Glaze mixing can be daunting to the novice, especially when the recipe contains an ingredient that is unfamiliar or unavailable. But with a few simple melt tests, you can learn a lot about what materials do at your firing range and start making educated guesses as to what might make a good substitute for the unfamiliar ingredient. In this excerpt from her Pottery Making Illustrated article, “Love the Frit You Have,” Deanna Ranlett walks us through the testing she did to find substitutes for some frits. By following her lead, you can figure out substitutes for the materials you might be missing in a glaze!
It’s fundamental that you know what effects your raw materials have when fired to your ideal firing range. To test raw materials, label a test tile for each material, mix a small amount of each material with water, brush it on the tile, and fire it to your ideal cone. Once you know your materials, making educated guesses when mixing glazes becomes more intuitive and you’ll be able to understand what ingredients to use when making changes to your glazes. You’ll see that some materials have matte surfaces when fired, some glossy, and some satin, so you can start to understand each material’s role in a glaze. When trying to replicate a specific glaze, I normally use my materials knowledge to think of at least two potential recipes, and then I do a quick and easy line blend using those two potential recipes.
Weigh out and mix 500 grams of each recipe (this makes a bit extra). Line up 11 cups in a row from left to right. Put 30 grams of glaze A into the cup on the left end and 30 grams of glaze B into the cup on the right end. Next, you’ll fill in the cups in the middle with a mixture of the glazes you’re testing. Each cup will end up with 30 grams of glaze. Starting with the left cup, the progression in each cup is 10% less of glaze A (see chart below); using a round number makes things easier. You already have 30 grams (or 100%) of glaze A in cup #1, so you’ll want 27 grams (or 90%) in cup #2, 24 grams (or 80%) in cup #3, 21 grams (or 70%) in cup #4, and so on. Then work from the right side toward the left side, measuring out glaze B in the same progression. When finished, each cup should have a total of 30 grams of dry base glaze.
For the tests in this article, we focused on three glazes from Developing Glazes: Cu-38 (Copper 38) on p. 100 (figure 1), Co-11 (Cobalt 11) on p. 94 (figure 2), and Ti-19 (Titanium 19) on p. 122 (figure 3).
In addition to lead, the book also uses a soft borosilicate frit—some substitutes are researchable online, but often too many options come up, so my advice is to test the frit you have on hand first before investing in others. Ferro frit 3110 is a great substitute for a soft borosilicate frit as it has a low-melting point, supports great color, and can aid in crystal formation. It can be tricky in glazes for functional ware because it can craze when used in high percentages so some adjustments may be necessary to make the glazes food safe.
Glaze Recipe 1
There are also viable alternatives to some toxic ingredients, like barium. One I regularly use is strontium at 75% of the original barium content. For the Co-11 glaze below, I use a 1:1 ratio substitution of strontium instead of barium for Glaze A. And in Glaze B, I used the recommended 75% substitution but added more frit. I did this to show that the strontium doesn’t melt as easily as the original barium, thus necessitating the reduction in overall percentage. You can see the progression of the melt in the tiles, although the original barium softness is harder to capture—future tests might include using some strontium and some lithium carbonate to gain some of the softness back.
Glaze Recipe 2
For the Cu-38 tests, there is no direct substitution to replace the lead bisilicate frit. This is where the test on raw materials comes in handy. I know that sodium content is often important in color development and that some sodium fluxes have a lower melting point, making them good candidates for substitution. In reviewing the frit composition of lead bisilicate frit 3403, I found it has a very low melting point of 1350°F. This tells me that Ferro frit 3110 is a good candidate but it’s already a large portion of the recipe. One option was to go with increased Ferro frit 3110 and add in nepheline syenite (a powerful feldspathic flux high in sodium content) in Glaze A. And in Glaze B I added borax, which I know to be a low-melt flux common in many glazes. Regarding the frit content, I knew we also needed to add some silica, as frits are manufactured and are compositions that are fused or melted, then quenched to form glass and granulated and powdered. This process is helpful because it renders any soluble and/or toxic components insoluble by causing them to combine with silica and/or other materials. So, note the addition of silica where there was none in the original recipe. The original surface of this glaze was matte but my results are more semi-gloss with great color. The addition of a slow cooling cycle could further crystallize the surface and give even more similarity to the original glaze.
I find that when you are interested and have a little fun with glaze chemistry it doesn’t seem so daunting. Materials substitutions don’t have to be a headache—just make sound educated guesses! And remember, if you can’t have the frit you love—love the frit you have!
Note: All glazes are cone 6 and standard tests need to be run on all of these glazes to ensure they are safe for surfaces that will come in contact with food.
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