Jon Pacini on wed 27 mar 02
Greetings All--- Let's look at some common types of pin holing faults.
If the pin holing is on the surface of the Glaze, it can usually be
corrected by firing a little slower on the top end or soaking a bit longer
to allow everything to smooth out.
If the pinholes go down to the clay, its' usually a gassing related
fault.
Now--It's easy to blame the clay because it is high in organics, but the
organic content of clay is going to vary. No way around that, so if you've
got this kind of problem, the best thing to do is to get the organics burned
out.
IMHO the best way to resolve this dilemma is to slow the entire process
down so that the heat has a chance to soak its' way in, heating the entire
wall of the ware throughly, not just on the surface. Firing hotter may
resolve this in a round about way, but you will still get uneven results if
the firing is too hot, too fast. Soaking the heat evenly into the ware
should be what a potter is trying to accomplish in the Bisque as well as the
Glaze firings.
Something that needs to be addressed here is that electric kilns,
because the heat they generate is radiant, they tend to 'Shadow'. That is,
it gets a lot hotter on the outside edge of a shelf of ware than it does in
the center. This can best be illustrated by having someone stand between you
and an electric space heater. Yes, the room gets warm, but the person
blocking the radiant heat gets a lot hotter that you do. Pots do the same
thing in an electric kiln.
Kiln sitters and thermocouples are usually located out on the edge, the
hottest part of the kiln. They are mechanical devices that for the most part
reflect what is going on in their small segment of the kiln, the outside
edge, they hardly tell the whole story. Put cones all over the kiln, and do
it every 5-10 firings. You'll be amazed at the differences you see during a
single firing and over time.
.
In closing, if you are having pin holing problems, that are caused by
out-gassing, the best remedy is slowing the whole process down, let the heat
do the work.
Jon Pacini
Clay Manager
Laguna Clay Co
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