Floating Blue Cone 5-6 Original Glaze Recipe
Recipe
Notes
This has been used by thousands of potters over the years, it was originally popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. It is the common Gerstley Borate/Nepheline/Silica 50:30:20 highly-melt-fluid transparent with added colorants plus the magic ingredient: Rutile. That makes the colors dance as the glass solidifies during cooling. Colored areas appear to "float" in the transparent. Cappell says this effect is "reminiscent of a deep pool of water". This recipe actually produces a numbered of different mechanisms of variegation: It's color and opacity vary greatly with thickness (so it highlights irregularities in the surface). 'Phase separation' in the translucent matrix (areas of differing color, opacity, brilliance) makes the color 'swirl' as small rivulets of more fluid glass flow around more viscous phases. Titanium crystals grow in the matrix and make it sparkle. Opalescent calcium-borate boron-blue crystals grow within the glass. Bubbles of escaping gases create pools of lighter colored glass surrounded by darker rings (as the glass softens during approach to final firing temperature). Small black speckles are produced by unground or agglomerated particles of iron (common in most grades of red iron oxide). People also commonly employ methods to increase the variation of surface color (i.e. stippling a second layer, brushing on a wash of another coloring oxide, double dipping, applying a wash of rutile, etc.). Anyone who has used this glaze will testify to the fact that it is "fickle" (as Chappell notes). The fickle nature is due principally to the fact the Gerstley Borate is partially soluble and it makes the slurry gel. Among the recommendations he makes is: "Don't substitute any other chemicals for those given". Unfortunately, that is exactly what needs to be done to make this recipe more user-friendly. The boron needs to be sourced from another material (e.g. a frit, Ulexite). Floating blue is a testament to how unique Gerstley Borate is. This recipe depends on the GB to suspend it (since there is only 5% kaolin) and flux it. Common frits contain less boron. People who have tried to substitute frits have found their results lack one or more of the variegation mechanisms. That being said, there are successful versions of the mechanism (which depends on cobalt, iron and rutile) in other base glazes (e.g. Ravenscrag and Alberta Slip versions).
Description
Floating Blue is a classic cone 6 pottery glaze recipe from David Shaner. Because of the high Gerstley Borate content it is troublesome, difficult. But there are alternatives.