Casting Platinum Sterling – A New Alloy


One of the major reasons for attending Kraftwerks is to learn what is new in the industry. This year was no different.

Casting Platinum Sterling (925) – A New Alloy

Doc Robinson talking/new alloy

This year at Kraftwerks (2004) the refiners of a brand new alloy were there to demonstrate this new and very interesting metal.

This brand new alloy is called Platinum Sterling, and is mostly fine silver with a modifier of Platinum in it (4-6%). There is also a small amount of copper (a few %). At this point the alloy composition is still pretty much a secret because the refiner’s are still in the process of acquiring patents.

Pictured left: The inventor of Platinum Sterling is Marc “Doc” Robinson, he is the guy doing the casting.

He is currently the Technical Director of Far East Operations at ABI Precious Metals. As a note of interest Doc was the original founder of Kraftwerks.

As you may guess this new alloy is going to be a major new thing in jewelry manufacturing and currently the Asian jewelry manufacturers are working heavily in this metal.

Note: There is a solder made for this new alloy, in case any of you jewelers were wondering.

This alloy is still brand new and there are currently only a few people/jewelers working with it in the USA. However the refiners did have casting shot of this alloy available for people to purchase and try for the jeweler’s that attended Kraftwerks. (Contact Info at end of article)

Platinum Sterling has a very bright look to it and could be easily mistaken for white gold. Which is actually one of the uses this new alloy will really be used for in jewelry. It has the very bright white look of white gold and at just a small percentage of the gold cost.

This new alloy also has some other very unusual properties that make it unique. Both in milling (see my article on milling Platinum Silver) and in casting. It also has some unusual annealing and hardening characteristics.

Melting Crucible with casting shotA crucible filled with Platinum Sterling casting shot.

In this demonstration of casting as you will see Doc is going to cast a sprued ring tree. Think of a Christmas tree but made up of rings of wax.

Here is the basic process leading up to this actual casting demonstration. Some people already know this but I thought it should be explained for people who may not be familiar with the process.

The original ring (or any object that you want to mass produce) is molded into rubber and vulcanized using pressure and heat.

After the rubber mold has cooled the rubber is cut apart in a particular way (this is really an art) to not only remove the original metal ring but to leave the rubber basically intact so that hot wax can be injected into the impression of the ring remaining in the rubber mold to make copies. As many wax copies as desired can be made using this rubber mold and wax.

The wax rings are then sprued onto a wax tree (this is done so the molten metal can move freely to all the wax ring impressions in the plaster mold during casting). After all the wax rings are attached to the sprue, the whole thing is put into liquid plaster. The plaster with the wax rings in it is vacuumed to remove air bubbles and then left to dry/harden completely.

Once the plaster is dry, the entire thing is put into a “burn out” oven and heated to specific temperatures to “burn out” the wax leaving a plaster mold behind.

Right… Now we get to the molten metal.

Lighting the torchA crucible filled with Platinum Sterling casting shot is being heated to the melting point

There are many ways to heat metal for casting. For smaller casting applications a torch (pictured) is often used.

Torches work fine on small casting projects but many people prefer using ovens and kilns for heating larger amounts.

Note: There are specially made kilns of all sizes and a lot of casters prefer them because they allow for a finer control of heat.

Melting metal

Melting metal

Inserting the hot plaster mold into the vacuum casting machineDoc is putting the hot plaster mold/flask into the vacuum casting machine.
Hot plaster moldThe hot plaster mold/flask ready to be inserted into the machine.

Notice that the plaster mold/flask that has been “burned out” is encased in a steel cylinder with circular holes cut in to it.

In most cases the plaster is poured into a steel cylinder for support during the heating and casting process. There are two basic casting processes, centrifical and vacuum. In this demonstration Doc is using the vacuum method. The holes in the cylinder are there so that when the mold/plaster is placed into the vacuum casting machine and the vacuum is turned on the vacuum effect will suck the molten metal into the plaster mold through the holes.

Note: Vacuum casting is a more expensive process but also generally considered a more dependable method of casting.

Pouring/casting the actual molten metalDoc is pouring the red hot metal into the plaster mold/flask which is sitting in he vacuum casting machine. Actually “casting” the metal.
Plaster/flask in the machineThe hot plaster mold/flask is ready and has been inserted into the machine.

Notice that every thing is red hot at this point, so care must be taken.

Red hot metal buttonThe button of the cast, it is still molten metal.

After the molten metal is cast/poured into the top of the plaster mold the vacuum is left on.

Leaving the vacuum pressure on until the molten metal cools helps ensure a good cast and that all the “rings” are completely filled out with metal.

Notice that there is excess metal on the top where the entrance to the plaster mold is.

Having excess metal is very important for several reasons. The extra metal insures that there is of course enough metal for all the ring impressions in the plaster to be filled.

Also extra metal helps make a compacting weight in the actual casting and helps ensure that there will not be any porosity because of a metal shortage.

The extra metal also helps maintain heat during casting and in the casting making the casting process more temperature stable.

The excess metal in the top of the picture, which is generally the bottom of the sprue tree is called a “button” because of it’s obvious shape.

Doc is getting ready to cool the castingDoc has removed the plaster cylinder from the vacuum casting machine. It is still very hot.
Cooling in a bucket of water.Doc is submerging the still very hot plaster into a bucket of water.

Once the casting has cooled to the point where the metal has cooled enough to be completely set up the entire cylinder/flask is removed from the vacuum casting machine. The plaster at this point is still very, very hot.

Taking the hot plaster/flask and submerging it in a bucket of water will not only cool the casting but because of the temperature difference between the plaster and water the plaster will basically melt leaving the casting behind.

Note: The casting will still likely be very hot so care must be taken when handling it.

Red hot metal buttonThe finished casting “ring tree”.

Platinum Sterling has some characteristics that make it both very interesting to cast and also a big improvement over casting standard sterling.

As you can see in the picture the rings cast in this demonstration came out great.

The entire casting process in this demonstration only took 15 to 20 minutes. So as you can guess the actual casting part of this process is pretty fast.

Notice how clean and sharp the entire casting is and look at the rings. The casting is very good and smooth there will be very little clean up required on these castings.

For those of you who are jewelers or just interested in working with this new alloy.

I would suggest that you contact the refiner/manufacturer of this interesting new alloy and try some. I think this alloy may change the jewelry markets significantly.

Contact Info for Buying & Questions:
This is a patent pending product and owned exclusively by ABI.

Contact them at: Marc “Doc” Robinson
Technical Director
ABI Precious Metals
Box 11509
Carson, CA 90749
800-768-1566
310-768-1566 Fax
[email protected]
[email protected]