"-in a way you did pass along a myth but in another way,
there is merit. Let me explain. If you are casting with wet grain,
of course the water will evaporate when the metal temperature
reaches 212F in the absences of excessive pressure. The
vaporization temperature of water increases as pressure increases.
In the typical casting machine, this only happens when you have
water trapped inside of some popcorn shaped shot. What would then
likely happen is that you would eventually have a loud pop and some
splattering of molten metal out of the melting crucible.
My experience with trapped moisture is that it cannot be dried with
a hairdryer, paper towels, or even by warming on top of the oven.
This is moisture that has been sealed into a hollow in the shot. In
my experience, it always pops before the metal has formed a pool.
However, the liberated oxygen from the evaporated water
can cause reactive elements in an alloy to form oxides such as
cupric, silicon or zinc oxide during the melting process.
Here again, the oxygen is bound to the hydrogen. It is not really
free. There is a continual stream of water coming from the tip of
your torch. Why would you think that water in the shot differs from
the water streaming from your torch?
Oxides can cause harm in several ways but here are two
that can increase porosity. Scenario one, oxides prevent the
metallic portions of an alloy from bonding, similar to the way that
solder will not flow on an oxide. If your model already has stress
from a less than perfect design, then grain boundaries with oxides
are weaker than the metal around it and it is at the point of the
oxide that the structure will breakor
For sure, oxides are the damnation of every jeweler. That is why
most of us use flux. It helps to dissolve oxides and helps to coat
the metal to further protect it.
Scenario two, oxides in a molten metal stream can form
blockages that prevent proper metal flow in a casting. Imagine the
trash that you see under a bridge after a heavy rain. Any time you
get turbulent metal flow or a restriction in directional
solidification due to oxide trash in the stream you have a
opportunity for "porosity".
So, you should definitely use dry shot but mainly so as
not to pop something into your eye.
As pointed out above, even dry shot can pop. That is a good reason
to wear some type of eye protection.
Bruce D. Holmgrain
JA Certified Master Bench Jeweler / CAD/CAM Solutions