My guess is that the increase in alumina will simply encourage nucleation (resulting in more crystals than you may want to have in terms of the aesthetic), long before it impedes running enough to matter. Therefore, a focus on the specific gravity constant of your hydrated glaze(s), the peak temp (including any hold there), and application position/ thickness would seem more valuable to me. All of these things translate into how much glaze and where it is on your pot. Additions of what may become “refractory seeds”, which you’ll have to try to dissolve at a temperature when the glaze will be flowing rapidly (the variable you’re trying to minimize), may just put you into another realm of hurdles.

Having said this, John Tilton uses additions of clay (silica/alumina) to his glaze, adding benefits to application as well as the opacity and color effects he gets (marvelous stuff!). I believe John told me that he could add 3-4% EPK to a glaze w/o too much change visually. John goes for smaller crystals, so his pots don’t get as crowded as mine would if I had to deal with the same number. But, Derek Clarkson also incorporated 1-2% alumina hydrate into his glazes to balance a formula to his liking, and he achieved huge crystals on wide open ground. -So I certainly don’t discourage testing…

Using your glaze base, I’d just mix & fire a simple line blend, incorporating additions of alumina hydrate or grolleg kaolin at 1.5 - 3 - 4.5 - 6% for referencing where to proceed. Remember to pay attention to the variables above (specific gravity, thickness, etc), or you may not be able to reproduce any goodness that you find.

I want to begin testing glazes with increased viscosity to help minimize their flow off the work and have been considering the addition of alumina as a starting point. How do you feel about this approach? Can you suggest an addition range? Will the addition of alumina increase the refractoriness of the glaze to the point where I will need to add additional frit?

Phil Hamling

Have fun!

~jesse.