Gillespie Borate runnier than Gerstley Borate?

[EmilyK]

I am wondering if anyone has advice about how to help with the increased melting fluidity of the Gerstley Borate substitute, Gillespie Borate. It's an almost exact 1:1 substitute, but is slightly more runny in some glazes. I work at a large and busy community studio, and since 9 of our glazes use GB and now need Gillespie Borate, I worry that it will create a chaotic drippy mess throughout the kiln.  Luckily, other than the increased glaze movement, the glaze tests with Gillespie Borate seem otherwise great. Does anyone have any slight changes or additions/subtractions to help with the Gillespie Borate that won't drastically change the overall chemistry of the glaze?

[Min]

Simplest thing to try first would be to reduce the Gillespie Borate by 3-5% in some glaze tests and see if that does the trick.

[EmilyK]

Great, thank you for your help! Would you recommend replacing it with anything to round out the recipe?

[Min]

I don't think it's necessary to add anything.

To make a glaze runnier one either decreases silica + alumina or increases the fluxes. To do the opposite, that is make a glaze stiffer, you do the inverse; either increase the silica + alumina or decrease the fluxes. 

Looking at the data for Gillespie Borate we can see that it has some silica (just under 12%) + very little alumina (under 2%) the rest of the oxides are boron + fluxes. Now look at the total Gerstley Borate you will be replacing, lets just for an example say a glaze has a rather high 30% Gerstley. If you reduce the substituted Gillespie by 5% that would be 1.5 grams less. Now look at the amount of silica, this would be 12% of 1.5 grams for a grand total of 0.18 grams less silica per 100 grams of base. This is negligible. 

If you find a 5% reduction in the amount of Gillespie isn't enough to control the runniness then reduce it further. Do the math to see if you need to top up the silica. Chances are it won't be necessary unless your glaze has exorbitantly high levels of Gerstley you are replacing.

Gillespie Borate analysis from Digitalfire below.

Oxide	Analysis	Formula
SiO2	11.80%	0.34
B2O3	24.50%	0.62
Al2O3	1.70%	0.03
SrO	0.45%	0.01
Na2O	3.77%	0.11
MgO	3.90%	0.17
CaO	23.00%	0.72
LOI	30.90%	n/a
Oxide Weight		120.86
Formula Weight		174.90

Edited September 6, 2023 by Min

[neilestrick]

Gillespie is known to be a bit stronger than Gerstley. I've always reduced Gillespie by 3%.

[EmilyK]

Amazing, thank you all for your help!