Observation on converting glazes...Bentonite needed

[davidh4976]

Because of the availability issues for Gerstley Borate and Custer Feldspar, I've been converting our studio glazes to use frits and G-200/Mahavir feldspars (with some tweaks for other ingredients to get the chemistry right). One thing that is quite noticeable is that many of the new versions will hard pan without the addition of Bentonite to the recipe. Some need as much as 3 or 4% Bentonite.  I'm doing line blends with different percentages of Bentonite in order to find the minimum amount that will prevent hard panning and keep everything in suspension for a reasonable amount of time. I'm just offering this as an observation for anyone who is heading down this path.

I'm 90% done with our recipes that need conversion. Later, after I get all the recipes completely done, I may experiment with other versions of recipes to get to the same results without needing Bentonite.

[Callie Beller Diesel]

3-4 bentonite seems high. What’s the other clay content like in your revisions, and are you using epsom salts to help keep things suspended?

[davidh4976]

12 hours ago, Callie Beller Diesel said:

3-4 bentonite seems high. What’s the other clay content like in your revisions, and are you using epsom salts to help keep things suspended?

4-to-14% EPK depending on the recipe. I have not yet used Epsom salts because I am following John Britt's guidance to use up to 4% bentonite before using Epsom salts, and I am able to get rid of the hard panning without exceeding 4% bentonite. I don't really know enough about the trade offs between Epsom salts and bentonite, so I have to rely on people who seem to know what they are talking about!

[Kelly in AK]

2% bentonite ought to be enough, but I’m not arguing with John Britt. Perhaps a more plastic form of kaolin? EPK is popular. (Edit-I was reading at unsafe speed and didn’t notice you were already using EPK).

Epsom salts, try it before going to great lengths to avoid it. I’ve spent too many precious minutes of my life scraping the sludge off the bottom of a bucket and get it in suspension. I never have that problem anymore. I sincerely mean never. I don’t know the dangers of using Epsom salts over increasing bentonite, I do know that I can adjust the viscosity of the glaze at will by flocculation and it works like a charm with every glaze I have. 

I use a glaze with 10% kaolin and 2% bentonite as the only plastic ingredients, frit and silica make up the balance. Epsom salts keeps it in suspension, doesn’t hardpan after months of sitting in the bucket. Another one I use is Selsor red, no clay in that one at all. The Gerstley borate helps with suspension, but if you leave out the bentonite (2%) it will hardpan in a day. Without Epsom salts it’ll still settle quickly. With Epsom salts it doesn’t hardpan over months. 

(Edit- I see you were offering an observation, not seeking a solution. It’s much appreciated. I’m down to fifteen pounds of Gerstley borate and I’ve got a couple glazes to reformulate myself. Been putting that off.)

Edited May 23 by Kelly in AK
Revision and added additional information.

[Min]

10 hours ago, davidh4976 said:

4-to-14% EPK depending on the recipe.

If you do decide to work on the suspension I would suggest that replacing some or all of the EPK with OM4 ball clay would help. Given there is less alumina and more silica in the ball clay that leaves room to increase the clay, provided there is silica added to the recipe that in turn would need to be reduced. Just a thought, but on the other hand if your bentonite works then that’s good too.

[C.Banks]

John Britt suggests 1-2% bentonite in the reference I can find on page 30 of TCBtHFG.

Any more is in reference to leather-hard glazing.

I have a project that settles over time so I'm curious too - I'm thinking maybe I forgot the flocculant(epsom salts).

The biggest issue I notice with epsom salts is the glaze remains tacky for longer. I forget  and wind up sticking my fingers on wet pots.

 

Edited May 23 by C.Banks

[davidh4976]

13 minutes ago, C.Banks said:

John Britt suggests 1-2% bentonite in the reference I can find on page 30 of TCBtHFG.

Any more is in refernce to leather-hard glazing.

I have a project that settles over time so I'm curious too - I'm thinking maybe I forgot the flocculant(epsom salts).

 

 

Interesting... in John Britt's video, he says 2-to-4% Bentonite.

[C.Banks]

10 minutes ago, davidh4976 said:

Interesting... in John Britt's video, he says 2-to-4% Bentonite.

I'm also not one to argue against J.B. but will echo the sentiment that anything over 2% bentonite sounds like a lot.

A friend helped me understand the value of flocculants a few years ago. I try now to work with them as best I can.

Edited May 23 by C.Banks

[High Bridge Pottery]

13 hours ago, davidh4976 said:

4-to-14% EPK depending on the recipe. I have not yet used Epsom salts because I am following John Britt's guidance to use up to 4% bentonite before using Epsom salts, and I am able to get rid of the hard panning without exceeding 4% bentonite. I don't really know enough about the trade offs between Epsom salts and bentonite, so I have to rely on people who seem to know what they are talking about!

Anything after 8-10% kaolin should be easy to flocculate. There's not really any trade offs, you can use one or both to help things stay suspended. No reason to avoid flocculating things. 

[Callie Beller Diesel]

What I’ve noticed about flocculation in general is that either bentonite or epsom salts alone work a little, but the 2 together work better.  Personally I’d rather have enough clay in the recipe and use the epsom for suspension. I dislike using bentonite because dispersing it properly is a nuisance. I don’t find the drying time from Epsom to be a problem with my workflow, but that’s a personal preference.
If the 4 % bentonite works for you, there’s no need to fix it. It’s just a different solution to the problem of how to keep non-gerstley glazes suspended .

 

[davidh4976]

On 5/23/2024 at 6:57 AM, Min said:

If you do decide to work on the suspension I would suggest that replacing some or all of the EPK with OM4 ball clay would help. Given there is less alumina and more silica in the ball clay that leaves room to increase the clay, provided there is silica added to the recipe that in turn would need to be reduced.

Min, you may be on the right track. Playing around with the formulas in a calculator, it looks like I can increase the clay in the recipes by using OM-4 instead of EPK.  Things that also seem to help appear to be using Mahavir Potash Feldspar instead of G-200EU, and Frit 3124 over some of the other frits.  Now, this is just from playing with the formulas a short while, and I need to take a closer scrub on the recipes, and the proof will only come with testing.  But, by focusing on using OM-4, Mahavir, and 3124 in the calculator, in one recipe I was able to increase the clay content from 3.5% to 15.5% which could make a huge difference.  Thanks!

[Min]

2 hours ago, davidh4976 said:

in one recipe I was able to increase the clay content from 3.5% to 15.5% which could make a huge difference.

Don't forget though to take into account the higher shrinkage that comes with using ball clay which if too high can lead to glaze crawling. Testing might show a compromise between the 3.5% and 15.5% by using part kaolin and part ball clay works well. 

[Callie Beller Diesel]

Frit 3134 can also be useful if you need to make room for more alumina to be supplied from clay. It’s very similar in composition, but with less alumina. I think I read somewhere on digitalfire that it was formulated to do just that.