|Monthly Tech-Tip |
Modified: 2018-03-22 21:55:20
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.
|Iron Oxide Red||2.00|
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:
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).
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.
M340 stoneware fired to cone 6 (drop-and-hold schedule). The L3954B engobe fires deep black (it has 10% Mason 6600 black stain instead of the normal 10% Zircopax). It was applied inside and partway down the outside (a much less messy process than using a black clay body). They were bisque fired and glazed inside using the base GA6A Alberta Slip amber clear (using Frit 3195). The outside glaze is Alberta Slip Rutile Blue (you are seeing it on the bare buff body near the bottoms and over the black clay surface on the uppers). To learn more about how to make the engobe and start making black pots click "Product Data Sheets" at PlainsmanClays.com and go to the section on Medium Temperature.
Cone 6 oxidation. GR6M Ravenscrag version is on the left. The Alberta Slip version (GA6C) is more fluid, but that also means it will run more during firing and blister more if too thick or on re-firing. Generally, the Alberta Slip version appears better on dark bodies and the Ravenscrag one on lighter burning clays. The Alberta Slip version gets its color only from Rutile (and thus requires a special drop-and-hold firing scheduel), the Ravenscrag one produces blue in any firing schedule (although the color will be better in the drop-and-hold schedule).
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 (more information 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.
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.
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.
Cone 6 Floating Blue Glaze Recipe
A discussion by Jonathan Kaplan on dealing the with fickle nature of this glaze
Subsitute Gerstley Borate in Floating Blue Using Desktop Insight
Use Desktop Insight to explore ways of calculating substitutes for Gerstley Borate in the popular Floating Blue cone 6 glaze recipe while maintaining or improving the other raw and fired properties of the glaze.
Plainsman Cone 6 Electric Standard
Used in the Plainsman lab to fire clay test bars in our small kilns
Floating Blue page at gerstleyborate.com
Tons of information on Gerstley Borate
GR6-M - Ravenscrag Cone 6 Floating Blue
Plainsman Cone 6 Ravenscrag Slip based version of the popular floating blue recipe. It can be found among others at http://ravenscrag.com.
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