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Glaze Additive/s

bentonite EPK suspension hardener

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#1 glazenerd


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Posted 15 March 2017 - 06:36 PM

Need some input, point/s of reference. 


1. Do you add bentonite, EPK, or gums to your glazes?


2. In what amount do you add to the final (dry) batch weight?


3. Do you add more for high magnesium or sodium glazes?


4. Have you had problems with bentonite clumping?


5. Do you rely on additives to achieve viscosity?



#2 Dick White

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Posted 15 March 2017 - 07:01 PM

I add bentonite when the recipe calls for it, and not if the recipe doesn't. Some people add 2% bentonite to everything. Why, dunno, just because. Some recipes require it (high feldspar/frit low clay) or the slurry will hard pan. Other recipes don't need it, but apparently don't suffer by adding it, so they just add it to everything. Bentonite will clump when added unless it is seriously blunged. Even then, it will take some time to plump up. Consequently, I include it (when listed) only as I make a new batch. Adding it later has never worked for me.


I have not differentiated high Mg or Na glazes for additional bentonite. Should I? I welcome your insight.


EPK as an additive? That would change the UMF of the glaze.


I use gums (CMC) for brushability when that is required. Most of my glazes are dunking glazes, only rarely brush. CMC also promotes sticking, so that helps when brushing a reglaze.


I get my viscosity through flocculants such as epsom salts or vinegar.

#3 Magnolia Mud Research

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Posted 15 March 2017 - 07:33 PM

This is from my archive of Clayart discussions:



On Wed, Jan 28, 2009 at 6:24 PM, mel jacobson  wrote:

i use bentonite in every glaze i have.
even the chinese glazes and shino.

here is a tip.

take a blender, fill half full of water.
add about a cup of bentonite.
blend the hell out of it.
add more water, blend some more.

i mean for minutes.

it will turn to jello.
place this jello in a large empty skippy peanut butter jar.
(my god, don't eat the peanut butter, it is more toxic
than barium.)


store for some time.

add a quarter cup to your 5 gallon glaze.
stir like crazy.
it will dissolve.
   god i hate adding dry bentonite to glaze.
gobs up.
i know many stir it into the dry mix..but often that
does not work.  lumps.
wet bentonite mixes well.


end quote


My jar is a Reese peanut butter jar, but it works OK (so far).



#4 Mark C.

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Posted 15 March 2017 - 08:25 PM

I have a few glazes that call for small amounts of bentonite. I never had a problem with it. I just toss all ingredients into a 5 gallon bucket with water in it and power mix it up(jiffy mixer on 1/2 inch drill) then sieve it thru my Tailsmen sieve.I think many here do not use these tools?as they make all this a non issue.

I do use a small blender on tough materials like zinc oxide and blend them wet and add to bucket then sieve.

PS EPK is common material in my glazes no special treatments.


Gums are totally different issue.I use magma not gums for settling issues-It does not go off like gums and stink.

I own lots of gums but rarely use them. I do use them in some soda salt sprays for bricks but I use it all up and it cannot stink.

Mark Cortright

#5 neilestrick


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Posted 15 March 2017 - 09:42 PM

1. I rarely add bentonite. I always make sure my glazes have at least 10% kaolin/ball clay so that they stay suspended well.

2. If I do add bentonite, it's 2%

3. No

4. Not if I dry mix it well with the kaolin.

5. I add epsom salts to most of my glazes, 1/2% by dry weight. I don't measure viscosity, just go by feel.

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#6 glazenerd


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Posted 15 March 2017 - 10:30 PM



The chemistry involved in a clay body, slips, and glazes are exactly the same: the amount of water is what changes. (UMF aside) Magnesium however introduces a whole new set of reactions. The end result of magnesium reacting is seen in crawl glazes when they dry: flaking off in large chunks. I will get into to the differences a bit later.



#7 Diesel Clay

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Posted 15 March 2017 - 11:01 PM

1) Like Neil, I prefer to have at least 10% clay ( EPK or other) in a recipe as a source of silica and alumina. That way I don't have to mess with bentonite.
2) see above.
3) don't really work with magnesium, and I've not *noticed* the need with anything I might have that's fluxed with sodium.
4) See answer number 1
5) I will use a saturated Epsom salt soloution in both my cone 10 celadon and my current cone 6 clear. They're both quite stiff in the melt, and any drips or runs made in the application don't smooth out, so I try to avoid them in the first place.

#8 MatthewV



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Posted 16 March 2017 - 01:32 AM

1.Bentonite if needed. EPK comes in with the recipe most of the time.


2. 1 or 2% Bentonite


3. Never noticed


4. Sure, but it easy to avoid. If a glaze is hard panning I add a note for next time instead of trying to add it afterwards.


5. No

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#9 High Bridge Pottery

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Posted 16 March 2017 - 10:25 AM

I like glaze with some kaolin/ball clay in there too so I don't bother with bentonite. Might use vinegar or epsom salts for some extra control.

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#10 oldlady


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Posted 16 March 2017 - 02:48 PM

nerd, i always put 2% bentonite in a glaze because i do not understand the chemistry at all.  i add it dry to the dry mix and never have a clumping situation.  know nothing about the other questions so can only say N/A.


i started mixing glaze recipes and having made glazes that hardpanned, it was a revelation to find that adding bentonite to the original recipe made the glaze stay suspended.  then it just became a part of making glaze.

"putting you down does not raise me up."

#11 glazenerd


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Posted 16 March 2017 - 06:30 PM



Sodium is the primary reason glazes hard-pan. Just gathering info on how potters prep their glazes. So the question (I am asking myself), is there a better way of dealing with hard-panning, suspension, and rheology. Is there a single additive that will do all these things? Will explain what I am looking at after the rest respond.



#12 Joseph F

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Posted 16 March 2017 - 06:51 PM

I only add bentonite if the clay % is low in the glaze. Otherwise, why bother with it? It is a pain in the butt to get mixed up well(problems clumping). I usually try to mix it with the small amount of clay in the recipe ahead of time. Seems to help to dry mix it before hand reduces clumping significantly. 

#13 glazenerd


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Posted 16 March 2017 - 09:20 PM

A very simple rule in clay chemistry: positive molecular charge attracts, and a negative molecular charge repels.

Feldspars, and silica produce positive charges: kaolin/s neutral, and ball clay/s, bentonites, and plasticizers are negative.

Sodium causes dead-panning because it is hydrophobic (repels water) and because it is positively charged. Being positive, it creates a positive charge in adjacent particles; causing separation. (clumping.)


So then you come to "sodium" bentonite.  Positive charges and negative charges in the same particle. The sodium (positive) is what makes it clump. Calcium bentonite would be much easier to work with, but you rarely see it offered. While it described as a suspension agent: it is more accurately a coagulant. It suspends by creating a gelatinous viscosity in lieu of suspension created by a negatively charged mixture.



Also tagged with one or more of these keywords: bentonite, EPK, suspension, hardener

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