Mastering Cone 6 Glazes
by John Hesselberth & Ron Roy

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It may not be well known, but I have been an advocate of mid-fire clays and glazes since my first experiments in this range during college days, so it was with particular pleasure that I received a copy of John Hesselberth's & Ron Roy's "Mastering Cone 6 Glazes".

The subtitle to this publication is "Improving Durability, Fit and Aesthetics" and it certainly delivers on the first two points at least.

Now I admit that I am not a great glaze technician. I have been one of those potters happy to try out glaze recipes from various sources and modifying them to my needs. However, this is a 'hit and miss' method that may or may not yield good results as far as glaze fit is concerned and gives no information on durability and food safety of a glaze. This is where Mastering Cone 6 Glazes comes in. Few glazes, even fully melted, high gloss glazes in the mid-fire range are food safe. They may not stand the test of time in this respect, nor as far as durability goes. Over time and through contact with food acids such as juices or vinegar, glaze components may leach from the glaze, slowly accumulating in the body and possibly eventually contributing to disease. As potters, we should bear responsibility for the products we make and the trusting consumer buys from us.

Here is a case in point. Very recently a friend of mine asked me to reproduce a mug he had bought from a Swedish potter some time ago. It had what I suspect to be a barium or zinc matt glaze on it. I was aghast to see that all color had leached from the interior, after years of use. The outside was a beautiful matt blue, but the inside, underneath tea and coffee stains, was a dirty cream color. I told my friend to immediately stop using those mugs, to contact the potter responsible and tell him that his glaze was probably a health hazard.

Mastering Cone 6 Glazes explains why glaze components leach, which ones are hazardous, how to test for leaching and what to do about it. With the help of Glaze Calculation Programs and Seger Formulas, glaze stability and fit can be improved to an acceptable level.

Another issue is resistance to scratching, knife marking, chipping and microwaving. Some glazes will only show such wear after months of use. How to test for such durability issues? As it turns out, methods for improving glaze stability will often also improve durability and glaze fit.

The authors present 4 Rules for Stable Glaze Fit, which if followed will help considerably to formulate a stable glaze. They also offer a further 3 Guidelines, which will also improve glaze characteristics. (Maybe the book could have been called "Zen and the Art of Mastering Cone 6 Glazes"? )

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