See the example on page 11.

FlocculantsFlocculants are less often used in industry, but are widely used in the studio. The figure on page 9 shows that flocs (loosely bonded aggregates of particles) settle to less dense coatings. Hence, floc-culants can be used to control coating density. Second, and more important, ions (charged particles) can be leached from most glaze materials, given sufficient time. These ions tend to be the alkalis (soda and potash) that thin the slip to a viscosity below that needed for application. This often occurs during storage. Flocculants can counteract this trend and restore a glaze to the viscosity needed for the application process. Flocculants are generally very powerful, so they are used in very small quantities, from 0.005 to 0.1 percent. For specific types and amounts, refer to the chart on page 10.

Suspension AgentsIf a glaze is to be applied from an aqueous slip, its formulation must include an amount of colloidal material (material with plate-like shape, and particle size less than 1 micron) that provides the means to suspend the other heavier-than-water components in the slip. The most common suspending agent is clay. Clays come in three general classes: kaolins, ball clays, and montmorillonites.

Kaolins are white burning and are comparatively pure kaolin-ite. They are moderately powerful suspending agents. They find use primarily in white and light-colored coatings, where the im-purities in ball clays cannot be tolerated.

Ball clays are less pure, often containing substantial free silica and/or micas in addition to kaolinite. Many contain substantial concentrations of iron oxide and titania, and are thus darker burn-ing. Therefore, they can alter the color and color purity of the

glaze. Since 70–80 percent of their total particles are less than 1 micron, they are more powerful suspending agents than kaolins.

Clay additions (either kaolin or ball clay, or some combination) up to 12 percent by weight are often used. If no other suspending agents are used, at least 3 percent by weight of clay is required. However, excessive clay additions are to be avoided! Too much clay will cause excessive glaze shrinkage on drying, leading to crawling defects.

Bentonite and hectorite are the names given to a class of mont-morillonite clays that have higher-than-normal water content and very fine particle size. They are somewhat difficult to disperse in water, but once dispersed they collect a very high water concentra-tion around the particles, forming strong gels that are up to five times more effective in suspension power than normal clay. As a result, they are effective at concentrations of 0.5 to 2.0 percent, well below the 10 percent or more used with more conventional clays. Bentonite is particularly useful with a fully fritted glaze, which can be formulated in many cases with 99 percent frit and 1 percent ben-tonite. Unlike organic agents, bentonite does not degrade due to bacterial action. Combinations of 0.5 to 1.0 percent bentonite with 3–7 percent kaolinite or ball clay are also possible.

Most of the materials previously discussed as binders also have some suspending power and can be considered as suspending agents as well. Similarly, the various clays have some binding capa-bility, particularly when present in large quantity.

An additional complication to flow behavior is time dependence. Some materials, catsup for example, require force to get moving, but little or no force to keep moving. If the flow rate in such ma-terials is reversed, the material does not immediately require force to continue moving. It continues to flow for a while with little or no external force. This time dependence is called thixotropy. It’s a fascinating phenomenon—but it’s a different article.

This article was excerpted from Understanding Glazes, by Rich-ard A. Eppler with Mimi Obstler, published by the American Ce-ramic Society. For further information and other resources, go to http://ceramics.org/acers-bookstore/whitewares-glazes/.


Kiln Firing Chart

temperature °c °f 1400 2552

1300 2372

1200 2192

1100 2012

1000 1832

900 1652

800 1472

700 1292

600 1112

500 932

400 752

300 572

200 392

100 212

cone(approx.)

141312111098765432010203

04

05

06070809010011012013014015016

017

018

019

020

021

022

incandescence

Brilliant white

White

Yellow-white

Yellow

Yellow-orange

orange

Red-orange

cherry red

dull red

dark red

dull red glow

Black

event end of porcelain range.

end of stoneware range.

end of earthenware (red clay) range.

1100–1200˚c: Mullite and cristobalite (two types of silica) form as clay

begins to convert to glass. Particles start melting together to form crystals,

and materials shrink as they become more dense. Soaking (holding the

end temperature) increases the amount of fused material and the amount

of chemical action between the fluxes and the more refractory materials.

800–900˚c: the beginning of sintering, the stage where clay particles be-

gin to cement themselves together to create a hard material called bisque.

300–800˚c: carbonaceous materials (impurities in the clay along with

paper, wax, etc.) burn out. the kiln requires ample air during this stage

since after 800˚c sintering begins and the clay surface begins to seal off,

trapping unburned materials and sulfides, which can cause bloating and

black coring.

573˚c: Quartz inversion occurs where the quartz crystals change from an

alpha (a) structure to a beta (b) structure. the inversion is reversed on cool-

ing. this conversion creates stressses in the clay so temperature changes

must be slow to avoid cracking the work.

Between 480–700ºc chemical water (“water smoke”) is driven off.

upon cooling, cristobalite, a crystalline form of silica found in all clay bod-

ies, shrinks suddenly at 220ºc. fast cooling at this temperature causes

ware to crack.

Water boils and converts to steam at 100ºc. trapped water causes clay

to explode so keep the kiln below 100ºc until all water has evaporated.

Firing converts ceramic work from weak greenware into a strong, durable permanent form. As the temperature in a kiln rises, many changes take place at different temperatures and understanding what happens during the firing can help you avoid prob-lems with a variety of clay and glaze faults related to firing.

This article was excerpted from 2010 Buyers Guide to Ceramic Arts and published by The American Ceramic Society.


Heatwork describes the measurement of changes that have been effected on clay and glaze. It is a function of a combination of effects including temperature, duration of firing, kiln atmosphere, volume and mass within the kiln, and volatiles in the kiln. Under-standing how heatwork helps measure the progress of a firing, and also understanding the limitations of the concept of heatwork, is important to achieving successful kiln firings.

Measuring HeatworkWhy, if a perfectly good thermocouple is installed in a kiln, would an artist also want to put pyrometric cones in a firing? What do those cones accomplish? Why use them? Each cone within a tem-perature range has a separate chemical and physical composition designed to permit the cone to bend over under the force of grav-ity at a particular temperature when heated at a specific rate.

For practical purposes, a pyrometric cone is simply a sophisticated blend of finely ground glaze ingredients. As those ingredients begin to melt, the force of gravity lets individual particles slide past each other allowing the cone to bend in a reliable, predictable manner.

This deformation of the cone due to grain boundary slip occurs because part of the cone melts to a glass phase. The relatively weak liquid glass is not strong enough to keep the cone erect any longer so the cone, which is manufactured with a slight lean, begins to bend. This bending does not begin at a specific temperature, but rather at a combination of temperature and time.

to Soak or not to Soak?A soak is a period of time in a kiln firing during which temperature is held constant. Some processes in a firing are temperature depen-dent while others are time dependent. Achieving a uniform tem-perature in ceramic ware is a time dependent process. A soak may be used at any point in a firing to reduce the difference between the surface and internal temperature of ware in a kiln.

Temperature dependent processes include organic burn off, driving off chemically bound water, mullite formation, and silica melt. A soak at any of the different temperatures critical to these processes may be useful. Additionally, once a glaze is melted, it will become less viscous (flow easily), as temperature increases. The viscosity of a molten glaze is temperature dependent. A soak may be used to hold the viscosity of a melted glaze constant while time passes so that the glaze may flow or smooth out. This type of soak is often used to improve the final surface appearance of a glaze. In kilns for which the atmosphere is controlled, different degrees of oxidation and/or reduction may be used during the soak to further influence the appearance of the glaze surface.

How does one soak without overfiring? Once a particular cone is down, and temperature is held constant for 15 minutes, the next numbered cone will go down. So, adding a 15 minute soak at peak temperature will have the effect of increasing the firing by one cone. This means, of course, one should start the soak before the

Heatworkby Dave Finkelnburg

Assuming an initial heating rate of 300°F/hour up to 1725°F, the graph above shows that cone 6 will drop at very different temperatures depending on the heating rate during the final hours of a firing. At a rate of 270°F/hour the cone drops at 2269°F after 2 hours (red line); at 108°F/hour the cone drops at 2232°F after 3 hours (green line); and at 27°F/hour, the cone drops at 2165°F after 3.5 hours (blue line). Data: Edward Orton Jr. Ceramic Foundation.

CONE 6 TIME/TEMP VARIATIONS

5 6 7 8 91700°F

1800°F

1900°F

2000°F

Hours

2100°F

2200°F

2300°F

2 4 6 8

The area below the line represents this firing’s total heatwork, a combination of time and temperature. If one of these factors is increased, the other is decreased to maintain the same heatwork. Note that heatwork continues into the cooling part of the firing.

0

500

1000

1500

2000 TOTAL HEATWORK OF A FIRING

Hours

2000°F

1500°F

1000°F

500°F

0

maximum desired cone drops, using the above rule of thumb to estimate when to begin soaking. Adjust the kiln burners or digital controller to slow or arrest the temperature rise. By the end of the soak, the target cone should be fully bent to a 90° angle.

Because an infinite combination of kiln heating rates and times can occur in any kiln, a cone can only be expected to bend at a spe-cific temperature if it is heated at precisely the rate prescribed by the cone manufacturer. Thus, given the variability of firings, a cone cannot be considered a scientific measure of temperature. Rather, it can be inferred from the appearance of the cone that a particular combination of time and temperature has been reached in the kiln.

This article was excerpted from Ceramics Monthly by Dave Finkelburg and published by The American Ceramic Society.


left and below: don’t you hate this? this kiln wash chip melted into the glaze could have been prevented.

The Many Layers of Kiln Washby John Britt

Some people might think that kiln wash is the place where you take your car kiln to get it cleaned. Well, that may be a good idea for a lot of kilns I have seen, but kiln wash is really a necessary and valuable tool for potters. It protects kiln shelves from glaze runs, drips and other accidents that occur in red hot kilns, like pots that tip over, bloating or melting clay bodies, etc. It is also used to protect shelves from volatiles in atmospheric kilns like wood ash or sodium oxide in salt and soda kilns.

Most potters don’t give it a second thought and grab any recipe or just use anything that is in the bucket labeled “kiln wash.” How-ever, in order to make a good kiln wash you need to select materi-als that have very high melting points and that, when combined, do not create eutectics that cause melting. Knowing a bit about the properties of materials and the principles of kiln wash allows you to choose the ingredients that make the best kiln wash for your specific situation and avoid costly problems.

Kiln wash is used in the full range of ceramics firing from cone 022 to cone 14 and every where in between. The type of kiln wash


needed varies for each specific situation because some potters work in electric kilns at low-fire temperatures, while others work with fuel-fired kilns at very high temperatures.

Understanding the structure of a glaze is helpful when selecting or creating kiln wash recipes so you can understand how not to create a glaze on your kiln shelf. Very simply, a glaze is composed of a glass-former (silica), a flux (sodium, potassium, lithium, cal-cium, barium, magnesium, zinc, boron or lead oxide) and a refrac-tory (alumina, usually sourced from clay/kaolin). Historically, what potters did was to leave out the flux in their glaze recipe to make their kiln wash. That meant that only silica and alumina (kaolin/clay) were used as the kiln wash.

One of the first kiln wash recipes I used was:

BaSic Kiln WaSH EPK Kaolin . . . . . . . . . . . . . . . . . . . . . . 50 % Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 50 %

This means you use 50 grams of silica and 50 grams of kaolin. In everyday practice, potters rushing to load a kiln, often just use a scoop of kaolin and a scoop of silica. This is not technically accurate because silica weighs more than kaolin, but it is close enough to work.

Silicon dioxide has a melting point of 3100°F (1710°C) and alu-mina (aluminum oxide) has a melting point of 3722°F (2050°C). Since potters fire to temperatures between 1100°F (593°C) and 2400°F (1315°C) a mixture of these two materials will not melt, will not form a eutectic, and will protect the kiln shelves. (The source of alumina in kiln wash is often kaolin, but it can also be alumina hydrate or alumina oxide. The source of silicon dioxide is usually 200 mesh silica.)

This is a good kiln wash for low and midrange electric firings. The only problem is that it contains silica, which is a glass-former. So, if a lot of glaze drips onto the shelf, it can melt the silica in the kiln wash and form a glaze on the shelf. Also, when you scrape your shelves to clean them, you create a lot of silica dust, which is a known carcinogen. So using silica in your kiln wash is not always the best choice.

Another drawback of this recipe is that, if it is used in salt or soda firings, it will most certainly create a glaze on the shelf. This is because silica, as noted above, is a glass-former. When sodium oxide, which is a strong flux, is introduced atmospherically, it can easily melt the silica in the kiln wash into a glass. This is why silica should not be used in a kiln wash recipe for wood, salt or soda kilns.

For these types of firings this kiln wash is better:

BaSic Salt Kiln WaSH Alumina Hydrate . . . . . . . . . . . . . . . . . 50 % EPK Kaolin . . . . . . . . . . . . . . . . . . . . . 50 %

Kaolin has a melting point of 3218°F (1770°C) and alumina, which has an even higher melting point, will not melt, even in a cone 10–13 firing. These ingredients are called refractory because they are resistant to high temperatures. The refractory industry, which includes bricks, kiln shelves, posts, etc., relies heavily on these materials.

This kiln wash recipe can be used at all temperatures and in all kiln atmospheres. It can also be used as a wadding recipe to set the pieces on in wood, salt and soda kilns. Just mix it up thicker than the kiln wash—like bread dough—and roll it into wads. The recipe can also be adjusted to 60% kaolin and 40% alumina hy-drate, which produces similar results but costs less. Since alumina hydrate costs about $1.44 a pound and EPK kaolin costs $0.32 a pound, tilting the recipe toward EPK kaolin quickly reduces the price. Another high-temperature wadding recipe that is cheaper is:

SHane’S Wood firinG WaddinG Alumina Hydrate . . . . . . . . . . . . . . . . . 5 Fireclay . . . . . . . . . . . . . . . . . . . . . . . 16 Sand . . . . . . . . . . . . . . . . . . . . . . . . . 16 Sawdust to taste (Parts can be measured by scoops, or cups, etc.)

This wadding is easily removed from the bottoms of pots be-cause the sawdust burns out and the wad becomes very fragile and smashes easily with pliers or small hammer. Once you understand the principle of kiln wash you can easily substitute other refrac-tories like zirconium oxide (or Zirconia, ZrO2), zircon (ZrSiO4 a.k.a zirconium silicate, zircon flour, or Zircopax), kyanite, sand, fireclay, ball clay or calcined clay to make variations in your wash recipe if you have problems (See recipes on page 18).

For example, a common problem with kaolin-based kiln washes is that they crack off the shelf. The reason for this is that clay has the physical property of shrinkage. When you put it on the shelf, it looks really uniform and smooth, but then as it dries it cracks like Texas soil in the summer sun. After several firings, you typically just scrape off the glaze drips and the pieces that have chipped up, apply more kiln wash to hide that firing’s issues; and then that new layer cracks and the crevasses just keep getting worse. This can cause your pots to crack when they get hung up on the uneven wash during periods of expansion/contraction. Or, when using


Most potters apply kiln wash with some

kind of brush. if you are coating the whole

shelf, use a 4- or 5-inch house-painting brush,

but if you are touching up bare spots after

scraping off glaze drips, use a small 1–inch

glaze brush and just dab it on in the spots

that need it. if you use a brush, work very

fast because the shelf will suck up the wash

as soon as the brush touches it, making areas

of uneven thickness.

Mix up the wash about as thick as heavy

cream and paint on several thick layers to

protect your shelves, allowing each to stiffen

before applying the next coat. then clean

the edges with a wet sponge. Some potters

leave a bare ½–inch or ¼–inch band at the

edge of the shelf so that chips don’t fall onto

the shelf below.

if you have a lot of shelves to kiln wash all

at once, one of the best and fastest ways is

to use a spray gun. lay out all of your shelves

in a row and coat them all very quickly and

evenly. depending on your spray gun, you

may need to adjust the nozzle spray pattern

and the thickness of the wash to get it to spray

properly, but once you get that figured out

you will be very happy with the consistency of

the results. Any overspray on the sides of the

shelves can be wiped off with a damp sponge.

if you don’t have a spray gun, another

excellent method of coating the whole shelf

is to use a paint roller with a short nap length.

Just fill the rolling pan with kiln wash and roll

on the wash for a smooth, even coat. Allow

it to get tacky to the touch and then apply

another one or two coats, depending on the

thickness desired.

At my studio, i have a lot of students

working and testing glazes, so the shelves get

really beat up and have a lot of glaze drips.

once or twice a year i grind my shelves clean

and re-apply the wash. Since i don’t have a

spray gun, i prefer to use a roller because it

gives a smooth even coat very quickly.

Kiln wash should be mixed to the consistency of heavy cream.

work quickly when brushing kiln wash on new mullite shelves.

rolling kiln wash on new mullite shelves produces an even thickness.

Kiln WaSH aPPlication


porcelain, the foot can even become warped and uneven as it flux-es and conforms to the uneven surface of the shelf. Another more insidious problem with cracked kiln wash is that the turbulence created by the burners blows some of the kiln wash chips up into the air and they inevitably land in your favorite bowl, ruining it.

The best way to avoid this is to calcine the kaolin or buy calcined kaolin called Glomax. You can calcine kaolin by putting some in a bisque bowl and firing it to red heat (or just put it in with your bisque firing.) Calcining will eliminate the physical property of shrinkage but leave the chemical refractory properties of kaolin intact. What you have made is very fine ceramic grog. So you can adjust your kiln wash recipes by substituting half the kaolin with calcined kaolin or Glomax.

no cracK Kiln WaSH Alumina Hydrate . . . . . . . . . . . . . 50 % Calcined EPK Kaolin . . . . . . . . . . 25 EPK Kaolin . . . . . . . . . . . . . . . . . 25 100 %

You can add more calcined kaolin—like 35%—if you want. You just want to keep enough kaolin in the recipe to suspend the other materials so that it goes on smoothly and doesn’t settle out.

I discovered a small refinement of this recipe after visiting the Homer Laughlin China Company in Newell, West Virginia. There, the Chief Ceramic Engineer told me that, because they have high air turbulence in their kilns, he adds approximately 1% feldspar to help “stick” the kiln wash together. They determined how much feldspar to add by trying to rub it off with their finger after the fir-ing. If it rubbed off, then there wasn’t enough flux. More flux was then added until it took a fingernail to scrape it off. If it took a key or screwdriver blade to scrape it off, there was too much flux. So, the recipe then becomes:

SuPer aWeSoMe no cracK Kiln WaSH Alumina Hydrate . . . . . . . . . . . . . 50 % Calcined EPK Kaolin . . . . . . . . . . 25 EPK Kaolin . . . . . . . . . . . . . . . . . 25 100 % Add: G-200 Feldspar . . . . . . . . . 1–2 %

Although it seems crazy to add flux to a kiln wash, this very small amount actually is just enough for the kiln wash to stick it lightly to itself and to the shelf, preventing the kiln wash chips from flying around the kiln and getting onto pots.

As you can see in the recipes on page 18, there are many kiln wash variations. However, it is essential to know the melting prop-erties of ingredients to make sure that they don’t melt on your shelf. For example, zirconium oxide is a refractory and melts at 4892°F (2700°C) and zirconium silicate, which goes under various names like Zircopax, Ultrox, Superpax, milled zircon, zircon flour, etc., has a melting point of 4622°F (2550°C). So these can make excellent additions to a kiln wash recipe. The only drawback is that zirconium silicates can cost from $1.33 to $3.00 a pound, depend-ing on the amount you buy.

To illustrate the wide variety, some potters just dust alumina hy-drate on their shelves to protect them, while some wood firing potters use 100% silica and wall paper paste to make a very thick (½-inch) coating that protects their shelves from excessive ash deposits. Still others, who have the new advanced nitride-bonded silicon carbide shelves, don’t even use kiln wash at all because the glaze drips shiver off when the shelves cool. Other potters, who are very neat and don’t share their space with others, may not even use kiln wash so that they can flip the shelves after every firing to prevent warping.

Kiln wash is such a ubiquitous material in the ceramics studio that we take it for granted. Potter’s make a significant investment in their kiln shelves but rarely take more than a few minutes to mix up two scoops of kaolin and alumina to protect them. They also spend countless hours making and perfecting their work only to suffer unnecessary breakage and loss of pots because they just don’t know that a kiln wash doesn’t have to crack or fly off into the bottoms of pots. There are many kiln wash recipes to choose from and many solutions to common kiln wash problems if we just take the time to learn about the materials we use.

The article was excerpted from Ceramics Monthly by John Britt and published by The American Ceramic Society. The author John Britt lives in Bakersville, North Carolina and is author of The Complete Book of High-Fire Glazes: Glazing and Firing at Cone 10. See www.johnbrittpottery.com.


barium carbonate BaCO3—alkaline earth—active high tempera-ture flux, but also promotes matt glaze surface. Unsafe for low-fire functional glazes. Often used as additive in clay bodies in very small percentages to render sulfates insoluble, reducing scum-ming.

bentonite Al2O3•5SiO2•7H2O—formed from decomposition of airborne volcanic ash. Suspension agent used in quantities no more than 3% of dry materials weight.

bone ash (calcium phosphate) Ca3(PO4)2—high temperature flux—opacifier in low temperature glazes—translucence in high temperature glazes.

borax (sodium tetraborate) Na2O•2B2O3•10H2O—a major low temperature alkaline flux, available in granular or powdered form. Gives smooth finish, bright colors. Water soluble, so often used in fritted form.

chrome oxide Cr2O3—standard vivid green colorant—often soft-ened with a little iron or manganese. Very refractory. With tin produces pink.

cobalt carbonate CoCO3—standard blue colorant for slips and glazes—5% will give dark blue in glaze or slip. Will cause crawling if used raw for underglaze brushwork.

copper carbonate CuCO3—a major glaze colorant to produce greens in low temperature and high temperature, copper reds in high temperature reduction, and greens and metallic effects in raku.

dolomite MgCO3•CaCO3—high temperature alkaline earth flux, promotes hard, durable surfaces and recrystallization/matting in glazes.

feldspar High temperature alkaline fluxes—insoluble aluminum sili-cates of potassium, sodium, calcium, and/or lithium—inexpen-sive flux for glaze.

frit Fluxes that have been melted to a glass, cooled, and ground in order to stabilize soluble and/or toxic components during han-dling of unfired material.

ilmenite An iron ore with significant titanium—most often used in granular form to produce dark specks in clay or glaze. Higher iron concentration than in rutile.

iron oxide, red (ferric oxide) Fe2O3—refractory red in oxidation, converts to black iron (flux) in reduction and/or high-fire. Low quantities in clear glaze produces celadon green—high quantities produce temmoku black or saturated iron red—powerful flux.

kaolin; china clay Al2O3•2SiO2•2H2O—very refractory white pri-mary clay. Source of alumina in glazes.

lithium carbonate Li2CO3—powerful all temperature alkaline flux, especially with soda or potash feldspars. Promotes hardness and recrystallization in low temp glazes.

magnesium carbonate MgCO3—alkaline earth—high temperature flux, promotes mattness and opacity in low temperature glazes, smooth, hard, buttery surface in high temperature glazes—pro-motes purples/pinks with cobalt. Used to promote controlled crawl glaze effects.

manganese dioxide MnO2—flexible colorant—with alkaline fluxes gives purple and red colors—by itself gives soft yellow-brown—with cobalt gives black. Used with iron to color basalt bodies. Concentrations of more than 5% may promote blistering.

nepheline syenite K2O•3Na2O•4Al2O3•9SiO2—a common feld-spathic flux, high in both soda and potash. Less silica than soda feldspars, and therefore more powerful. Increases firing range of low-fire and mid-range glazes.

rutile Source of titanium dioxide, contains iron, other trace miner-als—gives tan color, promotes crystallization giving mottled multi color effects in some high temperature glazes, or in overglaze stain.

silica (silicon dioxide, flint, quartz) SiO2—main glass-former—vitrification, fluidity, transparency/opacity controlled by adding fluxes and/or refractories.

spodumene Li2O•Al2O3•4SiO2—lithium feldspar—powerful high temp alkaline flux, promotes copper blues, good for thermal-shock bodies and matching glazes.

strontium carbonate SrCO3—alkaline earth, high temperature flux, similar to barium, slightly more powerful—gives semi-matt sur-faces. Nontoxic in balanced glaze.

talc 3MgO•4SiO2•H2O—high temperature alkaline earth flux in glaze, promotes smooth buttery surfaces, partial opacity—similar composition to clay.

tin oxide SnO2—most powerful opacifier, but expensive—inert dispersoid in glaze melt—5–7% produces opaque white in a clear glaze.

titanium dioxide TiO2—matting/opacifying agent. Promotes crys-tal growth, visual texture in glazes.

whiting (calcium carbonate, limestone) CaCO3—alkaline earth, contributing calcium oxide to glaze—powerful all temperature flux—major high temperature flux for glazes—gives strong du-rable glass.

wollastonite (calcium silicate) CaSiO3—In some cases, it is used in place of whiting.

zinc oxide ZnO—high temperature flux that promotes brilliant glossy surfaces. Can encourage opacity, with titanium in low-alu-mina glaze can encourage macrocrystalline growth.

zirconium silicate ZrSiO4—zircon opacifier—low-cost substitute for tin oxide—use double the recipe weight of tin. Includes Zir-copax, Opax, Superpax, Ultrox.

Glossary of Common Ceramic Raw Materialsby Vince pitelka

Excerpted from Clay: A Studio Handbook by Vince pitelka.


Primary Function of Common Ceramic Raw materialsBarium Carbonate Flux Strontium carbonate Bentonite Suspension agent Ball Clay Do not exceed 3%Bone Ash Opacifier Borax Flux, glassmaker Boron frits Chrome Oxide Colorant GreenCobalt Carbonate Colorant Cobalt oxide BlueCopper Carbonate Colorant Copper oxide Greens, copper redsCornwall Stone Flux, opacifier Custer Feldspar Glaze core Potash feldspar (G-200) Dolomite Flux, opacifier Whiting Many brandsEPK Kaolin alumina, opacity Kaolin Ferro Frit 3110 Glaze core, flux Pemco P-IV05, Fusion F-75 Crystalline glazesFerro Frit 3124 Glaze core, flux F-19, P-311, Hommel 90 Boron fritFerro Frit 3134 Glaze core, flux F-12, P-54, Hommel 14 Boron fritFerro Frit 3195 Glaze core, flux Hommel 90, Fusion F-2 Complete glazeFerro Frit 3269 Flux, glaze core Pemco P-25 Ferro Frit 3278 Flux, glaze core Fusion F-60, Pemco P-830 G-200 Feldspar Glaze core Potash feldspar (Custer) Green Nickel Oxide Colorant Black nickel oxide Blues,. tan, browns, greens, graysKentucky OM4 Ball Clay alumina, opacity Ball Clay Kona F-4 Feldspar Glaze core Soda feldspar Lithium Carbonate Flux Magnesium Carbonate Flux, opacifier Promotes crawlingManganese Dioxide Colorant Purple, red, yellow-brownNepheline Syenite Glaze core Red Iron Oxide Colorant Celadon green to brownRutile Colorant Ilmenite Silica glass former, glaze fit Flint Use 325 meshSpodumene Lithium glaze core Strontium Carbonate Flux Barium carbonate Talc Flux, opacifier Many brandsTin Oxide Opacifier Zircopax Titanium Dioxide Opacifier Whiting Flux, opacifier Wollastonite, Dolomite Many brandsWollastonite Flux, opacifier Whiting, dolomite Wood Ash Glaze core, flux, colorant Whiting Results vary by type.Zinc Oxide Flux, opacifier Zircopax Opacifier Superpax, Ultrox

Material Glaze Function Substitute Comment

Notes:1. Substituting glaze ingredients may alter color, texture, opacity, viscosity, and/or sheen, as well as create pinholing, crazing, black spotting, and/or pitting. in most cases, additional adjustments to other ingredients need to occur when substituting.2. Test and record your results.3. Materials vary from supplier to supplier and batch to batch. This article was excerpted from Out of the Earth, Into the Fire, by

Mimi Obstler and published by The American Ceramic Society.

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