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The Art of Crystalline Glazing: Basic Techniques

Introduction

TYPES OF CRYSTALLINE GLAZES

There are two major types of crystalline glazes: microcrystalline, which have crystals so small that you need a microscope to see them (and are the basis for most matt glazes), and macrocrystalline, which have crystals large enough to be seen with the naked eye. This book discusses the most popular macrocrystalline glaze—that based on zinc with its large, beautiful crystals. When people speak of a crystalline glaze, this is the particular glaze they mean. The crystals have been identified as willemite, a crystal also found in certain limestone deposits.

HOW CRYSTALS FORM

If a glaze contains the proper ingredients it can form zinc crystals. The process is as follows:

  1. As the temperature of the glaze is increased, all of the components begin to melt together.
  2. When the glaze is at the proper high temperature, "seeds" begin to form in it.
  3. As the glaze reaches its maximum temperature, it begins to flow, and many of the seeds dissolve.
  4. The kiln temperature is then lowered.
  5. When the temperature reaches the correct range, the remaining seeds, acting somewhat like magnets, attract the appropriate minerals in the glaze and the crystals grow on the seeds. The longer the temperature is held there, the larger the crystals grow.

All four of the following are important in attaining crystals and are discussed in detail in this book.

  • Clay body — its composition, texture, and bisquing temperature.
  • Glaze formulation — exactly what ingredients are in the glaze, how much of each, and in what form (fritted, not fritted).
  • Thickness of the glaze application.
  • Firing — maximum temperature, soaking (crystal-growing) temperature, and how long it is held there.

 

Zinc crystalline glazes can be formulated to fire from cone 3 to cone 12. Those firing at cone 8 and above are frequently called high-fire glazes. In general they give the nicest (largest and most interesting) crystals. But for those not able (or willing) to fire that high, there are several low-fire (cone 3 to cone 7) possibilities.

Since most potters using zinc-crystalline glazes use the high-fire recipes, that is where we have placed our emphasis. However, the low-fire glazes are covered in Chapter 10. If you are interested in those glazes, be sure to read all the other chapters also, since most of the procedures that are used for the high-fire glazes are also used for low-fire ones.

Must you use an electronic kiln controller when firing crystalline glazes? Some (like one of us) feel that the convenience and reproducibility that controllers offer are a real advantage. Others (like the one of us who uses them only for bisque firing) feel that they make the process too "mechanical" and remove much of the spirit of randomness from the experience. (Also being much younger he does not mind getting up in the middle of the night to tend his kilns.)  The point we wish to make here is that whether you use an electronic kiln controller or not, you can produce beautiful crystalline glazes.

ABOUT THIS BOOK

One of our goals is to debunk a number of myths regarding crystalline glazes. Why bother? Because belief in some of these is what prevents many from even trying to use these wonderful glazes. It is surprising how many experienced users of crystalline glazes believe some of these myths.

For instance, do you know which of the following are true and which are false with respect to zinc crystalline glaze?

  • The ratio of zinc to silica cannot deviate by more than 10%.
  • Alumina is bad for the glaze, and even minute amounts can prevent crystal growth.
  • The rate of heating is very important.
  • The rate of cooling is very important.
  • The coloring oxide you add can have a significant effect on the melting and crystal-growing temperatures of the glaze.
  • The glaze must be sifted.
  • The glaze must be ball milled.
  • The quality of the water used in the glaze is very important.

 We address each of these statements in the following chapters.

GLAZE PHOTOGRAPHS

How many times have you looked at the photograph of a glazed pot and said, "How did they do that?" And how many of those times did you find complete (or even partial) details of its production? Our experience is that all of the important information is never given.

In every photo of a glaze, we give you all of the details of how it was made. That is, we tell you the clay body, the bisquing temperature, the exact composition of the glaze, thickness of application, and the glaze-firing schedule. That information appears at the ends of the figure captions. The key to decode the information is in Appendix A on page 147.

Of course, given the fickleness of crystalline glazes, even if you (or we) exactly reproduce all of those factors, there is little chance that the piece will come out looking like the photograph. But it gives you a good, reliable starting point.

EXERCISES

People learn best when they actively do something, rather than just reading a description of how it is done. Thus we have developed a series of step-by-step exercises to help you gain not only the experience needed to make beautiful crystalline-glazed ware, but also a basic understanding of the process.

GLAZE RECIPES

We give many recipes. For each one that comes from the published literature, we state its source. Many of these recipes appear in several publications, and we have tried to give the first (oldest) publication listing it.

We have modified some of the recipes from their original forms. The major reasons for such modifications include the following:

  • To make the sum of the ingredients total 100, so you can immediately see the percentages of each item. This permits easier comparison of ingredients of one glaze to another.
  • Colorants have been removed. All of the glazes in Chapters 3 and 10 are "base" (uncolored) glazes. Coloring glazes is discussed in Chapter 9.
  • Rutile has been changed to titanium dioxide. (Rutile is an impure form of titanium dioxide and often adds a creamy color to the glaze.)

The notes that follow some of the recipes are from the source of the recipe or someone else who has used it.

Several recipes give one of us as the source. In those cases it is a recipe that we have developed, usually based on one or more published recipes.

WHAT OTHERS SAY

In addition to the results of our own experiences, we present overviews and summaries of information that appear in the many articles, theses, and books that we have found on crystalline glazes.

We feel this additional information is one of the most important features of our presentation. Much of this information can be found at the end of each chapter in sections titled "What others say."

We believe you will find it interesting that authors come to such different, and often opposite, conclusions about some of the important details. Understanding these differences in attitudes is a big help in mastering these glazes.

FURTHER INVESTIGATIONS

For the potter just beginning to explore crystalline glazes, we strongly recommend that you follow the procedures that we present in the Exercises. In this way you will be able to come up to speed the fastest.

However, after you have some experience with producing these exciting glazes, we urge you to begin your own investigations regarding the body, the glaze composition, and firing schedules.

Why? Because there is much yet to be learned about these fascinating glazes. And by conducting your own investigations, you may turn up something very unique, and very beautiful. Don't be afraid to experiment. Yes, many (perhaps most) experiments will yield results that are not of particular interest to you. But from each experiment you will learn something. And combining that with what you already know may lead you to a very interesting and productive path.

To get you started, we occasionally identify what we call a "Topic for Investigation." These are set off from the other text so you do not have to read them on your first time through. Some of these topics we are investigating, others we hope to investigate when we get the time. We encourage you also to look into these questions. There is no reason to believe that all of us will find exactly the same results or come to the same conclusions.  Please don't be limited by our few suggestions. For instance when you read that something is either good or bad for crystal growth, or that such-and-such is the right way to do something, you may want to test it yourself to see if it is so. (However, never take chances with your health or safety.) You may be pleasantly surprised at the outcome.

Experiment—it's fun. And who knows what wonderful things you might find.

SYSTEMS OF UNITS

By a system of units we mean the units used to measure quantities.  In the United States the British system is most commonly used: volumes in ounces, weights in pounds, and temperatures in degrees Fahrenheit. In most of the rest of the world the metric system is used: volumes in liters, weights in grams, and temperatures in degrees Celsius.

The U.S. is (slowly) moving to the metric system. Until that move is completed both systems will be used in the U.S.

So what system of units do we use? Basically the metric system. However, we realize that many of our readers will not be familiar with that system so we usually present both systems. We state the metric value and follow it with the approximate value in British units in parentheses.  However, when discussing the mixing of glazes we will always use grams without giving the (very inconvenient) British equivalents. We believe that most scales for weighing out glazes that are found in ceramic supply stores are calibrated in grams.

RECORD KEEPING

If you are to make progress in your quest for crystalline glazes, you must keep complete, accurate records. Why? Because you will be trying many different glazes and firing schedules. And it is only by recording the results that you will be able to keep track of what works, what works only a little, and what does not work. So make up a logbook or diary in which you record, in detail, what you do. These records can save you many days (even weeks) in your quest for nice crystals. Including photographs of your fired pieces in the log is a great help.

ONE LAST REMARK

We are confident that, following our directions, potters with access to a suitable electric kiln and the perseverance to make many tests can create attractive crystalline-glazed ceramics. On the other hand, it does take attention to detail, cleanliness, curiosity, patience, and good record keeping. A sense of humor about failures is also helpful. If you are the type of person who gets easily frustrated, these glazes are probably not for you.

Do we guarantee that you will have success? We wish we could, but there are so many variables, as we will discuss, that depend upon exactly how you do things and exactly how your kiln fires, that we cannot make such a guarantee. On the other hand, if you carefully follow all of the procedures we give, we believe your chances of success are very great.

Will you have success on your very first try? We hope so, but realistically, it can take several firings before you see that first (usually small) crystal.