search  current discussion  categories  teaching 

need lesson on additives to glazes--suspension

updated mon 26 dec 05


Dave Finkelnburg on tue 27 dec 05

I realize this is an old post of yours I am responding to, but I am just now having a chance to read through some of the last 5,000+ Clayart posts I haven't seen this last few months.... :-(
Your question is a good one, one that's troubled a lot of us. I'll try to answer briefly what is a very complicated subject.
For starters, a glaze with 10% kaolin or ball clay will suspend and apply reasonably well at a density of 1.4 to 1.6 grams/cubic centimeter.
A glaze with more clay will do well toward the lower end of this range. A glaze with less clay will do better at the higher end of the range. The reason is clay naturally flocculates, and the more clay you have in the recipe the more flocculated your glaze is.
IF there is no or very little clay, then you need some ingredient in a glaze with a lot of surface area. Such ingredients include bentonite (Wyoming is good), Veegum T, etc. No, don't automatically add these. Add these if the glaze won't suspend on its own well enough at the density you want to use.
Epsom salts (magnesium sulfate), works to coagulate the flocs in a glaze. The glaze MUST contain enough clay or bentonite or other high surface-area material or the magnesium Mg++ ion will not work! CAUTION: if you add too much of any "salt," which includes sodium, potassium, Mg, calcium, you can cause the glaze to form a stiff gel. One-fourth of one percent by dry weight of Epsom salts is a good place to start. 0.7 percent (7/10 of one percent) is about the upper end of what you need. IF you overdose a glaze, the only cure is to add more of the entire glaze ingredients EXCEPT the Epsom salts.
Ron Roy taught me some years back, and I think he has mentioned recently, that replacing kaolin with ball clay reduces the tendency of a glaze to "dust" when it dries. The ball clay dries to a harder surface. Bentonite is even more effective in this regard. HOWEVER, these finer clays are slower to dry, so the more of them you use in a recipe, the longer you have to wait for the glaze to dry. That's why with a high-clay glaze you don't want to also have bentonite! And, you really don't want to add a coagulant to such a combination. It will really dry slowly. I have made this mistake. :-(
Finally, Darvan. Any deflocculant will reduce the viscosity of a glaze-water slurry. That also means it is easier for the solids in the glaze to settle out. So, if you want the glaze to suspend well you need to balance the deflocculant by using less water in the glaze, or use extra clay, bentonite, or agitation to keep the ingredients well-suspended. I like a deflocculant to make a very fluid, very dense glaze with little water--say a glaze to spray on very dense bisque or already-glazed ware. It doesn't make sense to me to deflocculate a glaze and then add clay to thicken it.
To use Darvan or another deflocculant properly, you should add it on the basis of the Specific Surface Area (SSA) of the ingredients in the glaze batch. This is usually expressed in square meters, and is calculated by multiplying the SSA in m^2/gram X grams of clay. Add Darvan at about 0.1 mg/square meter of surface area of the glaze ingredients (a milligram is one one-thousandth or 0.001 gram). You can assume the SSA of EPK is about 25 m^2/g. Ball clays are usually a little less, 20 to 25 m^2/g. 25 is a good average number for all your clay. The SSA of bentonite depends on the source, so check with your supplier. Since feldspars, frits, whiting, dolomite, anything ground up to use in the glaze has an SSA of only about 1 m^2/g, you can ignore them.
I have not discussed gum solutions because I do not know enough about how they affect glaze suspension. I do know they are excellent at making a glaze surface harder, but I think they can make a glaze difficult to dry if too much is used. 0.2 to 0.7 percent of the recipe on a dry weight basis is a good amount to test. The more clay or bentonite in the recipe the less gum you want to use. Test, though, because you don't want too much gum.
This only scratches the surface of this subject but I hope it gives you a little information to build on. Good potting!
Dave Finkelnburg, thinking that whether you celebrate Christmas...whether you celebrate the New Year on January 1...whatever you believe...what brings us together though Clayart is our common interest in clay...that what we have in common is so much greater than the differences we perceive...and that for me, personally, life is so much richer because of the people I have met through clay...thank you for all you have done for me. :-)

Mary/Adams <> wrote:
What do you do with what and when? Gum solution, Bentonite (do you add it
automatically to every recipe and then how much), Darvan 7, epsom salts,
vinegar, on and on......

Yahoo! for Good - Make a difference this year.