=?iso-8859-1?Q?Nisse_Holmstr=F6m?= on tue 20 feb 01
Some notes on the noble art of drying plaster-moulds:
Freshly made plaster moulds should not be exposed to temperatures above =
42 degrees C (sorry . . ) since modern plaster=B4s are mixtures of alfa- =
and beta-hemihydrates which have different hydration-times - =
beta-hemihydrate is fully hydrated after 15 min; alfa-hemihydrate after =
90 mins. If the moulds are exposed to external heat this will give rise =
to tensions in the moulds, which will weaken them considerably. =
Air-drying for 24 hrs is recommended for freshly made moulds. If this is =
not practical a high humidity dryer is essential and it is allways =
important not to exceed 49 degrees C (again sorry . .) when drying the =
moulds. At higher temperatures the plaster starts to decompose. It is of =
great importance for a long mould life to fully dry out the moulds =
before and between castings. Another tip: allways dry the fresh moulds =
face down or even cover the moulding-surface with a plastic shower-cap =
to force the water beeing driven off through the "back"of the mould; =
this to increase the permeability of the plaster and a good way to avoid =
loss of first cast=B4s due to the surface-skin that allways results when =
drying out plaster moulds (skin develops as a result of the slight =
water-solubility of the calcium-sulphates and these salts will form a =
less permeable surface on the plaster when the water is evaporating from =
the drying plaster).=20
Take it easy - but take it !
/nisse/
http://w1.510.telia.com/~u51002415/
Snail Scott on tue 20 feb 01
At 11:51 PM 2/20/01 +0100, you wrote:
>Freshly made plaster moulds should not be exposed to temperatures above 42
degrees C...
>...it is allways important not to exceed 49 degrees C...
>/nisse/
Notes for the metric-impaired:
42 degrees C = 107.6 degrees F
49 degrees C = 120.2 degrees F
-Snail
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