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INSIGHT Software, the best way to deal with this: Watch the Gerstley Borate video here.

Floating Blue

Floating Blue (also called Blue Hare's Fur) is probably the most well known and popular cone 6 pottery glaze. It was popularized by the book The Potter's Complete Book of Clay and Glazes by James Chappell (in the current edition it is on page 210 under the heading "Floating Glazes"). The author calls it "spectacular and extraordinary". In fact, the vase that graces the cover of the book uses this glaze. Like most other popular glossy GB base glazes, this one also uses nepheline syenite, silica and kaolin. Its borate content is about middle-of-the-road compared to the range of recipes that we have studied. Like the Butterscotch glaze, the fired effects produced by this recipe are a testament to the variegating effects that 4% rutile imparts to colored non-opacified and well melted boron glazes.

This glaze was both famous and infamous! Because the base contains two partially soluble materials (the nepheline and GB) many people had gelling issues with the slurry. Also, the recipe contains a token 5% clay, so the burden of suspending of the glaze was on the Gerstley Borate, settling issues were thus common. Also, there has been alot of questions around the supply of Gerstley Borate and the consistency of its chemistry. In addition, it was susceptible to blistering during firing. Finally, the high feldspar content (this has almost 50%, that is alot!) means high K2O and Na2O, these are the oxides that contribute to higher thermal expansion (and therefore crazing). It was a love-hate relationship for many people, the pull of the appearance was enough to make many endure all of its problems.

The term 'floating' could refer to two possibilities in our observation: 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. More likely, however, it refers to a white opalescent 'boron-blue' layer that often appears to float over the cobalt blue background (boron-blue calcium borate crystals form in borate glazes when calcium is present and the glaze is very fluid). This effect can best be amplified by melting a ball of glaze on a tile to get a very thick pool of glass. Cappell says "the colors seem to float on a surface of a darker background of great depth, 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:

No wonder this is such a popular glaze among potters!

Case Study Using INSIGHT Software

The is a video tutorial at that shows you how to substitute GB for frit while maintaining the same overall chemistry in the glaze. The principles shown apply to substituting Gerstley Borate in any recipe, the challenge is mainly one of chemistry.

Using Boraq

Floating Blue with GB
 Amount% Anyone who has used this glaze will testify to the fact that it is "fickle" as Chappell notes (this is an understatement, see above). He makes a number of recommendations on using this recipe: Use distilled or low mineral water, force all material through an 80 mesh screen, stir thoroughly before and during use to prevent settling out of the iron content, apply the thickness of a dime, fire to cone 6 oxidation exactly, and cool normally. One interesting comment Chappell makes is: "Don't substitute any other chemicals for those given". Since that is exactly what we plan to do I might also comment that we have had success at lower temperatures and slower cooling should enhance the effect.

People who use this glaze employ a variety of methods to increase the variation of surface color (i.e. stippling a second layer, brushing a wash of another coloring oxide, double dipping, applying a wash of rutile, etc.).

Nepheline Syenite 47.90 44.5
Gerstley Borate 27.00 25.1
Silica 20.30 18.8
EPK 5.50 5.1
Iron Oxide Red 2.00 1.9
*Cobalt Oxide 1.00 0.9
Rutile 4.00 3.7
Floating Blue with GB
Floating Blue with Boraq 2
CaO 0.42 5.7 7.4
MgO 0.09 0.9 1.6
K2O 0.10 2.2 1.7
Na2O 0.39 5.9 6.9
TiO2 0.19 3.7 3.4
Al2O3 0.51 12.8 9.1
B2O3 0.43 7.3 7.6
SiO2 3.48 51.1 61.4
Fe2O3 0.05 2.0 0.9

CaO 0.43 5.8 7.4
MgO 0.09 0.9 1.6
K2O 0.10 2.3 1.8
Na2O 0.37 5.6 6.4
TiO2 0.19 3.7 3.3
Al2O3 0.54 13.1 9.2
B2O3 0.47 7.8 8.0
SiO2 3.56 51.4 61.3
Fe2O3 0.05 2.0 0.9


As you can see by the glazed tiles, Boraq 2 produces a visual effect that is very close to what GB does in this recipe. The flow test (show here) shows a melting comparison, again the two are identical in color and fluidity (clearly this is a very fluid glaze). However the runoff pool flow test shows that the Boraq version has slightly less boron-blue clouding (slower cooling should help if this proves to be a problem for you). We have not found an improvement with Boraq 3 in a flow test, although this was expected since it has more CaO. If you know why, please let us know? However we have had good results using the Boraq 2 version of the glaze on pottery, the visual effect is very similar (click on goblet for larger picture). It takes a little practice to learn how to apply the glaze to get the right thickness. Like the GB version of the glaze, it does tend to form dimples in the surface if the ware is cooled too quickly (due to the 20%+ gases of decomposition that come off during firing). Remember also that you can deal with slight color variations by adjusting the amount of iron and cobalt.

Substituting a Frit

There has been a lot of discussion about how to remove the Gerstley Borate from this recipe and replace it with a frit. However you must remember a couple of things.

Does your floating blue run off the ware?

The Boraq version of Floating Blue does go on thicker so you have to dip pieces quicker. If it is thicker it will run more. Much of the visual effect of Floating Blue is due to the fluid nature of the melt, it is 'on the edge' and it does not take much to make it start running. There is another possibility also. Boraq has a silica content that is a little lower than GB. We did this for good reasons (as explained elsewhere on this web site). Try adding 5% silica if the glaze runs.

Ravenscrag Floating Blue

The magic of this recipe is actually the iron:cobalt:rutile addition. You can put that into another transparent recipe (preferably one that has none of the problems of the 50:30:20 Gerstley Borate clear base). You will need a base that melts well and creates a fluid melt where the rutile can dance! A good example is the Ravenscrag Floating Blue recipe on the cone 6 glazes page.

Floating Red (Oxblood Red)

Gertley Borate 54.88
Talc 14.63
Silica 30.49
RIO 21.95 (that is alot of iron!)

This is a dark chocolate brown with streaks of red. It is very fluid at cone 6 and tends to bubble if fired too fast between 700 and 900C (1292 to 1692F) or reduced. More information coming soon.

Tony Hansen

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