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Firescale Prevention During Vacuum Casting

Firescale has been the scourge of sterling silver casters since the
first silver was poured into a mold. It has been a thorn in the side
of silver fabricators. Fortunately there are ways to prevent fire
scale while silver soldering. This paper describes a simple,
inexpensive method of preventing fire scale during vacuum
casting…

Complete Story:

Dear Lee,

I have a question.

The main reason for our cast to get oxidized is the contact with
oxygen in the air, right?

What happens if I use the Vacuum Bell on my casting machine to keep
my casted flask from the air in the first minutes of the casting?

I think it may work, but what concerns me is that the plexiglass of
the Vacuum Bell could melt by the heat of my just casted flask.

What do you think?

Thank you and best regards,

Juan Pablo Martenez Mansilla
CEO
Grupo Rex Nisi, S.A.
Calzada Roosevelt 33-86 Z.7 Edificio Ilumina Of. 801.
Guatemala Ciudad, Guatemala, Centroame9rica.
Tels.:++(502) 2439 7003 - 2439 7007
Fax.: ++(502) 2439 7005

Juan,

You are correct in wanting to keep oxygen from around your casting.
No oxygen, no firescale.

Right off the top I see two possible problems when creating a vacuum
around your hot flask while its under a bell jar. There is a good
possibility the heat from the flask could melt or weaken the bell
jar. The second problem is the affect of sucking heated air into
your vacuum pump for an extended period of time while the flask
cools.

I would suggest you not try using vacuum and a bell jar to keep
oxygen away from your hot flask.

Lee Epperson

I know this question was for Lee, but in case he’s busy, I’d like to
chime in before something bad happens to Mr. Mansilla

    The main reason for our cast to get oxidized is the contact
with oxygen in the air, right? What happens if I use the Vacuum Bell
on my casting machine to keep my casted flask from the air in the
first minutes of the casting? 

There is still air (and oxygen) in the bell vacuum, so no, there
wouldn’t be an advantage.

    I think it may work, but what concerns me is that the
plexiglass of the Vacuum Bell could melt by the heat of my just
casted flask. 

Worse, the bell may implode or explode, sending shards of hot,
melted plastic everywhere. Also, as the pump tries to create the
vacuum, the heated air will be trying to expand at the same time.
I’m not sure exactly what would happen, but this is one of the most
dangerous ideas I’ve ever heard. Please don’t try it!!!

James in SoFl

Hi,

One source of oxygen/fire scale is the airborne oxygen. However,
there are other elements that get involved from the plaster
ingredients, and the high heat. Sulphur dioxide for one example.

When we use a reducing atmosphere either hydrogen/nitrogen or heaven
forbid-carbon monoxide, we still see fire scale. So we still use
deoxidisers of one kind or another for best results. As further
proof, use a good oxygen free atmosphere to cast an ingot in a steel
mold. You will notice far less fire scale/oxidation compared to
plaster cast gold or silver.

Daniel Ballard

There is some excellent on page 76 of the complete
metalsmith by Tim mccreight

Andy " The Tool Guy" Kroungold
Tool Sales / Technical
Stuller Inc
Phone 800-877-7777 ext. 94194
Fax 337-262-7791