Pottery In Australia Vol 35 No 1 Autumn 1996

Deflocculated slip
- Clay Adhesive
'I learned of using deflocculated slip
as an adhesive compound from John Dermer'.
Article by IVOR LEWIS.
Opposite: Oval dish. Ivor Lewis.
J
ohn recommends its use and insists that the parts
being joined should NOT be scored. He contends that
scoring may leave cavities which en-trap air when the
slip is applied. Expansion of the air as it heats causes
pressure to increase. This stress can initiate cracks and
possible fracture of the joint.
I have accepted his suggestions. I make a thick, dense
deflocculated slip from the clay body I am using. Soak
dried, crushed turnings, stand overnight, the excess fluid is
decanted off, or mopped up with a damp sponge. When
the mixture is blunged and sieved, allow the water to
evaporate then reblunge, so the mixture is thickened. A
consistency should be similar to thick (double) cream,
does not readily run, and peaks when an object is pulled
from it.
Parts to be joined should be leather hard, firm, but
pliable. I mark locations with a liner brush and red ink.
Both pieces are liberally coated with the deflocculated slip
on their mating faces. I try not to obscure my location
marks. There is a need for speed without undue haste. At
this stage it is important not to apply pressure which will
cause the pieces to lock. Instead, the lubricant properties
of the slip are exploited, allowing the parts to be precisely
positioned. When the parts are accurately located, gently
apply pressure to each side of the weld and vertical to the
plane of the joint, causing the slip to exude, flushing out
any trapped air.
When this has been done gently along the joint, excess
slip is sponged away. The damp sponge should be rinsed
and squeezed regularly. The pressing process is repeated
with increased force to ensure that the high and low points
between the contact surfaces meet. Butted joints should be
given a slightly convex profile. This provides a central
point of contact and slip exudes easily out of the wedge
shaped gap.
Lawrence and West ("Ceramic Science for the Potter"
1982, Page 68) give a comparison between flocculated
(ordinary throwing slurry) and deflocculated slips and
describes their qualities. Their information shows quite
clearly that deflocculated slip has properties which make it
ideal as an adhesive. My own experience leads me to
believe that it behaves in a way analogous to a high
temperature brazing alloy. Consider these qualities: High
density because it is made from the minimum volume of
water containing a high weight of solids, so providing a
high bulk of clay to fill or bridge the voids; low viscosity,
providing lubrication for movement during alignment of
parts; high bulk to volume means reduced shrinkage,
hence reduced susceptibility to hair line cracks; high green
strength, reducing potential for damage prior to firing;
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70 POTTERY IN AUSTRALIA+ ISSUE 35/ I AUTUMN 1996