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.

B. Glazes

Click here to watch this at youtube.com or click here to go to our Youtube channel

Let me show you what the slurry of Worthington Clear, a 55% Gerstley Borate recipe, is like: A dreadful bucket of jelly!

You'll hear the frustration in my voice about how anyone could use this and about two poor options to endure it:
-Adding water - No good because it will dry even slower.
-Add Darvan - No, it just gels again later.

Using an iPhone and my account at insight-live.com I show you how explore a couple of other options:
-Switching the EPK to calcined kaolin hoping that reducing the total clay content (Gerstley Borate is a clay), will make it gel less. That does not work!
-Switching Gerstley Borate for Ulexite (calculation done off-camera).

For that last option I demonstrate:

How I check the specific gravity.
How I add water to tune the slurry to 1.42 specific gravity.
How I flocculate it with a little vinegar (to be thixotropic), and then enjoy seeing how well it applies to test tiles.
How I use code numbers to organize testing.
How I flip back and forth between my PC and phone when doing the work.

The next day, using my tablet to get into my Insight-live account, we look at the fired GLFL balls, glazed terra cotta tiles and glazed L210 mugs.

Ulexite is difficult to obtain, I used it here just to demonstrate one way option. And even better option is using a frit.

Links

URLs	https://insight-live.com/insight/share.php?z=HzyNzj9ELs Replacing the Gerstley Borate in recipes containing 50% or more of it
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?
Materials	Ulexite A natural source of boron, it melts at a very low temperature to a clear glass.
Materials	Gillespie Borate A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.
Articles	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.
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.
Typecodes	Gerstley Borate Glaze Calculation examples Examples of how we use glaze calculations (in an Insight-Live.com account) to replace Gerstley Borate with other materials, especially frits, in various glaze recipes. In doing so we take the opportunity to improve the recipe in other ways (e.g. reduce thermal expansion, improve slurry properties, reduce bubbling and crawling).

A cure for long-time Gerstley Borate sufferers

Glazes using frits instead of Gerstley Borate

These are various different terra cotta clays fired to cone 04 with a recipe I developed that sources the same chemistry as the popular G2931 Worthington clear (50:30:20 GB:Kaolin:Silica) but from a different set of materials. The key change was that instead of getting the B₂O₃ from Gerstley Borate I sourced it first from Ulexite (G2931B) and then from a mix of frits (G2931K). All pieces were fired with a drop-and-hold firing schedule C03DRH. Fit was good on many terra cottas I tried (pieces even surviving boiling:icewater stressing). Where it did not fit I had thermal expansion adjustability because more than one frit was sourcing the boron. Frits are so much better for sourcing B₂O₃ than Gerstley Borate (the latter is notorious for turning glaze slurries into jelly!). Of course, a little glaze chemistry is needed to figure out how to convert a recipe from Gerstley Borate bondage to frit freedom, but there is lots of information here on how to do that.

Why would a glaze turn into a jelly like this?

This is one of the things Gerstley Borate does to glazes (when its percentage is high enough). This slurry has a high water content and should be far more watery. It is also highly thixotropic - this can be stirred vigorously to thin it and yet within seconds it turns back to jelly again. This was deflocculated with Darvan yesterday and it was stable enough to dip bisque ware - but overnight salts have gone into solution and it is no longer dispersed. It also dries very slowly on bisque ware. What can be done with a mess like this? Replace the Gerstley Borate with something else. Gerstley Borate sources B₂O₃, it can be supplied using frits or Ulexite (glaze chemistry calculations are needed to juggle the recipe), this can be done in an account at insight-live.com.

Colemanite and what its decrepitation does in glazes

Decrepitation refers to a decomposition accompanied by scaling, delayering, even disintegration of the glaze layer. Moving rightward these glazes have increasing percentages of colemanite. At its worst (far right) the glaze is spattering off the sample and onto the kiln shelf. The others are crawling, first pulling away from the corners (far left) moving toward pulling away on the flat surfaces (center). Gerstley Borate and Ulexite, similar minerals, are far less likely to do this (but they have other serious issues also). A much better solution is to use frits to source the oxide B₂O₃ (easy to do in your account at Insight-live.com). Photos courtesy of Nigel Hicken.

Click here for case-studies of Insight-Live fixing problems

You will see examples of replacing unavailable materials (especially frits), fixing various issues (e.g. running, crazing, settling), making them melt more, adjusting matteness, etc. Insight-Live has an extensive help system (the round blue icon on the left) that also deals with fixing real-world problems and understanding glazes and clay bodies.

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).

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

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.

Ferro Frit 3134 is NOT A SUBSTITUTE for Gerstley Borate

This frit, or any of similar chemistry (e.g. Fusion F-12) IS NOT A 1:1 SUBSTITUTE for Gerstley Borate, their chemistries are too different. That being said, the frit sources lots of B₂O₃, that makes it a candidate to weave into recipes as the source of boron. To show the difference I have put 100 parts of each in side-by-side recipes in my Insight-live.com account, set the calculation type to non-unity formula and increased the frit until the B₂O₃ in the two match. Notice it takes 118g of the frit to source the same amount of B₂O₃ as 100 GB. Notice also that the frit sources almost triple the amount of Na₂O per weight unit (that is a big deal because it means the recipe containing the Gerstley Borate to be substituted needs an Na₂O-sourcing material that can be reduced to compensate). And the frit sources 3.5 times the SiO₂ (other SiO₂-sourcing materials in the recipe will need to be cut to compensate). And the Gerstley Borate has significant MgO while the frit has none (so an MgO-sourcing material like talc will be needed). Minor tweaks will also be needed to reduce other sources of CaO (since the frit has quite a bit more). The recipe will also need enough flexibility to do the final matching of Al₂O₃ and SiO₂. The GBMF test confirms the difference at 1700F, these 10-gram balls melted down onto the tiles very differently.

By Tony Hansen
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