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
This paper describes a simple, inexpensive method of preventing fire scale
during vacuum casting.
Firescale is cuprous oxide, the combination of copper in the silver or gold
with oxygen. Firescale forms whenever silver or gold containing copper is
heated above 1000 degrees F. Unlike cupric oxide, the black coating on silver,
firescale cannot be removed in the pickle pot. Normally it is removed by
abrasion, electro striping or left as a surface finish.
Firescale usually shows up, as a purple or copper stain in the metal, during
the polishing process. It prevents sterling silver or gold from being
polished to its fine brilliant luster.
The method of preventing firescale during the casting process consists of
preventing oxygen from reaching the metal while it is cooled in the investment
mold. The use of a reducing flame and graphite crucible during the melting
process will help in keeping oxygen from the metal as it is melted. Oxygen
must be removed during the cooling process in order to prevent firescale.
The method described in this paper to prevent the forming of firescale
during the cooling process is very simple. It allows the metal to cool in a
reducing atmosphere. Castings, when removed from the mold, are bright silver.
They do not have to be pickled to remove the black coat. In some instances they
can be polish with jewelers rouge after removal from the quench water.
I vacuum cast my projects. I use an electric Handy Melt Furnace. The
process requires two smooth soldering pads, scrap wax shavings and an empty flask
the next size larger than the mold flask.
I place some wax shavings on a solder pad before I start the pouring. Once
the metal is poured I place the mold flask on the wax covered solder pad. I
then place the larger flash around the mold flask. Wax is then thrown on the
sprue button and the second solder pad is immediately placed on the cover
flask. The wax inside the cover flask absorbs the oxygen as it attempts to
burn. The flask should be cooled for a longer period of time then normal as the
heat does not dissipate as rapidly because of the cover flask.
When the flask is first quenched the wax forms seals with the investment and
prevents it from breaking up. The flask has to be removed from the water
and the surface of the investment scrape to remove the wax/investment skin. The
flask is then quenched as normal.
This is a sample of a sterling silver casting that was cooled in the
reducing atmosphere created by the process described in this paper. The casting
was quenched seven minutes after the pour. It was brushed with a brass brush
to remove the investment. Investment can still be seen under the bear. The
casting was not pickled and does not need to be pickled before polishing it.
It is free of both cupric oxide, the black oxide, and cuprous oxide,
Step one Place about a tablespoon of wax shavings on a solder pad prior to pouring
Step two. Place the hot mold flask on the wax shavings. The wax will immediately
melt and seal the bottom of the flask to the solder pad, which prevents air
from entering the investment mold and metal from the bottom side.
Step three. Place the cover flask around the mold flask. The wax will seal the cover
flask to the solder pad.
Step four Throw wax shavings on the sprue button.
Step five Place a soldering pad on the cover flask. The wax shavings will attempt
to burn and will absorb all the oxygen around the mold flask and the metal it
contains. A reducing atmosphere will be created around the mold flask. There
will be no oxygen remaining in the cover flask to combine with the copper in
the sterling silver to form firescale. The cooling of the casting flask will take longer due to the fact it is
inclosed in the cover flask. The cooling time before quench should be
increased by a minute for small castings to several minutes for larger castings.