Potters & Sculptors - Making Rock from Mud
I've been trying to work out a raw glaze recipe, after reading a few books suggesting ball clay works better for raw glazing leather hard pots. This glaze is intended to fire to ^8 and will be in an electric kiln, although perhaps gas firing in the future. I'm planning to make up a kg or so to test. Just wondering if anything sticks out as problematic to anyone? It is withing the green and copper limits for this cone range, and expansion seems ok according to a glaze simulator I used.
|HP 71 Ball Clay||32.00|
|BPS Soda Feldspar||32.00|
|BPS Calcium Borate Frit||10.00|
I have some bentonite I could add if needed after a bit of testing
HP71 has 0.80% iron with 19% alumina. Old Hickory #5 has 0.88% iron! but nearly 30% alumina. Just as alumina gives glaze strength, it likewise does the same for clay. In short, #5 will make your glaze stronger and more resistant to cutlery marks. At cone 8, the 32% soda spar is probably right on the border for a melt; most cone 6 recipes typically have 50%.
By the way, with that much ball clay, bentonite for suspension should not be required.
Thanks Tom, my local UK supplier only sells hp71 ball clay so I'm stuck with that for now. I guess I could reduce the quartz if alumina is too low? But I suppose that will result in Matt glaze. When your say right on border you mean only just melting and ^8 ?
Look at the glaze this way: a standard porcelain clay body is 50% kaolin, 25% silica! and 25% feldspar. Your glaze recipe has 32% clay content; the most I have seen prior to is 20%. If cone 6 glazes average 50% spar, I would guesstimate 40% minimum at cone 8. With that amount of clay content, it will need some added flux to melt it. 32% clay content will require more flux and heat! and will tolerate more flux and heat.
i looked up some info on Potclay's site in the UK. hVA/R 3408 has 27% alumina, 0.90% iron! and zero carbon content. You have pay closer attention to carbon content when using clay in a glaze. Carbon plays a role in pin- holing and bloating: so 0% carbon is well suited for a glaze recipe.