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Argentium Silver – Investment Casting – Production Guidance

Author Steve BernstromCategories Alloys, Casting, MetalsTags ,

The following article offers guidance to the best practice to be followed when using Argentium silver casting alloys.

FLASK BURN-OUT

  • The flask should stand for 2 hours before the burn-out cycle proceeds.
  • The furnace used for the burn-out should not be overloaded; this is to ensure that the centre of all the flasks will reach the required temperatures for complete burn-out and that there is sufficient air flow to remove the moisture from the cured investment powder.
  • Raise the flask temperature to 270°C over the period of 1 hour.
  • Hold at this temperature for 4 hours.
  • Raise the temperature (banking rate) at a rate of 160°C an hour to a maximum of 730°C.
  • Hold at this temperature for 4 hours.
  • Lower the temperature to the flask casting temperature.
  • Stabilise at this temperature for 1 hour before casting.

CASTING CONDITIONS

For 935 Argentium (Original) Silver

Size of objects                                     Metal casting (°C)                         Flask (°C)
thin from 0,2 to 0,5 mm.                        1020-1010°                                  560-600
medium from 0,5 to 1,2 mm                  1000-99                                   520-560
thick > 1,2 mm.                                        990-980                                      480-520

These are for guidance only and your own experience and knowledge of your casting machine will allow you to judge if you need to alter either the metal temperature or the flask temperature for specific castings.

The cast flask should be held under protective inert gas cover (either nitrogen or argon) on the casting machine for 3 minutes before removal from the casting machine.

INVESTMENT REMOVAL

  • Before quenching the flasks in water to remove the investment powder the flasks should be allowed to stand for 20 minutes in total after the metal has been poured.
  • This includes the 3 minutes under gas cover on the casting machine.
  • For stone in place castings the flasks should be allowed to cool for a minimum of 1 hour 30 minutes before quenching in water to remove the investment.
  • To remove any remaining investment powder after quenching a water jet should be used.
  • This is to be followed by immersion in 10% phosphoric acid or 10% sulphuric acid at 40°C to remove any remaining investment powder or water staining.
  • The cast trees should then be washed in water before any further operation takes place.

HARDENING ARGENTIUM SILVER
Argentium silver can be hardened without the need of a high temperature heat treatment.

  • The cleaned and dry cast tree should be placed in an oven with an air atmosphere.
  • The complete tree should be heated to 300°C and held at temperature for 2 hours.
  • After heat treatment the tree should be removed from the furnace and allowed to cool to room temperature.

POLISHING AND CLEANING
The polishing and cleaning operations are of critical importance in producing a quality product. It is important to emphasis that Argentium silver alloys, because of their high degree of tarnish resistance, will in most cases be sold without having undergone a plating operation (either silver or rhodium). Where it is not the intention to subsequently plate a more rigorous polishing operation will be required to achieve the level of finish suitable for release to the customer.

FIRST STAGE POLISHING

  • The first stage of the polishing process is the surface stripping and burnishing operation which creates a gloss finish on areas of the casting which are hard to reach by traditional polishing methods.
  • This requires the use of magnetic pin type media in accordance with the media manufacturer’s recommendations.
  • All pieces should be removed from the cast tree.
  • The remains of the sprue which connected them to the tree hand-filed flush with the main body of the piece.
  • Then a minimum of 20 minutes processing in this media before then proceeding to the grinding or cutting polishing operation.
  • Ensure that the water and soap mixture used in this operation has a pH of between 6 to 9.

SECOND STAGE POLISHING

  • The second stage of the polishing process is the cutting operation which dresses the surface of the piece and smoothes the surface, while removing the cast skin present on the surface of all investment castings. This can either be a mechanical polishing operation involving the use of ceramic or plastic media and utilising water as a vehicle to enable the optimal flow of the polishing media over the pieces in accordance with the media manufacturer’s recommendations or it can be the more traditional hand polishing process involving the used of abrasive wheels and emery paper.

 FINAL STAGE POLISHING

The final stage of the polishing process is the finishing operation which gives the piece its deep lustrous surface finish. This final polishing stage is a mechanical operation it should be a ‘dry’ process involving the use of a media such as crushed walnut shells in accordance with the media manufacturer’s recommendations.

Manual polishing involving the use of polishing compounds and polishing mops requires strict quality control

Between each polishing stage the pieces must be:

  • Cleaned using an ultrasonic tank using an alkaline, aqueous cleaner at a temperature of between 40-60°C and a pH in the range 7 to 9.
  • The pieces should be immersed for 2-3 minutes to remove oils, greases and loose particles before rinsing in tap water, drying and progressing to the next polishing stage.
  • Cleaners which give good results include Gesswein 851-5300 and Brulin 815QR.

DE-IONISED WATER and ELECTROLYTIC CLEANERS

It is important that deionised water is not used to rinse the pieces at any stage as its reactivity can affect the final tarnish resistance of the Argentium alloy.

Electrolytic cleaning systems should not be used to clean this alloy as they will strip the germanium from the surface of the Argentium.

PASSIVATION OPERATION

  • Argentium silvers work by developing a protective germanium oxide layer on their surface.
  • To assist the formation of this layer on to promote this reaction in more aggressive environments this passivation treatment must be used.
  • After the pieces have received their final polish and cleaning operations they are heated in an oven with an air atmosphere at 100°C for 1 hour.
  • No further polishing should be required after this process (provided the oven is clean).