Lyn Packer on wed 13 jan 99
Hi, I am having a problem with hairline cracks that run from the rim,
vertically down the mug. If I explain my procedures, would anyone have any
ideas whether I'm to blame, or whether it could be the clay, or the glaze.
I've checked out Hamer's article on cracks. One of his suggestions was
readsorption. When I bisqued them, I put them in the kiln with other pots
that weren't quite dry. I then turned the kiln on, heated it up to 100oC,
and left it at that temp for several hours {bungs out of course}. I then
turned it off and left it overnight. In the morning I turned it on again,
fired to 900oC, slowly until 400oC. Then, put the bungs in and turned the
kiln up to full. {an old electric kiln, very faithful}. I didn't have a full
load, so they weren't stacked one on top of the other. Also I am very
careful with my raw pots. I have been doing pottery on and off for 18yrs,
and the thickness of my pots is pretty even. The drying was even and not
rushed. I have been using the same glaze for about 10ys. Also because of
that, I know the thickness of glaze my pots need. The batch was not a new
one. The clay that I used is a type that I haven't used for some time. It's
a white stoneware body. I think I've covered everything I did. I would like
to be able to blame the clay, but if I can't, then does anyone know what I
can do to remedy the situation? This is the base glaze I use. It's always
been a stable and reliable glaze that I use to decorate over. Fires to Cone
9 or 1280oC.
Potash Feldspar 7lb 8oz
China Clay 6lb 4oz
Silica 8lb 8oz
Dolomite 5lb
Thanks, Lyn { enjoying a lovely summer here in New Zealand.}
lyn.packer@clear.net.nz
Tom Wirt on thu 14 jan 99
Subject: Hairline Cracks In Mugs
>----------------------------Original message----------------------------
>Hi, I am having a problem with hairline cracks that run from the rim,
>vertically down the mug. If I explain my procedures, would anyone have any
Lyn....
We had a problem with the same kind of crack in bowls. It tended to show
more on the inside than the outside. Finally figured out the young lady
who was helping load bisque, occasionally bumped one ever so gently against
something. It couldn't be seen until after bisque and sometimes after glaze
fire. As someone on the list pointed out at the time, if you can hear or
feel the bump, even setting it down, it's probably cracked. We probably get
away with a lot more than we realize.
Also, be sure you're not distorting the leather hard cup when applying
handles. That can do it too.
Tom Wirt
Randy O'Brien on fri 15 jan 99
>When I bisqued them, I put them in the kiln with other pots
>that weren't quite dry. I then turned the kiln on, heated it up to 100oC,
>and left it at that temp for several hours {bungs out of course}. I then
>turned it off and left it overnight. In the morning I turned it on again,
This could be your problem, Lyn. Once you start a bisque you shouldn't
turn it off until it reaches temperature. Heating greenware to 100
degrees C and then letting it cool can definitely cause hairline cracks
to form. I've experienced hairline cracks on pots that were put in a hot
box and then cooled to room temp before being bisque fired.
Randy O'Brien
euphoria@mcn.org
Richard Ramirez on sat 16 jan 99
Lynn,
Two things..you say you're careful to dry them slowly, yet you put them with
not so dry pots...why..Heat them, then their off...why..then heat them
again...sounds like alittle stress, might show up with newer clay body...Who
knows, maybe you cross the "kiln gods" Richard Ramirez "The Clay Stalker"
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