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Will Lowering Specific Gravity Always Make A Glaze Less Likely To Run?

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Recently we changed bentonites and our newly mixed glazes look way thicker than they used to even while I'm

adding more water.  The specific gravity measurements are a little lower than they used to be and the glazes look

flocculated.  Is it possible that a watered down glaze could run more, simply because it's over flocculated or had too much bentonite added?  All of this seems very counterintuitive to me. 


I always kind of assumed that for a given mass of glaze material, adding water would thin the glaze and

make it less likely to run.  Taking out water would thicken it up and make it more likely to run.  That is, if I drop the  

specific gravity, there's going to be more water, and less chance of the glaze running.


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Recently we changed bentonites "


The beginning of your post is the answer to your question. Most bentonite sold in the pottery trade is sodium based. Sodium bentonite has a high cation exchange rate: it becomes gelatinous quickly/ instantly. So what seemed thicker to you with less water was actually electrophoresis. Fancy word for " man that stuff got thick quick!" After stirring awhile and some timed passed: not so thick no more. My guess you have purchased calcium bentonite, which has a much lower cation exchange rate. Now it seems it requires more water to hit the same viscosity. Calcium bentonite clay (smectites actually) will hold more water in its particles without becoming gelatinous. So what you perceive as a change in viscosity is actually a change in chemistry.

Calcium bentonite is a much more stable product than its cousin,just make the adjustments in water and go from there. You will also find calcium bentonite a much better suspension agent.


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Thinning a glaze with water will make it less likely to run only if you now apply it thinner. If you hold the pot in the glaze longer to make up for thinner than you will get a thicker layer and it will be more likely to run. Whats important is the glaze thickness on the bisque ware not so much on what bentonite you used-the most important thing is glaze thickness .

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Calcium bentonite is usually a tan to light green/gray color: but it can also be the classic gray color. It is often referred to as fullers earth. Typically it runs $4-5 per pound. Sodium bentonite clumps (congeals) as soon as it hits water. Calcium bentonite will disperse uniformly. Cation exchange is also temperature sensitive, so using warmer water will help the clumping problem that sodium bentonite has.


Sodium is just a flat nasty element, very corrosive with high reaction rates. Sodium based clay will corrode the wheel head, pit a pugger, and crystallize in a bucket of glaze. It will eat fire bricks during a salt fire, eat elements, and should not be used as a flux in a clay body-- but it is. Hydrochloric acid is processed from sodium chloride; that should say enough about sodium.



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I'm going to make things a little more concrete.


Matte Gray Glaze


Custer Feldspar 6 %

Silica 8 %

Wollastonite 20 %

Kaolin 40 %

Frit 3134 12 %

Talc 14 % : 100%


Mouse Gray M. Stain 8 %

Bentonite 2 %


This stuff is the thickness of wet concrete. Today, I added water till I got down to a 1.25 specific gravity.

It is still crazy thick - thicker than heavy cream. Can I just keep adding water? Should I deflocculate?






Panama Blue



Custer Feldspar 44.1 %

Silica 15.8 %

Whiting 2.6 %

Kaolin 2.6 %

Dolomite 7.7 %

Strontium Carbonate 4.2 %

Frit 3110 10.7 %

Frit 3134 9.7 %

Zinc Oxide 2.6 % : 100%


Tin Oxide 2.6 %

Copper Carbonate 1.75 %

Bentonite 2 %




I've chucked in 4% bentonite (that's a lot) and added a tablespoon of epsom salts to this glaze. It looks heavy as

whipping cream, but it still gives me a thin covering, curtains badly, and dries crazy slowly. Should I just add more bentonite or epsom salt? The specific gravity is 1.46. That's about in the range for our cone 6 electric oxidation

dipping glazes. If I take out any more water, I'm afraid I'll have to put it on with a putty knife.


Any thoughts or suggestions?

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With 40% kaolin in that first recipe there's no way you should need to add bentonite to it. Really wouldn't add more epsom salts solution to it, just going to make it thicken up even more. I'ld pull out about a cup of glaze, then add some darvan to it, just a drop or two at a time, give it a good stir then dip a test tile. Add more darvan a little bit at a time until the glaze is at a good consistency. I'm guessing it's going to take longer to dry so drips are probably going to be a problem. Have to fettle them down when the glaze is really dry. Use the small test glaze as a guide to fix the rest of it.


Second glaze, I can see 2% bentonite in this one, not the 4. Again, I would try a small amount of glaze with a titch of darvan and see how the glaze hangs on a test tile. Go by the specific gravity, not how it looks. Think of thickening a sauce when you are cooking, you add a cornstarch slurry to thicken a sauce, water content doesn't change but the sauce is thicker. The bentonite and epsom salts did that, you don't want a watery sauce / glaze, you "thin" it down with darvan.

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2% bentonite is the norm many times 4% is a lot

You say the coating goes on thin

With 20% fit and 44% feldspar does this settle like a stone??

how are you measuring the specific gravity?and what scale are you using if its a hydrometer?

I took some photo today as it was my weekly glaze day and will post on specific gravity as a separate post-I'll put a link here-after glazing and loading my two kilns and mowing a 1/2 acre I'll get to this later

My wife retired today  with the state as well so maybe on Sat.

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