Craig Martell on mon 14 may 01
Hey:
The thing that makes thickly applied slips crack is shrinkage
differential. In other words, the slip and claybody are shrinking at a
rate different enough to cause cracks. Adding bentonite or other
plasticizers will cause the slip to shrink more thus crack more. One needs
to adjust the plastic to dry shrinkage of the slip. An easy thing to do is
deflocculate the slip with Darvan or some other sodium type
deflocculant. This cuts the amount of water added to the slip and will
lessen the shrinkage. This will also allow the slip to be thicker, or
fatter, as some like to call thick slip.
The other thing to do is adjust the clay content of the slip to reduce the
amount of shrinkage. If the slip has kaolins you could use some Kaopaque
20 in place of more plastic kaolins such as Grolleg, Tile #6, or EPK. You
can also cut the ball clay if there is any of that in the recipe. You can
also increase the feldspar and silica a bit and lower the total clay
content. I just use my porcelain body for decorating slips and since slip
and body are the same recipe I can apply the slips very thickly without
cracking. If you use commercial clays, just save trimmings and when they
are dry, weigh out what you need, add color if you want and mix it up. You
can also calcine some of the clay in the slip and this will cut wet to dry
shrinkage.
later, Craig Martell in Oregon
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