Janet Kaiser on thu 11 sep 03
Plaster of Paris (dehydrated gypsum) was known to the ancient
Egyptians and Greeks but this "modern" name is a result of it
being prepared from deposits in the Montmartre district of Paris.
As to the precise date this name "caught on", I have no idea.
I am not conversant with the term, "slipocasting". I am also not
sure that it is even possible to say which historic ware was cast
using a liquid body (slip) and which was cast using soft clay
pressed or thrown into moulds... Anyone?
As slip has to be made of very fine clay to cast successfully,
surely we are not talking of "primitive" times or work methods
here? After all, it would need a lot of extra work to produce a
slip compared to just making. I quote Ernst Rosenthal from
Pottery and Ceramics, page 95:
"The body used for slip casting is prepared from plastic body by
adding a small amount of sodium silicate (water-glass) or sodium
carbonate, the free OH ions of the alkali depriving the clay
particles of their plasticity and turning the plastic body
liquid. Slip casting can only be used if the clay is not too
plastic. Very plastic clays are too sticky and form an
impermeable skin over the plaster of Paris mould, making it
impossible for the plaster to absorb more water from the slip.
Casting slips have to be extremely fine and must be carefully
prepared. Very coarse particles would settle and the final body
would not be homogeneous."
As Rosenthal was writing in 1949, it is also interesting to hear
this take: "Slip casting is used in pottery, earthenware and
china manufacture for articles which, owing to the irregularity
or special nature of their shape, would be difficult or
impossible to produce by other means".
I think that is a clue to why some of us have or had a certain
attitude toward slip casting... We are simply more in accord with
that position than others. Slip casting being fine or even the
only way to go for the intricate Dresden shepherdess type of
production but not for "simple" ware like mugs, teapots or other
work.
Even in my reformed state, I still personally cringed when I
heard of someone making a mould to produce ten vases... They had
made one, but were not able to reproduce another nine... Just
shows that that maker was not the skilled craftsman of my
dreams...
Sincerely
Janet Kaiser
*** IN REPLY TO THE FOLLOWING MAIL:
>Another history question, guys.
>Does anyone know when, where, and by whom plaster of Paris was
invented,
>and when potters first started to use it for molds? And when
slipocasting was
>invented?
*** THE MAIL FROM Paul Lewing ENDS HERE ***
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