Borax again
I found this seller talks about the wonderful benefits of borax to ceramics glaze :
Benefits of Borax:
For glazes and enamels, boric oxide is unique in that it acts simultaneously as a glass former, as a flux, and as a viscosity stabilizer that prevents the glaze from running too much while it is being fired. Perhaps most important of all, boric oxide reduces the thermal expansion of the glaze so that it can be matched to the expansion of the underlying ceramic or metallic body. At t
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The problem with solubles in a glaze slurry is that the dissolved flux materials (e.g., borax, soda ash, sodium leaching out of neph sye or some frits) moves into the interior of the bisqued ceramic with the water as the glaze is applied. Then it stays there. Because it is a flux, the ceramic body is fluxed (and thus overmelts), not the glaze. It you want to use a soluble material in a glaze, you have to use the glaze right away, before it has a chance to fully dissolve into the water, or used i
