PPP: The Perils and Rewards of Glaze Testing

or How I Learned to Make Glaze Testing Work for Me

Cast and slab-built test tiles.

One way to put ceramic students immediately to sleep is to talk about glaze calculation, glaze testing, or materials science. It happened to me many years ago, in the days before handheld calculators, computers or glaze software. In fact, we used slide rules and the most humble of input methods-the pencil! I did not continue to fall asleep. In fact, glaze testing is an important part of my studio practice.

I guess I was lucky in that my inquisitive nature led me to want to know about ceramic materials and how they melt. High school and college chemistry provided me with a pretty sound understanding of the foundations of materials science, which, of course, is the basis for silicate science.

My first clay and glaze class at Alfred University was an eye opener, especially when it was coupled with hands-on work in the glaze laboratory. While it certainly seems like that was in another world or another century, the basic fundamentals have not changed since then; the means to arrive at the desired results have just become more efficient.

We now have kiln controllers that can provide repeatable firings, whether in electric or fuel-burning kilns. Glaze calculation software can show us the relationships of materials on an oxide level and calculate important ratios and coefficients of expansion. But the basic fundamentals still remain the same: an understanding of ceramic materials and what oxides they provide, and how these oxides function in a glaze when subjected to heat. The numbers really become meaningless unless we have a basic comprehension of the building blocks of a glaze (or really, a glass).

This short article is not intended to be a primer on glaze calculation or silicate science. Rather, its aim is to provide a methodology that makes glaze testing approachable, even easy.

The mixed glaze is screened into a small plastic cup.

Glaze Testing System

I have honed a logical and repeatable system over many years that provides concise and researchable results for years after the initial testing is completed. My glaze testing system comprises four components: test tiles; glaze selection; glaze mixing, application and coding; and firing.

Test Tiles

I work with two clay bodies: one is a plastic throwing and hand-building mixture, and the other is a casting clay body. I make test tiles from both. It takes a few days but provides a good quantity that will last for many months. Since I make work that is both slip-cast and thrown/hand-built, I want to see how the glaze looks on each clay body.

The easiest method of making test tiles from a plastic clay body is to throw a low semi-bottomless cylinder and angle the walls inward. Use a serrated rib to texture a small section as a last step in throwing the low cylinders. Once these have dried slightly, use a potter's knife to cut the cylinder into small tiles.

You can also roll a slab and cut out small rectangles. Or, you can pour a smooth plaster slab with raised sides full of casting slip. After it sets up so that it can flipped over, cut the large slab into smaller workable tiles.

The test tile should provide a host of information, not only about the fired glaze but about how it sits on the clay body. By texturing the tile, you can see how the glaze pools in thick and thin areas. Even more information can be gleaned when you include a bead of white or black engobe.

It may be useful to put a hole in the top of each tile so you can hang or otherwise display them for reference. The tiles need to be fired vertically, so the low-cut cylinders are perfect for showing how a glaze flows. I usually fire my cast tiles vertically by supporting them from behind with a small piece of kiln furniture.

Kaplan's glaze lab in Denver.

Glaze Selection

Thousands of glaze recipes/formulas exist in books, on the Internet and in your own glaze notebooks. I'm sure that most potters, after gaining a degree of fluency, end up using the same perhaps 6-12 glazes. While this is certainly acceptable, adding a few test tiles to each glaze firing might just expand the repertoire of glaze possibilities and result in a small surprise that makes your day!

I look for glazes that have something of interest in the formula/recipe. Depending on the atmosphere, one might look for glazes that contain oxides such as strontium, lithium, barium, boron or others. I place a high value on these materials, as my current firing (cone 6/7 in an oxidizing atmosphere with a cooling cycle) may promote crystal growth. I also look for glazes that have a high calcium content, since calcium promotes brightness in a glaze. These are just simple criteria that work for me and my studio practice; there are certainly many more to choose from.

After I find a few glazes that spark my interest, I enter them into Hyperglaze and look at some of the oxide relationships in the unity molecular formula. If I wish to alter or change the formula, I use Insight Ceramic Software.

Mixing, Application and Coding

Hundreds of books on glazes are available and can provide a great deal of information. Some of those that I use are Robin Hopper's The Ceramic Spectrum; The Potter's Palette by Christine Constant and Steve Ogden; and, perhaps the quintessential reference, Clay and Glazes for the Potter by Daniel Rhodes. These texts include enough material for a lifetime of glaze testing.

First, I dry-mix 2000 g of glaze base into a plastic bag. Then I fold the top of the bag over a few times, shake it and transfer it from hand to hand, changing its orientation many times so that the powders are well dispersed. I now have enough base glaze for 8-10 200-g tests.

I use 200 g for each test because that amount provides enough material to enable the tiles to be dipped or sprayed, and there is enough left over for additional testing. Also, from a statistics point of view, the margin of error when using a 200-g sample when weighing out smaller amounts of coloring materials is much less.

Select two base glazes that have several different materials in different amounts, and mix up 2000 g of base. Perhaps hundreds-if not thousands-of coloring oxide and stain combinations can be added to a base glaze. For simplicity, it might be best to select combinations of coloring oxides; they can provide a more interesting result than using just one material.

Pick a selection of colorant additions that are of interest to you. For the sake of brevity for your first tests, select four combinations of differing percentages of iron oxide, rutile, cobalt oxide or carbonate, and manganese dioxide. You will add these to each of the two base glazes that you have mixed to make four tests for each glaze base.

Weigh out 200 g of base material and add the first combination of coloring oxides. In your notebook, write down the base formula and the four combinations of coloring oxides that you have selected. Add this first combination to 220 ml of water. Remember to always add dry materials to water, not in reverse. It is easier to mix this way.

Using an inexpensive fountain drink mixer, mix the glaze and water together for a few seconds until well dispersed. Then, using a small 80-mesh test sieve (from your ceramic supplier), screen the mixed glaze into a small plastic cup. Adjust the water content, if necessary.

You will make eight tests, four of one glaze base and four of the second glaze base. Using a marker, label the cups so you know that you have four tests of base A and four tests of glaze B (see coding instructions below).

Take your test tile, dip it into the test glaze and count slowly to 10. This should deposit about .12 mm of glaze onto the tile. If you are using a flat cast clay tile, use a fettling knife and then a sponge to remove the glaze from the back of the tile. (Remember that any glaze touching the kiln shelf or support furniture will fuse the tile to the furniture.)

Tools of the trade.

Mix some black stain or iron oxide with a very small amount of ball clay and feldspar to make a fluxable marking compound. A commercial black underglaze can also be used. On the reverse of the test tile (the surface that you just removed the glaze from), code/label the tile as follows:

mm/dd/yyyy (Date) #nn (Test Number)

For example, 02132011 #01 would refer to test number one on February 13, 2011. Then write this code/label number into your glaze notebook next to the glaze formula and oxide addition so you can then cross-reference the fired tile with your test work.

It's also possible to test the combinations of these tests. Do a simple volumetric line blend with one test of one base with another test of the other base. Try them in the following percentages: 75A:25B, 50A:50B and finally 25A:75B. You will then have 16 tests. (You have already done the 100A:0B and 0A:100B.)

With additional work, you could run a quad-axial blend. Or take three glazes and run a simple tri-axial blend. There are instructions in many books on how to run these combinations. (Volumetric tests are easily done using 1/8- or 1/4-cup measuring devices, combining the wet materials, mixing, and applying to a bisque tile.)

Try combining all of the cups together. You will then have a slop glaze for a specific testing sequence. Combine all the cups into a bucket and date that bucket. That is your day's slop glaze. Dip a test tile. You just might have the hottest glaze since sliced bread. But you will only have a limited quantity of this special blend!

You have already mixed "shop glazes" that you use in your studio practice on a regular basis; it is very easy to run a wet volumetric line blend on any two glazes to see what happens. Measure out 1/4 cup of each of two glazes and combine in a small cup. Or take equal parts of however many glazes you wish and combine them. Keep accurate records! Mix thoroughly and apply to a test tile and fire. This finishes your testing regimen, unless you have selected other glazes to test.

The loaded kiln is ready to fire.


I have a Nabertherm Top 25 kiln with a Bartlett CF6 controller. The kiln is round, 1.58 cu ft and nicely insulated. I set one shelf in the middle of the kiln and load the test tiles vertically, with small 1-in.-square pieces of kiln furniture to support the tile from behind. If I am at all concerned with the glazes running, I have some small setter discs that I use to place the tiles.

I program the controller for three segments, as follows:
  • 100°F/hour to 2200°F, with a hold of 15 minutes
  • 250°F/hour to 2080°F, with a soak of 10 minutes = cone 6
  • 150°F/hour to 1500°F
Note that this end-point is specific to my kiln and for the firing of one shelf of test tiles supported by small pieces of kiln furniture. I use Orton standard size three-hole cone plaques that hold two witness cones and the firing cone. The last segment is a cooling segment to encourage crystal formation.

When I see the fired results and find a glaze that may be of interest for further testing, I mix up a 1000-g batch of glaze, put it on actual pieces and then fire. I also layer the test glaze with other glazes to see the result. I both dip and spray glazes. Many interesting surfaces can result by layering glazes with these two methods.

It's important to keep in mind that, due to the influence of thermal mass, glazes will look different on a test tile after being fired in a small test kiln vs. fired in a regular-sized kiln on actual ware. When I want to put the new glaze into use-after all of the testing is done and the results are repeatable-I will mix a 5000-g batch, which is approximately 3 gal of liquid glaze.

Accurate record keeping is essential.

Thinking Ahead

Yes, glaze testing is ponderous and time consuming. It can try the patience of the best of us, but it should be an essential part of any ceramic studio practice to always be proactive and think ahead. Customers' color preferences change from year to year, and we may become bored with a glaze we have been using for a long time. Or maybe our new work just requires something different.

Beyond using commercially prepared glazes, glaze testing encourages us to understand our materials and how they work with each other when heated. Your work is important; it's important that your work look its best, don't you think?

About the Author

Jonathan Kaplan has enjoyed a 40-year career as a professional potter, ceramic artist, designer, manufacturer, author and educator. He developed the ceramics program at Colorado Mountain College in Steamboat Springs, Colo., where he also operated Ceramic Design Group, a contract design and manufacturing business. In 2006, Kaplan moved from the Colorado high country to Denver to reinvent himself as a ceramic artist and kick-start his studio practice. Concurrent with this move, he established Plinth Gallery in Denver's River North Art District. In addition to Pottery Production Practices, his writings have appeared in Ceramics Monthly, Pottery Making Illustrated, and Ceramics Technical. He can be reached at jonathan@plinthgallery.com.


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