Every business needs to have some bookkeeping, and the business of glaze development is really no different. Records are for your own benefit and should be kept as simple as possible.
Recording Glaze Development
It’s amazing how many students and full-time clay workers go about making glazes with a lengthy and time-consuming process, only to forget to clearly write down just what they’ve done. It seems to be quite normal to work with scraps of paper and the odd hieroglyphic on the back or base of the test tile. I know from bitter experience that the finest glaze ever developed was, of course, the one where the scrap of paper got lost or the hieroglyphic got obliterated by an accident of firing, if any record was made at all. Somehow, that one gem that might have been a crowning glory was lost forever. I’m sure this has happened to every potter at some time, but it doesn’t have to. It’s very easy to start in the glaze-making process with good working habits such as:
1. Keep an accurate notebook.
2. Develop a serial numbering system.
3. Work as neatly as possible.
4. Write down everything in your notebook.
5. Take basic health precautions when handling materials.
1. An accurate notebook is very important and can be set up in many ways. It’s best to get a hardback book simply because these have a greater longevity in the wear and tear of studio life. The book can be sectioned for glaze development at various temperatures. It can also be vertically lined for tabulation of procedures followed, as in the accompanying illustration. Starting with this or a similar format, it’s easy to keep records.
2. Developing a serial numbering system or code is quite simple, and if you use a combination of letters and numbers, it is easy to follow. For example, the process is done as follows. The basic entry is MFV, which stands for Matte Flux Variations (as I was testing glaze results using various fluxes). This is followed by a number that represents the particular flux used in this series (helpful to add if you’re doing variations on one glaze base). The fourth flux tested, for example, would appear as MFV4. This gives you all you need to record the basic glaze. To this you add colorants, which I designate with a lowercase letter (a, b, c, etc.) to save time writing out the full name of a chemical symbol over and over again, and a number (1–6), which tells what percentage of color, (0.625%, 1%, 2%, 3%, 5%, 10%), is in the glaze, with 1 being the lowest percentage (0.625%), and 6 being the highest (10%). You end up with a number, both on the test and recorded in the book, along the lines of this: MFV4a4. Put simply this means Matte Flux Variation, made with the fourth tested flux, and 5% of a colorant. If testing is being done in both oxidation and reduction, it is helpful to append an O or an R as well, as the differences in the same glazes from the two atmospheres is often very marked. Note: It’s important to keep a key that lists what your letters, abbreviations, and numbers stand for, and the order you list them in on the tile.
Numbering tiles or other test pieces should be done on a section that will remain unglazed, with either a black underglaze pencil or pen, or by painting with a mixture of equal parts of iron and manganese mixed with a little gum and water (1). The gum lessens the likelihood of smudging the numbers. Some gums, such as gum arabic, should be mixed with alcohol, but other powdered gums may be mixed with hot water and allowed to soak overnight. This mixture can be kept in a jar and thinned with water when needed.
3. Neat and efficient working habits are essential to gain the most from your efforts. From accurate measuring and weighing of materials, through to serial numbering, and to recording observations and results, care and a certain amount of patience pay great dividends. The old fishing story of “the one that got away” need not become a reality if disciplined work habits are started from the beginning.
4. Make sure you write down everything in the notebook, even variations in firing, because over a period of time you tend to forget some important details. If it’s on paper, and the paper isn’t lost, recall comes more easily.
5. Safeguard your health with basic protection from materials that can be toxic, carcinogenic, or silicotic. Use a properly fitted respirator while mixing glazes to prevent breathing airborne dust particles. For some materials, such as wood ash, lead, and barium, rubber gloves are also helpful to prevent problems. Washing hands after using materials is also a good practice, and you should never eat or smoke in areas that may have dust contamination, or chew fingernails when glazing or mixing glazes.
Making Glaze Tests
Glaze and material testing can be done in a variety of ways, using tiles (1), small bowls or crucibles, extruded test
forms, sections of thrown forms (2), or even clay squeezed between your fingers (3). The most important features to include are some alteration of surface, either throwing ridges or stamped, impressed patterns, and enough flat space on the unglazed portion to write or paint the serial numbers. The alteration of the surface gives some idea of how the glaze will behave on a textured background, or on thin edges, as the glaze generally runs slightly. Because of the tendency of glazes to flow, many people prefer to do testing on sections of thrown forms or some other vertical test surface. When testing new glazes or doing material fusion tests, it’s wise to make sure that the kiln shelves are well protected with a kiln wash (equal parts kaolin and silica, with added water to make a creamy mixture) or powdered silica or alumina hydrate to diminish the likelihood of ruined shelves. If tests are being done on small bowls or similar objects, a saucer or slice of insulating brick can be placed underneath to eliminate damage from running glazes.
It’s generally preferable to do glaze tests on clays that are as white as possible so the true colors of the glazes will show. I usually put a stripe of medium iron slip down one side of the tile to indicate the behavior of the glaze over a dark clay (see 1). On dark clays, I usually apply a stripe of white slip for the opposite reason (see 2, 3). It’s also useful to make an impression in the clay to indicate the quality of the glaze when pooled (see 1). The mixtures are tested on bisque-fired clay, except where the tests are for the development of glazes for work that will be fired only once.
Mixing Test Batches of a Glaze
The quantity of powdered materials required are measured out using a variety of methods. Pinches and handfuls, spoonfuls and kitchen measuring cups, and very sophisticated electronic scales used in chemistry laboratories are among the possibilities. Perhaps the most convenient method is somewhere in the middle of this range in the form of a triple-beam balance scale using grams as the unit of measurement. This makes it much easier to calculate percentages for adding colorants, opacifiers, and other textural additives. However you do the weighing out, do it as accurately as possible for consistent results. A small error in compounding a test batch of glaze mixture can easily become a giant one if the glaze later gets made up or mixed in a large volume. A few grams out in the test could mean a kilogram out in a large batch of glaze.
In most cases, all glaze guidelines are intended to be made up as “parts by weight” mixtures. The materials should be weighed out first. Then they can be mixed with water until the mixture is of a creamy, brushable consistency. The usual ratio of dry mixture to water is four to three for most glazes. When glazes are being made up in larger volumes, a convenient measure of dry mix to water is with kilograms and liters, where one kilogram (2.2 pounds) equals one liter (1 quart). Therefore, four kg of dry glaze mixture can be added to three liters of water for approximately the right consistency. This approximation can then be adjusted by the addition or removal of a small amount of water. Once an optimum thickness for a given glaze is established, its specific gravity can be recorded with a hydrometer.
Applying Glaze to Test Tiles
The mixtures are painted on the tiles with a variation of thickness from one end to the other, to give an indication of the behavior of the glaze with differing thicknesses. These variations can produce remarkably different results from the same glaze, and a glaze test should yield a good deal of information.
Glaze tests that are initially promising should be retested on pots, as they often seem to change character. These changes can be due to various factors: thickness, firing time, temperature and atmosphere variables, and even the more complete interaction of materials when used in bulk glaze. Before covering all your favorite pieces with inadequately tested glazes, it is advisable to spend a little more time with the groundwork. Glazes, like people, need to be lived with before they reveal their full potential. Good first impressions don’t always produce the best friends!
When you’re doing the work of glaze development (and are probably heartily sick of weighing out little bowls of powders), remember that a few days or weeks of careful testing can be enough to set you up with a lifetime of glazes, developed for your own personal requirements. The important part of the process isn’t just arriving at a good recipe or formula, but the search that led you there.
Excerpted from Robin Hopper’s book The Ceramic Spectrum, which is available at www.ceramicartsnetwork.org/shop.
This article can be found in the September/October 2017 issue of Pottery Making Illustrated.
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