Martin Howard on thu 12 oct 00
Having settled on a 3% copper glaze, because of its blue/green colour, and
then read about copper being a poisonous substance and how we should all be
careful about our glazes used for food, I did a simple test.
At 4pm I half-filled an uncrazed 3% copper green glazed bowl with malt
vinegar.
By 9.30pm the bottom half of the inside of that bowl was completely crazed.
The upper part was unaffected.
I had found that glaze to craze occasionally, and was just going to
reformulate it so that it didn't. But this effect of vinegar finding the
known weak point of that glaze in such a short time has really surprised me.
So, what is the chemistry behind it?
Martin Howard
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
England
martin@webbscottage.co.uk
Norman van der Sluys on fri 13 oct 00
I think it likely that the crazing was already there, just hard to see. The vinegar
caused a reaction that stained the tiny cracks. I'll leave the chemical reation to
the chemist-gurus.
Martin Howard wrote:
>
> By 9.30pm the bottom half of the inside of that bowl was completely crazed.
> The upper part was unaffected.
>
> I had found that glaze to craze occasionally, and was just going to
> reformulate it so that it didn't. But this effect of vinegar finding the
> known weak point of that glaze in such a short time has really surprised me.
>
> So, what is the chemistry behind it?
>
> Martin Howard
> Webb's Cottage Pottery
> Woolpits Road, Great Saling
> BRAINTREE, Essex CM7 5DZ
> England
> martin@webbscottage.co.uk
>
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--
Norman van der Sluys
by the shore of Lake Michigan
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