Why Textbook Glazes Are So Difficult

Section: Glazes, Subsection: General

Description

The trade is glaze recipes has spawned generations of potters going up blind alleys trying recipes that don't work and living with ones that are much more trouble than they are worth. It is time to leave this behind and take control.

Article

We have been heading down the road of textbook glaze recipes for many years now and it is time to change direction.

If you are like me you have racks of test samples that never quite worked.

Currently moves are under way to put these recipe into computer databases so we will have access to thousands that don't work instead of hundreds. There is compelling evidence to make a drastic change in direction. It is time to do things differently; we have new tools, and easy access to information

It is time to know why. In fact, more and more people are saying "I am at the stage now where I want to know why". I believe there is no doubt, making your own glazes either from scratch or adjusting an existing base is the best way to go, it is the only way you will get what you really want.

It can take new potters years to get textbook glazes out of their system. Admit it; like me you went through a stage where you were on a quest, a quest to find that perfect recipe; and you mixed up hundreds of textbook glazes to find it. However, experience soon teaches us that the effort and expense involved are not really worth it. The fact is, these glazes seldom behave, and they are frequently touchy and introduce more problems than they solve.

Don't get me wrong. Sometimes people have found useful glazes that work well in their circumstances. But how many, and how long did it take, and how expensive was it? And do they know how to adjust these? Do they know why the glazes exhibit those little quirks? Do they know the function of each material in the recipes? Perhaps you will agree that textbook quests don't normally breed intimate knowledge of how glazes work.

A good glaze is a little like a good dog. It is best to raise your own from a pup rather than trying to adopt someone else's full grown hound. Let's consider some of the reasons why a textbook glaze might not "travel" very well. As we will see, much more goes into achieving a particular glaze effect then just weighing it, throwing in some water, and slapping it on the ware. I'm not trying to discourage the use of all glaze recipes but I am saying a few words of warning to anyone who might have delusions about their use.

If your raw materials are different, then your version of the glaze can hardly be expected to match the author's. Remember that chemical or mineral names can be, and are used very loosely. There are thousands of different distinct minerals and grades of minerals available under an umbrella of a few hundred different names. For example, the feldspar available to the glaze author may be completely different than yours. Ball clays vary substantially across one quarry site; never mind what they do in different parts of the country. Even for kaolins, seeing one definitely does not mean you have seen them all. Very common materials like talc, whiting, and dolomite are available in high and low quality versions, and these vary in purity and loss on ignition. Even silica has many grades and sizes, each of which will respond differently. Likewise, the particle size of materials like nepheline syenite and harder feldspars will have a marked effect on their reactivity. Clearly, the potential for problems is great, especially if the glaze depends on a critical balance of oxides for a eutectic, crystal development, or color response.

The glaze recipe may be volatile or touchy. This means that a certain critical visual or tactile property may depend on a tight balance of oxides, a certain mineral phase, or particle size profile in one of the materials, a particular firing schedule, or some special preparation or application technique. The author may be unaware that the glaze is "on the edge" in any respect, and has never seen reason to test various temperatures, material substitutions, or oxide variations. Inabilities of a glaze to survive these kinds of variations will show up in your studio or factory.

Most glazes are dramatically affected by the nature of the clay they are applied to. Porcelain reacts little with overlying glazes, whereas stonewares, especially those with iron and added fluxes and silica, can react strongly with a glaze, changing its appearance considerably. Typically, the whiter, smoother, and more mature the clay body; the more vivid and brighter a glaze will be. Conversely, the darker, coarser, and more immature the body; the more subdued and dry the glaze will be. If there is no information about how the glaze reacts to different types of clay bodies; then just about anything could happen when you try it on yours.

A glaze can be very sensitive to temperature and cooling cycle. Different types of kilns have their own characteristic cooling curves and if your kiln is different from the author's, your version of his or her glaze will be to. What are the chances that you fire the same or even have the same interpretation on how to read cones? This is why it is logical to try a new glaze at different temperatures, and heating and cooling rates to note any volatility in these areas. For example, if a cone 6 glaze fires a perfect tan crystal matte with a slow cool, yet produces a glossy deep brown with a cone 6 1 /2 faster cool, it sounds like something to stay away from unless you like angry customers trying to match sets of your variable ware.

Some glazes are very sensitive to thickness of application and whether they are on a vertical or horizontal surface. Therefore, you will want to develop a glaze testing tile that provides planes of both, as well as some incised lines and sharp edges to see how well the glaze covers these, and whether it highlights or masks the relief. Also, be aware that different application techniques like spraying, pouring, and painting can produce fired surfaces on the finished ware that are very different in character from what the small dipped samples indicated.

Is the author ball milling the glaze? If so, how long and how efficient is the mill? Many glazes are profoundly affected by milling, especially if they contain materials of coarser particle size.

Even if you find a glaze that looks great, it could well craze or shiver on your clay body, or else fit just poorly enough to considerably weaken fired strength. Doubtless, the author has not documented how to adjust its expansion. It is even possible that the author doesn't really care or even know about issues involving fired hardness or solubility of things like manganese or barium. What if the person has an undeveloped eye for fired quality in glazes and thus tolerates a measure of pinholing, bubbling, blistering, or immaturity? What if he has the recipe so loaded with clay that it crawls half the time, or uses no clay and is so powdery you can hardly handle or decorate the ware? These are all things that you may miss at first, but which later become painfully evident.

Unfortunately, these differences don't line up and politely hit you one at a time. It is likely that most of them will come into play in any attempt to transplant a glaze recipe into a new environment. The trouble is this: If you know how to "level the playing field" to get useful tests, then you can probably formulate glazes from your own native materials anyway.

"What about calculation?", you might ask. Every glaze has a formula of oxides, and it is true that oxide formulas travel a little easier than recipes, especially when accompanied by adequate documentation. Still, most of the above problems will come into play anyway because you still have to derive a recipe of materials to supply those oxides, and if you are smart enough to adjust a textbook glaze, then why bother. It is just easier to make your own glazes using materials native to your area.

Yes, there is something that travels even better than oxide formulas. It is the knowledge of what each oxide type contributes to a glaze, what each mineral type does to give it fired personality, and how they affect physical working properties. Each new recipe either confirms or fine tunes your existing oxide-effect knowledge, or educates you in the effects of its special purpose source materials (i.e. colorants, opacifiers, crystal forming agents) or unique formula (i.e. crystals from high CaO and low Al2O3 ). Each can be adapted to your own proven base recipes.

Add a smattering of line blending and trial and error adjustment, and you can make anything you want; adjust it any way you want. You can tell the glaze what to do. So why jump through hoops trying to pacify temperamental recipes that are always throwing ceramic tantrums in your kiln? You don't have to put up with that any more.

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R-47

Authors

Tony Hansen (Owner)

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