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Glaze Without Fear

By Ihada Vishun
February 23 2009


Firing is very costly. Likely to get more so. It is a serious matter, a budget breaker, and you must be very sure you can afford it you before you commit yourself to these expenditures.

Move slowly and carefully. It was very "in" some twenty, thirty years ago to build one’s own kiln! Gas was cheap, wood easily obtained, and there were, as yet, few regulations.

For those with rural property the building of a fuel burning kiln still is feasible. Costly, time consuming, but feasible. In suburban areas and cities permits and the like are required, and a gas company, for instance, may refuse you a big enough tank in a residential area. Check, double check, check again!

"Artistic" defiance of rules and regulations can lead both to tears and lawsuits. So it is only after you have checked all zoning and fire regulations and talked to your insurers--and, if you rent, your landlord--that you go kiln shopping.

Electric kilns --long used all over the worlds--became more popular, and more “respectable” in the US in the past decade .

Manufacturers, models, and features have increased. Again check and double check before you buy.

No matter what size kiln you get: there are several excellent brands and a great many models to choose from . There is a division among kilns: the standard, and the heavier, more insulated kilns. The first are known as "hobby" kilns, although many professional potters use them. The latter are seriously heavy duty kilns often with steel outsides, and "extreme" insulation. Today both kinds come with or without computer controls. The controls have many advantages--but add greatly to the kiln's cost.

A kiln is very much of a "deal breaker". While they last "forever" if treated well, they are very costly, and, it should be added, do NOT benefit from being moved.

It is the cost of the kiln and the fuel to fire it that is the determinant in whether you set up your own studio. Write away for all possible information. Visit as many showrooms as you can. Try to check with schools and potters in your area before you shell out that kind of cash.

1. Buy a test kiln kept for tests, and fire your actual pots in a kiln where you rent space. Many test kilns use regular household current--110v.

2. Buy a medium-sized kiln in which you can fire tests as well as your pots. Medium-sized kilns are best for beginners. You want a kiln you can fire once a month--especially if you are working on glazes and need to fire tests. Such kilns require 220v current, which requires enough electricity coming into the house. That may demand a bigger “entrance” (the place where the electricity comes in). You must check with then utility before you buy the kiln!

3. Get a big kiln. Fire maybe every two months. If you make large pieces a large kiln is your only choice. But you certainly do not want to fire a kiln half empty! Firing half empty not only wastes money, it does not produce as good results. The kiln need not be in your glaze room. It must be in a place where it can be properly vented, and in which it is not a fire hazard. There are different venting systems and to find the right one takes a lot of checking. Do not expect to make your kiln decision and purchase in less time than it takes to buy a car!

Record keeping is the heart of glaze development. No matter what approach you take -- empirical or more scientific--you must keep track of what you are doing, what you did.... and why.

Records can be kept on 3 x 5 cards, in notebooks, on the computer.

Details do not matter. You must set up a system that works for you, allowing you to access, with reasonable speed, glaze recipes and notes that you made last week--or ten years ago.

Everyone develops a personal way of naming and identifying glazes and glaze tests. Any system will do, if kept simple and clear! Upon your record keeping depends your success.

The Public Library can supply you with a great many books on clay. Daniel Rhodes "Clay and Glazes for the Potter" and "Stoneware Glazes" are readily available, and, while outdated as far as health issues go, are well-written, excellent sources of information. 

You need a good current book on safety issues. It should be the first book you buy! We recommend Monona Rossol's "Artist's Complete Health and Safety Guide“, the latest edition.

As your basic texts: "Glazes for the Craft Potter," and "Ceramic Faults and Their Remedies", both by Harry Fraser: clear, exemplary books, that will prove your constant references.

Emmanuel Cooper has several books on glaze and electric firing--but these can be had only on the used book market and are costly. 

Robin Hopper's "The Ceramic Spectrum" is an excellent book, becoming more affordable second-hand. Libraries are likely to have it.

If you plan to fire at c. 6 electric both "Mastering Cone 6 Glazes" by John Hesselberth and Ron Roy, and Michael Bailey's “Glazes Cone 6" are primary texts. They are quite different, both very good.

And "Glazes: Materials, Recipes and Techniques", a collection of Ceramic Monthly articles, edited by Anderson Turner is a wonderful source for all sorts of glazes and excellent advice.

ALWAYS REMEMBER that safety standards and regulations have changed and become stricter over the years. Remember that older books, or older editions of books still in print, or older magazines, may be contain unacceptable recipes, and suggest materials we no longer use. ALWAYS double check recipe contents in a book such as Rossol’s.

1. Read several of these books and decide what you want to do. You may want to test a whole variety of glazes. After all you have read you will know that high magnesium, zinc, nepheline syenite, strontium all affect glaze results differently. You may want to start by testing some glazes in each group.

Or you may want to achieve a certain look. A certain color. And you will need to make tests and more tests to get that look just right.

Likeliest will be that, for starters, you want to check out glaze recipes that fit in with the pots you are making. You "inherited" some recipes and want to make sure they work correctly on the clay body you are using.

2. MAKE TEST TILES. Every potter seems to have a preferred shape test tile. Some throw or extrude cylinders--approximately 2" in diameter—that then are cut into 2" rings. (They look a lot like napkin rings)

Others throw wider cylinders and cut them into rings, which then are halved, forming arches. Some make very small bowls. The most common in this country seems to be the L or T shaped test tile--which can be thrown (a wide, low ring sliced up like a pie) or extruded.

If you have neither wheel nor extruder, try this. Roll out a slab of clay about 1/4 inch thick and 3 or so inches wide. Drape it over a 2" mailing tube, or an empty paper towel roll. Let it get leather hard, and cut into 1 1/2 inch slices. Clean them up so that they stand properly on "their own two feet" and you have excellent U shaped test tiles!

Carol makes her like this: She throws a small sphere off the hump, a shape like an orange and about that size. When the "orange" is leather hard, she cuts it into wedges--exactly as one would an orange. Each wedge acts as its own "saucer"--as the curved shape contains any glaze that runs!

It does not matter what shape you choose. But the test tile must be big enough so you can see what is going on. Again, methods vary. Some people mark their test tiles when leather hard. This can be done by scratching in an ID or by stamping in the ID with a rubber number stamp (about $5 at a stationer's) or stamping in letters from a child's "printing" set. The test tiles then are bisqued. When the tile is used, its ID is entered next to the test description on the chart.

Other people bisque their test tiles, and marks them with an underglaze pencil as they are used. You need to test tiles for the body you are using and for the other three bodies as well. You will NOT need to test every glaze on all the bodies. But if a "given" glaze recipe gives you trouble, testing it on other bodies will help identify the problem.

Test tiles can be striped with white and black slip. Ridges on them allow a glaze to "break" and reveal potential patterns. Besides test tiles you might make some tiny test cups by the following method, we learned from a ClayArter who prefers to remain anonymous. Roll out some clay, and with a large sewing thimble make a series of depressions in it. You can cut the slab up as you wish. The depressions are filled with materials to be tested for fusion.

You will, alas, soon enough have a damaged shelf you then reserve for tests.

Meanwhile, make yourself a number of "biscuits"--pancakes of clay, preferably grogged, to put under test tiles when you do not know how much the tested materials will run. You may prefer to make the "biscuits" for your first tests with a rim--like little pie-pans--in case the glaze REALLY runs!

Alright. You have your glaze room, your table, your materials, your test tiles. You are ready to go.

Now what? Where do you begin? You have read several of the books suggested--or others--and you have an idea what you want to do.

You may want to test different materials. For this you would place a small amount of the material in one of the little thimble cups and fire to the chosen cone/temperature. You may want to test different glaze recipes you have. For this you weigh out and mix up the materials and apply the mix to test tiles.

You may want to see what difference replacing one material by another makes. Again you weigh out and mix up your materials and make up a glaze you apply to a test tile. Now there are several methods, ways, of doing this. And these should be tried out till you find one you like, can stick with, and that works for you.

One of us prints out the recipes--or mixes-- to be tested on her computer. She pastes each recipe to the container where the test will go, crosses off the ingredients as they are added to the pot.

The other uses paper with little squares printed on it. Across the top of the sheet she writes the names of the materials to be used. Down the left margin: the names or ID of the glazes or tests. On a line with the glaze name the amount of material that glaze wants is entered in the material’s column. As the material weighed out and added to the test container, it is checked off on the paper.

What containers you use to mix your tests in are up to you. Old yoghurt or cottage cheese containers work well, as do disposable plastic “glasses” The important thing is to have solid containers, that you mark clearly! Here too systems vary. Some use letters and numbers to identify glazes being tested. A test series might be Jan/09/A, Jan/09/B with numbers added for the next (Jan/09/A1) and next and next test--and so on. Others prefer actual names. This is mostly done when one tests another person’s glaze. Cooper 163, or Cowles Mope, or Thorpe Cornwall Stone. As said letters or numbers can be used to identify tests in series. The only important part of this is that you mark your tiles clearly, that you keep good records, that you do the work diligently.

After the tests come out you need to sit down and examine them.  Both good light and a good magnifying glass are a must. We suggest a magnifying glass with 10x magnification, as well as one with 30x magnification.

Look at each test tile carefully. Ask the questions. Did this test mature properly? Did it underfire? Overfire? Craze? Bubble? Write your observation down in whatever record “book” you keep.

Assuming the glaze turns out perfect, test it on an actual pot. One of us makes 200 gram test batches, the other 100 gram. The latter never is enough to test on a pot--so a new and bigger batch is made up. While that sounds like extra work, it is an excellent control on whether the original was weighed out correctly! Assuming the glaze has defects. Hit the books and try the remedies! Keep excellent notes all along the way, and for every step. That is how you learn! In a few years you will know “instinctively” what is what...and need to consult your notes less often. Till then it is upon your record keeping that your success depends.