Since my base glaze is almost half gerstley borate, I had to look for a substitute when the price shot up due to the raw material being used up. I have used my base glaze for years, saving on cost and space by mixing my own. Plus, the recipe was short and had few raw materials, and without oxides would fire almost clear. Many have used the recipe as a clear glaze and have been happy with it. The base recipe is as follows:
Gerstley Borate – 50
EPK. – 20
Flint. – 30
I have used this base in both electric and in gas reduction and worked great as a cone 5/6 glaze.
As you can imagine any change in the material makeup will have changes and testing for melt, fit, and durability should be done on your clay body before using any advice from myself or others. At times issues have come up with raw material that are labeled the same but from a different mine or from a different supply company. So be sure to test everything.
So…RIP Gerstley Borate!
Substitutes And Replacements
To me, substitutes and replacements are two different things.
A substitute can replace a raw material in equal amounts with little or no change to the glaze melt, fit, or hardness.
A replacement is using a raw material or materials to make up for the chemical structure or the raw material it is replacing. Therefore, you have a new a different glaze.
Below are a few raw materials that can be used in recipes as substitutes for gerstley borate, and have been known to work well in most cases when substituted in equal amounts. But as stated above these all need to be tested for melt, fit, and durability on the clay body you use.
Gillespie Borate has been shown to be a good substitute for Gerstley Borate. It almost has the very same melt and flow as gerstley borate. It is known to have fewer impurities and can make certain colors lighter or brighter. It seems to not settle as quickly in the pail as gerstley does. It seemed with Gerstley I had to stir the glaze every ten pots to keep the glaze in suspension in the pail.
With this, use only a little water or take some out when it settles, as it’s not as thick as gerstley when mixed. I still need to get a specific gravity number for this and test different bisque temperatures, as with my clay body, I bisque lower and fire slower in the gas up drafts. You may read more on Gillespie Borate Here.
Laguna Borate has the same fluxing ability as gerstley botate and can be used as a good substitute. While far less expensive than gerstley borate at the time of this writing , it is still more expensive than the Gillespie Borate mentioned above. If you have some of this raw material, test and use it for equal amounts. I have not seen it for some time on the Laguna website, and they may have discontinued it for some reason.
I accidentally got a bag of this way back when testing a glaze. I mixed it up and it worked out great. Frit 3134 is Frit 3124 without the Alumina. Frit 3134 is can replace Gerstley borate in a recipe. But if you do use it, know you have a different glaze and be sure to test it with your clay body. You may see the results of a glaze where Frit 3134 replaced Gerstley borate and worked out fine by clicking here.
Ferro Frit 3134 50.0
If you would like a more modeled and satin look, you could try a test with Frit 3195. Here if the test melts too much for a cone 5/6 add both Kaolin and Silica in 10% increments evenly to fire hotter. Using Frit 3195 it may need to be fired a bit slower as it melts sooner and may need time to allow the gasses to escape before it heals over. You may see a cone 5/6 glaze here using Frit 3195
Wood Ash at mid-range can be used as a flux. However, it is not a good substitution but can act as a replacement as a flux when mixed 3 to 1 ratio with a frit. Here, a bit of Whiting is added to make the glaze melt more durable. But as you can see it is quite close to the recipe of my base above, and I have ash from my wood stove already.
FF 3134 20.0
So as seen in the recipe above, you can eliminate the gerstley borate by a combination of other fluxes to cause the ash to melt properly at cone 6. A nice article on how to process ash glaze to use it as a flux can be found here.
I think it best to just say rest in peace, Gerstley Borate! You got us all through the loss of Colemanite back in the day, and we thank you! But all in all I’ll have to mix less often and will not need to worry about glaze crawling as much.
Have you made the transition from Gerstley Borate yet? Let me know in the comments. Also, feel free to give feedback on anything you would like to add!
Creek Road Pottery L.L.C.
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