Shelley S on mon 21 dec 98
Hi,
I'm wondering if anyone can help me figure out why I'm getting carbon
pinholing in my porcelain clay body. I'm firing to about cone 9 and a half
in reduction, starting the heavier reduction when 012 is down. Am I
starting too early or would this be a sign of over-doing the reduction?
Its not affecting the stoneware but, the carbon seems to get trapped under
the glaze rather than burning out on porcelain. I learned to reduce at
this temp for reds but it has been fine for other colors as well.
thanks in advance,
Shelley
Karen Gringhuis on tue 22 dec 98
Shelly - I don't know exactly what "carbon pinholing" is but
sometimes pinholing can result from outgassing of materials which
were not burned out int the BISQUE. In which case slowing down the
bisque firing from 1000F up to temp might help - let temp rise
at about 100 degrees per hour, no faster. Helps to have
a digital pyrometer. Just a possibility. Karen Gringhuis
Louis Katz on tue 22 dec 98
Hi Shelly,
When I went to school we started reduction at 012.
Once out of school I gradually started to reduce latter and latter finally
settling in at around 06.
I recently started reducing at 012 again TO TRAP CARBON.
Are you sure you need to start reducing this low for your copper reds?
If you do need to reduce perhaps you are reducing too hard.
If you fire with forced air try turning up the blowers, and closing the damper
and get more primary air and less secondary air. If you have too little
primary air you will get a lot of carbon but not necessarily get good
reduction.
Louis
Shelley S wrote:
> ----------------------------Original message----------------------------
> Hi,
>
> I'm wondering if anyone can help me figure out why I'm getting carbon
> pinholing in my porcelain clay body. I'm firing to about cone 9 and a half
> in reduction, starting the heavier reduction when 012 is down. Am I
> starting too early or would this be a sign of over-doing the reduction?
> Its not affecting the stoneware but, the carbon seems to get trapped under
> the glaze rather than burning out on porcelain. I learned to reduce at
> this temp for reds but it has been fine for other colors as well.
>
> thanks in advance,
>
> Shelley
--
Louis Katz
lkatz@falcon.tamucc.edu
NCECA Director At Large
Texas A&M-CC Division of Visual and Performing Arts Webmaster (512) 994-5987
Craig Martell on tue 22 dec 98
>I'm wondering if anyone can help me figure out why I'm getting carbon
>pinholing in my porcelain clay body. I'm firing to about cone 9 and a half
>in reduction, starting the heavier reduction when 012 is down. Am I
>starting too early or would this be a sign of over-doing the reduction?
Hi:
Most porcelains don't start to tighten up until you are above the cone 06
range, which is about 1830F. Of course, this is not true of all high fire
porcelains. My clay is a mix of 6 tile kaolin, kaopaque 20, and a very
small amount of ball clay. I have to bisque this body to cone 03 to get it
tight and strong so that large platters and other large flatware pieces will
survive the quartz inversion during cool down. So, I would say that in all
probability, if you are reducing heavily from cone 012, there may be quite
an excessive carbon inclusion happening with the ware. I would think that
most kaolin based clays will be very soft and receptive to carbon inclusion
below about cone 06. I start reduction at cone 06 and get very good results
with copper reds. I sometimes think that a really early reduction is not
necessary for the development of reds. This is an opinion and I have no
solid evidence other than my own results to give any weight to this opinion.
Carbon trap shinos are an exception to this as they seem to work very well
with an earlier reduction.
Stoneware clays will start to fuse earlier than porcelians so the above rant
is only meant for porcelains and high kaolin bodies. If you fire stoneware
and porcelain together in most firings, you may need to find a "happy
medium" to achieve the best results for both.
regards, Craig Martell in Oregon
Shelley S on wed 23 dec 98
Craig, Louis and Karen,
I'll definitely try starting reduction later. I may also need to work on
the primary air - I'm firing with venturis. I do bisque very slowly though,
so I think the problem is in the glaze firing.
Craig - your comments about bisquing (sp?) the porcelain higher (to 03)
sound interesting. Does this help keep platters from warping as well? My
porcelain is grolleg based - I don't have cracking but do have problems
with warping occasionally.
thanks again,
Shelley
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