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G2826R - Floating Blue Cone 5-6 Original Glaze Recipe

Modified: 2022-11-12 22:34:15

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.

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).

Related Information

The classic cone 6 floating blue? No, it is Alberta Slip blue.

And it contains no cobalt! Fairly close in appearance to the classic cone 6 Floating Blue recipe used across North America, this is a variation of the Alberta Slip Rutile Blue glaze (except this adds 1% tin oxide, 1% black copper oxide and 2% ceramic rutile, it is GA6-C1). Because of the melt fluidity, it thins on the edges of contours and breaks to the color of the underlying body. It looks best on dark bodies, but if thick it is OK on light ones also.

Better melting gives Ravenscrag Floating Blue more zip!

GR6-M Ravenscrag cone 6 Floating Blue (center) on Plainsman M340, a buff burning body. On the left is a version having 80:20 Ravenscrag:Frit 3134 (no extra 10% Frit 3124). On the right is GR6-M on porcelain (where the floating effect has been largely lost). It appears the effect benefits from the iron it finds (albeit not much) in the stoneware body.

A much better Cone 6 Floating Blue

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 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.

A black engobe transforms the floating blue glaze over it

This is M340 stoneware glaze fired to cone 6 using the C6DHSC schedule. The L3954B engobe fires deep black (it has 10% Mason 6600 black stain). The engobe was applied by pouring and dipping at leather hard stage (inside and partway down the outside). After bisque firing the piece was glazed inside using the base GA6-B Alberta Slip amber base. The outside glaze is Alberta Slip Rutile Blue GA6-C (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers).

Close-up of Floating Blue on cone 6 dark/buff burning bodies

Originally popularized by James Chappell in the book The Potter's Complete Book of Clay and Glazes. It is loved and hated. Why? The high Gerstley Borate content makes it finicky. But the magic ingredient is not the GB, it is the rutile, Rutile makes the cobalt and iron dance. This recipe actually produces a number of different mechanisms of variegation. Color and opacity vary with thickness. Small rivulets of more fluid glass flow around more viscous phases producing micro-areas of differing colors and opacities. Titanium crystals sparkle and calcium-borate creates opalescence. Bubbles of escaping gases (from GB) have created pooling. Small black speckles from unground or agglomerated particles of iron are also present. Surprise! This is actually Ravenscrag Floating blue. All the visuals, none of the headaches.

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