Many ceramic artists find their studios filling with glazes and materials used for a specific project that they no longer have an interest in. Or they find themselves in a rut, using the same glazes over and over again, but wanting to open their color palette and surfaces to fresh ideas while maintaining a reliable finish.
There are many ways to alter a commercially made glaze even without access to a well-stocked materials room. Also, this type of experimentation can teach you a lot about the chemical properties of each material, giving you a better foundation should you want to transition into mixing your own glazes one day.
Note: Adding ingredients can make a recipe less stable or change its food safety rating. Always test before using on functional ware.
In the Lab
Since you don’t know the dry weights or the exact materials in commercial glazes, measuring by volume is the best approach. Start with cup of wet glaze and add dry materials in 5–10 gram increments. Samples were fired to cone 05 for low-fire glazes and fired to cone 6 for high-fire glazes. The following materials were added to commercial glazes with the following results:
Rutile: Rutile, which contains titanium dioxide and iron oxide, can form small microcrystals, which can result in a floating glaze or rabbit’s-fur look to the surface.
Results: On Mayco’s SW209 Charcoal glaze fired to cone 6, the rutile turned the glaze to a mottled brown color (1). Rutile added to Mayco’s FN-51 Strawberry glaze changed the bright red glaze to an earthy brick red color (2).
Zinc Oxide: Zinc can increase crystals and cause a matte effect.
Result: Zinc oxide was added to Mayco’s SW132 Mirror Black glaze and fired to cone 6. Mirror black on its own is very glossy and metallic, but with the added zinc, tiny crystals formed, turning the glaze into a beautiful oil spot glaze (3). Lithium: Lithium carbonate is a beautiful flux giving a soft, pearl-like surface.
Note: Use less than 10% due to its low expansion and contraction rate (CTE).
Result: Lithium carbonate added to Mayco’s SW205 Coral glaze, fired to cone 6, overfluxed the glaze and burnt out the stain leading to a clear crackle but I would love this contrast on a bigger piece (4).
Feldspar Flakes: We had some feldspar flakes (commonly used to create a Shigaraki-type effect in clay bodies) and I decided to try those as well to see if I could create little pockets of extra melt.
Result: On AMACO’s PC25 Textured Turquoise glaze fired to cone 6, the feldspar flakes added texture and surface interest without changing the glaze color (5).
Magnesium Carbonate: Magnesium carbonate can make the most interesting crawl or lichen texture glazes.
Result: On AMACO’s C-41 Pear glaze, fired to cone 6, magnesium carbonate only made small white bits appear in the glaze, but gave a mottled effect (6).
Alberta Slip: Alberta Slip is a dark slip clay, and we thought this might give a more muted, or jewel-like appearance to some of the glazes.
Results: On Mayco’s SW136 Weathered Blue glaze fired to cone 6, I couldn’t see much change with the Alberta Slip other than a slightly higher level of crystallization in the glaze. I did notice that the glaze brushed on very silkily with the addition of the Alberta (7). On Mayco’s FN-11 Light Blue glaze, the Alberta Slip gave a delicate speckle like a robin’s egg (8).
Mason Stain Mango: Mango #6030 is a beautiful new orange stain from Mason Color Works. We wanted to add stain to a high-fire glaze that was very neutral to show how dramatic the change could be.
Result: Adding the Mango stain to Mayco’s Matte Maycoshino SW124 and firing it to cone 6 had a very dramatic result. It made a soft, beautiful coral-hued glaze (9).
Note: I didn’t sieve for the test and a slight texture on the surface appeared. I think you could sieve it for a smoother result. Either would be very nice.
The zinc oxide, rutile, and magnesium carbonate tests were successful in all of the colors I tried and I would consider using them in any commercial glaze. Additional chemicals to try might be Zircopax, titanium, and additional commercial stains. This experiment produced useful results and is something I encourage you to do in the future before getting rid of old commercial glazes.
Deanna Ranlett is a frequent contributor to Pottery Making Illustrated. She owns MudFire in Decatur, Georgia, (www.mudfire.com).