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Newsgroups: rec.crafts.metalworking
Subject: Re: Casting metal in Forge
From: (J. Kimberlin)
Date: 28 Aug 1995 18:18:55 -0700

In article <41spdc$>,
BrnzCaster <> wrote:
>Yes, regular plaster-of-paris is a NO-NO for metalcasting. The investments
>for metalcasting not only contain plaster but also alumina, silica,
>zircon, etc. I typically burnout these investment molds at 1300F before
>pouring bronze.
>Most of my experience with melting various metals is with propane-fired
>crucible furnaces. It is my limited experience with melting of
>brass/bronze in a forge is that you get too much heat lose if you go
>bigger than a #2 crucible (6 pounds of brass). Even in a crucible furnace
>with the top removed during melt, it is difficult to get the proper
>superheat for thin item pours when using a #8 pot or larger.
>My two cents....
>Winston W. Harness, Metalsmith

I generally agree, but isn't it true that uncombined water is driven out 
by 300 deg C?  Water of hydration (chemically combined water) is released 
at about 900 deg C.  If you analyze inorganic materials such as plaster 
of paris, silica, perlite, obsidian, etc., by TGA (thermo gravametric 
analysis) you find two weight loss periods, one about 300 and the other 
about 900 deg C, give or take.  Water of hydration is generally not 
explosive, but water that is just included in the mold can be 
devistating.  You burn out your molds at 1300 deg F, which takes care of 
the one real problem.  It seems to me that plaster of paris would be good 
for some molds, providing a person properly prepares them using a heat 
cure above 300 deg C for an appropriate period of time - dictated by the 
mold thickness, etc.  Aluminum, lead, zinc, cerrobend, woods metal, and, 
of course plastics and rubber can be cast in plaster.  I wouldn't go 
bronze as there are better compositions to buy for that.


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