May Luk on mon 10 nov 03
Hi all;
I have started making some tube vases with the circumference of about 280 cm
(14 in?) The height is from 12" to 20". The diameter of the circle about 5".
The thickness of the slab 6 mm. The height does not matter. I have been
having problem with cracking while drying. The crack is usually at the
opposite end of the joint. I am very careful with the pots. I wrapped them
up in plastic sandwich bags in the first 4 days. Then I poke small holes in
the bags to slow the drying. The weather in London is quite damp and I don't
have heat on. I also move the drying pots to a safe spot when the kiln is
on. I am aiming at having the drying time for about 3 weeks- 4 weeks. I had
made 14 vases and I have cracked about half of them. It really hurts when I
take great care to roll out a 340 cm x 300 cm slab, plus the time for
stamping and sprigging.
The clay is Earthstone extra smooth, which is good for stamping and
sprigging.
The analysis is:
Sio2___67.41
TiO2___1.04
Al2O3__21.1
Fe2O3__0.6
P2O5___0.07
CaO____0.35
MgO____0.35
K2O____2.72
Na2O___0.99
Loss on drying:19.47
% Total Contraction:1200-13.5
Pre Fired Total: 1280
Thermal Expansion:
500 deg: 0.240
600 deg: 0.300
Will this analysis tell me anything? I don't know how else to work with the
clay that would make a difference [for a tube vase].
T.I.A. for any suggestions.
Regards
May
London, UK
Russel Fouts on tue 11 nov 03
May
>> I have started making some tube vases with the circumference of about 280 cm (14 in?) The height is from 12" to 20". The diameter of the circle about 5". The thickness of the slab 6 mm. The height does not matter. I have been having problem with cracking while drying. The crack is usually at the opposite end of the joint. I am very careful with the pots. I wrapped them up in plastic sandwich bags in the first 4 days. Then I poke small holes in the bags to slow the drying. The weather in London is quite damp and I don't have heat on. I also move the drying pots to a safe spot when the kiln is on. I am aiming at having the drying time for about 3 weeks- 4 weeks. I had made 14 vases and I have cracked about half of them. It really hurts when I take great care to roll out a 340 cm x 300 cm slab, plus the time for stamping and sprigging. <<
On a positive note, if the pieces are cracking on the side opposite the
join, you know that your joins are good. ;-)
Are you drying them upright all the time? Is the crack all the way down
the piece or does it start at one end? Are you turrning them over from
time to time?
When you wrap them in plastic are is the plastic tight? If your clay
comes in bags the bags are probably stiff enough to stand up by them
selves. Use them as tents over the pieces and just leave them like that.
Maybe occasionally take the tent off and turn it inside out to get rid
of some of the water that has collected. You might also turn the piece
over at that time.
If this is the body I saw when I visited you, it was very smooth,
doesn't seem to have any grog in it. The analysis would also show some
grog content if there was any so I think it doesn't. Maybe if you wedged
in a little bit of really really fine grog, it might help.
Also, how you make the slabs, how you form them and how dry they are
when formed might also be factors.
Like David says "everything matters"
Russel
--
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Brussels, Belgium
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May Luk on tue 11 nov 03
Hi Russel;
The crack is in the centre-like when you make a slit cutting a salmon. It
looks like the clay wants to go back to being flat--very good memory and
stubborn!!
It's the clay you saw in my studio. It's been very soft coming out of the
bag these days. So when I finish the vase, I could still leave finger marks
on it so I don't touch it and then wrap it with thin plastic bag-tight
because the cheap sandwich bags are very thin. The first few days, there's
condensation. I would open the bag a bit and air it. But I don't even get to
turn it around and it's cracked, when it starts changing colour-from med
gray to lighter gray and the surface starts to become more powdery in
texture.
Actually, the vase and the jug that you saw were all cracked, but I patched
them anyways to see how the magic water and paper clay fix works.
You just reminded me and I called Valentine Clay and it turned out there's
no grogg in the clay. I had made the mistake with the clay number ES/10 and
I thought there's 10% grogg in it. Oh, dear, but they are sending me some
other samples to try. No real help for my open studio next month though.
Thanks and regards
May
London, UK
Last time I bought clay from Valentine, they sent the biggest truck [Like
the one Thelma and Louise burnt down] ever to Nothing Hill and the driver
could not park or stop anywhere. He had to go back to Stoke-on-Trent with
the clay. It was only 5 hours drive, I think.
P.S. You are in Morley, just in case nobody notify you yet.
on 11/11/03 9:24 am, Russel Fouts at Russel.Fouts@skynet.be wrote:
>
> On a positive note, if the pieces are cracking on the side opposite the
> join, you know that your joins are good. ;-)
>
> Are you drying them upright all the time? Is the crack all the way down
> the piece or does it start at one end? Are you turrning them over from
> time to time?
>
> When you wrap them in plastic are is the plastic tight? If your clay
> comes in bags the bags are probably stiff enough to stand up by them
> selves. Use them as tents over the pieces and just leave them like that.
> Maybe occasionally take the tent off and turn it inside out to get rid
> of some of the water that has collected. You might also turn the piece
> over at that time.
>
> If this is the body I saw when I visited you, it was very smooth,
> doesn't seem to have any grog in it. The analysis would also show some
> grog content if there was any so I think it doesn't. Maybe if you wedged
> in a little bit of really really fine grog, it might help.
>
> Also, how you make the slabs, how you form them and how dry they are
> when formed might also be factors.
>
> Like David says "everything matters"
>
> Russel
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