Common Gerstley Borate Glossy Glazes

Project Name

Gerstley Borate Transparent Glossy Base Glaze Recipes

Project Codenumber

UnAssigned

Notes

Gerstley Borate was found in amounts up to 50% of the total recipe, creating very fluid base glazes (at both low and middle temperatures). Depending on the amount of feldspar present, recipes had from 5-20% kaolin (the two together supplying the needed Al2O3). Silica was almost always present, typically 20-30%.

These recipes are intended as example of base clear recipes (although some show pigment and variegator additions).

GB by itself melted well even at cone 06. So, as you can imagine, recipes having 50% would have a very fluid melt at cone 6. No wonder some reactive visual effects were achieved (e.g. variegation, crystallization, varying degrees of milkiness, boron blue and opacity according to thickness). Additions of rutile (for yellowish and brown colors) and titanium. Being fluid transparent it could support significant amounts of iron and yet still be quite light in color.

The melt fluidity was also a benefit in that darkly colored overglazes feather into lighter colored matte surfaces. Of course this is an issue with underlglaze decoration. And there are solubility and hardness issues with very high boron.

While many people believe that Gerstley Borate is the key to the variegation of glazes, this is not true. The visual character was most often because of titanium, rutile and lithium additions into an already very fluid-melt high-boron base. Fritted transparents can also be fluid-melt and they can host these same additions and produce variegation also.

Low percentages of kaolin (EPK) can be remedied by sourcing KNaO from Ferro Frit 3110 (instead of feldspar or nepheline syenite). That will produce a slurry that does not have issues with gelling (the way GB slurries did).

50:30:20 Gerstley Borate Cone 6 base

Total:100.00

Auto Unity Formula

Notes

*This is the base for many recipes (e.g. Butterscotch (Rust) added: 4% rutile, 4% tin oxide and 1% lithium carboante.

This fires with plenty of boron blue and bubble clouding, but that was seen as part of the aesthetic of the surface. It has a low thermal expansion and fitted most clay bodies.

Mixing this 50:50 with water produces 1.44 specific gravity, but it will be pure gel. Adding 0.5% Darvan helps to thin it for long enough to dip bisque samples (but the glaze goes on very thin). The slurry re-gels within seconds.

For making flow tester balls this slurry dewaters very slowly on a plaster bat.

GB is a clay, capable of suspending and dry hardening a glaze at only 25% concentration. Yet this has double that plus adds kaolin which would further increase drying shrinkage! It is amazing that it was even possible to make a useable slurry out of this!

Pictures

G2826A vs G2826A1 low test

A1 supplies boron using frit 3134.
Since the frit contains less boron the full B2O3 cannot be supplied and it has a little more Al2O3 and SiO2.
But the A1 has 6 times lower LOI.
G2826A is a good recipe for comparing GB substitutes.

G2826A Gerstley Borate 50:30:20 base as jelly

G2826A on M390 - lots of boron blue

You might think this looks nice. But it comes at a cost (bubbling, jelling, running, cracking on drying, crawling). Boron blue is considered a glaze fault.

That being said, many people like gerstley borate for this very reason.

Fired at 1700F G2826A 50:30:20 GB

Fired at 1700F
G2826A 50:30:20 GB
G2826A1 same chem using frit
G2826A2 50:30:20 using Gill Borate (should be crawling??)
G2826A3 50:30:20 makeover using GillBor
G2826A3-2 same but with calcine kaolin

Pattis Crystal Clear Cone 6

Total:50.00

Auto Unity Formula

Notes

*The glaze does have a fairly low thermal expansion.

It is quite similar to our G2926B M370 clear glaze (although having lower SiO2). So it is not clear how it can be melting as well as it is.

Pictures

Anna Rasmussen pitting Pattis Clear

Decal firing. M390.

Floating Blue Original Cone 6 Glaze

Total:107.70

Auto Unity Formula + Analysis

Notes

*This recipe has been used for many decades. Its secret is the high melt fluidity (because Gerstley Borate is such an active melter) and the colorant addition. The rutile variegates the color imparted by the iron and cobalt.

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: "Don039;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.).

This recipe is coming to end-of-life since Gerstley Borate (GB) will soon be unavaiable. While substitutes are available it is better to source the B2O3 from frits (a classic example of the value of glaze chemistry).

This calculates to a very high thermal expansion, crazing is likely.
The high Gerstley Borate content means it is likely to gel.

Pictures

G2826R Floating Blue Original Recipe glaze sample

G2826R Floating Blue melt flow test

Compares Gerstley Borate and Boraq 2
This is a good recipe for comparing Gerstley Borates substitutes.

Randy's Red Original Cone 6 Glaze

Total:117.10

Auto Unity Formula

Notes

*The most widely known iron red recipe. But it is also one of the most widely hated. The combination of Gerstley Borate and high iron turns the slurry into super-jelly. Thinning it with water just gels again. It also shrinks and cracks on drying and crawls on firing. People endure it in a quest to get the red it can produce.

This recipe may have a new life since the above problems are eliminated or much less when using Gillespie Borate.

Pictures

Example of a picture search of Randy's Red on google

All of these people suffer using this!

Gerstley:PV Clay low fire clear

Total:100.00

Auto Unity Formula

Notes

*The Plastic Vitrox : Gerstley Borate combination was very popular and used for many years as a base glossy glaze for cone 6. This recipe doubtless originated in California since PV Clay is mined there and was in common use in the 1970s and earlier (i.e. the California Artware casting body contained PV Clay). Since this recipe is comprised entirely of clay-containing minerals it shrinks and cracks off ware when applied too thick (actually, it is hard to understand how it is even possible to apply it without cracking). On the other hand it makes a good slip to apply to leather hard ware since it will shrink with the clay as it dries.

To create a white from this most people add 10-20% zirconium silicate.

Pictures

G2933 Gerstley:PV while blistered glaze

G2933 Gerstley:PV Clear