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It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
If you live with two children in the house like I do, you soon learn never to set important papers down on the kitchen table without first checking for greasy, wet, or sticky patches lurking there. When I say "checking", I don't mean just a quick visual to see if it looks okay. You have to turn a light on or open the curtains; then get down at just the right inspection angle and all those little trouble spots become obvious. For many potters and even industries who mix their own glazes, if a glaze looks attractive on the surface, then they use it without a second thought, without checking any deeper.
Sometimes it is easy to lose sight of the reason why one bothers getting his hands dirty instead of just buying the glaze from a glaze company. Isn't it for the control and flexibility? True, you may not have some of the fancy equipment glaze manufacturers use but there are simple mental, mathematical, and physical tests you can do to really improve your glazes and raise your standards.
First, consider three things I think are the key:
Unfortunately, when it comes to glaze quality, the standards that establish it are less than obvious.
I would like to set forth my expectations for an ideal glaze. As I have repeatedly said, it really is time to forget combing textbooks for the perfect recipe. You can create it. No one is asking every glaze user to become a scientist but there is a need to understand the materials we use; a need for certain standards in the interests of the customer and the manufacturer alike. When you begin to feel the control that a better understanding gives, your demands on the final result will likewise increase. Hopefully, better quality will follow.
I am proposing what I feel is necessary to produce a quality product at the lowest practical cost in terms of effort, time, and money (notice I said 'practical', not 'possible'). I'm tired of using fifty different recipes, each of which are deficient in some way. As much as possible, I now feel that my efforts should be directed toward developing one quality base glaze for each temperature range I work at and I can modify this as needed. Far better to understand one and keep perfecting it than tolerate fifty others that are never quite right (or worse yet, have a database of 500 you could never possibly control).
You might be thinking right about now, 'how can I modify the same base to make a shino or crater glaze, the two are fundamentally different!'. True, there is a huge variety of special purpose glazes and I am being reasonable here. I am talking about the glazes you use large quantities of, the ones you put on your bread-and-butter production functional ware. However, do not underestimate the huge number of effects possible with just one base recipe.
Here are some of the properties I want from a base glaze. In this particular case, it will be useful for functional ware on stoneware or porcelain. You are probably thinking that I am living in a dream world with this many expectations. I treat them as goals to be pursued and it is surprising how many can be achieved with little trouble.
Many people assume that high-temperature firing is necessary for ware that is strong and durable, not so, fired strength is a product of the body's maturity. Maturity means that significant glass development has occurred and that the body is dense (as demonstrated by low measured porosity). I have found that as long as you fire above Orton cone 2, very high strength is easily achieved by using a body that has the necessary flux. Conversely, very low strength can occur at cone 10 with bodies that lack flux (e.g. iron brown speckle stoneware and fireclay bodies). While the development needs to aim at a specific temperature, I find it is usually possible to move the melting point down a little at the end of the process. Another major hurdle to lower firing is ease of fit to the body. The lower the temperature a glaze melts at, the less Al2O3 and SiO2 will be present.
For a glaze to act as a base to be used in all sorts of circumstances, it needs some adjustability. "What do you mean by that", you ask.
As already stated, the most common glaze problem is body fit. Ware strength and fired quality are fundamentally affected by this property. Lead was so popular, and still is, because it has a low expansion and is an extremely active and effective flux. In years past, it seemed like the perfect answer to optimizing firing temperature and dealing with crazing. Now that we have learned the hard way, preventing crazing is a matter of minimizing high expansion oxides of (e.g. sodium and potassium) in favour of their low expansion counterparts and using as much SiO2 and Al2O3 as possible.
Also, the use of B2O3 is invaluable to fuse a glaze at a lower temperature while keeping its expansion down and silica up. The perfect base glaze should allow for movement in the SiO2 , B2O3 , or Al2O3 content without adversely affecting visual character. It should not depend on high soda or potassium oxide content making it a likely candidate for crazing. The same can be said for solving shivering problems, can it be moved toward higher expansion?
Another critical area is the glaze's suspension and drying properties. Does it have enough clay to suspend and can it be adjusted to source Al2O3 from extra clay rather than frit or feldspar?
What about its maturity? Can the recipe be altered to move the melting point up or down without disrupting other properties or is it a "one temperature glaze?" Or is it a "one man glaze" due to its touchy nature? How about the gloss? Can this be adjusted to produce a matte or semi-matte and is the method of adjustment, which is sometimes less than obvious, documented?
Frit is expensive, not to mention difficult to get at times. Ideally, high-temperature glazes should be frit-free. Also, a wide selection of quality raw materials are available for high fire. But as firing temperature is reduced, material choices narrow to those which are much higher in flux. Unfortunately, the money saved on the glaze itself can quickly be spent on higher reject rates and periods of crisis in production. Thus, there is a balance.
It is not surprising that the best industrial glazes are fritted. At the same time, potters and smaller companies frequently achieve unique effects imparted only by raw materials. Armed with the formula viewpoint, made possible by glaze calculation, it is possible to at least minimize the frit in low and medium temperature glazes. Many have found it a good compromise to employ frits mainly for B2O3 , since raw materials which source this oxide (e.g. gerstley borate) are unreliable or insoluble.
Forget about using exotic and hard-to-get materials unless they are employed in smaller quantities to produce a specific effect. If production depends on a hard-to-get material, then you will be shut down if your supplier is out-of-stock. Make each material in the glaze justify its existence in favor of an easier-to-get and more economical one. As an example, consider a glaze which specifies an opacifier, which is difficult to obtain in your area. How about a middle or high fire glaze that uses magnesium carbonate-do the calculations necessary to substitute talc or dolomite to source magnesium instead and test fire to see if the more expensive material is justified. The same goes for frits, expensive feldspars, stains, etc. If a material is expensive, that's usually a signal that your supplier does not stock large amounts or that it is shipped from some distance away.
Most glazes have a network of fine bubbles suspended in the glass. This is sometimes intended and necessary for the desired effect but typically it is just tolerated. The problem is somewhat insidious in that most people do not even know they have it or realize that other problems with glaze surface defects are related to bubbles. It can be difficult to know what improvements would result if the bubble population could be reduced. Benefits include fewer surface imperfections, better strength, and greatly increased clarity in transparent glazes. If you want a basic glaze to act as a starting point for others of different color, opacity, or surface quality, it makes sense to start with one that is perfectly clear. It is easy to introduce bubbles if they are needed.
Glaze bubbles have many causes which originate with the body, the firing methods, application habits and material quality. In any case, a glaze has to pass bubbles generated during firing and its ability to do so is under your control. Certain glaze materials release considerable levels of gases of material decomposition (CO2 ). For materials like whiting and dolomite, up to 40% (by weight) of the raw material just goes up the chimney of the kiln and much of this can contribute to glaze bubbles. To test for yourself, try sourcing MgO with talc and calcia with wollastonite. An article on this subject is available on our BBS.
Crystals can turn an otherwise visually boring glaze into something quite exciting and appealing. They are great when you want them but a real pain when you don't. In many cases, crystals will grow in very small sizes and congregate together to completely cover an area with spots or a dirty looking patch. This problem can come and go with certain glazes depending on firing schedule, thickness, and particle size. So, it is reasonable to move the chemistry to provide a margin for safety in this area. Provide enough SiO2 and Al2O3 to avoid the high melt fluidity that encourages crystallization; watch out for imbalances where certain oxides are at the saturation point (e.g. zinc, titanium) and test fire at a variety of temperatures with slower cooling to emulate a densely packed kiln. Check the textbooks for the kind of chemistry that encourages crystallization and move your formula away from it.
If the only difference between one line of product and another is the color, then it is logical to use the same proven glaze on both. It is also logical to use stains to achieve the desired color. They can be expensive, but then the metal oxide systems you would use to create the same color are also expensive and require more milling. Unfortunately it is not just a matter of putting the stain in any clear glaze and getting the desired color. While some color systems (e.g. cobalt blue, chrome green) are quite stable for diverse glaze types, others are sensitive to the chemistry of the glaze. Certain colors choke in the presence of zinc, others (notably chrome-tin pinks) are sensitive to the amount of calcia and magnesia. Stain manufacturers publish the oxides used in each stain (e.g. chrome-tin pink, manganese-alumina pink) and they provide information on what chemistry a color cooperates with. So why not concentrate development on one good transparent glaze recipe base, which makes allowance for as many of these as possible?
Is your glaze touchy? Does it contain barium? Heavy metals? Is it unbalanced and saturated with one oxide? Does it oxidize during use? Is it matched to your firing temperature, or is it under or over fired? Who gave you the recipe and do they care about safety and balance or have they publicly stated that aesthetics are all that matters?
Most glazes in use by potters and small companies today have never been examined in this respect. Even though a chemically unbalanced glaze contains no dangerous materials, the first time someone decides to add a heavy metal colorant there is a problem. As a result, some hard questions are being asking by governments and the public. Think about it for a minute. If your glaze is "touchy", it never looks right unless the temperature, atmosphere, thickness, etc. are just right. If its appearance, which you are closely monitoring, is difficult to control, then what about other properties like solubility which you are not monitoring? If the appearance is a product of the chemistry of the glaze and the glaze is touchy, then is it not likely the chemistry is off as well?
Many glazes are known to be non-functional. If so, then don't give the recipe to others who will in turn circulate it and quickly lose the non-functional label. In educational institutions, teach the students what chemical balance is, what limit charts are, how to do the calculations necessary, and how to appraise the results. Be sure to document any glaze recipes you give to others (e.g. print a FORESIGHT report after entering key information). Really, I feel that a glaze recipe is just not fully documented without a report that explains some of the things we are considering here.
For a clear base for functional glazes, it is logical to go with a middle-of-the-road chemical balance and use common materials which do not have a questionable reputation on the matter of glaze safety. This eliminates flux saturates and it requires adequate SiO2 and Al2O3 to produce a strong and full bodied glass.
A volatile glaze is like a volatile person. It is fine sometimes but bouncing all over the place at other times. I use the term 'volatile' in contrast with 'touchy' to refer to a glaze which is chemically balanced and safe but exhibits erratic behavior for other reasons. The most common area is narrow range of firing. You can easily check this by firing the glaze in a flow tester four cones below and two cones above the temperature you are working at. Some glazes are sensitive to thickness of application, ball milling time, brand name of specific materials, firing temperature, cooling curve, application method, etc. For example, a certain property could depend on a unique mineral phase in one material (e.g. it seeds crystal growth) and substitution of other materials to source the same oxides adversely affects that property. A unique distribution of particle sizes could produce an effect which is difficult to maintain. For example, an important flux could be a coarse size, thus slowing its melt time enough to allow higher temperatures at a faster firing rate. Or it could have a particle shape or size that speeds the melting process. Thus, try to be aware of mineralogical and physical properties of the material and what these might do to increase glaze volatility.
A glaze, like a clay body, can be strong or weak. "Cutlery marking" is the term used to describe glazes which are soft and easily marked with metal table ware. While higher temperature glazes are generally harder, it is also true that the high-temperature ones can be soft and low-temperature ones can have considerable hardness if designed and tested carefully. As you can appreciate, this is not a difficult property to test, all of us have hard metal tools we can use to attack a glazed plate to check it. The most common cause of softness in glaze is an imbalance in SiO2 :Al2O3 compared with the fluxes. If a glaze lacks SiO2 glass former or Al2O3 to give it body, then it cannot be expected to develop into an inherently hard glass. Moreover, you can also expect problems with hardness in glazes which are over or under fired or lack flux diversity.
A poor fitting glaze can severely affect the strength of tableware, especially when it is applied in a thick layer, a proper fitting glaze can greatly enhance it. Proper consideration of the chemical balance will produce a glaze with flexible expansion and with a base expansion low enough to avoid crazing on most common clay bodies.
If the glaze is to be a starting point, then it should be the lowest common denominator, namely, colorless, transparent, glossy, and bubble and crystal free. It is true that the body can affect whether the glaze is bubble free or not, but I am working on the basis that we evaluate the glaze on its own merits. If, for example, it is on a clean white porcelain, would it be ultra clear or not?
Glaze users should be aware of why each material is there. I see a number of problems with the way things are now. Obtaining a glaze from a magazine or stranger is somewhat like getting a piece of software without any documentation. It is probable that you will end up with a lot of frustration and wasted time.
First, consider a recipe which contains more than one material sourcing a particular fluxing oxide. The user should know why, for example, a glaze contains dolomite, whiting, and wollastonite, all three of which source CaO. Is it to achieve a special effect rendered by the mineralogy of each material? Is it to diversify sources to achieve better consistency? Or is the recipe simply the product of a line blend between two others and no one has had the time to remove the duplication?
Second, if the recipe contains expensive alternatives to cheaper materials, why? For example, if tin oxide is used to opacify, have zircon materials been tested and found wanting? If an expensive stain is used for a color, which is normally very easy to achieve using raw oxides, what is the reason?
Third, if a material is used in abnormal amounts, why? For example, we expect 30 or 40% feldspar in a stoneware glaze but we don't expect 70%. Talc or zinc at 5% is normal but 30% is not. The existence of only small amounts of clay would suggest trouble with a powdery application and difficult suspension in the slurry. Such unusual situations should be documented. Don't forget other unusual situations: excessive metal oxide colorants, which could saturate the melt and yield an unstable glass, high sodium minerals, which would cause crazing or high zircon, which can stiffen the melt and produce crawling and surface imperfections.
Fourth, if materials are used which seem inappropriate, why? For example, we all expect to see clay in a glaze to act as a suspending agent, plus a source of Al2O3 and SiO2. But what about a transparent recipe employing ball clay instead of kaolin? This introduces needless iron which affects the clarity, so what is the reason? Likewise, little or no clay or silica in a middle or high-temperature recipe is a definite abnormality that you would want a reason for. The presence of B2O3 materials in high-temperature glazes could be questioned, since other much cheaper raw fluxes are very effective. The use of an organic binder, when there is already plenty of clay to provide hardness and suspension, would be unusual and worthy of an explanation.
Fifth, what happens if the glaze is over or under fired? Has it been checked with a melt flow tester at a variety of temperatures to confirm that it is not too volatile for the recommended range? Or does it turn out perfectly at the intended temperature, but if even slightly over or under fired have a completely different appearance?
Casual potters are often so preoccupied with fired results that they do not appreciate the importance of a glaze's physical qualities. Industry is painfully aware of their importance. Sometimes the behavior of the glaze slurry is the biggest source of problems in a plant.
There are usually an infinite number of recipes that will yield a given formula, so we have considerable flexibility in choosing which source materials are used. A glaze needs, most of all, kaolin content (or ball clay) to suspend the slurry, to prevent settling, and to harden the layer on the ware after application. On the other hand, excessive clay will produce high shrinkage which will introduce other problems. A glaze should apply in an even layer, should dry reasonably quickly, and should be fairly immune to viscosity changes that can result from solubility of mineral ingredients.
If anything, I hope I have raised your expectations about your recipes and your ability to learn to evaluate them. We should all be exercising more responsibility in exchanging recipes and we should demand more from magazines, textbooks and seminar/workshop presentations. Make your life simpler by employing one or two good base recipes and trying to understand them well enough to alter each as needed. In the article that follows, I will give you an example of one that has worked for me.
If you would like an example of a recipe which has been documented from these viewpoints, then fire up FORESIGHT and print a recipe report of number G1213G. It is a cone 5 transparent.
We all have enough trouble trying to get glazes to fire properly without needing to experience trouble with application or the slurry itself. Unfortunately, it often seems that the glazes that fire the most beautifully are the ones that won't stop settling in the tank, are too powdery after drying, crack on the bisque or greenware after application, flake off after drying, don't stick to dense bisque ware, dry too slow or too fast, etc.
Industry simply adds organic materials to slurries to crowbar them to the desired working properties without impacting firing results (i.e. supplier directories list many companies which market additive materials like binders, hardeners, etc.). On the other hand, not many individual potters and smaller companies are exploiting these materials as well as they could. Still, if anything, industry relies so heavily on these additives that they sometimes don't take advantage of raw ceramic materials that will accomplish the same thing. Most companies just take a frit and throw in some kaolin and a binder; it is convenient, reliable, and they can blame someone else when there is a problem! In some cases, they employ mixtures that contain no plastic clay ingredients, something that might seem impossible to a potter uninitiated in the use of binders and other additives.
It seems strange that individuals and potters put so much emphasis on the fired properties of a glaze to the exclusion of other considerations while industry, on the other hand, puts emphasis on the working properties of materials. Many people are willing to tolerate glazes that are downright difficult to use just to experience the buzz of an occasional great firing! Yet, most of us have had that one 'perfect slurry' that applied to the ware like silk, that flowed well yet gelled ever so delicately, that went on perfectly even and dried to a wonderful hard surface. It is interesting that experiences like this demonstrate that it is possible to create glazes, which work very well, without using any additives.
Wouldn't it be great to have the best of both worlds: wonderful working and fired properties. Strangely, the former may be the more difficult of the two to achieve on a consistent basis. However, there is one approach that makes it very possible.
'Many individuals are willing to tolerate glazes which are downright difficult...'
G2571 % BY MATERIAL WEIGHT ----------------------- WHITING 4.00 *POTASH FELDSPAR 29.00 EPK KAOLIN 25.50 SILICA 18.50 DOLOMITE 19.00 GERSTLEY BORATE 4.00
This glaze happens to work very well with colorants to produce soft colors (i.e. blue with 1% cobalt, oatmeal with 3-5% manganese, etc). and it has a very pleasing silky surface. More important, with about 50% water it produces a slurry that is beautifully thixotropic and that will hang onto almost anything in a nice even layer. EPK kaolin is well known for its contribution to slurry properties; the Gerstley Borate flocculates the suspension just slightly. Other factors such as water chemistry, how well particle sizes complement each other, % water content, temperature, etc. come into play also.
You might wonder whether I am promoting one base glaze to replace each family of glazes or cheering the use of glaze binders. I am doing the former. Be on the lookout for a base which has good natural working properties and then you can control its and color, speck, opacity, gloss and other properties with recipe adjustments. In this way, you can focus your learning, testing, and development; you can reduce material inventories and costs, and have the benefit of good slurry properties.
Let's review again what a simple reliable general purpose base glaze needs to exhibit good physical working properties:
The clay suspends the slurry and hardens it after drying. Normally kaolin is used and it should be 15% at least, if it is the only clay. Ball clay is more effective than kaolin and it should be at least 10%, if no other clay is present; however, it is much lower in alumina and introduces iron which will muddy colors. Bentonite may be used as a last resort, if no other clay is present it probably needs to be 5%. If the clay content is so high the shrinkage after application causes the surface to crack, then part of the clay should be replaced by calcined clay substituted with a less plastic one of the same chemistry (e.g. kaolins are available in a wide range of plasticities). If you can't get calcined material, then make your own by bisque firing some powder to destroy its plasticity.
If a glaze slurry is too watery or runny, it may settle or separate despite clay in the mix. It will go on too thinly, dry too slow, shrink and crack off the ware, and require extended dipping times. If it is too heavy, then it will go on too thick, not drain properly, and go on in variable thicknesses.
A glaze should have a gelling property that helps it to hang onto the ware rather than drain and drip for a long time before finally drying. The tendency of a glaze to gel is a factor of materials in the mix, water content, the choice of kaolins and other clays, binders which might be present, and acidic additions to enhance this property (e.g. vinegar, calcium chloride). A glaze which has ideal thixotropy will hang onto a piece of glass and dry without running off. To some this might sound impossible, but the above glaze base and many industrial glaze bases will do exactly this.
A good glaze base should employ more than one of each key material. Thus, it would benefit from two kinds of clay, two feldspars, or two sources of key oxides like CaO. If materials known to be variable are used, their amounts should be minimal. If these principles are followed, changes in any one material will have less impact on fired glaze properties as a whole.
So think about it. If you use many recipes to get different colors, maybe it is time to investigate ways to combine groups of existing ones to employ a common base. It will require some testing and dreaded ceramic calculations to accomplish but consider the benefits.
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