Red clay as a clay body is most often called earthenware or terra cotta. It is usually bisque fired to 1742°F (950°C) and glaze fired to a maximum temperature of 2012°F (1100°C). It is a matter of degrees before the clay slumps and eventually melts. This marginal temperature increase is the difference between good pots and ruined shelves. The local red clay I use fires to 1832°F (1000°C) and after that, I would not fire it higher unless I want it to melt. And I do. I want to use it as a dominant glaze material for firing at cone 6 (2232°F (1222°C)).
My work is always fired to cone 6 in oxidation and I build glazes that include 20–25% red clay and mature at cone 6. This article could be very short, because red clay melts well before cone 6. That was easy. However, just firing the clay high enough until it flows does not yield the glaze surface I want for cone 6. It needs to fit my clay body, stay on the pot, and perhaps be colored with oxides for added surface interest.
Test 1: Making Clay Balls
The first idea I had when testing the possibilities of using red clay as a primary glaze ingredient was to observe the melting properties of the clay. I wanted to see how the red clay alone and also combined with typical cone 6 materials holds up in a cone 06 bisque firing (1828°F (998°C)). This is about 212°F (100°C) short of what I would fire the red clay to before it starts to become soft and melt. The following tests were made out of clay balls, which are slightly moistened and pinched together combinations of 50% dry red clay and 50% of another material (EPK kaolin, frit, feldspar, or whiting). EPK kaolin was added for more alumina, whiting was added for calcium and flux, frit was added for boron and flux, and feldspar was added to provide a complete glaze core (flux, glass former, and stabilizer/alumina).
The materials I chose each have an individual influence in a glaze base. The goal is to make a balanced glaze base with simple ingredients, acting in their most obvious ways. The final test was a mix of equal parts of all five materials. Post-firing adjustments to create a suitable glaze should therefore be straightforward. The clay balls were fired to cone 06. Expecting a little melt in some of the tests even at this temperature, I stamped each clay ball so that I could clearly see any signs of melting.
The top row of tests from left to right all fired to a vitrified clay body. They did not adhere to the bisque stoneware clay test bowls they were formed in. The stamped design in all three tests is still high relief and unchanged from the wet-clay stage (1).
The second row of tests in figure 1 gets more interesting because all the tests fused to the test bowls. The frit, acting as a flux and adding boron (bottom left), is by far the most melted, with a visible slumping of the stamped texture. The bottom middle tile with feldspar (a complete glaze core on its own) is melted just enough to round the edges of the stamp. The bottom right tile with the mixture of equal parts of all 5 materials, is fused, but less melted than the other two tests. The addition of EPK kaolin (alumina) has stiffened the mix.
Test 2: Making Slips
Next, the same 50%/50% red clay/material combinations were made into slips that were poured on the next set of test tiles and fired to cone 06.
The slips all adhered to the clay, but resulted in nothing more than dry to very dry smooth slips (2). It is evident, however, that the same observations from the clay balls are true for the slips. The addition of EPK kaolin makes the slip quite dry and the frit test has a slight sheen, melting significantly more than in any of the other tests. The mixture is flat and smooth, but underfired. With these results, I was one step closer to a glaze.
Now, with these tests repeated and fired to cone 6, I have visuals to tell me where to go. These tests were a logical progression, using materials with obvious characteristics. The final glaze will be predominantly red clay but not over saturated, so that the glaze can benefit from the attributes of the other ingredients. A total of 25% red clay and 18.75% each of the other 4 ingredients are added, which makes the base equal 100.
Test 3: Getting Closer to a Glaze
In order to keep the tests going from start to finish in a logical progression, I am still using the five materials I started with. Now, I am stepping up the mix from two ingredients to three. Hoping to get closer to a glaze, I used equal parts of three materials, with the constants in each test being red clay (the hero, or dominant material) and the feldspar (which is a glaze core on its own). The variables are the materials I have left, whiting, kaolin, and frit (4).
In the cone 6 firing of these three test tiles, the variables have an interesting influence on the constants. Calcium oxide has a bleaching effect on the iron content in the red clay (see left tile in figure 4). I have experienced this when decorations made using an iron-oxide wash faded or fully disappeared under clear bases and later learned it occurs with bases containing calcium. In figure 4, the middle tile with kaolin is as expected, underfired but fused, and the right tile with frit is melted. Now, with these cone 6 tests, I have visuals again to tell me where to go to next, as they have all the attributes I want to include in a final glaze.
Final Recipe Base and a Few Color Tests
Checking this mixture in a glaze calculation program, I added some silica for durability. I use the guidelines for stable/durable glazes established by Ron Roy and John Hesselberth in their book Mastering Cone 6 Glazes. With some more adjustments, the glaze base has enough amounts of silica and alumina to meet these criteria. The final recipe is as follows: 25% red clay; 15% feldspar; 15% whiting; 15% Johnson Matthey frit 169 (can substitute Ferro frit 3134); and 15% EPK kaolin. Satisfied with that, I mixed a bigger batch and colored the base with various oxides for more surface interest (5).
A very nice attribute of adding red clay to a glaze recipe is that the resulting glaze slurry clings to the bisque ware evenly and dries quickly. Second and third dips can be made with a minimal pause in between. When the glaze is dry, it is easy to handle and the raw glaze surface is quite stiff, not powdery or sensitive to handling. It makes glazing quick and forgiving.
When testing the addition of red clay to other materials to develop a glaze, it is important to work in a logical progression and record every action and result, so that materials and tests do not become overwhelming. Limit the materials used in the tests to predictable oxides commonly found in recipes formulated for the cone and atmosphere you are firing to. Keep on track with the progression toward a usable glaze by eliminating bad tests and developing the promising ones. Use each set of tests as building blocks for the next set. Limit your framework for testing by including a constant or two, as well as limiting the variables. Making glazes with simple steps is a confidence building exercise, which helps to raise curiosity, an important ingredient for glaze development work.
the author Alisa Liskin Clausen is US born and lives in Southern Denmark. She has a BFA in ceramics from Syracuse University, and has concentrated on cone 6 oxidation glazes since 2000. Her extensive glaze tests are shared on several ceramic forums and databases including the Sankey glaze database, Glazy.org, Flickr, and ceramicrecipes.org.