Bare Spots in Slumping Molds
Slumping molds made from ceramic (or bisque, as potters would
say) are generally easy to kiln wash, but on occasion these molds
contain a small area that remains white and resists being covered by
kiln wash. These bare spots tend to remain even after
thoroughly cleaning the mold and then reapplying kiln wash.
The bare spots, which are known as "hard spots" or "hot spots" by
potters, are caused by the mold-making process. When the
liquid clay suspension is poured into the plaster mold used to
produce the slumping mold, the force of the clay hitting the surface
of the mold causes a change in the orientation of the molecules of
the clay. This change results in the creation of a hard spot
in the clay. The spot resists water based liquids (such as
kiln wash), and results in an area of the mold with different water
The spot can be minimized or eliminated by sanding with a
Scotchbrite or similar pad. Alternatively, full coverage can
be achieved by applying a thick coat of kiln wash over the area,
allowing it to dry completely, then brushing lightly over the area
when applying further coats.
Even easier, in most instances it's ok to just ignore the bare
white spots and use the kiln washed mold anyway. So long as
your slumping takes place below 1300F/700C (the lower the better),
the bare spots will not stick to your glass, and the mold will work
just as well as if it were perfect.
Copyright 2006 Brad Walker.
All rights reserved.
Special thanks to Tom in Texas
for his assistance with this writeup.