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Sputty    73

Colemanite in glazes is notorious for causing problems, spitting/blowing off the pot surface. I understand that this is due to rapid decomposition of the material when heated (is that right?).

 

So, is there a case for calcining colemanite to reduce this problem? If so, at what temperature?

 

I'd be happy to use a calcium borate frit instead, but don't have easy access to such a beast in my little European corner.

 

Anyone tried it?

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Mark C.    1,800

I have used Colemanite /gertsley boratefor over 4 decades-1st in Raku glazes then in high fire glazes./Cone 10

Its been a small % always and I have not had many issues with it although its composition has always changes and it been far from consistent.

I read about all the woes but my take is most are at cone 6.

I actually bought a ton of it (gertsley borate)when it was going to be taken off the market due to mine issues. I still have lifetime supply. I only have a few glazes that call for it and it alway as stated is in small amounts.

I'm not a fan of Frits-only used them in Raku and low temp works.They settle like ground rocks.

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Marcia Selsor    1,301

I use gertsley borate as a sub for Colemanite. I bought a lot of that when it was going to disappear. I have no problems with it even though it has a lot of complaints against it.

Marcia

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neilestrick    1,381

Test it out and see what you think. It may or may not be a problem. If needed, reformulate for whatever you have access to, be it Gerstley or Frit. Frits do not settle badly if you have a well formulated glaze with enough clay to keep it suspended. The problem is that a lot of old glaze recipes relied on Gerstley to keep the glaze suspended, which it does very well. So if you substitute a frit in those cases then you can have suspension problems.

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glazenerd    816

I have been using Calcium Borate for a little over a year or so. Primarily produced from Turkish colemanite: 50% boron and 30% calcium (+/-): roughly twice the bang of gerstley borate. I started a thread about it back in Dec. 2015, and I seem to recall some posts about boron/calcium frits being mentioned. I have had no problems with it for cone 6 use: and I have used it (need to check this) up to 27% of batch weight. A borosilicate glaze with the right level of alumina can be highly abrasion resistant. I use I for a couple of floor tiles I make; I found it to be more durable, and a high satin sheen.

 

Nerd

 

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Sputty    73

@Mark C. - Thanks - I'd be only too happy to use frits, to be honest, if I could get the ones I wanted! I've used frits a lot with earthenware glazes, and am very happy to get them working properly. The real problem in the backwaters of Europe is availability - here in France there is only a small range to choose from, and inevitably not what you are looking for at the time. Hence the question about colemanite, which I can get, and which I would substitute for the more reliable (British) calcium borate frit I am about to run out of.

 

@Marcia Selsor - Thanks - Gerstley Borate is no more available in my bit of Europe than half the frits I'd like to use!

 

@Neil Estrick - Thanks - (see my response to Mark C) - I'm good with frits, if only I can get them!

 

@Glazenerd - Thanks for the link to that thread - I must learn to do a (proper) search on these forums as a first step.

 

As it happens, overnight I've just tested a glaze (for porcelain, ivory, silky smooth satin) which I didn't think would work as I wanted, but lo, it did. And it has no colemanite, calcium borate, ghastly borate or any other difficult thing in it. That's what I call a win-win situation!

I expected it to craze, it defied me - I don't mind being wrong!

 

Thanks again to all.

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I remember doing a bit of research into this after trying colemanite and it jumping off every pot. The only temperature I can find is 800c for getting rid of the chemical water. No idea if it will still be a powder after but melting temps seem around 1000~. Maybe you could just slow down a glaze firing through there.

 

post-23281-0-09154800-1490017633_thumb.png

 

Digital fire also talks about the possibility of gypsum impurities.

 

You could melt a load into your own frit  B)

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Sputty    73

You could melt a load into your own frit  B)

 

Don't think I haven't thought of it! If I had access to a ball mill, I'd be giving it a go!

I fancy something like these guys use to grind bottle glass down to a (sort of) frit:

 

 

(Jump to 18:45 for the hard-core ball-milling action...)

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Great video :D

 

I remember reading that you melt whatever you need for making frits and then pour it from molten into water and that does most of the smashing up of the frit. No need for a ball mill. Could be wrong.

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I got some Colemanite a few months ago, and tried putting it through a bisque firing. It didn't melt, but it shrunk and formed a hard mass.

IMG 0002

 

Judging by the sheen on the lid and sides of the container it was in, some of the boron volatized.

IMG 0003

 

It took quite a bit of work to grind it down fine enough to pass through a 120# screen, using a mortar and pestle, and then I discovered it was slightly soluble. You can see the crystals that have formed inside the cup as the water evaporated.

IMG 0002

 

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