Platinum Guru

Casting can be done with several alloy combinations, of which platinum/cobalt (Pt/Co) and platinum/iridium (Pt/Ir) are the most common. In the Pt/Co alloy, the cobalt acts as a grain refiner and gives this alloy the ability to fill very fine detail. Casting employs the lost wax process and can be accomplished with either the torch or an induction machine. Other alloy combinations can also be cast.

The extremely high melting point of platinum makes it possible to do bi-metal casting, where another metal is cast onto a finished platinum piece, thus creating colour contrast.

When casting platinum with a torch, hydrogen/oxygen is the preferred fuel combination for the melt. Acetylene is not useful for melting platinum as it expels carbon in the flame and carbon will contaminate the metal through absorption. While it is very hot and melts the metal rapidly, platinum will become brittle if acetylene is being used. Propane/oxygen can be employed successfully. It is also important to get the proper torch for this job.

Due to the characteristics of platinum, the high temperatures and the variety of applications, there is a substantial learning curve to become successful with induction casting platinum. It is recommended that small manufacturers send casting-to-casting houses.

Modern laser equipment has proven to be very helpful in removing defects such as excess porosity on a platinum casting. It also allows the repairing or welding close to stones, without causing damage to them. With a laser welder, repairs on vintage jewellery can be performed that are not possible with conventional methods.

  ~Jurgen

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Removing the castings from the investment can be a challenge.  The investment, which is baked at very high temperatures, turns into a glass-like substance that can be difficult to separate from the cast pieces.

Most manufacturers use hydrofluoric acid, which is extremely dangerous and must be handled with utmost caution.  Always use rubber gloves, a rubber apron, and a respirator that filters small particles from the air.

A small shop may wish to consider a safer devesting agent for platinum investment as a substitute for hydrofluoric acid.  Ask your tool supplier; there are many brand names. You can also mix your own substitute using the following formula (percentages are given by weight): 25 percent sodium hydroxide, 25 percent potassium hydroxide, and 50 percent deionized or distilled water.  (The sodium hydroxide and the potassium hydroxide should be in pearl or flake form.)  Combine all three solutions in a stainless steel container, and heat (do not boil) for 25 to 40 minutes.  Be careful mixing, since the three ingredients will naturally heat up when combined.

  ~Jurgen

Since wax must be completely burnt out to ensure a clean and trouble-free casting, the burnout oven (or kiln) is a crucial part of the process.  The kiln must be able to reach the high temperatures required for platinum casting, hold the temperature it is set to achieve, and do heat ramps as programmed to eliminate the wax from invested flasks.

One factor to consider is that steam dewaxing often used in gold casting, cannot be used in platinum casting because it will cause most platinum investments to break down.  That means all wax must be eliminated in the kiln, making it crucial to have adequate air flow through the heating chamber so the wax vapor can be carried out the exhaust.

Because of this need for good air flow, gas kilns have some advantages.  Gas jets distribute the heat more evenly and, unlike electric coils, require a great deal of oxygen to bum; consequently, good air flow is always part of a gas kiln’s design.  For a small shop, however, a good electric kiln will work fine.  There are even electric kilns made especially for platinum casting that offer excellent air flow.  (If your kiln does not have adequate air flow, you might be able to drill a few holes in its top or bottom.  Check with the kiln’s manufacturer.)  In addition, be sure to place the flasks at the center of the kiln, leaving enough space around each one so the temperature is as evenly distributed as possible.  (This will also help you avoid hot spots caused by having the flasks too close to the heating spiral.)

  ~Jurgen

 Using the right investment is especially critical when casting platinum.  Imagine a metal entering your flask with 20 to 40 G’s of force, at temperatures exceeding 3,500 F into a material that is expected to give you an exact replica of the wax model without problems or flaws.  You can see why choosing the right investment is a key to successful platinum casting.

Platinum investment, which is much stronger and can take higher heat and pressure than gypsum investment, comes in two basic types: phosphate-bonded and acid-bonded.  Typically, acid-bonded investments give you better reproduction, but they have much longer mixing and burnout cycles.

Some of the acid-bonded investments contain acid powder (usually silicic acid or organic acids such as oxalic, malaeic, or lactic), which activates when mixed with water.  Others require the addition of liquid phosphoric acid to work.  Usually the acid is mixed with distilled water at a specific ratio, with the powder then added to the mixture.  The acid powder formulations are easier to prepare because you only add water, but powder particles can settle to the bottom of the investment in the drum, requiring careful mixing of the investment before use to ensure a homogenous mix.  Whichever one you choose, mixing should take about 1 to 2 hours.

Either type of acid-bonded investment will provide excellent reproduction, but both take a long time to set and burn out.  For that reason, many smaller casters are turning to phosphate-bonded investments.  Widely used in the dental industry, these investments set quickly.  Unfortunately, they are also expensive and, because of the very short setting time, cannot be mixed in large quantities for high volume production.  They are well suited for the small shop, however.  While their reproduction quality is not as good as those of other investments, they will serve for most general designs.

The choice of investment is really a personal decision.  Try several brands under comparable conditions before deciding on one; most manufacturers will be happy to provide a small quantity of their brands for you to sample at no charge.

  ~Jurgen

To cast platinum your torch should have a multiport or rosebud tip that’s screwed on rather than soldered.  Solder could melt in the platinum’s reflected heat, causing the tip to fall into the molten metal and splash platinum.

In addition, your torch must have a built-in flashback arrestor, to prevent gas from flowing back into the tank in case of a pressure change.  This is an important safety feature.

  ~Jurgen

Check out this article on Platinum Casting:

http://platinumguru.com/articles/platinumcasting.html

For torch casting, a vertical centrifugal casting machine is the safest, most efficient, and most reliable way to cast platinum.  Vertical machines have high torque, produce a rapid centrifugal force, and require very little maintenance.  They are also safer than horizontal centrifuges – which, if a spill occurs, can fling molten metal in a waist-high circle around the shop.  A vertical machine has a straight centrifuge; major spills are very rare, and if one does occur, the flying metal is confined to a narrow vertical area.

The vertical casting machine should be mounted on a sturdy base so that one person can load the flask from the back while another person melts the metal at the front.  This two-person approach is important, since with the eye protection required to melt platinum, the operator sees virtually nothing but the glow of melting metal.  A machine that’s freestanding and bolted to the floor offers the best access.

Finally, be sure the crucible used for melting the metal is specifically designed for the higher temperatures required by platinum.  Use crucibles made from fused crystalline quartz or silica Si02+2.  Avoid graphite crucibles, since the carbon can contaminate the platinum.

~ Jurgen

Platinum Jewelry,Fabrication,Casting

Did you know that it takes 10 tons of ore to produce one ounce of platinum?

Selecting the proper fuel to cast platinum is of utmost importance.  Do not use acetylene, since it has a high carbon content and expels carbon in the flame.  The platinum will absorb the carbon, leading to contamination and brittle castings.

Although propane, or LPG, is also a carbon-based fuel, it does not have the high carbon content that acetylene does, and therefore can be used for platinum casting.  Be aware, though, that even when mixed with oxygen, propane does not burn as hot as hydrogen and thus requires more time to melt the platinum.  This longer melting time can lead to porosity caused by gas absorption or debris – a direct result of keeping the metal in the melting crucible too long.  If you do use propane, pay particular attention to the flame: It should be no larger than 6 inches with a high oxygen setting.  A sample regulator setting would be 5 lbs. of propane with 40 lbs. of oxygen.

The most efficient way to melt platinum is with hydrogen combined with oxygen.  This fuel is carbon-free, and the high heat created by a proper hydrogen/oxygen mix melts platinum in seconds.  But even with hydrogen, a proper flame is crucial: If it’s too big, the flame will heat the surrounding crucible, adding to the melt time-and creating the same problems as those with propane.  Use as much oxygen as necessary to make a relatively small but oxidizing flame.  A sample regulator setting here would be 50 lbs. of hydrogen and 50 lbs. of oxygen.

~ Jurgen

Platinum Jewelry,Fabrication,Casting

Proper eye protection is important when melting platinum, since the ultraviolet light emitted by the glowing melt can damage your eyes. Use only lenses that are approved by the federal Occupational Safety and Health Administration and comply with American National Safety Institute standards.  A #10 welding goggle is the absolute minimum protection for the caster, and we generally recommend a #11 or #12 lens to be safe.  Do not, under any circumstances, use regular sunglasses or other dark lenses!  Any jewelry tool supplier or welding supply house can provide you with the proper eye protection, as well as gloves and a leather apron-two other important safety items.

~ Jurgen

Platinum Jewelry,Fabrication,Casting

All fuel gases are dangerous, and you should have a professional install your torch systems and fuel tanks.  In addition, the regulator on the fuel tank should have a directional flow restrictor, which allows gases to leave the tank but not re-enter.  For safety, use only regulators designed for the fuel you’re using.

~ Jurgen

Platinum Jewelry,Fabrication,Casting