Kim, the problems with acetylene are unique to working with
platinum. Air/acetylene is a wonderful fuel to use with silver, and
for many things where the soft reducing flame of that type of torch is
useful, with gold too. oxy/acetylene has the drawback of being a very
hot flame. With larger tip sizes, it simply is much too easy to
rapidly overheat a piece in many cases. but torches with very small
tips, like the little torch or it’s copies, using oxy/acetylene, can
give you very tiny flames that can be quite useful in many
situations.
However, here’s the problem. Acetylene burns, especially when not
supplied with an excess of oxygen, with a somewhat sooty flame.
Normally, with silver and gold, this excess carbon in the flame acts
as a wonderful deoxidizer, part of why these torches are so nice with
silver fabrication. The metal tends to stay nice and clean with an
air acetylene flame. But with an oxygen/acetylene flame, as an
example, turn off the oxygen so it’s just the yellow acetylene
flame, and you’ll see lots of floating little bits of actual carbon in
the air. Burned with a bit of oxygen, those visible carbon floaters
go away, but there will still be unburned carbon in the flame. Until
you’re using an actually oxidizing flame, one that’s sharp and
hissing and VERY hot, there will be this higher level of carbon
available in the flame.
And that carbon is the problem with platinum. At the temperatures
needed to melt platinum, or the higher grades of platinum solder, the
carbon can cause contamination and brittleness in the platinum. The
old conventional wisdom is that the carbon is actually absorbed into
the platinum, much as carbon can be absorbed in iron to make steel
(platinum is quite close to iron in the periodic table, so
similarities can be expected). But with platinum, the carbon does
not form beneficial alternate structures, but rather simply makes it
weak and cracky. I’ve seen an alternative explanation for this that
frankly makes sense to me. That is that at the temperatures involved
with melting platinum, carbon becomes an exceptionally active
reducing agent (this part is of course true), and the explanation
suggests that silicon dioxide, the common silica found all around us,
can be reduced at these temps by carbon, to silicon metal. Metallic
silicon would then alloy with the platinum, and this is what causes
the brittleness. This mechanism would explain why the contamination
problem is sometimes sporadic and unpredictable. Sometimes it
happens, sometimes not. But silicates are all around a goldsmiths
bench. The soldering blocks we usually use for platinum are fused
quartz, or silicon dioxide, for example. Fluxes contain silicates in
various forms. Even house dust contains silica. Thus the need to
simply not allow platinum at melting/soldering temps to come into
contact with carbon, as it risks contamination, whether by the carbon
itself, or from reduced silicon. I don’t know which scenario is the
case, but I and any other platinum smith can tell you the situation is
one to be avoided. It ruins the platinum and makes it cracky and
unworkable.
For the other traditional jewelry metals, air/acetylene is
wonderful, and with small torch tips, oxygen/acetylene can be very
useful. The main drawback to oxygen acetylene is the difficulty of
lighting and extinguishing the torch without going through at least a
short time span with just the yellow sooty acetylene flame spewing
little carbon floaters into the air, and from there, all over
everything…
Note that all hydrocarbon fuel gasses, natural gas and propane, also
contain carbon. So with them when working platinum one also needs to
be sure to use an oxidizing flame for most purposes. But there’s less
carbon, and it’s much less of a problem and risk.
cheers
Peter Rowe