Bisque SlipsA Tale of Materials and Application

Liz Ashley Martin

Introduction This is an investigation into the application, chemical composition, and other properties of bisque slips. By nature, bisque slips are intended to be used exclusively in post-bisque application, allowing for better experimentation. Compared to slips that are applied in the greenware stage, bisque slips have a significantly lower specific gravity (1.2), giving them wa-tery, thinner consistency.

TestingIteration 1: Starting from Found Recipes

The six initial recipes:

Helmar Slip (cone 04)Nepheline Syenite 15Grolleg 20Helmar 65+ Veegum 2+ Bentonite 1

Tile 6 Slip (cone 04)Nepheline Syenite 20EPK 20Tile 6 50Silica 10+ Veegum 2+ Bentonite 1

70/30 Tile 6 Slip (cone 04)Nepheline Syenite 30Tile 6 70+ Veegum 2

PSU Bisque Slip (cone 04)EPK 10OM4 Ball Clay 5Grolleg (Kaolin) 15Silica 15Frit 3124 30Borax 5Zircopax 5Talc 10

Pete’s Forgiving White Slip (cone 04)Talc 40Nepheline Syenite 10OM4 Ball Clay 40Silica (Flint) 10

Betty’s Bisque Slip (cone 04)Silica 90Nepheline Syenite 60OM4 Ball Clay 60Zircopax 15Frit 3195 15

Findings:

From the initial stages of testing, I found that I preferred the recipes that contain whiter ball clays such as Grolleg and Tile 6. I also liked the more fluxed surface of the PSU Bisque Slip. I selected Pete’s Forgiving White Slip, 70/30 Tile 6 Slip, and PSU Bisque Slip for further testing.

Iteration 2: Revising the Clay ContentRevisions for 70/30 Tile 6:#2 - Replace Tile 6 with equal amounts of Grolleg#3 - Replace half of Tile 6 with Grolleg (35g)

Revisions for PSU Bisque Slip:#2 - Replace EPK content with Grolleg for a total of 25g#3 - Replace half of the EPK with Grolleg for a total of 20g Grolleg and 5g EPK

Revisions for Pete’s Forgiving White Slip:#2 - Replace OM4 with equal parts Tile 6 (40g)#3 - Replace OM4 with equal parts Grolleg (40g)#4 - Replace OM4 with equal parts EPK(40g)

Findings: I preferred the surface of the 70/30 and Pete’s revisions, and chose to continuing my testing using the Pete’s #3 and 70/30 #3.

Iteration 3: Adding TalcRevisions for 70/30 Tile 6:#4 - Add 10g of Talc to 70/30 #3#5 - Add 20g of Talc to 70/30 #3

Findings: The revision (revision #5) that had the great amount of Talc was easier to brush on in layers. Everything else was fairly similar between the two revisions. I chose to continue my testing using the 70/30 #5 for the next iteration.

Why add Talc?

I add talc because I want to bring the 70/30 and Pete’s recipe closer together in chemical composition. The Pete’s recipes had 40g of talc from the get-go and the 70/30 had none. While doing my initial research, I found that it is common to have a substantial amount of talc in a bisque slip.

Iteration 4: Adding Borax

Findings: I did not like the affect the borax had on either slips. I actually found that it made it harder to brush onto the test tiles, and the resulting test tiles showed signs of crawlings or the beginnings of potential crawling. Therefore, I did not continue with any of the revised recipes from this iteration, and instead, went back to 70/30 #5 from iteration 3.

Revisions for Pete’s Forgiving White Slip:#5 - Add 5g of Borax to Pete’s #3#6 - Add 2.5g of Borax to Pete’s #3

Revisions for 70/30 Tile 6:#6 - Add 5g of Borax to 70/30 #5#7 - Add 2.5 g of Borax to 70/30 #5

Why Borax?

It is common for bisque slips to have up to 5g of Borax. It is a low range flux and “hardener.”

Iteration 5: Fluxes

Findings: While the addition of the Frit helped in all revisions, I immediately eliminated revision #10, which contained Gerstley Borate, due to it’s absorption when coated with Worthington Clear Glaze. For my purposes, I need the bisque slip layer to remain fairly opaque with glaze applicaton. I selected revision #9, which contains Frit 3195, due to it’s brushability.

Revisions for 70/30 Tile 6:#8 - Add 15g of Frit 3124 to 70/30 #5#9 - Add 15g of Frit 3195 to 70/30 #5#10 - Add 15g of Gerstley Borate to 70/30 #5

Why 15 grams? This amount was based off of the Betty’s Bisque Slip recipe. That recipe contains 15g of Frit 3195.

Why Frit 3124, Frit 3195, and Gerstley Borate? I chose these fluxing materials based on their common use in cone 04 glazes.

Iteration 6: Opacifiers

Findings: Both results were beneficial and could be used depending on desired application, but if I were to choose which I wanted for a true white, I would select the revision with Tin oxide. Tin oxide is commonly found in white low-fire glazes.

Revisions for 70/30 Tile 6:#11 - Add 5g of Zircopax to 70/30 #9#12- Add 5g of Tin oxide to 70/30 #9

Iteration 7: ColorantsTriaxial Blend:

Following Jon Brit’s example, I completed a triaxail blend using the following criteria:

70/30 #9:Nepheline Syenite 30Grolleg 25Tile 6 25Talc 20Frit 3195 15Veegum 2

A: Red Iron Oxide (5%)B: Yellow Ochre (5%)C: Cobalt Carbonate (5%)

SURPRISE SURPRISE.....The Cobalt Took Over

From the seven iterations completed, my favorite slips are all made from the base of revision #9 of the 70/30 recipe. The colorants are 5% tin oxide, 5% yellow ochre, 2% red iron oxide and 3% yellow ochre, and 1% red iron oxide, 3% yellow ochre and 1% co-balt carbonate, respectively. Each of the final slips selected had a specific gravity of 1.25-1.26, and had proportional equal weight water to dry mix when hydrated.

The Winner’s Circle

White Bisque Slip (Tin Oxide)

Camel Bisque Slip (Yellow Ochre)

Light Rust Bisque Slip (Yellow Ochre/Red Iron Oxide)

Blue-Grey Bisque Slip(Yellow Ochre/Red Iron

Oxide/Cobalt Carbonate)