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Mastering Cone 6 Glazes, by John Hesselberth and Ron Roy

Mastering Cone 6 Glazes: Improving Durability, Fit and Aesthetics, by John Hesselberth and Ron Roy

“‘Mastering Cone 6 Glazes’ is probably the number one clay bestseller in the US. It is addressed to beginners eager to try well-tested, bright-colored recipes. While cone 6 electric kiln recipes abound, having been with us long before high-fired reduction became the US ‘norm,’ Hesselberth and Roy designed their glazes for digitally controlled kilns.

“The strong point of this book is that the authors tested their recipes for leaching and like that and give clear instructions for making one’s own safety tests. A trail-blazing book that no doubt has changed the way potters think about safety.”--Lili Krakowski in Constableville, New York USA

John Hesselberth and Ron Roy are two of the most well-known glaze experts in North America. Their book has developed a "cult following" and is considered by many to be indispensible.

Do you want your glazes to be stable, durable, and attractive? Do you want to be sure they will not leach significant quantities of metals into food or drink? This is the first book to address these questions in-depth. A wide variety of extensively tested glaze recipes are included as well as detailed guidance on formulating your own glazes.

Do you want to be sure your glazes don't craze on coming out of the kiln or after extended use? Or perhaps you do want glazes that craze for a decorative effect. This is the only book that gives an in-depth discussion of how to prevent (or cause) crazing and the related problems of shivering and dunting.

Do you want to know how you can test your glazes and pottery to be sure they are suitable for their intended use? A number of practical, inexpensive, in-studio tests are described that you may want to use.

All of this information and more is explained in terms potters can readily understand. This is a ground-breaking book that studio potters, ceramics teachers, students and others will find to be an invaluable source of useful reference information on cone 6 glazes.

Mastering Cone 6 Glazes has 168 pages, 42 color photos, 15 graphs and is softbound in a durable, water-resistant cover.


Review by Charles Moore of Sacramento, California

"Mastering Cone 6 Glazes" is fondly known in clay circles by its abbreviation MC6G. I bought MC6G in 2002 just shortly after it was published. Before beginning this book, I had no background in chemistry, never having taken a chemistry course in high school or college. I confess that as I began the book, I did not understand much of the glaze chemistry in the book.

I was able to understand John and Ron’s terms like “fit” and “durable”; I also could comprehend the causes of “crazing” and “shivering.” I had made glazes from glaze recipes for years, but I had no knowledge of the value of the “unity formula” or what it meant.

Like many potters I know, I turned to Chapter 6, “Stoneware and Porcelain Glazes,” and began to use the stable glaze recipes that the authors provided before carefully examining the principles of good glazing in the book. I did, however, begin to learn how glazes are created and how they may be revised.

In this review, I will not pretend that I will clarify all of the concepts in the book; in fact, I will intentionally simplify in the hope that I can interest other potters to buy into the principles of durability and fit promulgated by John and Ron. Because there is so much more than I can cover, I will pick the concepts that I have found most useful.

MC6G is the most responsible glaze book that I have ever encountered. Most other books devoted to ceramic glazes simply provide numbers of glaze recipes (e.g., James Chappell’s "The Potter’s Complete Book of Clay and Glazes"). Some books devoted to glazes do provide information on glaze calculation (e.g., Daniel Rhodes’ "Clay and Glazes for the Potter"). But no other glaze book gives so much emphasis upon safety in the studio and the durability and fit of glazes.

The Introduction notes that many glazes “circulating among potters or printed in books” are unsatisfactory for several reasons: “(1) a lack of reliability due to glaze ingredients that are notoriously inconsistent in composition [e.g., Gerstley Borate], (2) a lack of stability or durability to the point that the coloring oxides leach out over time or the glaze surface easily cracks, chips, accumulates metal marks or becomes dulled after repeated dish washing, (3) a glaze surface that is pitted, pinholed or crazed so badly that it is not easily cleaned or maintained after each use and (4) glazes that are so ill-fitting that they shiver and leave sharp splinters of glaze ready to injure the user of the pot” (p. 13).

Chapter 2, “Basics of Glazing and Firing,” deals with safety issues in the studio, purchase and storing of glaze materials, weighing and mixing, glaze suspension, and other topics of interest to the potter new to glaze making. Of particular interest in this chapter is the matter of “glaze suspension” which deals with “flocculation” and “deflocculation” of glazes. Ideally, a glaze is flocculated; that is, the glaze is characterized by “(1) slow settling of the glaze materials in the container and (2) rapid absorption of water into the bisque surface which results in a thicker glaze layer and more rapid ‘drying’ of the glaze on the pot so it be handled more quickly” (29). These conditions make for an ideal glaze for application; that is, the glaze remains well mixed and allows for a thick application.

Most of us have encountered the problem of a glaze that is not flocculated. Deflocculated glazes show “(1) more rapid settling, (2) hard-panning of glaze ingredients on the bottom of the container, and (3) slower absorption of water into the bisque pot resulting in a thinner layer of glaze and slower drying of the glaze on the pot” (29). A deflocculated glaze makes for a difficult glazing process and often results in a fired product with a too thin glaze on the pot. The authors offer a series of remedies, but the one that I have found most helpful is the use of Epsom salts: “about ½ gram dissolved in a small amount of warm water per 1000 grams of dry glaze ingredients” (30).

The authors feel that the term “food safe” often has little real meaning as used by many potters, who believe that if they avoid dangerous materials, like lead, they have met all the criteria for food safety. The authors prefer to use the terms “stable” or “durable” in referring to glazes that merit the use of “safe.”

The issue of using stable glazes goes beyond simple safety: “Making durable glazes is an integral part of good craftsmanship….even if there is no safety issue, certain glaze materials (e.g., copper) can impart a bad or bitter taste to food” (p. 15). What makes a glaze “durable?” The authors say that a durable glaze is resistant to acid, alkalis, thermal shock, knife marking, scratching, and chipping. Chapter 3 offers tests for each of these negative factors in glazes.

The authors present ideas that are most appropriate for functional pottery that may be used for storing food. They suggest, however, that the potter also consider using durable glazes on functional pieces that are not intended for use with food. “Who can tell when a vase might be used as a sake decanter or what bird droppings might do to an outdoor tile mural? Many times our pottery ends up being exposed to a much harsher environment than we had imagined” (p. 18).

Though the authors understand that many potters “do not have a strong foundation in ceramic science,” they feel that some knowledge of the Seger molar formula (also known as the unity formula, or simply the formula) is important. Most glaze recipes give the weight percentages, but ignore the molar percentages. This is unfortunate since the recipe, as opposed to the formula, does not provide sufficient information to judge a glaze.

A quick look at a glaze analyzed by the Insight glaze calculation program may well illustrate the difference between a glaze recipe and a glaze formula. I chose a cone 6 recipe known as 5 X 20 by Tony Hansen, creator of the Insight Glaze Calculation program; it contains 20% of 5 ingredients each and has been employed by thousands of potters around the world. The recipe: G1214 Cone 6 Transparent Base Glaze or “5 X 20”

Wollastonite 20

Frit 3134 20

EPK 20

Custer Feldspar 20

Silica 20 __________ Total 100

This is a simple straightforward glaze to which one might add colorants (red iron, copper, cobalt, etc.) and an opacifier (zircopax, tin, titanium, etc.). But the recipe does not even hint at the durability of the glaze.

The formula (simplified):

CaO (Calcium) 0.79*

MgO (Magnesium) 0.00*

K2O (Potassium) 0.07*

Na2O (Sodium) 0.14*

These first chemicals marked with an asterisk [*] are called the fluxers or melters and add up to 1.00 or unity.

Al203 (Alumina) 0.35

B203 (Boron) 0.21

SiO2 (Silica) 3.35

These elements are the other basics of a cone 6 glaze. Boron acts as an additional melter, very useful at ^6 and found in Ferro Frit 3134 in this glaze. There is sufficient Alumina and sufficient Silica. (See Chapter 4, “Making a Stable Glaze” for lower and upper limits of Alumina and Silica and making a good “melt.”)

Some additional information, again simplified:

Si:Al 9.65

This is the percentage ratio of Silica to Alumina in this glaze.

Thermal Expansion 7.03

This indicates the expansion/contraction rate of this glaze in the firing/cooling cycle. The expansion rate for the glaze and the clay should be close to get a good “fit.” (Unfortunately, most clay companies do not provide the thermal expansion rates for their clays. We are forced to make educated guesses and use trial and error to test our glazes with our clays.)

In brief, the formula analysis tells us that the 5 X 20 glaze will probably fit and be durable. Further testing needs to occur after the glaze is applied and fired.

If the glaze does not fit the clay, the results may be “crazing” or “shivering.” “If the glaze contracts more than the clay, i.e., it becomes too small for the clay it is on…it crazes. This is because the glaze is being stretched and has to break apart. This is also called a glaze under tension…” (p. 61).

Shivering is the opposite effect: “If the glaze winds up too big for the clay it is more difficult to see except when the difference is excessive and we get shivering…. In this instance the glaze is under compression—it’s being pushed together by the clay. If we have the ‘right amount’ of compression, we can make glazes that never craze [or shiver] and in fact make our ware stronger” (p. 62).

Chapter 5,”Fitting Glazes to Your Clay Body,” gives some very helpful hints about choosing an expansion level in a glaze to fit a cone 6 body. There are a number of scales to measure expansion levels. Since our 5 X 20 glaze formula used the Insight scale, let’s look at the expansion rate of our glaze on that scale. The Insight scale goes from 6.26 (low) to 8.40 (high). Our 5 X 20 glaze shows an expansion rate of 7.03; it is below the mid range, what one might call “medium low.” Low expansion (near the 6.26 range) is likely to shiver on some clays; medium expansion (around 7.38) will craze on many cone 6 bodies; medium high and high expansion will craze on most cone 6 bodies. Therefore, the choice of medium low expansion makes the best sense.

Of course, not all professional potters will agree with Ron and John about the importance of glaze fit, especially in the matter of crazing. Many potters wish to achieve this effect and consider it aesthetically pleasing. (I don’t think that anyone would choose to create a shivered glaze, but one never knows.) Our authors would answer something like this: If you choose to use a crazed glaze on functional work, you should at least use a durable, well fitted “liner glaze” on the interior. A crazed interior on a bowl or vase, for instance, may encourage disastrous seepage of liquid on fine wooden furniture. And be sure that a cone 6 matte glaze is not simply an underfired glaze intended for a higher firing.

At this point, I feel a moment of panic: there is far too much that I am omitting from this review to do justice to MC6G. But there simply is not enough time or space. Let me just mention Chapter 6, “Stoneware and Porcelain Glazes.” Our authors present a number of tested recipes beginning with a couple of Semi-Matte High Calcium glazes and then through a series of glossy glazes, each glaze illustrated with photographs with differing percentages of coloring oxides and opacifiers. I have two favorites: (1) High Calcium Semi-Matte Base 1 with 6% Rutile, 3% Copper Carbonate, 1.5% Cobalt Carbonate to produce “Variegated Slate” (pp. 86-87) and (2) Glossy Base 1, which combines with an assortment of coloring oxides and opacifiers to produce a very wide variety of glazes that give an even application over all the clay bodies that I have used with this glaze. Finally, there are some excellent specialty glazes: “Waxwing Brown,” “Waterfall Brown,” and a cone 6 “Maiolica” glaze.

If you glaze fire to cone 6, you need this book. Even if you glaze fire to ^10, MC6G can give useful and clearly presented insights about durability and fit and ways to test your glazes.

Out of a possible five stars, I give this book five.
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