Popular Platinum
Customers love platinum for many reasons: The most popular being that platinum and its alloys are inert, won’t discolor fingers, or cause allergic reactions. Its alloys are not susceptible to stress corrosion, as is white gold, and platinum is 70% heavier than gold. Its specific gravity is 20.1, while white gold’s is 13.1.
The Platinum Group
Members of the platinum group include: rhodium, ruthenium, iridium, palladium and osmium. But, platinum can also be alloyed with other metals such as cobalt, tungsten, gold, and copper.
When choosing to work with a member of the platinum group, here are some characteristics to keep in mind:
- Platinum ruthenium – Excellent general purpose metal, machines well
- Platinum iridium – Great for casting, rolling, die striking
- Platinum tungsten – Used when a spring alloy is desired
- Platinum cobalt – Fluid, produces superior castings
- Plat cobalt and alloys – Can be used together, but not welded only soldered, using 1700° C seamless solder
- Platinum cobalt – Dense with less porosity and longer lasting, the choice of many manufacturers for cast pieces. Its surfaces are smoother than platinum iridium and ruthenium immediately after casting. Gives a better success rate than other platinum alloys.
TIP #1: To determine the difference between platinum cobalt and platinum iridium, touch the piece with a magnet, and, if it is cobalt, it will react.
Tip #2: Cobalt is ferromagnetic, so keep it away from magnetic bench shavings.
Tip #3: Clean slight oxidation on platinum cobalt with a pencil eraser.
Tip #4: For heavier oxidation, dip it in a boric/alcohol (denatured) solution, heat, dip it in 900° C pickle and then neutralize with baking soda.
Handle With Care
Because it contaminates easily, you’ll need to designate a set of tools exclusively for working with platinum. Use ceramic solder pads, as charcoal will contaminate it, ceramic tweezers, tungsten picks, and tungsten burnishers. Also use separate bench pins. If you try to use steel tweezers, you’ll end up with black spots.
When milling, clean your rolling mill with alcohol, to remove any traces of other metal on the mill, and avoid contaminating your platinum. Oil the rolling mill surfaces, and dip the platinum sheet in a solution of 10% nitric acid to, ensure that all contaminants have been removed.
Anneal platinum at 1000° C for 1 minute at 1” square per 1mm thickness. If you are using a torch, bring it to bright orange. You may quench platinum after annealing. Following this process will prevent your platinum from becoming work hardened.
With its low thermal conductivity, platinum requires more direct heat and higher temperatures to work. Use hydroflux with torches because the fuel is carbon free, and it will not affect platinum. An acetylene torch is not recommended since the fuel has high carbon content and is likely to contaminate platinum.
Platinum Soldering Tips
TIP #1: Clean your platinum before you solder. Using copper tongs, dip your piece in heated nitric acid for 10 minutes then soak in water and baking soda for 10 minutes, to remove foreign metals.
Tip #2: Keep in mind: Most platinum solder contains palladium and silver. The higher melting solders contain up to 12% platinum. Cobalt and iridium melt at different temperatures. This is why they cannot be fused together.
Tip #3: Re-tip prongs with 14KW cadmium-free solder, and then come back with platinum wire on top of the white solder.
Tip #4: Remove gold melted onto a platinum surface with a ball bur and fill it with platinum wire.
Tip #5: Use 14KW cadmium-free solder when soldering platinum to gold (as in the case of wedding bands).
Tip #6: Solder platinum to platinum with 1200° C platinum solder.
Tip #7: Eliminate the dip in a solder seam by using a 1700° C seamless solder.
Tip #8: When in doubt always solder. Except when: Joining two metals – platinum iridium to same or platinum ruthenium to same, you can weld. If joining platinum cobalt to same – use 1700° C seamless solder.
It’s important to know all you can about the precious metals you’re working with, and hope this article refreshed your memory or mentioned something new. Find out more on working with platinum in our articles on downsizing a platinum ring and how to approach refinishing.
[Adapted from: September 2005 Stuller Standard]