Investment casting

Argentium 930 and 970 Silver casting alloys   
Hard and durable castings  
Casting temperatures  
Technical recommendations  
Investment casting report - Franklin Cox
Related documents

Description of casting alloys  

Argentium 930 Silver casting grain:  a general purpose alloy that is firescale free and highly tarnish resistant.  This alloy is easy to finish and has a lustrous colour, whiter than regular sterling silver.

Argentium 970 Silver casting grain:  a premium alloy that is highly tarnish and perspiration resistant.  This alloy has a high melting range, casts firescale free and is easy to finish to a beautiful white lustre.

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Hard and durable castings   

Germanium imparts unique hardening properties to Argentium Silver alloys.  Castings can simply be left to air-cool to room temperature to develop good hardness and durability.  Even greater hardness can be imparted by applying heat treatments at temperatures achievable in domestic ovens.  

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Casting temperatures for Argentium 930 & 970 Silver alloys:

	925	970
Casting pour temperature	1706 - 1742°F 930 - 950°C	1778 - 1814°F 970 - 990°C
Solidus	1409°F 765°C	1522°F 828°C
Liquidus	1610°F 877°C	1680°F 916°F

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Technical recommendations  

Accurate temperature control:  Argentium 930 Silver displays a paler colour than standard sterling silver when it is heated or molten.  Accurate temperature readings are important to prevent overheating.

Flask temperature:  1005-1255°F / 540-680°C

Crucibles:  Use separate crucibles to avoid contamination from other alloys.  Clay graphite or pure graphite crucibles are recommended.  Silicon carbide crucibles are not recommended.

Protective atmosphere/fluxing:  It is advisable to use a protective atmosphere (e.g.: argon or nitrogen).  If a protective atmosphere is not available, flux can be used (boric acid is recommended).  Skim any oxides off the surface before stirring.

Hardness - dry investment removal:  Leaving flasks to air-cool to room temperature before removing castings from the investment, will give the following hardness figures:
Argentium 930 Silver - approximately 100HV/DPH
Argentium 970 - approximately 90HV/DPH

Hardness - wet investment removal:  Leaving flasks to cool for 15-20 minutes before quenching, will give the following hardness figures:
Argentium 930 Silver - approximately 60-70HV/DPH (similar hardness as regular sterling silver)
Argentium 970 Silver - approximately 50-60HV/DPH. 

Increasing hardness using heat treatments:  To raise the hardness of castings, follow the 'quenching' or air-cooling' procedures described in the instructions for heat-hardening Argentium Silver.  

Remelting:  Use at least a 60% fresh to 40% scrap mix.  Using more fresh metal will yield superior results for high-quality pieces.  It is important that used trees and buttons are free from investment powder residue to avoid contamination.

Finishing:  Use separate polishing wheels, mops and media for Argentium Silver, to prevent contamination from other alloys.

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Franklin Cox describes  

I use 2 ½ inch flasks, casting between 10 to 40 grams of silver per flask.  (all custom work, small items ).  I centrifugal cast using a torch (propane & oxygen) to melt.  I use a hotter flame for Argentium Silver than for regular sterling.  The flame I use for regular sterling is somewhat reducing, therefore, I use more oxygen in the flame when casting Argentium Silver.

Image: 'Cherries', made in Argentium^® 930 Silver, by Franklin Cox

1. I find that I need a mix of new grain to old buttons of 60% new and 40% old.  I make sure the old material is clean.

2. I coat the material to be cast with boric acid dissolved in alcohol, then burn off the alcohol, I use no other flux.

3. I am careful to not overheat the metal (molten Argentium Silver looks different from molten sterling).  I cast shortly after the metal has become fully molten.

4. The flask temperature I use is slightly higher than what I use for regular sterling (1150°F/621°C for fine items down to 850°F/454°C for larger pieces).

5. I always let the flask cool to about 250°F-300°F / 121°F-148°C or right down to room temperature before I break out the cast.  I do not quench hot flasks in water, as I have experience cracking by quenching too quickly.

6. I ultrasonically clean off investment for a few minutes only, pickle in hot sparex for a few minutes, then ultrasonic again for about one minute, then repeat if needed.  This procedure gives me clean, white, porosity- free castings to start to work on.  Castings cooled to room temperature are harder than those broken out of a hot flask.

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Related documents  

'Investment casting Argentium Silver' data sheet (pdf download)

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