In today’s post, our own Holly Goring not only includes some versatile three-ingredient base glaze recipes, but she also gives simple straightforward explanations of the chemistry behind them. If you have always wanted to experiment with your own glazes, but didn’t know where to start, this post is just the ticket. And even though these are low fire recipes, you might be inspired to experiment with the ratios of ingredients to come up with higher temperature glazes.- Jennifer Poellot Harnetty, editor.
I’m all about simple when it comes to glazes. First, my studio is small and I don’t want to sacrifice room for a lot of glaze materials. Second I don’t want spend time repeatedly testing glazes when I could be making work. While I enjoy the experimental side of making my own glazes, I want to eventually have consistent glazes that I know will work every time.
With those requirements in mind, I started by defining my basic needs: glossy, matte, and satin base glazes for a cone 04 clay body. They needed to be consistent and stable, and work well with both stain and oxide colorants. They also needed to work well over underglazes and slips.
Starting From Scratch
To determine what to start mixing I began with the fundamentals of what’s in a glaze. In its most basic form, a glaze is composed of a glass former, a flux, and a stabilizer. A flux is any ingredient used to help melt the glaze or to lower the melting point. A stabilizer is any ingredient used to stiffen the glaze and hold it onto the clay body, as well as to extend the melting range of a glaze in some cases. In ceramics this is typically alumina. The glass former is the main ingredient of a glaze and is supplied by silica. When combined in various proportions, these three basic ingredients will yield a wide variety of base glazes. The melting points and ratios of the three basic ingredients to each other determine the firing temperature.
Ratios and Limits
Next I determined the ratios of each component. For a glossy glaze, the silica must be at least 8 to 10 times greater than the alumina. For a satin glaze, the silica is 5 to 8 times greater than the alumina. A glaze needs only 1 to 5 times more silica to alumina to yield a matte surface. The flux ratio balances the above two components. With a dozen or so possible sources of fluxes, many combinations are possible—this is where the real experimenting comes in. Fluxes are the main determiners of texture and how much the glaze will flow on the pot.
Knowing the chemistry and characteristics of your materials is important when deciding what to toss in the bucket. I’m generally not one to sit down and fine tune the science. However, I am willing to do a triaxial blend of three ingredients based on the information above.
I started with EPK kaolin in each of my glazes as my stabilizer, as it supplies a good deal of alumina. It also provides the silica I need for the glass to melt. It’s fairly plastic and is considered the best kaolin for use in suspending glazes.
I added Gerstley borate as a flux in the glossy glaze. It’s a generous contributor of boron, which can decrease crazing and intensify colors. Gerstley borate is a bit controversial among ceramic artists—it has good application and melt properties but can cause glazes to settle over time.
For the flux in the matte glaze, I added whiting (calcium carbonate) which is known to work as a matting agent at low temperatures. It can also harden glazes and prevent pinholing.
I used dolomite for my satin glaze. Dolomite forms crystals upon cooling, and these crystals produce a smooth, silky surface.
I also added a Ferro frit to each glaze. Frits are premelted glazes ground into powder and some are formulated to melt at low-fire temperatures. Frit 3124 is an all purpose frit for developing strong colors. Frit 3195 is also high in boron and is very fluid. Frits add both glass formers and fluxes to the mix.
Finally, colorants were added in small amounts to the base glaze in the form of metal oxides or commercially-made stains. Oxides generally give you a transparent glaze while stains yield an opaque surface. I used a combination of both.
After running a triaxial blend of my three ingredients and narrowing the results, I came upon three suitable recipes to test with colorants.
The glossy glaze recipe is very clear and stable but exhibits a small amount of crazing and could benefit from the addition of more silica. It produces excellent color with oxides and stains while maintaining the white underglaze underneath.
The matte glaze recipe produces a wonderful clear matte. It works well over textured slips and also produces excellent color with oxides and stains. Apply slightly thicker than the average glaze for good coverage.
The satin glaze recipe makes a beautiful buttery, waxy surface. It is opaque and works best when applied thick. Both oxides and stains work well in the recipe.