Janet Kaiser on fri 13 sep 02
Dear Tom
Just as a matter of interest... Lime popping or lime spitting (whichever
you prefer to call it) is usually because the contaminants were not known
to be present. Who would knowingly use contaminated clay, right? (I do not
like to shock a Canadian Gent by saying exactly what I think of the potter
who "just remembered" why they were ripping off a fellow maker... :-)
But in your opinion, where it is a recognised problem *before* the making
process, wouldn't biscuit firing and then soaking in vinegar before the
glaze firing not be a reasonable solution without doing all that tedious
sieving? Or is that not a viable solution with porcelain...? Would the
biscuit body be too dense?
Or what about ball milling? I notice no one suggested that as a solution.
But I have no real working knowledge of porcelain, so I don't want to speak
out of turn.
Hope you are missing the deluges reported on the news tonight...
Best wishes
Janet
*********** REPLY SEPARATOR ***********
On 12/09/02 at 11:34 Tom Buck wrote:
>Plaster of Paris starts off as gypsum in the ground, Calcium sulfate
>duohydrate (CaSO4.2H2O). the mineral is heated to change it to
>CaSO4.1/2H2O (demihydrate). and this is sold as PoP. in time the PoP
>reverts to duohydrate, and this is what would be in Kelly's clay. If she
>cleans it well, so the gypsum particles are very small, then yes the
>CaSO4.2H2O becomes CaO at high heat, and it will flux the body a bit. the
>problem with spitting is that the gyspum particles in the body are too
>big, and just become demihydrate again which suck up water and pop.
> later Peace. Tom B.
Janet Kaiser
The Chapel of Art =95 Capel Celfyddyd
8 Marine Crescent, Criccieth LL52 0EA, Wales, UK
Tel: 01766-523570 URL: http://www.the-coa.org.uk
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