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,
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.|
other notes on pottery