There has been some discussion about flask temperature for silver,
but what about gold? I have only cast gold a couple of times, but
it didn't dawn on me to use another flask temp than I use for
silver. The gold castings (14k) seem to come out ok at 900f, but is
there a better flask temperature?
There really isn’t any one flask temperature for any given metal. It
depends on how you are casting (centrifuge, vacuum with perforated
flask, less aggressive vacuum, etc) the size of the flask, weight of
metal you’re casting, type of metal (karat, color, exact alloy type),
thickness or detail level of the models, how much of a superheat
you’re giving the metal (if the metal is barely above it’s melting
point, the flask will need to be hotter than if the metal is a bit
hotter than it’s melting point, for example)
In general, silver won’t need quite as hot a flask as the same items
in golds, but 14K usually needs cooler than 18K, white golds and
yellow golds differ, and even just within 14K yellow golds, the
difference between silicon deoxidized alloys and zinc deoxidized
alloys will be noticable.
Ideally, you should run some tests for the types of things you cast.
The goal is to use the lowest flask temperature that still gives you
a fully filled casting. Usually, that will give you the best results.
(but not always. Some alloys will give you more porosity if you are
casting too cool, while others will give you more problems if you’re
casting too hot.)
900F is a decent starting point for your 14K yellow castings if
vacuum cast, and items are of typical weight, not very detailed, not
too heavy either. But test it from there to see what changes and
improves with lower or higher flask temps, and more or less
"superheat" (the amount the metal is heated above it’s melting
point). Lower flask temps mean the metal solidifies more quickly,
giving a smaller grain size. This generally is good, giving stronger
metal. Too low a temp though, and you’ll find your castings coming
out glossy and shiny. To an extent, this isn’t bad, but is an
indication that the metal was solidifying before it fully "wetted"
and conformed to the investment surface, which would have left it
matte. And too cool a flask can lead to some shrinkage porosity just
under the skin of the casting. Too hot a flask gives you larger
grain size, more oxidation of the casting surface, and more chance
for porosity caused by breakdown of the investment, or gas absorption
of the metal during the casting process.
You might also consider simply asking your refiner or metals
supplier for the recommended flask temps for their alloys. Usually,
casting alloys are well enough known that the dealers will have an
idea of what temps work best, both for melting/pouring the metal, and
for the flask temps.