Glaze adherence PRE-firing

[davidh4976]

I mean during the time between the dried glaze application and putting it into the kiln. I’m not talking about glazes that crack and/or flake off before or during firing.

It seems that some glazes, if you just touch them, chunks come off.  Other glazes seem to stick on pretty well and can be handled without problem prior to firing.  Any ideas about what makes some stick and others fragile? (all similar application and thickness, etc.)

[Kelly in AK]

It usually has to do with how much and what kind of clay is in the glaze recipe. Ball clays have higher green strength than kaolins so they’re less powdery, but they also shrink more and aren’t white. The worst glazes for this have very little clay at all and are really tender. Some people add CMC gum to mediate it. I’ve heard of people spraying hairspray on their freshly glazed (dry) pots, but I’ve never tried it. 

[Babs]

I think also it can be remedied by calcining half of the Epk.

I had a glaze I tried which had a ridiculous amount of Bentonite in it, as the glaze dried it just cracked and dropped off.

How much EPK does the glaze in question have in it.

Not an expert but there are folk here , post the recipe of the culprit glaze and I am sure you'll get the answer to your prob.

[Min]

Apart from clay content I've found that how dry the raw glaze is can make a difference with the adhesion also. If I wait a couple days before loading the kiln the glazes are more powdery but less likely to have a chunk knock off if I'm a bit clumsy loading. I try and never touch a rim or handle edge etc while loading. Most of my glazes have approx 20% raw clay in them.

[davidh4976]

I’ve just noticed this generally and don’t have good records to remember which glazes have this problem and which don’t. However, I have two raku glazes that I recently used.  One cracks off easily upon touch (with 15% EPK) and the other is not coming off unless I sponge it off (zero EPK).  Both have CMC + propylene glycol for brushing. Here are the recipes:

Raku Blue Copper Matte went on smoothly and is not cracking off easily:

Gerstley Borate - 80.00

Bone Ash - 20.00

Copper Carbonate - 7.00

 

Raku Crackle Base easily knocked off, particularly edges:

Ferro Frit 3110 - 85.00

EP Kaolin - 15.00

[Kelly in AK]

The Gerstley is pretty clay like, I can imagine it’s a tougher unfired surface. 

The CMC will get harder the drier it is, propylene glycol slows the drying down, maybe that’s a thing. Ron Meyer’s Clear is a glaze with lots of frit and only 10% kaolin, but it’s also got 2% bentonite, and that improves green strength (as well as keeping glaze in suspension).