LOWELL BAKER on tue 30 jan 01
Over the course of many years of firing salt and wood kilns I have,
on occasion, encountered a crusting of the surface. This lizard
skin is or seems to be associated with heavy reduction in the very
early stages of firing. This is particularly the case in salt kilns. It
used to happen in my salt kiln when the burner would backfire and
soot up the kiln over night. This crusty surface is often very black
and sometimes has a fragile crystal like surface. It is normally on
the lower portion of the piece.
We got some of this last firing and it didn't go away even with the
cone 12 temperature. The firing temperature went up and down
between 1800 and 2000 degrees several times during the firing and
my graduate students added some water to the kiln while I wasn't
looking. (No wonder they lost temperature every time they stoked)
This crustiness was also found on porcelain bodies but it was not
dark.
I haven't a clue, can anyone help?
W. Lowell Baker
the University of Alabama
philrogers pottery on wed 31 jan 01
Lowell,
This problem has plagued me for two years. I have done a lot of work to get
to the bottom of it and have noticed a number of conditions that seem to
have to be in place for it to happen.
1. High alumina slips. Any slip high in feldspar or silica generally is
O.K.
2. Certainly free carbon contributes to the blackness but I'm not so sure
about the crustiness.
3. Oxidation. I think that an early reduction BEFORE the rise of any
residual salt is important.
I would be interested to know a little more about the slips you were using
and the firing path. I have recently changed my kiln from oil to gas to
overcome this problem but the first firing wasn't wholly successful. I
believe that what is happening is that certain pots are overheating to soon,
sealing over from residual salt and then for some unknown reason the surface
doesn't flux in the usual way. The body looks reduced but I think this is
happening from the inside where there is a conventional glaze and therefore
stays open and receptive to reduction for longer than the outside. I have
also noticed many times that the crusty surface is always the side of the
pot facing the salt.
Let me know what you think and any further information/details about the
firing would be helpful. I spoke at length with a mate of mine, Michael
Casson, only two days ago. He fires a kiln similar to mine with gas, similar
slips, similar body with no problem. He thinks it is a lack of salt issue, I
think not. Ruthanne Tudball, another close friend, has the same body, fires
with gas, Soda and not salt, same slips, no problem. So it is a firing
procedure issue and entirely correctable if we can only pin down exactly
where the problem occurs. In short , I am convinced the crustiness is an
Ox/Red problem, the blackness is carbon. Read Nils Lou on the issue of free
Carbon and reduction - interesting.
Phil.
Phil and Lynne Rogers,
Lower Cefn Faes,
RHAYADER.
Powys. LD6 5LT.
Tel/fax. (44) 01597 810875.
philrogers@ntlworld.com
www.philrogerspottery.com
----- Original Message -----
From: LOWELL BAKER
To:
Sent: Tuesday, January 30, 2001 9:57 AM
Subject: Re: Serious wood fire/salt question, really
> Over the course of many years of firing salt and wood kilns I have,
> on occasion, encountered a crusting of the surface. This lizard
> skin is or seems to be associated with heavy reduction in the very
> early stages of firing. This is particularly the case in salt kilns. It
> used to happen in my salt kiln when the burner would backfire and
> soot up the kiln over night. This crusty surface is often very black
> and sometimes has a fragile crystal like surface. It is normally on
> the lower portion of the piece.
>
> We got some of this last firing and it didn't go away even with the
> cone 12 temperature. The firing temperature went up and down
> between 1800 and 2000 degrees several times during the firing and
> my graduate students added some water to the kiln while I wasn't
> looking. (No wonder they lost temperature every time they stoked)
> This crustiness was also found on porcelain bodies but it was not
> dark.
>
> I haven't a clue, can anyone help?
>
> W. Lowell Baker
> the University of Alabama
>
>
____________________________________________________________________________
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> Send postings to clayart@lsv.ceramics.org
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>
Dennis Mummert on wed 31 jan 01
This may seem sacreligious, but if you've got one of those pieces that
you're not really in love with, and you have a moderate sized university
close to you with a decent physical chemistry department, ask them if they
have an SEM (Scanning Electron Microscope) and a quadrupole mass
spectrometer. The SEM can actually do most of the tests, so the quad isn't
a requirement. Bust up the pot and take a small piece of the glaze surface
in. By small, I mean about half the size of the area of your little
fingernail. Don't hammer on the crystals though - try to preserve them. Be
prepared to spend some money on film, the universities usually have a modest
budget when it comes to allowing the grad students to snap pictures on
freebie jobs.
-----Original Message-----
From: Ceramic Arts Discussion List [mailto:CLAYART@LSV.CERAMICS.ORG]On
Behalf Of LOWELL BAKER
Sent: Tuesday, January 30, 2001 12:58 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Serious wood fire/salt question, really
Over the course of many years of firing salt and wood kilns I have,
on occasion, encountered a crusting of the surface. This lizard
skin is or seems to be associated with heavy reduction in the very
early stages of firing. This is particularly the case in salt kilns. It
used to happen in my salt kiln when the burner would backfire and
soot up the kiln over night. This crusty surface is often very black
and sometimes has a fragile crystal like surface. It is normally on
the lower portion of the piece.
We got some of this last firing and it didn't go away even with the
cone 12 temperature. The firing temperature went up and down
between 1800 and 2000 degrees several times during the firing and
my graduate students added some water to the kiln while I wasn't
looking. (No wonder they lost temperature every time they stoked)
This crustiness was also found on porcelain bodies but it was not
dark.
I haven't a clue, can anyone help?
W. Lowell Baker
the University of Alabama
____________________________________________________________________________
__
Send postings to clayart@lsv.ceramics.org
You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
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