Specific Gravity and Bentonite

[Robin Cherfane]

Does anyone know what a good specific gravity is for VC satin matte? Also how much Bentonite is too much? I put 2% in this glaze and it is still hard panning. Do I add another 1% or try something else?

[Norm Stuart]

Robin -- Val Cushing Satin Matte is a "magnesium matte glaze" so adding more bentonite like Veegum is not likely to change the chemistry, until you get to the point there's enough magnesium to cause crawling in the glaze. - See the comparison below with 5% bentonite.

Bentonite is mostly silica (in a five to one ratio with alumina) with magnesium and/or calcium.

The 6 to 1 Silica to Alumina ratio of Bentonite is typical for a transparent glaze, which is what makes it an ideal additive. Kaolin is a ratio of only two parts silica to one part alumina, so adding more kaolin without also adding more silica would change the character of the glaze. The magensium in bentonite does denature low-fire cadmium sulfide reds, turning them black.

With only 10% clay there's not really enough to keep the whiting, frit and silica suspended. The feldspar you use may also leak sodium and potassium over time. In addition to adding bentonite you may want to try adding calcium chloride (ice melt crystals) to help the limited amount of clay maintain its suspension. Adding Epsom salts (magnesium sulfate) may add to the magnesium level enough to leave you with a crawling glaze.

[Robin Cherfane]

Hi Norm

The recipe I used is as follows: Whiting 20, Frit 3124 - 25, Neph Sye 25, EPK 15, Silica 15, Zircopax 10

I made a 5000g batch, I may have screwed it up. I had added 2% Bentonite, that did nothing, so then I added some Epsoms Salts. For now it seems suspended. Hopefully it won't crawl. I wish I new more about the chemistry. If crawling does happen how can I fix it, if at all?

[Norm Stuart]

Robin - Nepheline Syenite is a notorious source of free sodium and potassium ions which cause a glaze to hard-pan. 

You get an flush of these atoms as the water leeches them from the particles of nepheline syenite -. and an ongoing source of sodium and potassium as bacterial action breaks down this raw material further.

This is far less of a problem when using other feldspars.  All glaze containing a big percentage of nepheline syenite will hard-pan over time, even if they contain 20% clay. Sodium and potassium ions make a conductive electrolyte which neutralize the charges on clay particle which make them repel each other.

The easy solution to your problem is the addition of calcium chloride, or epsom salts to flocculate the 15% clay (EPK). 

When you buy a deflocculant like Darvan or Sodium Silicate, they work by locking up calcium and magnesium ions and replacing them with a sodium ion. So these two flocculants add back the calcium or magnesium ions normally present in most water.

The recipe you're using is a calcium semi-matte which has for some reason been fluxed way down, I'd guess, to a Cone much lower than Cone 6. I bet you could get a nearly identical look with half as much zircopax. Zircopax is somewhat refractory, so at 10% that may explain all the low temperature fluxes flux used.  The added bentonite will not have changed the fired look of the glaze at all as the chemistry either way is almost identical.

Just add a tablespoon of calcium chloride or epsom salts to the bucket.  Of course you'll still need to use manual force with a scraper to pull this glaze off the bottom of the bucket and get these new ions mixed with it.

[Robin Cherfane]

Thank you Norm, your information and explanations are invaluable!!  I added the epsom salts, all is nicely suspended and silky smooth. I'll be sure to let you know how it fires.