Original glaze with Gerstley Borate vs. improved version with frit

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).

Related Pictures

Gerstley Borate is passing on to a better place. With a 300% price hike.

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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.

This is how bad Gerstley Borate glazes can blister

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The words "blister" and "Gerstley Borate" are very often used in the same sentence. This recipe is 60% Gerstley Borate, 20% Silica, and 20% OM4 ball clay. The piece was fired a second time to try to heal the blisters but they only got worse. While it is possible to fire this without blisters beginners often end up with this result.

Videos

Media	Getting Frustrated With a 55% Gerstley Borate Glaze I show you why people love/hate this material and how I substituted it for Ulexite in this crazy recipe to make a far easier-to-use slurry that fires identical.
Media	Use Insight-live to substitute materials in a recipe We will substitute wollastonite for whiting and a frit for Gerstley borate in the G2571A cone 10 matte while maintaining the chemistry of the original recipe.

Links

Glossary	Glaze Gelling 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.
Glossary	LOI Loss on Ignition is a number that appears on the data sheets of ceramic materials. It refers to the amount of weight the material loses as it decomposes to release water vapor and various gases during firing.
Materials	Gerstley Borate 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?

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