Remove Gerstley Borate and Improve a Popular Cone 6 Clear Glaze

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. Insight-live

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

I show step-by-step how I killed the troublesome Gerstley borate material and fixed the crazing in the popular cone Perkins 6 glaze recipe (this method would also work with Gerstley Borate substitutes like Gillespie Borate). I also show how I used my account at Insight-Live.com to manage a testing project on this glaze and the surprizing things I learned and the unexpected benefits the recipe accrued on improvement. Following is an outline transcript I used to make it.

This caught my attention on Facebook so decided to test it
-Copied from Facebook: create a new recipe, import it, name it, code number it as G2926
GB not linking, fix it.

Mixed 2000 grams (mix ticket)
Glazed a mug and fired it
-It is very transparent, didn't craze out-of-the kiln on my porcelain. Good? Not quite.

Appraise it, its materials:
-Need to kill GB (dont have it, it gives amber to clears, causes blisters, gels slurry)
-Click GB (sources boron, a little MgO)
-Duplicate it, edit the copy, name/code#. Replace the GB with Frit 3134, Save and Done

Where are we?
-KNaO higher, B₂O₃ lower, MgO gone
-Edit and add a little Talc to source MgO, Save and Done

Juggle the recipe to match the chemistry
-Enter calculation mode, set increment at 0.5, enter MgO,

-Set Increment to 2 and add frit to match B₂O₃
-Al₂O₃ low, we can add clay, great! But not yet.

-Reduce Nepheline to match KNaO

-Correct B₂O₃
-Switch back to 0.5 and correct MgO
-Switch to 2 and add kaolin
-Adjust B₂O₃ again, then SiO₂
Retotal recipe

-Glazed a mug: good
-A flow test to compare with original GB version (will attach it here)
-Did 300:icewater on a mug, crazed: expansion too high
-Another to compare to a standard I already know. Running too much.

Will accept silica.
Add 10% silica and retotal again
This was tested. Result was excellent

Do white variation (will be harder, flow even less, lower expansion)
-Print recipe for 2000 g

The lessons are:
-Test your glazes thoroughly
-Replace the problem materials
-Make sure they fit your clay
-Keep a good audit trail of your tests, use code numbers and pictures. Do it at insight-live.com.

Links

Articles	A Low Cost Tester of Glaze Melt Fluidity This device to measure glaze melt fluidity helps you better understand your glazes and materials and solve all sorts of problems.
URLs	https://digitalfire.com/university/insight-live/overview/Insight-Live%20Overview.html Insight-Live.com Overview Video
URLs	http://m.youtube.com/user/tonywilliamhansen YouTube channel for Tony Hansen
Oxides	SiO2 - Silicon Dioxide, Silica
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	Gillespie Borate A Gerstley Borate substitute that became available during the early 2000s and is still available in 2023.
Recipes	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.
Glossary	Transparent Glazes Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
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).
Troubles	Glaze Crazing Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.

Use a low silica porcelain to craze test your glazes

Cone 6 transparent glaze testing to fit Plainsman M370: Left and right: Perkins Studio Clear. The far left one is a very thick application. Center: Kittens Clear. The porcelain for all is Plainsman P300. Why? Because P300 is much more likely to craze the glaze because it has a lower silica content (about 17% and only kaolin whereas M370 has 24% silica plus the free quartz that comes with the 20% ball clay it also contains). If a thick layer works on P300 it is a shoe-in to fit M370. If it also passes the oven:icewater test.

How can you test if a different brand of tin oxide will work?

This is a melt fluidity test comparing two different tin oxides in a cone 6 transparent glaze (Perkins Clear 2). The length, character and color of the flow provide an excellent indication of how similar they are.

I improved a cone 6 transparent glaze by removing the Gerstley Borate

The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at insight-live.com. In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt-flow GLFL test comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica (see the G2926B recipe that adds it and switches it to 325 mesh). This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear.

An example where adding silica really helps a glaze

The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater IWCT test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.

Frit made this Gerstley Borate glaze much better

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.

Adding silica will fix crazing, right? Not here.

Thee porcelain mugs, the glaze on two are crazed

G2926B (center and right) is a clear cone 6 glaze created by simply adding 10% silica to Perkins Studio clear (a glaze that had a slight tendency delay-craze on common porcelains we use). Amazingly that glaze tolerated the silica addition very well, continuing to fire to an ultra gloss crystal clear. That change eliminated the crazing issues on most of our bodies. The cup on the right is one of them, that body is vitreous, near-zero-porosity, and fits most glazes. Why? Because it has 24% silica in the recipe. The center porcelain is also dense and vitreous, but it only has 17% silica, that is why it is crazing this glaze. Then I added 5% more silica to the glaze, it continued to produce an ultra smooth glossy, and applied it to the 17% body on the left. Why did not fix the crazing? That silica addition to the glaze only reduces the calculated expansion from 6.0 to 5.9, clearly not enough to fix the problem. So, the obvious solution seems to be use the porcelain on the right. Are you wondering why adding silica to a body raises its thermal expansion, and adding it to a glaze lowers it? Mineralogy is the reason.

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.

By Tony Hansen
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