Platinum Casting with Gemstones

Today casting with gemstones is no more a new technology for jewelry production, since it offers many advantages in comparison with hand setting. Several companies in many countries worldwide already use this method for producing gold and silver jewelry.

15 years ago I had the chance of knowing and learning the process of casting with stones, when in Germany I attended a seminar on the latest technologies and processes for investment casting. Product quality and field of application were still limited, but the enormous advantages offered by this technique convinced me and I was induced to get a deeper understanding of the stone casting process and to try to improve it.

Platinum casting with gemstones in place has been the latest development before the end of the second millennium. It has been a world first to give fully successful results. The development of this new process required several years of uninterrupted experimentation, and in the spring of 1999 I obtained the first good results that encouraged me to continue the study.

This process has been specifically developed for the industrial production of large quantities. Later the know-how of this innovative process has been introduced in some important factories in USA and in Asia with in factory training courses. Now these factories are using it successfully for mass production.

What advantages does this technique offer?

1. The most obvious advantage is the lower labor cost for gem setting. Savings can range from 50 to 90%, depending on the setting, on model design and on the type and quality of cut. For example, a ring with 20 princess cut channel set diamonds, requires about 4 hours of work for hand setting, while 20 minutes are sufficient for setting in wax.
2. The setting is stronger, because the metal cast around the stones shrinks slightly during the cooling phase and forms a homogeneous tightening fit around the stones.

What are the setting methods?

1. There are two principal methods for setting stones in wax patterns:
   • The stones are positioned in the rubber mould and the injected wax flows around them. When the wax pattern is extracted from the mould, the stones are already set.
   • The stones are set directly in the wax pattern. This method is suitable for setting any type of cut and also for different types of setting.

What types of stone can be cast in place?

On the basis of the results obtained up to now, only the following types of gemstones can be used:

1. Diamond. Very important: the diamonds should be of natural color, untreated and their purity should not be lower than the "S2" grade (small inclusions).
2. Ruby. It should be of "good" quality and not submitted to treatment of any type.
3. Sapphire. The requirements are the same as for ruby.

What is the process for casting platinum with stones?

To obtain a good result, some adjustment is required to nearly all steps of the casting process.

1. A correct preparation of the master model is essential, and it must be performed by someone experienced in stone-in-place casting.
2. The correct choice of the rubber and proper rubber cutting are important for the final result
3. Only some injection wax types are suitable for this process.
4. The right tools must be used, to set the stones in wax correctly and precisely.
5. When calculating the weight of the metal needed for casting, the weight of the stones must be excluded from the weight of the whole wax tree.
6. Some adjustment to the powder to water ratio is required for the preparation of investment. It is necessary to add Pro-Tech: Pt-D, a protective liquid for the stones, developed for this casting technique.
7. Burnout temperature should be slightly lowered, to avoid damaging the stones.
8. The traditional platinum alloys can be used for casting, but it is much safer to use the Pro-Tech: Pt-S alloy, that has been specifically developed for this use.
9. An induction heated melting/casting machine equipped with a high precision optical pyrometer is required to obtain very good results consistently.
10. After casting, the flask should be allowed to cool to nearly room temperature before removing the investment.
11. Presently, a solution of potassium hydroxide (KOH) is available for removing the investment residues from the cast tree, as a substitute for the toxic and harmful hydrofluoric acid that has been used up to now.
12. To obtain best quality products, there are specific practices also for finishing and polishing.

What developments may be expected for this technique in the near future?

In Asia and in America I have noticed the need for a cost reduction in the production of platinum jewelry with diamonds and invisible setting. Now, thanks to further research and experimentation, I succeeded to make the casting process suitable for this type of setting.

By Hubert Schuster, 2004

Jewelry Technology Institute

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