Marvin P Bartel on mon 12 oct 98
>----------------------------Original message----------------------------
>Okay, have consulted all my reference materials and still need help. I
>am however gratified that after (about) the 15th reading of Rhodes "clay
>and glazes for the potter" it is beginning to make some sense to me.
>Whew!
>
>1. I have cobalt carbonate. In glazes requiring cobalt oxide is the
>ratio 1 to 1? or can you use less of the cobalt carbonate.
We find a 1 to 1 substitution works fine even though the oxide is a bit
stronger than the carbonate. I know a potter with a bad case of cobalt
poisoning. It is hazardous if inhaled. Wash glaze off of hands before
smoking. Do not spray unless precautions are taken. I believe cobalt in
fired glaze is okay. mb
>
>2. Right now I am doing some tiles intended for kitchen backsplashes and
>bathrooms. Is there a need to avoid toxic materials in the glazes when
>the likelihood of food being on the tiles is remote if not impossible?
>What is the conventional wisdom here?
Hmm. I can't see how this could be a hazard, other than for the potter
working with the raw materials. mb
>3. I have a large quanitity of bisqued tiles and have started actually
>glaze firing a few. I noticed that even though the bisqued tiles were
>flat, upon completion of the glaze fire they were slightly warped. I
>guess the obvious answer is to try some different clay that has more
>grog, but for all these bisqued tiles, is there something I can do
>during the glaze firing to prevent the warping? I am using electric
>Skutt at medium speed to cone 6.
In my experience, this warping is caused by the release of residual stress
left in the clay when they formed and dried. Clay does get soft at cone 6,
so be sure they are on flat level surfaces. mb
>
>Thanks in advance.
>
>Diane Chen
>
***********************************************
Our November, 98, Visiting Artist, Mary Ellen Mark is at:
http://www.goshen.edu/art/DeptPgs/MarkProg.htm
Our current gallery schedule:
http://www.goshen.edu/art/exhsc98.htm
***********************************************
Marvin Bartel, Ed.D., Professor of Art
Goshen College, 1700 South Main St., Goshen IN 46526
Office (219) 535-7592 Fax (219)535-7660
Studio (219) 533-0171
***********************************************
My October, 98 exhibition: www.bartelart.com/exhibit.htm
The show runs through Oct 25
In the Goshen College Art Gallery, Good Library
Fax (219) 535-7660
e-mail marvinpb@goshen.edu
http://www.goshen.edu/art/
http://www.bartelart.com/
***************************
http://www.bartelart.com/acorn.htm
"Plant trees. It's a good thing."
David Hewitt on wed 14 oct 98
In message , Marvin P Bartel writes
>----------------------------Original message----------------------------
>>----------------------------Original message----------------------------
>>Okay, have consulted all my reference materials and still need help. I
>>am however gratified that after (about) the 15th reading of Rhodes "clay
>>and glazes for the potter" it is beginning to make some sense to me.
>>Whew!
>>
>>1. I have cobalt carbonate. In glazes requiring cobalt oxide is the
>>ratio 1 to 1? or can you use less of the cobalt carbonate.
>
This question made me do some reading as I wasn't sure of the answer and
I found the following in Hamer's 'Potter's dictionary of Materials and
Techniques' which may be of general interest.
'Cobalt oxide is usually purchased as cobalto-cobaltic oxide (cobaltosic
oxide, Co3O4) which is black. This is a stable form at room temperature
which liberates its excess oxygen during firing above 800oC to form
cobaltous oxide (CoO). Cobaltous oxide is the stable form at high
temperatures and the one which combines with the silicates and borates
to give a blue colour. Other convenient forms are cobaltic oxide
(Co2O3) which is grey but contains only 90% effective stain against the
93% of the black oxide, and cobalt carbonate (CoCO3) which is mauve in
colour and is only 63% effective stain content.'
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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