Nikom Chimnok on mon 4 oct 99
Hello,
Recently I wrote contending that casting rates have more to do with
the permeability of the clay than of the plaster. Here is some anecdotal
evidence.
Occasionally I blunge some pure clay into slip, screen it, then dry
it out by pouring it into large plaster molds meant for jiggering flower
pots. The clay is very, very fine, and after a few hours, the plaster stops
absorbing and I have to rely on evaporation, which takes practically
forever, to complete the process.
I have found, however, that after a few hours, the clay stuck to the
plaster is plastic enough that I can get a fingernail beneath it and peel it
off the mold, wadding it up into a lump which I leave in the bottom. And
then the plaster starts absorbing again. In a few more hours, it will again
build up a layer I can peel off. Eventually the plaster is completely
saturated, and water is running right through it, puddling on the concrete
floor. In this way, I can dry slip with a specific gravity of 1.3-1.4 in
only a couple of days, whereas it might take two weeks if I just let it sit
there.
I wish I knew how large the pore sizes are in plaster, but they
don't seem to be clogged at all by this clay whose median particle size is
3.4 microns. Makes me think that the pore sizes must be measured in
fractions of a micron.
So much for now,
Nikom
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