Intro to Ceramics/Clay

• Clay Makes up 75% of the earth’s land mass!!

3 Properties that make up clay

1. Plasticity (ability to form)

2. Porosity (ability to hold moisture)

*Similar to a sponge

3. Vitrify (ability to harden and keep its shape)

3 types of clay

Clay is characterized by it’s iron content (color), vitrification process, and firing temperature.

Earthenware

-low fire temperature (1800 degrees), red in color, often used to make Mexican pottery and flowerpots.

Stoneware-medium fire temperature (up to 2200-2300 degrees), tan or buff in color, often used to make everyday dished, mugs, etc.

Porcelain-high fire temperature, white in color, often used to make dolls, China dished, toilets, sinks

Wedging clay

-removes the air bubbles-prepares and aligns the platelets in the clay-develops a uniform texture-----When building objects with clay, it is important not to trap air inside the clay. This will cause the piece to explode in the kiln.

3 steps to attach clayScore- lightly put in hatch marks on both pieces to be attached

Slip- (liquid clay) add this muddy substance to both of pieces ofclay you are attaching

Weld- put the two pieces together and blend, it is sometimes necessary to add an additional coil to the seam to make it stronger

Greenware

- Any clay that has not been fired in the kiln is considered greenware

3 stages of greenware

Workable- clay is very moist and pliable

Leatherhard- clay is still somewhat moist and easily be carved but breaks easily when bent

Bone dry- clay is lighter in color and warm to the touch

Clay shrinks 5-15 % when it dries and fires.

Cracking and Warping--Your clay piece can crack or warp when it dries.1. Uneven moisture2. Uneven drying3. Uneven thickness4. Drying too quickly

Machines Kiln

Pottery Wheel

Pug-mill

ToolsBat Needle Tool

Loop, Carving, trimming Wire cutter

Firing Pottery

Kiln - oven like machine that “fires” (bakes) the potteryWood FireElectric Fire

Gas Fire

Pit fire

Raku fire

Firing Pottery cont…Bisque Firing- 1st firing, about 1750-1800 degrees. When it comes out of the kiln it is called bisqueware

Glaze Firing- 2nd firing, anywhere from 1800-2300 degrees. When it comes out it is called glazeware.

--It takes about 2 days to go through a full firing cycle.--Clay looses it’s plasticity after it has been fired, and can NOT be (recycled) or reused at this point.

5 stages in the firing cycle

1. Water smoking- occurs when the temperature in the kiln reaches 212˚F, all remaining surface water is burned off, enough steam is often generated to be seen. Hence the term water smoking. Water boils and turns to steam at 212˚F. If the steam leaves the clay too fast, the pot explodes just like if you boil eggs too fast, they crack.

2. Dehydration- 660˚F, the chemical water, starts to burn off, by 950˚F the clay is completely dehydrated. It is now a chemically different material than it was when put into the kiln. Now it is aluminum silicate known as mullite. The change is non reversible.

3. Quartz Inversion - 1000°F. The quartz crystals in the clay undergo a change called the quartz inversion. The crystals grow and change in shape.

4. Oxidation - the burning off of organic materials occurs at 1600°F to 1700°F. This has no chemical effect on the clay, but it does leave the clay more porous.

5. Vitrification - Partial vitrification begins as the temperature rises. This will be determined by the type of clay body. Clay with a high alumina content (stoneware and porcelain) will vitrify more slowly and at a higher temperature than clay high in fluxes such as iron or talc (earthenware). Vitrification, you will remember, is melting of the clay platelets. Complete vitrification results in a glasslike material; therefore, clay ware is never completely vitrified.

Glazing and Finishing

Glaze- glass like coating put on pottery to make it waterproof, melts when fired, hardens as it coolsGlaze application techniques:1. Dipping

2. Spraying

3. Pouring/dripping

• Brushing/painting

…a little art history• The Chinese discovered wood ash floating through the kiln and when it landed on thepottery it created a shinysurface (but isn’t considered real gaze)

• The Egyptians were the first to use true glazes since about 5000BC. They dug their clay from the Nile river and used sand from the desert to make glaze.

Ingredients of glaze

• Glass former – (silica or sand) creates the glossy surface, silica forms glass all by itself. However, silica melts at about 3100⁰ F, which is much too hot for ceramic kilns. As such, it cannot be used on its own.

• Flux – aids melting, keeps surface of pottery from oxidizing so the glaze can attach, lowers the melting point of the silica.

• Refractory (alumina) – resists melting, aids in high temperature firing, stiffening agent.

Glazing Tips

1. Keep a “dry foot”. (no glaze on the bottom) Why???

2. Use a pencil (regular #2, not a mechanical) to draw a thick line around the bottom of the glaze edge. Why??

3. Glaze “takes” best on the first firing. Why??

4. Dip glazes once, (unless it’s thin enough, or you are crossing colors). Why??5. Brush 2 or 3 layers. Apply coats as soon as the sheen of the coat before isdisappearing. Why??

..Other ways to finish your piece

• Burnish- “to polish byrubbing to a high sheen” Often with pottery, a slip is applied to the surface of the clay when it is in the late leather-hard stage then polished with a smooth rock or spoon.

• Sgraffito- a colored slip is applied and often burnishedto the pottery surface then a design is etched or carved out of the applied slip.

…other ways to finish your piece cont...

underglaze

Photo transfer

Slip trailing Glaze transfers