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Platinum
is the most important of the Platinum group of metals. The other
members of which are Ruthenium, Rhodium, Palladium, Osmium & Iridium.
Platinum is a greyish-white metal. It has a high fusion point,
is malleable & ductile, expands slightly upon heating and has
high electrical resistance. It is a chemically inert metal and
resists attack by air, water, single acids and ordinary reagents.
It reacts in presence of halogens. Platinum melts at about 1772oC
(about 3222oF), boils at about 3827oC (6921oF)
and has a special gravity of about 21.45. Its average weight is
195.09 giving it high density.
The
properties of the platinum group metals in general, and platinum
in particular, are ideal for jewellery (although, like gold and
silver, pure platinum is too soft to be used by itself). In its
pure form, platinum is harder than pure gold or silver, but once
precious metals are alloyed, they're all pretty much the same.
Because platinum is usually alloyed with other platinum group
metals, it retains its density and it's resistance to corrosion.
It has little or no "memory", which means that, if it is bent,
it doesn't spring back to its original position. This makes platinum
ideal for prong settings. It also opens numerous possibilities
for designers.
Platinum is a little less malleable than gold and silver.
This makes a number of usual manufacturing processes- drilling,
sawing, polishing-more difficult. It also means that, once you
polish it, the finish is very durable. Platinum has to be annealed
often when it is worked. Cold working distorts the metal's crystal
structure. The
crystal become elongated and compressed. Annealing, which simply
means heating the metal until it's just below its melting point
and keeping it there for a while, reverses that process, the crystals
realign themselves. Platinum's low heat conductivity keeps the
heat from the torch from spreading.This is important in welding,
which involves heating two or more pieces of metal to the point
where they melt and fuse together. Because the heat is localized,
there's little risk of melting the whole piece, unlike gold and
silver. Platinum's higher melting temperature makes it more difficult
to cast too, because it's harder to keep it in a molten state.
The larger the piece, the Harder it is to cast, so designs that
can be assembled from several small castings, rather than one
or two large ones is sensible. Working in platinum requires different
skills. Even an experienced goldsmith or silversmith needs some
training and some time to adjust. Making and repairing platinum
jewellery is more expensive than silver or gold. There are, however,
some intrinsic savings, too. Platinum prong rarely need retipping
or replacing.
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