Raw glaze recipe

[Tom Humphries]

Hi,

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. 

Thanks! 

Tom

BallClay2
HP 71 Ball Clay	32.00
BPS Soda Feldspar	32.00
Quartz	14.00
Wollastonite	12.00
BPS Calcium Borate Frit	10.00

[Tom Humphries]

I have some bentonite I could add if needed after a bit of testing

[Tom Anderson]

Tom:

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%. 

Tom

[Tom Anderson]

By the way, with that much ball clay, bentonite for suspension should not be required.

[Tom Humphries]

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 ? 

[Tom Anderson]

Tom:

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.

Tom