It is unlikely that one glaze can cause several glaze defects over the years and still be in current use by potters. Floating blue is a cone 6 (2232 0 F.- 1222 0 C.) oxidation glaze that is popular for its variegated mottled milky blue gloss surface qualities. One reason for its popularity is that it mimics the results obtain in cone 9 (2300 0 F. - 1260 0 C.) reduction glazes. However, due to the relatively large percentage of Gerstley borate in the floating blue glaze potential problems can develop.

Due to the large quantity of bound water associated with Gerstley borate it can cause high shrinkage rates as the water is driven off during the firing. A fast heat increase during the first part of the firing can cause Gerstley borate glazes to crawl, peel, crack or jump off the pot, landing on the kiln shelves in a circular pattern. Such defects are caused by excessive shrinkage or steam created by the bound water being driven off too fast in the first part of the firing.

Gerstley borate was the basis for many low temperature glaze formulas and Raku glazes because of its considerable ability to melt and pull other oxides into a vitreous glaze. The formulas were transferred from teacher to student and then to other potters without a critical evaluation of the raw materials contained within. Today many published formulas in books and magazines list Gerstley borate as an ingredient. Gerstley borate does work for long periods in glazes, which was complicating factors in discovering their limitations. If Gerstley borate glazes formulas failed every the time they would not be passed along to other potters.

Gerstley borate in glazes can create fired surfaces that are “soft” or can easily be abraded. A characteristic of a soft glaze surface occurs when knife scratch marks are observed on the surface of dinner plates. Fired glazes containing Gerstley borate can leach in the presence of acidic food or liquids such as tomatoes and vinegar, causing a discoloration on the fired glaze surface. Additionally, glazes which contain Gerstley borate can become unstable in their dry or liquid form due to their solubility. Gerstley borate is hydroscopic drawing atmospheric moisture in storage conditions of humidity and temperature. On a practical level the material can weigh more when subjected to high moisture conditions in the studio causing an inaccurate weight when used in a glaze formula.

Why is Gerstley Borate Unique? – Gerstley borate is composed of a mixture of the minerals Colemanite (Ca2B6O11·5H2O) and Ulexite (NaO·2CaO·5B2 O3·5H2O).
The most noticeable characteristic of natural borate ores such as Gerstley borate results from surface tension changes as the calcium and sodium contained within the ore enter into a melt in the presence of boron and silica which are glass formers. During the firing process a mechanical turbulence takes place, as calcium and sodium move back and forth to equalize their chemical potential much like oil and water in the presence of certain detergents. The result of this interface turbulence in the liquid glaze produces variegated unique qualities in the subsequent fired glaze surface.