The truth is, I was a nerdy ceramics undergraduate student. I wanted to learn everything, right away—and I loved my glaze calc class. No, really, I did. I took a ridiculous amount of notes and then put them all in plastic sleeves in a binder. I’m sure I tested every recipe I could find or invent. After discovering commercial underglaze, I was sure I could make that too.
In this post, I am sharing my adventures in homemade underglazes, and my underglaze recipe too! – Holly Goring, Editor, Pottery Making Illustrated. This item originally appeared in the January/February 2012 issue of PMI.
Smooth, silky, perfectly opaque, commercial underglaze is that wonderful substance that coats and colors both greenware and bisque ware with ease (I’ve even seen it work on mature cone 04 earthenware), and without flaws. And, in terms of color, what you see is what comes out of the kiln, no guessing, no hoping. They are dependable as well; covering large areas quickly with smooth and consistent brush painting. They take light-colored, transparent or clear glaze very well without dissolving into the glaze during the firing. Finally, commercial underglaze fires into a hard, unscratchable surface without pinholing or flaking from cone 04 all the way to cone 10.
Today, most commercial underglazes are formulated using frits, which reduce shrinkage, allowing them to be applied to both greenware and bisqueware. They are produced using a colloidal process. A colloid is a substance microscopically dispersed evenly throughout another substance (think mayonnaise or hand cream). Underglaze manufacturers use a chemical process that employs a high-shear mixing technology to create colloids. The substance created does not settle and cannot be separated out by ordinary filtering or centrifuging like those in a typical suspension. This allows for complete integration of all raw materials, including the colorant, during the base mixing stage.
Developing the Recipe
Variations of underglaze recipes were available on the Internet but not much could be found in textbooks, beyond iterations of slips (clay suspended in water, formulated to fit either to wet or dry greenware) and engobes (generally a lower clay content, most often fits greenware and bisque ware ). Few of these recipes encompassed all the characteristics I was looking for—something similar to commercial underglaze. Not too much to ask, right?
I started by choosing a clay and a flux. I needed a fairly heavy clay content for adhesion to the clay body, and an equal amount of flux to lower the melting point of the silica, and to create a hard surface. My clay choices included: EPK kaolin, calcined kaolin, OM-4 ball clay, and talc, all fairly white firing as to not add to the color, and each contributing something different to the adhesion needed. My fluxes consisted of: Ferro frits 3124, 3134, and 3195, a good place to start in terms of readily available frits. Then in a radical move, I ignored all previous instruction and treated the colorant as one of the base ingredients. This allowed for full incorporation of the color with the other two ingredients during the mixing of the base recipe. I used commercial stains in order to get an opaque quality (I later tested variations with oxides that resulted in a somewhat transparent underglaze.)
I mixed 1000 gram batches in a thousand variations (or something close to that). I actually mixed three batches of each recipe to testing gums and suspenders, without which, any substance mixed from the above ingredients would settle to a rock-hard mess and be nearly impossible to brush onto any clay surface. I tested CMC gum (powder, premixed into a liquid), premixed bentonite, and Sta Flo laundry starch (a tip from one of the internet recipes).
Not having the ability to replicate the colloidal process, I sieved and ball milled each recipe in order to fully integrate the raw materials into each other and to reduce the particle sizes as much as possible. (Note: Do not ball mill recipes with zirconium-silicate inclusion stains, as ball milling will destroy the inclusion that renders these stains non-toxic.)
I tested each glaze on leather-hard, bone-dry, and bisqued earthenware and stoneware test tiles. I fired the earthenware tests to cone 04 and the stoneware tests to cone 6 and cone 10, all in an electric kiln.
After many firings and many eliminations, this recipe came very close to replicating commercial underglaze:
Sieve all materials with an 80 mesh sieve and then ball mill for at least 12 hours . Incorporate Sta Flo Laundry Starch until the mixture reaches a thin yogurt consistency and sieve the entire mixture again .
Pros and Cons
The results were good, very good—smooth, creamy, good adherence, versatile at all temperatures, a hard surface, and an intense color. The substance worked well on both clay bodies and in all stages, but was best on bisque ware. Brushability was best with the Sta Flo. I found that too thick of an application caused flaking and pinholing. The lack of sieving and ball milling did the same. If mixed to the correct consistency, one coat was sufficient while two coats often was too much. The colors became muted when fired to cone 10 but still held up in hardness and adhesion. Only small batches could be mixed at a time due to the inclusion of the Sta Flo, which is organic and caused mold to grow in the bucket within a few days. The mold could be skimmed off but added unwanted lumps if it wasn’t all removed. And of course the amount of commercial stain to produce the rich colors similar to commercial underglazes was ultimately very expensive. In the end, mixing this homemade underglaze was a lot of work for a product the manufacturers do just a bit better, faster, and cheaper. However, if I want colors that aren’t available commercially, now I know how to make them.
You can receive great technical information like this along with intermediate to advanced techniques in every issue of Pottery Making Illustrated.
**First published in 2012