We have researched many glaze types and provided detailed information about using Boraq instead of GB in each. Fired samples are displayed where appropriate and we provide information and invite comment to help isolate the cause of any differences. We invite you to continue to email scans of your test tiles to us with information so we can post them here for comment. If your recipe is unique we will not reveal it (we will show the oxide formula however).
It appears that a one-for-one substitution in all glazes is not likely to surface. Variations in the amount of Boraq may be appropriate. More important, we have created two adjustments (simple additions of whiting and/or dolomite) to the Boraq 1 recipe. Where these adjustments are appropriate it is noted.
At cone 6 Boraq 2 is working best (Boraq 1 is too fluid because it falls a little short on the CaO and MgO content in natural Gerstley Borate).
We have had very good success using Boraq 1 on a number of common recipes (Boraq 2 is too refractory for use at low fire).