Properties of platinum

Manufacturing Jewelry
Platinum ore
  • To get one ounze of pure platinum you have to process 11 tons of ore.
  • Platinum (Pt) is one of the heaviest precious metals.
  • Pt has an extremely low thermal conductivity, it doesn’t bend or twist even under the effect of heat.
  • Platinum doesn’t tarnish or oxidise on air.
  • Pt has a very good wear restistance.
  • It can disolve only in hot aqua regia.
  • Pt has catalytic properties. It’s always needed to start chemical processes, without participating in this process.
  • Pt and platinum group metals are the only material for the production of glasfibres and melting crucibles for glass.
  • Regarding on high melting point and resistance to attack, Pt is famous for the measurement of high temperatures.
  • Pt has an excellent skin compatibility.

Platinum casting

  • The weight of platinum castings will be about 30% higher than for 18 karat gold.
  • The weight of casting trees are between 100-3000 gr, in average 300 gr.
  • Different tree and sprue design, regarding on total different feeding behaviour.
  • Centrifugal spin-casting machine required, torch or induction melting. Ultraviolet lenses for melting platinum is recommended.
  • Special investment (phosphat or acid based investment) required.
  • High preheat temperature, up to 1000°C necessary. Special heat resistante flask required.
  • Oxidation and gas resistance of platinum is extremly good.
Platinum wires heat treated under normal atmosphere
Casted platinum rings show different surface roughness
Platinum casting tree arrangement

Casting

  • Even 100% scrap can be melted.
  • Special crucibles made of calcium or magnesium stabilized zirconia required.
  • Casting process allows no waiting time, melted platinum has to be poured quickly.
  • Rapid loss of heat during the casting process.
  • Use hydrofloric acid to dissolve the platinum investment from the cast product.

Handworking

  • Use a separat tooling kit for Pt, and keep them clean.
  • Avoid any contamination during the manufacturing process, work clean.
  • Use rolls and dies (tungsten carbide) highly polished.
  • Pt tends to smear or drag wear on working tools.
  • Pt can be extensivly deformed, without becoming brittle. Most platinum alloys allow coldwork 80% and more.
  • Reduction less than 30% should be avoided.
  • General annealing temperature is about 1000°C, a stress relieve at about 600°C is possible. (Guideline: for each mm thickness, heat min. 1 minute, bright orange heat)
  • Annealing with a torch: use a broad flame and heat up to a dull red.
  • Binary Pt alloys don’t age harden, so quenching is not necessary.
  • Metal contaminations of Pt surfaces has to be removed by pickling in hot 10% nitric acid.
  • Iron contaminations can be removed with hydrochloric acid.
  • Pt/Co oxides can be removed with hot hydrochloric acid 20%.
  • To enamel Pt is not possible, only Pt/Pd with limited special colors.
  • Pt doesn’t tarnish or oxidise on air.
  • Regarding on elasticity Pt is ideal for settings. Pt bends easily and does not spring back.
  • Tension settings are speciality for Platinum.
  • Always polish the setting, before you cut the seat for a stone.
  • Color combinations are easy to produce.
  • Etching of gold inlays are easy to produce.
Torch casting of platinum cobalt
Platinum ring with etched finegold inlays
Soldering of a platinum ring, using a hydrogen/oxygen flame

Joining techniques

  • Useful joining techniques for platinum are: Fusion welding, pressure welding, spot welding, laser welding, soldering, sintering, bi metal casting, puddeling, mechanical processes.
  • Principle: welding is better than soldering, solder seams to be polished out easier and are visible, they don’t match the colour.
  • Before joining operation, clean Pt in an ultrasonic cleaner to remove oil, grease and metallic residues.
  • During welding/soldering you may not need a flux.
  • If you weld, use a filler of the same alloy.
  • The wettability between Pt and solders are excellent.
  • Use a heat source with less carbon, like hydrogen/oxygen.
  • Pt has a relatively low thermal diffusivity, the heat stays near the working area.
  • Regarding on the low heat conductivity it is possible to repair without damaging stones.
  • Use a ceramic (alumina) brazing pad, no charcoal blocks.
  • Don’t use iron binding wire and dirty steel clips, this will contamine the Pt.
  • Use tungsten twisters and tungsten solder picks to place solder on hot platinum.
  • To bond Pt with gold there are special fluxes (free of borates) available.
  • Use welding lenses when brazing or welding Pt.
Interface area of a bi metal casting ring
Solder seam of a ring shank, soldered with platinum medium solder
Laserwelding seam

Finishing

  • No short cuts for filling and polishing
  • Choose the minimum grade of abrasive that totally remove the deepest defects.
  • Emery/polish across line defects or solder seams to reduce dragout.
  • Change always the working direction at 90° to the previous direction.
  • Frequently use fresh abrasive surfaces Avoid the transfer of coarser particles to the next finer process. Good housekeeping is necessary.
  • Before surface texturing, prepare a highly polished surface.
  • Electrolytical polishing and tumbling are not possible.
  • Plating with Rhodium or Pt is recommended, the coating increases the wear resistant and eliminates color differences at the solder seams.
  • If Pt will be combined with gold, finish the Pt before you join it to the gold, otherwise the gold will be overpolished, because it will finish faster.
Finishing of a platinum ring
Platinum salt after precipitation
Platinum powder after refining process

Refining

  • Keep Pt fillings and scraps separate from other precious metals.
  • Hold platinum/gold combination scrap separate.
  • Don’t seperate your fillings with a magnet, Pt/Co is magnetic.
  • Clean material from sheets, wire etc. and the same alloy can be remelted and used again, without problems.

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