Casting of platinum and
platinum alloys is best carried out by means of high frequency induction melting
and centrifugal casting. It is advisable to utilise very small "trees" as platinum
tends to solidify quickly and would not fully occupy a large or long cavity. Platinum
melts at 1,7731C but it is necessary to elevate the melt to about 2,400'C before
attempting to cast. This process must be carried out with the utmost rapidity.
Platinum is no friend of silica and borates, so preparation areas for platinum
casting must be scrupulously clean and clear of sulphate-bonded investments as
used for gold and silver. The specific gravity of the metal is 21.45. Metal
to wax ratio is 21-1. Weigh the wax patterns and multiply by 21 to arrive
at the weight of the charge of metal. The crucible used must have an extremely
high thermal resistance and also be proof against thermal shock. It must also
be silica-free. The metal to be cast must be cut into small plates roughly 8 x
8 x 2 mm or rods 7-8 mm diameter x 8 mm. Usually, platinum is alloyed with copper
in the proportion 950 Pt + 50 Cu. To render it more suitable for stone setting,
it is usually alloyed with Palladium, for wedding rings with cobalt and with tungsten
for brooches, springs, findings etc. Platinum should never be used if its
purity is uncertain. The melting crucible must be set very close to the flask
on the casting arm. Melting occurs in a very short time interval. Quenching
into water should be done about fifteen minutes after casting. The phosphate-bonded
investment is difficult to remove from the castings and it is usually necessary
to use some form of sand or bead blaster for this purpose. Sprue buttons and scrap
may be recast with 50% new metal. Precise flask burnout is, once again, very important
here. Flasks should first be dewaxed at 200'C for about three hours and then fired
in a burnout furnace having a time-temperature profile conforming to that shown
on the chart (figure 75). For certain applications, this investment may need to
be fired up to 1, 1 OO'C. 
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