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Glaze flaking off bisque - summer field pottery

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I have a base glaze that i use a lot, but it starts to flake off the bisque ware after it has aged about a year. i want to know if I can add something to counter act that and continue to use the glaze instead of throwing it away. I assume that there is combination of chemicals that is causing this. This glaze has whiting, zink ox, D.F. stone Kona 4 feldsparbentonite, EPK kaoin and titanium dio. any one who has some info. I would be glad to furnish the reciepy. I mix large batches of this because it is such a good glaze to add colorants to.

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Sodium from the F4 is probably deflocculating the glaze slurry over time. I would try scooping a cup or so of the glaze into a smaller container and adding a flocculant to counteract this. A few drops of saturated epsom salts solution to a cup or so of glaze slurry, don't add so much that the glaze considerably thickens.  Dip a test tile or two and see if this fixes the problem.

 ( @Summer Field Pottery,  I'm going to edit the title of this thread so it's more pertinent to your question. Welcome to the forums.)

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7 hours ago, Summer Field Pottery said:

I did try to add epson salt and it did not flake on the small pot that i used as a test. and yes I do add water as needed, my containers have rubber seals so not too much evaporation. 

Only asked if you add water because some glazes flocculate over time and may seem like they've evaporated off but really are just "thicker" and when you add water it will thin them out but also add a bunch of water as well. This can lead to cracking and peeling glazes.

The opposite can happen too.  Measure specific gravity instead and this won't happen.

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8 hours ago, Summer Field Pottery said:

I did try to add epson salt and it did not flake on the small pot that i used as a test.

Excellent! I'ld give it a try on a few more pots before committing the whole batch to epsom salts. If it doesn't work 100% then you probably need to add some macaloid (aka bentone ma) also. Would have to make it up as a gel and then add that to the slurry. If you're not doing it already then once the glaze is back to working properly measure the specific gravity of it so you can replicate it when necessary. (don't let the epsom salt / macaloid adjusted glaze trick you into thinking you need to add more water)

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On 9/25/2019 at 4:36 PM, Summer Field Pottery said:

This glaze has whiting, zink ox, D.F. stone Kona 4 feldsparbentonite, EPK kaoin and titanium dio. any one who has some info. I would be glad to furnish the reciepy. I mix large batches of this because it is such a good glaze to add colorants to.

Just curious the glaze recipe already calls for bentonite, my question is what is the current  percentage?  Maybe publish the recipe, From a suspension standpoint I would also be curious what percentage clay as well.

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19 hours ago, Summer Field Pottery said:

whiting 17.60, zink ox 8.02, corwall stone DF stone 21.99, Kona 4 feldspar 44.10, bentonite 3.25 EPK 4.98 this is the receipy 

Don’t like the bentonite over 2%, generally thats the limit and such a small amount of clay usually results in suspension problems. As a total quick fix  upping the clay should keep this more suspended on its own  but keeping your recipe fairly similar in texture, gloss, R2O,  etc... you could try: whiting 17.22 , zinc oxide 7.85, Cornwall 17.12,  Kona 43.14, bentonite 0 (Zero), Epk 14.67

No guarantees but maybe worth a 200 gram test. Clay content of 10 - 20% generally keeps things suspended better and glazes are a bit more well behaved than those that need excess bentonite.

Edited by Bill Kielb

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I agree with Bill, if you can get some additional epk in the glaze in place of some of the bentonite the glaze will suspend better. (might have to reduce the alumina from one of the other materials) Either way though, as is with 3.25 bentonite and 4.98 epk or an altered version, the sodium can deflocculate the glaze over time. Sodium ions from the K4 (and Cornwall stone) can deflocculate the slurry negating the effect of the clays ability to bind the glaze slurry to the pot therefore cause the issue you are having. BTW that recipe is low on silica.

 

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