David Martin Hershey on wed 6 aug 08
Hi Lesley and Ivor,
Ivor, your information confirms what Joe Koons at Laguna Clay once told
me was the cause of this problem. For me the problem is worse on molds
that are not that tight.
The idea of tapping the mold seams is interesting, and I will try it.
But intuitively I would think this would tend to align the particles more.
Grinding the bisque would not work for me, and is a silica hazard. I
tried soft bisque firing and wet sanding, and it didn't help- but I
didn't leave the flash on.
My pieces use up to 8 part molds, and cutting and filling the grooves
would be too labor intensive.
Lesley, I've gotten acceptable results by burnishing the seams when the
piece is just dry enough to handle. My pieces don't have texture, so
this is not a problem. I use a hand metal burnisher made for
printmaking. I use a burnishing motion to push the clay across the
seams. Most of the time this deposits enough of the unaligned clay in
the seam area to stop the problem.
I am also testing using a slip under the glaze.
Ivor is correct that Jonathan Kaplan is the master of casting. I don't
know if he is on the list right now, but he can be contacted at
http://jonathankaplanceramics.com
Also our great contributer John Rogers probably knows a lot about this.
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave #137
Hermosa Beach CA USA
90254-2447 310.379.6890
http://www.dmhstudio.com/
Ivor and Olive Lewis wrote:
> Dear Lesley Anton,
> Lawrence and West, "Ceramic Science for the Potter" attribute this
> fault to particle orientation effects.
> They draw on Norton for their information in suggesting that seams
> should be
> 1, lightly hammered to induce plastic flow.
> 2, Bisque fired and then ground to remove the proud material.
> 3, Cut a groove in the leather hard clay and refill with plastic clay.
>
> I think Jonathan Kaplan would have the best information if you can
> contact him.
>
Ivor and Olive Lewis on fri 8 aug 08
Dear David and Lesley,
My only brush with attempting to do slip casting left me standing in a
pool of rapidly setting plaster so I make no claims to knowledge about
the process.
But I have dubious feelings about people who tell me that clay
crystals align in parallel sheets, though the diagram L and W give
looks plausible if you mould edges are seriously worn.
When I have visited Hobby Ceramics workshops I
have been amazed at the degree to which their castings are sanded
once they have dried. It seemed there was no attempt to fettle the
seams at the leather hard stage.
If your research reveals anything useful, perhaps you would share you
new knowledge.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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