|Monthly Tech-Tip |
This is a GBMF test, it compares the melt fluidity of the Gerstley Borate based cone 6 Perkins Studio clear recipe original (left, our code number G2926) and a reformulated version that sources the boron from Ferro Frit 3134 instead (right, our code number G2926A). The latter is less amber in color (indicating less iron). The good news was that it melted so much better that we were able to add significant Al2O3 and SiO2 to really drop the thermal expansion (improving glaze fit on common clay bodies), which produced our G2926B base recipe. Every time I use it I think of how unfortunate we would have been had we continued to use the Gerstley Borate original.
Gerstley Borate has just become Costly Borate. The supplier, LagunaClay.com, likely raised the pice to wake us all up to take action in substituting it before supplies run out. It is a ceramic glaze flux, sourcing boron to melt far better than any other common raw material. It has been a foundation material in low and middle temperature pottery glaze recipes for many decades. Potters have a love/hate relationship with it: Enjoying its low melting point but enduring its problems (inconsistency, gelling of the slurry, crawling, micro-bubbles, boron-blue discoloration). Strangely most people have used it without knowing what it really was. And few realize how easy it is to replace. Yes, existing substitutes work sometimes - but it is better to adjust each glaze recipe to source boron from a frit (fixing other issues also). Please read that last statement carefully. It did not say that there is any frit that can substitute. It said that frits can source boron.
These pieces are fired at the same temperature. The glaze on the left is a popular recipe found online, Worthington Clear (our code number G2931). The Gerstley Borate in it is "farting" as the glaze is melting (the calculated LOI of the glaze as a whole is 15% mainly from that one material). Unless applied very thinly tons of micro-bubbles appear (this example is fired to cone 03). And strange, it is crazing badly also despite the low calculated thermal expansion. Using my account at insight-live.com I was able to source the B2O3 and MgO from a frit (actually two frits) to create the G2931K recipe. Although the thermal expansion calculates higher it is strangely fitting better (albeit not firing as white). As you can see, the new fritted glaze is very glassy and clear (thick or thin).
G2926B - Cone 6 Whiteware/Porcelain transparent glaze
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
GerstleyBorate.com - The best place for info on Gerstley Borate
Glaze slurries can gel if they contain soluble materials that flocculate the suspension. Gelling is a real problem since it requires water additions that increase shrinkage.
Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
Trafficking in Glaze Recipes
The trade is glaze recipes has spawned generations of potters going up blind alleys trying recipes that don't work and living with ones that are much more trouble than they are worth. It is time to leave this behind and take control.
|Oxides||B2O3 - Boric Oxide|
How I Improved a Popular Cone 6 Clear Glaze Using Insight-Live
How I found a ceramic glaze recipe on Facebook, substituted a frit for the Gerstley Borate, added the extra SiO2 it needed and got a fabulous more durable cone 6 clear.
A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.
Gerstley Borate was a natural source of boron for ceramic glazes. It was plastic and melted clear at 1750F. Now we need to replace it. How?