Cindy Strnad on wed 14 nov 01
Bob,
The problem with a clay that offers a wide firing
range is that the offer is a lie. Because the
manufacturer claims the clay is suitable for ^6 to
^10, you don't know where in hell it actually
matures. For the newbies who may be reading this,
mid to high fire clay is mature when it has
reached the point at which it absorbs the least
possible water, but has not yet begun to show
signs of over-firing such as bloating. We call
this vitrification, or becoming glass-like.
It's not that the clay offering a wide maturation
range is unusable, but that the manufacturer is
not being truthful. That clay vitrifies
*somewhere*, but it doesn't vitrify *everywhere*
between 6 & 10. Since your supplier is not being
truthful, it's up to you to figure out how hot to
fire the clay. If you can fire a clay to ^10
without bloating, it most certainly is not
vitrified at ^6. Fire the clay to your desired
cone, then test it for absorption. For the sake of
clarity and saving time, I include Snail's
instructions, cut and pasted from her post of this
morning, here:
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Make a test piece of clay, fire it, then weigh it.
*weigh it before it cools completely, as it will
begin to absorb atmospheric water.*
Put the fired test piece in a pot of water and
boil it. *make sure it is completely submerged
during the boiling--boil for about 20 minutes*
Let the water cool, then remove the clay and wipe
it dry. *or remove the clay immediately and cool
under running water if you're impatient*
Weigh it again. Convert the difference into a
percentage.
(Take the 'wet' number, and subtract the 'dry'
number
from it, then divide by the 'dry' number, and
multiply
that answer by 100 to get the percentage.)
Example: 260 grams minus 250 grams equals 10
grams.
10 divided by 250 equals .04, so the absorption of
this test would be 4%. *you're looking for an
absorption of 3% or less--preferably less*
-S
nail
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If this doesn't work for you because you don't
have a sensitive enough scale, you can test your
ware by treating the test piece as described
above, then placing it in a microwave oven.
(You're not supposed to run microwave ovens with
no food inside. To protect your microwave oven,
also place a cup inside containing a small amount
of water.) Nuke your test piece for 10 seconds,
then feel it. Be **careful**. It may be hot. If it
isn't hot, give it another 10. Keep this up until
it gets hot, or until you're certain it isn't
going to get hot. If it doesn't, then the clay is
well vitrified. If it does get hot, stop the test.
Your clay is not sufficiently vitrified.
My guess is that this ^6-10 clay will mature at
^10. But then, that would be logical, and there's
no guarantee of that.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
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