jr wrote on 26 Aug 2005:
I'm just trying to make a mold with plaster + kaolin
(25% + 75 %) to hold bronze. The mold works fine but
the final piece have a lot of big bubbles inside the
bronze , so the piece it's totaly unusable.
and on 28 Aug 2005:
The images has been renamed, follow this link instead the old:

http://www.metalworking.com/DropBox/last_cast_base.jpg
http://www.metalworking.com/DropBox/last_cast_lado.jpg

Is that 2nd image a side view, a different try, or what?

Anyhow, from the 'base' picture I imagine the mold
was wet. Plaster molds used for lost-wax casting are
heated to about red heat (to cook out all organics)
over a period of four to eight hours, and are poured
into almost immediately upon coming out of the furnace.
Also, such molds are made with "investment plaster",
which is not the same as "plaster of paris".

Where did you get your "plaster + kaolin (25% + 75%)"
recipe? I don't see how that would work, because AIUI
kaolin is a clay, so your mold would not have adequate
porosity to allow gases to escape when you pour. For
the simple cylinder you are making, perhaps you should
try a "green sand" mold. Green sand is fine sand mixed
with perhaps 10% clay and 5% moisture, although some
recipes use half or twice as much. The main idea is to
use just enough clay and water so the mold holds its
shape. Too much clay will decrease porosity and cause
bubbles; too much water gives a wet mold and bubbles.

-jiw