Cindy Strnad on mon 19 feb 01
Hi, Lili.
I can't quite picture the bend in your slab bowls, but I'll bet I can tell
you why they're cracking. I suspect that U-shaped bend. The more precipitous
it is, the more tendency toward cracking you'll have to deal with. You might
try rounding things off a bit--the edges and anything sharp--to prevent this
problem
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Lili Krakowski on mon 19 feb 01
1. Someone told me of someone adding muriatic acid to glaze. WHY?
Having used it with great precautions cleaning brick, I would not like it
around the studio (the supply of it, not a bit in the glaze.) What
benefits does it give?
2.Does anyone have recipe[s] for c.6 bone ash glazes using more than 10%
b.a. ? A very very nice person asked me because she cremated a pet
and wants to use the ashes in a nice glaze. (Before anyone giggles: many
decades ago I was told of a British potter who knew he was dying and
worked on b.a. glazes which he hoped his friends would put on special pots
he was making. The glazing project was happening when I heard of it...)
3. As I do not know if I will be able to throw any time soon, and for how
much longer, I have been making bowls in female molds. The mold make the
slab look a bit like a sail in the wind, or laundry on a line. THE
PROBLEM IS that the bowls often crack at the rim, where there is a "tight"
fold; really U shaped.
I have tried slowing the drying by latex or wax resist on the top 2". I
have tried rewetting the clay at the top and then "pasting" plastic wrap
on it. I always dry big pieces under styrofoam "cloches"; so I hav tried
all that. WOULD HUMP MOLDS BE BETTER/
Now I wonder: are there additives to my clay, or clays particularly good
for such flexibility.
Bows, hugs thanks to all helpful ones. (TGSDF there are m any.)
Lili Krakowski
Stephen Grimmer on tue 20 feb 01
Lili,
If you have a glaze that settles badly from lots of soda spar, frit,
neph sy, or lack of clay, muriatic acid (HCl) will flocculate the glaze and
help keep it from forming a rock in the bottom of the bucket. It also
thickens the slop and helps application problems like running, sheeting, and
slow drying. I like to dilute it 1:1 with water (add acid to water, not
water to acid). Usually a tablespoon of the solution will noticeably thicken
5 gallons of glaze.
Additive A is a sodium lignosulfate which is, I think, derived from
wood. It works as a particle lubricant, and can increase the flexibility of
drier clays. It is used in extruded brick bodies to lower the water content
of the clay and save wear on the machinery. It comes as a granular, sticky
powder, and you add it to your water before mixing clay. It smells like soy
sauce. I think I used to add 0.5% to my porcelain body.
Good luck,
steve
--
Stephen Grimmer
Southern Illinois University at Carbondale
> From: Lili Krakowski
> Subject: muriatic acid; bone ash glazes, "bendable" clay bodies
>
> 1. Someone told me of someone adding muriatic acid to glaze. WHY?
> Having used it with great precautions cleaning brick, I would not like it
> around the studio (the supply of it, not a bit in the glaze.) What
> benefits does it give?
>
snip
>
> Now I wonder: are there additives to my clay, or clays particularly good
> for such flexibility.
snip
> Lili Krakowski
>
Michael Tannock on tue 20 feb 01
Hi Lili
I can't claim to be an expert on slab slump moulding work but I think that
your best chances of getting work out of a mould without cracking to choose
the right clay.
I have spent some time making a series of work, about half of the peices
slump molded on the inside and half on the outside of a similar shape. They
both seemed to break the same amount, they were shapes that had quite thin
rims... problems. I too tried to latex bits. In the end I found that the
best thing was using slab clay, that is clay that is formulated for
handbuilding. I still had to do the careful drying thing.
I think that the good thing about it was that it had a proportion of natural
grog (feldspar and other sands) which limited the plastic to dry shrinkage
and so shrinkage problems. Come to think of it it was also really plastic,
kind of went more plastic when I moved it and it was difficult to throw, so
it may be other properties which made it good for the process apart from the
grog. I dont know what choice of clays you have but if you can pick and
choose, choosing a clay like this may help.
Good Luck
Michael Tannock
>slab look a bit like a sail in the wind, or laundry on a line. THE
>PROBLEM IS that the bowls often crack at the rim, where there is a "tight"
>fold; really U shaped.
>
>I have tried slowing the drying by latex or wax resist on the top 2". I
>have tried rewetting the clay at the top and then "pasting" plastic wrap
>on it. I always dry big pieces under styrofoam "cloches"; so I hav tried
>all that. WOULD HUMP MOLDS BE BETTER/
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