=?iso-8859-1?q?Ela=20Trent?= on thu 9 may 02
Hi
I am having real problems with Audrey Blackman porcelain from Valentines
Clay (in the UK). I'm biscuit firing to 1000c and then 2nd firing to 1270
(according to the manufacturers this is within the clays limits - in an
electric kiln).
Anyway, i'm finding that with smaller pots, after the 2nd firing,
horizontal cracks appear on the exterior of the pots. They are not
completely through the whole body. Sometimes they appear just on half the
circumferance of the pot (usually near the base) - sometimes all around the
pot (in small bottle shape forms with rounded, highly turned bases).
And then with large pots - big bowls, i'm finding these same kinds of cracks
but after the biscuit firing.
Help! I'm not sure what i'm doing wrong. I'm a beginner with porcelain. I
do try to let the pots all dry slowly and naturally. I'm using a bog
standard glaze recipe - i've been told you can use the same recipes for
porcelain as in stoneware - is this right? I'm not sure if the glazes are
the problems though, because i use a few different ones and the same
cracking problems occur on all of them.
Is this a characteristic of this clay? If it is, is there another porcelain
out there which will be a bit more reliable and less likely to cause nervous
breakdowns?
Any help is much appreciated.
Ela
---------------------------------
Do You Yahoo!?
Get personalised at My Yahoo!.
Paul Gerhold on thu 9 may 02
Ela,
Two things to check. I would suspect that you have the cracks in the bisque
but they are very fine and not obvious. I just was reading that if you wet
the area with kerosene they will appear but the kerosene will not hurt the
pot. Wear gloves and you can buy kerosene at any hardware store.
The other question is whether or not the cracks are only in the trimmed area
, at the junction of the trimmed and untrimmed, or over the entire pot.
Also try if possible a slow controlled drying to see if this will eliminate
the cracks.
Avril Farley on thu 9 may 02
Dear Ela
In conversation with various porcelain potters when looking for the most
suitable for my work (also small bottles/bowls at the time) I found that
generally Audrey Blackman is chosen for the fininished translucent quality
being a difficult clay to work with, and hardly any of the potters I spoke
to used it for large thrown items. Most came down in favour of Harry Fraser
or Limoge, and lately a number of people have changed to Southern Ice from
Australia.
Other than choice of clay, (I use Harry Fraser and Limoge) there are so many
reasons for biscuit cracking including opening the kiln too early - i.e. top
of bottle cooling quicker than the base on the kiln shelf, and post glaze
firing cracks, and I have experienced them all! I recommend you read Hamer
and Hamer, where 6 pages are devoted to the causes and cures for cracking,
and work your way through the possible causes based on your own
making/turning/drying/firing regimes. The main cause of my early troubles
with the turned bases of bowls and bottles were - unequal shrinkage, uneven
wall thicknesses from middle of bottle to too thick a base causing
undetectable biscuit dunting cracks that opened in the glaze firing; and
trying to turn the clay when it was too wet. I've pretty well got over
these problems now.
Whatever it is, good luck in finding the reason for this highly frustrating
problem, and do not hesitate to e mail me direct if you want to.
Avril in the Forest of Dean, UK
avril@soudley44.freeserve.co.uk
>
>
>___________________________________________________________________
> 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.
>
Timakia@AOL.COM on thu 9 may 02
Ela, there are already some wonderful advice out there for you to follow.I
want to add this: When trimming porcelain, your clay will be fairly dry and
your instruments must be very sharp.Make yourself a disk(bat) with a thin
layer of foam glued on there. Trim your work on it to prevent too much shock
to the piece. With sharp tools, you do not have to put too much preasure on
the work that can cause cracks.
If you talk about vertrical cracks, it sounds to me as if the problem begin
when you trim.
If you are a beginner with porcelain, you poberbly leave thicker walls to
support your pieces and you have lots of clay to trim away at the leather
hard stage. That means that you have trimmed the area that was compackted
during throwing,away. For that reason you have to compact that area again.
What I do, is wet the area again a little bit to give me a slippery area. I
then use a plastic card and with a smearing movement compact the clay
particles again.Take care, if you left it very thin, you might distort the
shape).
You will know if dunting appeared during bisque if the edges of the cracks
are "finished off" by glaze.If you have a sharp crack edge, the problem ins
in the glaze firing. That will also help you look for the mistake in the
right places.
Hope this helps.
Antoinette Badenhorst
http://hometown.aol.com/timakia
105 Westwood circle
Saltillo, MS
38866
Stuart Gray on thu 9 may 02
Maybe this will help.
Dry your pieces with a plastic bag loosely draped over them for the first
three days. At that point, (and depending on your humidity), the piece
should be sufficiently dry to allow trimming. After trimming, dry piece
upside-down, without plastic bag, for remainder.
Utsyarg
Paul Taylor on thu 9 may 02
Dear Ela
I use David leach porcelain I found it works well for bigger pieces and
ok for small pieces. It is not as good as any of the other clays at any
thing; but all the other clays I found always had a catch - some
idiosyncratic flaw that always made me go back to the David Leach or the
Potclays Standard porcelain - not that I have tried any others recently.
I liked the Blackman porcelain it was lovely to throw with but it seemed
very expensive when the others are cheaper the potclays stuff also seems to
reclaim well .
Porcelains! I treat them with the same lack of reverence as I do a stoneware
and the clay has always been the least of my problems. I wonder sometimes if
this "I use the 'la vondervite clay' is just a bit of snobbery.
My kiln is not very efficient and I fire to cone 8 (down) at about 40 to
50 degrees centigrade an hour from 1000 c in reduction. This inefficiency
means I do not get half the problems that other people seem to get with
porcelain. I make 15 inch dishes with out a hint of a crack ( touch wood).
All I do is paint some aluminum hydroxide on the bottom and a little bat
wash (as thin as possible) on the kiln shelf. The porcelain (Davids) is
translucent from cone six but like some others on this list I am suspecting
my cone sixes are really sevens.
I throw some pots thick and some turn to almost egg shell - makes no
difference.
I think a low biscuit 950c and slow finish is the secret to success.
Industry fires quickly for economic reasons. These economies should not be
too large a consideration for the art potter since labor is way our greatest
cost. Yet we are all very influenced by the fast fire hysteria . The little
extra spent if it avoids problems is well worth it, although it will take
some nerve to fire your kiln slower when every body is boasting about how
fast they go. If your kiln is well insulated the extra cost will be very
small if any.
The DL porcelain at cone five and six is translucent but is not vitrifyed
( turned Thin). I find this interesting; it's as if the glaze will fit a
porcelain clay at very low temperatures - but I suppose bellow cone 8 it is
still absorbent and should be considered a stoneware.
Your porcelain may respond better to the gentler firing schedule - bung
a cone in the biscuit to check the temperature - some pyrometers can be very
fickle. But the fault you mention usually happens with a lower biscuit
firing and/or a draft of cold air getting at some of the pots before they
have gone through the ceramic change about 500 to 700c. Sometimes pulling a
bung to get a look ,or let some air in to stop bloating, will crack a few
and you will not notice until the glaze firing.
I note you specify a temperature. If you use cones you can play about
with the firing times with greater accuracy. Most people on this list talk
'cones' as well. You will have to find out what cone your 1270 is at your
rate of firing before you start experimenting. Also take draw trials of your
glazes ( remember to turn the kiln off when you do (( I would not trust my
absent mind to do this on my own))) to see at what point the glaze starts to
vitrify and the porcelain go translucent - and be ready for a surprise.
--
Regards from Paul Taylor
http://www.anu.ie/westportpottery
I am writing to test my beliefs not to champion them ( I hope)
> From: Ela Trent
> Reply-To: Ceramic Arts Discussion List
> Date: Thu, 9 May 2002 10:14:30 +0100
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Porcelain - help me!!!
>
> Hi
>
> I am having real problems with Audrey Blackman porcelain from Valentines
Clay
> (in the UK). I'm biscuit firing to 1000c and then 2nd firing to 1270
> (according to the manufacturers this is within the clays limits - in an
> electric kiln).
>
> Anyway, i'm finding that with smaller pots, after the 2nd firing,
horizontal
> cracks appear on the exterior of the pots. They are not completely
through
> the whole body. Sometimes they appear just on half the circumferance of
the
> pot (usually near the base) - sometimes all around the pot (in small
bottle
> shape forms with rounded, highly turned bases).
>
> And then with large pots - big bowls, i'm finding these same kinds of
cracks
> but after the biscuit firing.
>
> Help! I'm not sure what i'm doing wrong. I'm a beginner with porcelain. I
do
> try to let the pots all dry slowly and naturally. I'm using a bog standard
> glaze recipe - i've been told you can use the same recipes for porcelain
as
> in stoneware - is this right? I'm not sure if the glazes are the problems
> though, because i use a few different ones and the same cracking problems
> occur on all of them.
>
> Is this a characteristic of this clay? If it is, is there another
porcelain
> out there which will be a bit more reliable and less likely to cause
nervous
> breakdowns?
>
> Any help is much appreciated.
>
> Ela
>
>
Craig Martell on thu 9 may 02
Hi:
Compress the surface of the pot with a rib. If you want throw rings in the
finshed piece put them on the surface gently, after you rib the piece.
regards, Craig Martell in Oregon
| |
|
