Pottery: Glaze Analysis

Here are some results of analysis of the Sankey Glaze Database.

It is generally believed that the expansion for a stable glaze must be 1-10% less than that of the clay. Glaze expansions much less than this can result in shivering - the breaking off of sharp pieces of glaze under thermal stress. Glaze expansions higher than that of the clay can cause crazing - fine cracks in the glaze surface that will trap contaminants, thus making the item unsuitable for containing food.

Analysis of successful glazes from the database calls this into question - glazes can have expansions well outside this range and still be successful. The mean COE of these glazes is 6.2x10-6/K as expected, but the range is far wider than 10%. It should be noted, however, that almost none of the testers performed any functional tests, such as a freezer to boiling water cycle.

Studies of copper leaching from glazes by Hesselberth&Roy suggest that a stable cone 6 glaze should have a silica Seger ratio above 3.0 and an alumina ratio of 0.25-0.45. H&R exclude boron and all materials they consider to be colorants from the Seger ratios on which they based their recommendations. The values at right were obtained using their definition (i.e. including solely SiO2, K2O, Na2O, Li2O, MgO, SrO, ZnO, BaO, CaO, Al2O3), for cone 6 gloss or semigloss glazes. Clearly, most glazes in the database that are considered to be successful by testers have much less silica than a glaze should have by their criteria. And, a significant number have alumina outside their limits as well.

Potters seem to be saying that the criteria of Hesselberth&Roy are stricter than necessary. It's true that H&R used 5% copper carbonate, a larger than normal amount of one of the most leachable colorants. Lower amounts of colorant leach much less. However, for centuries potters used large quantities of lead in glazes, even cadmium and uranium; all are now known to be dangerous and are legally prohibited in many countries. So, perhaps H&R are the voice of the future.

The resistance of a glaze to thermal expansion forces varies considerably with composition. So, one might expect that glazes with extreme coefficients of thermal expansion might tend to have high resistance to crazing/shivering. As the graph at right shows, there is a slight tendency in that direction at low COEs, but if anything the opposite direction at high COEs. Glaze fluidity seems to be the major factor in permitting the use of glazes with a high COE. Fluid glazes are applied more thinly than thick ones to avoid running, and glaze strength varies inversely as thickness.

John Sankey
other notes on pottery