Suzanne Wolfe on sat 7 dec 02
Dear Clayarters,
My student has been working with china paints on very large pieces, which
she has to fire in a gas kiln. Some problems are developing, and I hope
that some of you might be of assistance.
The china paint surfaces came out of the last firing all gritty and grey,
with substantial loss of color. She called the company from which she
purchased the china paints, and they called the problem "mildew", which
seems to be something that has arisen from the fact that the original
firing was in reduction. (This sounds really fishy to me....)
Previous firings were no problem, but this one was a disaster. She (in my
opinion) did the china paint firing over far too long a time period -- she
candled the kiln for about 12 hours, and the entire firing took about 30
hours! Since the firing was in a gas kiln (and since I was not there to
be watching what was happening), it is possible (no probable, given the
gas kiln) that some slight reduction occurred.
Can anyone offer some advice about this situation, based on your actual
experience/knowledge? (Actually, speculations would be welcome also!)
Please note that she is using Coleman porcelain to build her large pieces,
and spraying a clear glaze on top. In her first firing, she slightly
over-fired, and some of the pieces slumped. She rebuilt everything, and
for the next firing, she underfired a bit -- about cone 7-8. I hope that
this is enough information.)
I am concerned about the next step. Could she grind down the gritty
surface and re-do the china paint? (I admit, she is not much of a
technician -- she uses any medium she wants. However, on these pieces she
has used both water base and oil base china paints, and both have
exhibited this gritty, washed out surface.)
Does she have to completely rebuild everything and then do tons of tests?
Should she make everything smaller, or in pieces, to fit into our electric
kilns? Etc.
Thanks to all who can help.
Suzanne Wolfe
Snail Scott on sun 8 dec 02
At 07:26 PM 12/7/02 -1000, you wrote:
>
>The china paint surfaces came out of the last firing all gritty and grey
>...the company...called the problem "mildew", which
>seems to be something that has arisen from the fact that the original
>firing was in reduction. (This sounds really fishy to me....)
Don't take the 'mildew' name too literally; they don't
mean actual fungus, after all, just the appearance. And
it does sound like what happens in reduction.
>...she
>candled the kiln for about 12 hours, and the entire firing took about 30
>hours!
Damn! That's long! Remember that since china paints
are a very thin surface layer, you don't have to heat
the entire piece through; just the outside. There is
no reason for a long firing, since as soon as the
kiln air hits the right temperature, so do the china
paints. It's almost instantaneous. In fact, when I
fire my electric for lusters or china paints, I leave
the lid propped until after I turn up to medium,
and if I don't come back to shut the lid on schedule,
the sitter will often shut off while the lid is still
gapped, and I STILL get a successful firing! When I
do it 'right', the whole firing (on 2' high, thick
stoneware sculpture,) still only takes maybe two hours.
Fuel firing is a little different, and heating the
kiln itself can take longer, but unless the piece
(or the kiln!) is very porous and got wet since the
last firing (and you said matured porcelain, right?)
you shouldn't need to candle more than 20 minutes,
I'd think. Then fire it up. You should be done the
same afternoon, easily.
You may be able to abrade off the bad china paints;
maybe with a dremel-type tool, or some such. You will
probably have to refire the glaze to smooth it out
again before trying again. I'm just guessing on
this, though. Let us know what happens...
-Snail
Paul Lewing on mon 9 dec 02
on 12/7/02 9:26 PM, Suzanne Wolfe at swolfe@HAWAII.EDU wrote:
> The china paint surfaces came out of the last firing all gritty and grey,
> with substantial loss of color. She called the company from which she
> purchased the china paints, and they called the problem "mildew", which
> seems to be something that has arisen from the fact that the original
> firing was in reduction. (This sounds really fishy to me....)
You're right to be suspicious of that rationale, Suzanne. The temperature
or atmosphere of the original glaze firing makes no difference whatsoever to
how china paint comes out. Nor does it make any difference whether you mix
your china paints with water or oil.
I'd bet almost anything that you got that firing too hot and a little
reduced. And I'd bet it was the reds and oranges, and maybe the yellows
that gave you the problem. Those are the cadmium colors, and they're
incredibly sensitive to reduction and overfiring. If you have to fire these
in a gas kiln, I'd say to have some kind of forced air system on it, so you
knew you were really in oxidation all the time. And twelve hours is a
ridiculously long time for a china paint firing. Inn my 23" electric
top-loader, a really packed load of tiles and tile setters takes about 4
hours, and a loose one takes about 2. And the first hour is slow warm-up.
You're just wasting gas there.
Good luck, Paul Lewing, Seattle
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