How I Mix Bentonite / Store a Glaze That Settles

[Erik Evans]

If you ever used bentonite, you know it's a bear to mix into a glaze.  It tends to clump together in solution.  It doesn't want to play well with others.  I've tried a number of things over the years to get around this.  I thought I'd share this latest method that works for me:

I first look for the dry clay in my recipe, usually EPK.  Even if it's a small percentage of my recipe, I measure this out first, and place it in a resealable plastic bucket (I bought mine in the paint dept. at Home Depot).  If there's no clay in the recipe, I'll start with the silica.   Next, I measure out the bentonite and place it in the bucket with the clay.  I then seal the plastic bucket and shake the heck out of it for a few seconds, loosely mixing the dry bentonite and clay together.  I set the bucket down outside, open the lid, and walk away.  You don't want to breathe the dust that billows out. 

Next, I pour a little boiling water into a clean 5-gallon bucket.  (I boil my water in an electric kettle I picked up at Walmart for less than $20.  It's perfect.)  I take the hot water outside, pour in my bentonite/clay mix, then walk away.  You don't want to breathe this dust either.

I then go on to measuring out the rest of the ingredients in my recipe, adding each one to the 5-gallon bucket outside.  I work from the fluffiest, lightest materials to the heaviest.  Colorants are last.  Sieve and enjoy. 

Occasionally, I have a glaze in the shop that does settle out.  With five gallons or less, I just store it in a sturdy plastic bag inside a bucket.  That way, I can lift the batch up from the bag just enough to get my fingers underneath it and start breaking up the hard stuff from the bottom, through the bag.  Once I get it going, I can pour it in a bucket for application, then pour it back in the bag when done.  ∆

[Kimberly Wilbee]

What kind of bags do you use. Where do you purchase thick and sturdy?

[Kathy Ransom]

Bentonite is an interesting material.  I've used it to settle sediment in wine where it's stirred into the wine and allowed to settle for a few days, then bottled.

https://winemakermag.com/26-a-clearer-understanding-of-fining-agents

[Norm Stuart]

We've used lots of different smectites / betonites. Brand names like VeeGum from Vanderbilt Minerals, and Bentone-MA have less iron and fire whiter.

Chinese Bentone-EW from Trinity Ceramics is an interesting product which wets faster and I can use in very small quantities to suspend cadmium-sulfide frits which are complete low-fire red or yellow glazes. The addition of much of anything to these frits turns them black. With Bentone-EW I can use so little it darkens the color just slightly. But in other glazes this doesn't seem to suspend as well.

We also had an old 50 pound bag of cheap bentonite which was dark grey with far less suspending ability which is more typically used to seal man-made ponds, even if they're already filled with water, and used as a one of the sealant layers in landfills. That's one essential nature of smectites, even after years of submersion under water, the water doesn't infiltrate further than a relatively short distance - depending on the bentonite used. This cheap bentonite was fine for suspending high-iron glazes.

Essentially all of these high silica clay products structurally include a large number of flocculating ions, either magnesium or calcium or both. If the glaze has sufficient clay adding flocculants like ammonium sulfate, calcium chloride, or magnesium sulfate will create the same effect as bentonite. But in a glaze without sufficient clay it's really the only solution to maintain suspension. Gums are deflocculating and don't really suspend for long.

But since it takes such a long time for even finely divided bentonite to wet fully it's easier to to pre-wet bentonite at a known ratio with boiling water.  Over time it settles some being thicker at the bottom of the container, so we simply shake it up prior to dispensing.

[Erik Evans]

Thanks, Norm.  I had actually forgotten that about bentonite.  In fact, I now remember reading about that before.  It was as disappointing then as it is now.  It's such a stubborn material!  Do you avoid bentonite altogether, deflocculating exclusively with VeeGum?

And yes, Kathy, I'd still sieve.  I sieve everything through an 80-mesh screen.  Bags are helpful when I still have settling despite my best efforts. 

[Kathy Ransom]

I keep a fairly thick mixture of bentonite and water in a plastic container and add small amounts to hardpanned glazes.  I also use a heavily saturated epsom salts and water solution that I store in plastic containers and add it a few drops at a time.  I think I got this idea from a youtube video by John Britt and as I like a lot of glazes and keep most of them in smaller amounts I find this method very convenient and easy.  I also use gum in sprayed glazes as it strengthens the dry glaze but need to use a heat gun as it takes longer to dry and can slump on the pot before it dries.  I really like the bag idea but would still need to sieve the glaze?

[Norm Stuart]

The bag idea for glazes with settling problems is clever.  I keep VeeGum pre-mixed with boiling water in a closed container. Tony Hansen says bentonites mixed for several hours with hot water still is not fully hydrated for three days.