Jeff Longtin on sun 29 sep 02
Pat/Janet,
While the Brits have some funny names for ceratin physical properties they've
also done some great work in the area of slip-casting.
Most potters don't realise just how advanced slip-casting technologies really
are.
Different grades of sodium silicate exist because it was recognized years ago
that different slip ingredients deflocculate at different rates. If I recall
this is due to different molecular weights.
Porcelain clays are heavy, molecularly speaking, and so they need really
strong silicates. Ball clays and stoneware clays, on the other hand, are not
so molecularly heavy, and so they can be defloculated with weaker silicates.
Non clay elements, like feldspars and silica are heavy as well.
If you're making a small batch of slip and pouring a few pots the type of
deflocculant you use is not that big a deal. If, on the other hand, you're
mixing massive amounts of slip and mold wear is critical, if you've invested
in hundreds of molds, then how much, and how strong your silicate is becomes
a big deal. The more silicate you have to use the shorter your molds' lives
will be.
That's why R.T. Vanderbilt has two commonly used silicates, Darvan 811 and
Darvan no.7. Darvan 811 is for the heavier clay bodies, like porcealin, while
no.7 is for lighter bodies like stoneware and earthenware bodies. Earthenware
is typically made with lots talc and non porcelain clays so it deflocculates
easily.
(I think in practical terms R.T intended 811 for clay and no.7 for glazes and
such but since it works well for MOST clays no.7 is commonly used for clay
bodies as well.)
Years ago I found a little book that had David Leach's porcelain slip-casting
body printed in it so I thought I would give it a try. Could not deflocculate
that sucker for my life! If I had, however, a greater knowledge of "degrees
of twaddle" I'm sure I would of had much better luck.
Take care
Jeff Longtin
ps. A few years ago a friend was involved with a woman who grew up in a
little town near Stoke-On-Trent. Curious to see how close she had been to
that great pottery city I looked at a map. Lo and behold I noticed that there
is one district in Stoke-on-Trent called Longtin-On-Trent. I thought my
ancestors were French and German...but then again?
ps2 Do you think Tweedle De and Tweedle Dum have any relation to twaddle?
Could Lewis Carroll have been a closeted pottery enthusiast?
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