Of all the individual components of ceramics, glazes and glazing seems to be the one that frustrates people the most. Glazes are fickle, subject to the vagaries of firing, completely unobservable while shut up in the kiln and generally infuriating. For all that though, the best glazes can attire a pot in a garment that enables it to transcend the simple beauties of shape and form.
While most aspects of pottery are skills that can be learnt, glazes themselves are merely recipes. Certainly glazing techniques, including things like decorative brushwork are things that can be built up over time, but glazes themselves are the result of the application of heat to various mixtures of rocks and minerals. There are no physical skills to be learnt, no muscle memory to be acquired. Rather, it is an intellectual discipline, in which the role of each ingredient in the mix must be determined.
Judging by the number of different books, purely on glazes and glazing that are available, it is one of the areas of ceramics that most infuriates people. This is in part because the glazes that are most spectacular are those right on the edge of usability – whether because of their fluidity, or the way in which colourants disperse through the matrix, or the way in which they crystallize during cooling – and thus slight alteration of ingredients one way or another can cause their failure. Not only that but when developing new glazes, it can be hard to find that sweet spot.
In “Chinese Glazes” Nigel Wood declares that the history of Chinese glazes is the exploration of iron oxide as a colourant. By changing only the concentration of iron oxide that was used, and one or two ratios of the components in the base glaze, potters in China were able to produce glazes that ranged from clear sky blue celadons, through deep black tenmokus. Glazes that have been evocatively named: Oil spots, Hare’s fur, Partridge feather. All these glazes were produced, not with a multitude of ingredients, but just by varying the ratios of silica, alumina, calcium, sodium, potassium and of course iron.
It seems to me that many of the books on glazing that are available either pick a few base glazes and thoroughly investigate the effect different colourants have, or they are books that contain multiple recipes, with very little investigation as to why these recipes work as they do, or why changing the base glaze so totally changes the glaze itself. While developing a glaze may seem like medieval alchemy, there is no reason that modern empirical methods cannot be applied to the investigation of glazes. This blog will chronicle my attempts to do precisely that. One of the key points about empirical investigation is that failures, or experiments that yield no obvious result, can be just as informative as those that give spectacular results. I will try and publish those too, along with my speculations.
Over the course of the next few years, I hope that I (perhaps we!), will discover precisely how different components change the properties of the glazes we make.