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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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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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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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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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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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