Inspite of the fact it is very fickle, the floating blue cone 6 glaze is a good example of a recipe that displays many different kinds of variegation. Gerstley borate is one of the main reasons for its properties.
Floating Blue, probably the most well known and popular cone 6 pottery glaze, was popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. Like most other glossy Gerstley Borate based glazes, this one also uses nepheline syenite, silica and kaolin. The fired effects produced by this recipe are a testament to the variegating effects that 4% rutile imparts to colored non-opacified boron glazes.
The term 'floating' could refer to two possibilities: There are multiple 'phases' of glossy glasses of differing color, opacity and gloss; the blue cobalt colored surface appears to float on a translucent brown glass layer. This layer is visible where the color breaks to brownish hues on thinner sections at edges and irregularities in the surface. It either or also refers to a white opalescent 'boron-blue' crystalline layer that can appear to float over the cobalt blue background. Cappell says this effect is "reminiscent of a deep pool of water".
An amazing thing about this recipe is that it actually has the potential to produce six separate mechanisms of variegation:
In addition people commonly employ a variety of 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. Among the many recommendations he makes on how to use it he says "Don't substitute any other chemicals for those given". Well, that is exactly what we have done. Boraq 1 is a Gerstley Borate substitute available from Plainsman Clays and its dealers. It is thoroughly documented at www.gerstleyborate.com. Boraq 2 (84 parts Boraq 1 with 8 dolomite and 8 whiting) works well in floating blue. We have done a flow test to determine an exact melting comparison, the two are identical in color and fluidity.
Floating blue is a testament to how unique Gerstley Borate was. Floating Blue depends on the GB to suspend it (since there is only 5% kaolin). Try using a frit and it will settle to the bottom of the bucket like a rock. In fact, by using Boraq 2 you can not only achieve the same color but you will have a glaze that has even better application qualities.
Gerstley Borate was the main reason for the unique visual appeal of this glaze. People who have tried to substitute frits have found their results lack one or more of the mechanisms of variegation and are more color sensitive to firing temperature and thickness. This is a testament to the fact that while glaze materials do have a chemistry that can be matched by blends of other materials, they also have a mineralogy and particle size distribution that can play a big part in how they melt and behave in a melt of other things. A frit has a homogeneous chemistry across all the particles whereas GB was a mixture of particles of radically different chemistries and mineralogies.
For more information, visit https://digitalfire.com/gerstleyborate/recipes/floatingblue.shtml
GR6-M Ravenscrag Cone 6 Floating Blue on Plainsman M340 buff stoneware. This glaze also has this variegated visual character on porcelain. Because it has the GR6 base recipe (which is publicly available at ravenscrag.com), the slurry has very good working properties in the studio, it is a pleasure to use. This is an excellent showcase for the variegating mechanism of rutile.
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By Tony Hansen