Beth Armour on wed 25 sep 96
I am sending out this problem for a friend in the hopes that someone has a
suggestion to her dilema.
She fires to cone 06 in both a gas and electric kiln. Her pots have been
coming out of the kiln lately with a scum (her description) on the lips and
sides of her pots. It never appears on the inside of bowls or platters. She
hasn't changed her clay body or her glazes since this problem occurred. It
is happening on both types of clay bodies she is using. She uses some
commercial glazes and mixes others and seems to happen on all of them. It
does NOT happen when she fires in the electric kiln. She does a big volume
and can't just fire in electric and she has successfully fired in her gas
kiln many times. Could it possibly be bad fuel and how would you check that
out. Any other possible suggestions she could try to narrow the culprit
down?
Any help would be much appreciated! Please e-mail to me and I will pass on
your thoughts to her.
Thanks,
Beth
Louis Howard Katz on thu 26 sep 96
Sounds like the scum you have is plain old scum. USed to be that people
fixed it with Barium Carbonate. Still is that it is fixed with barium
carbonate, just that now some people won't use barium carbonate because
it is toxic.
Commercial clay mixers avoid Barium Carbonate as it makes getting thier
products certified as Nontoxic for use in schools more difficult, perhaps
impossible.
Scum is soluble salt carried to the surface of the ware by drying water.
Manythings affect how much scum is carried to the surface. I discussed
how to reduce scumming without bar. carb. a few weeks ago and I will
forward that post to you.
Bar. Carb. works by reacting with scum to produce insoluble compounds. It
only works on common calcium sulphate scum.
Another source of scum is Gertsley Borate. If it is in decorative slips
that get recycled with the clay it can cause scumming.
Louis
***************************************************
*Louis Katz lkatz@falcon.tamucc.edu *
*Texas A&M University Corpus Christi *
*6300 Ocean Drive, Art Department *
*Corpus Christi, Tx 78412 *
*Phone (512) 994-5987 *
**************************************************
Talbott on thu 26 sep 96
>----------------------------Original message----------------------------
>I am sending out this problem for a friend in the hopes that someone has a
>suggestion to her dilema.
>
>She fires to cone 06 in both a gas and electric kiln. Her pots have been
>coming out of the kiln lately with a scum (her description) on the lips and
>sides of her pots. It never appears on the inside of bowls or platters. She
>hasn't changed her clay body or her glazes since this problem occurred. It
>is happening on both types of clay bodies she is using. She uses some
>commercial glazes and mixes others and seems to happen on all of them. It
>does NOT happen when she fires in the electric kiln. She does a big volume
>and can't just fire in electric and she has successfully fired in her gas
>kiln many times. Could it possibly be bad fuel and how would you check that
>out. Any other possible suggestions she could try to narrow the culprit
>down?
>
>Any help would be much appreciated! Please e-mail to me and I will pass on
>your thoughts to her.
>
>Thanks,
>Beth
Sounds like she is OVER REDUCING (to put it mildly) in the gas kiln--not
enough oxygen is getting to the propane or methane so that it can
completely combust--maybe something is blocking her
flue/chimney????--Definitely not enough oxygen getting in--at least that is
where I would start----Marshall
Marshall Talbott
Pottery By Celia
Route 114
P.O. Box 4116
Naples, Maine 04055-4116
(207)693-6100 voice and fax
clupus@ime.net
Lawrence Anderson on fri 27 sep 96
Louis,
Interesting comment about scumming and the barium being a preventitive.
Is there another flux that will do the same thing that is less toxic?
Or, could the scumming be turned into a decorative effect as crystal
glazes? Actually, this is a round about way of asking you if you've
received my order for your video on thai potters? I just got "on" and
noticed your name. Later, Larry Anderson: larrya@csufresno
Fresno, Ca.
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