Wednesday, June 26, 2013

Casting fine silver...

So we had a production run of fine silver parts to cast for an enamel artist. Anyone who has tried, will know that casting fine silver is difficult to say the least. Also clean fine silver ".9999" is hard to find. Most of the time you can find .999 pure and most of the time that .0009 % can be a problem. I have seen it wreck large runs of casting when mixing with germanium based alloys....
We have been using mullite crucibles for vacuum casting gold alloys and fine silver, the small white ceramic ones you can find at pottery supply shops.
That turned out to be problem #1... It is an aluminum silicate oxide..... I'm not a chemist but I think it could have an issue with the dissolved oxygen hungry silver.






 Hmmm... Oxide.... we noticed an interesting effect happening on the bottom of the silver if allowed to cool in the crucible. Looks like the crucible is off gassing while the metal cools. What you see in the picture is not what the inside bottom of the crucible looks like...
The first run yielded surface and flow issues, second run we used a more accurate way to measure the temperature and threw a chunk of charcoal block in the melt.

 The other thing that really helped was to raise the flask temperature a couple of hundred degrees above what we would cast the same parts in deox or sterling. Because of the fast chill rate of fine silver.







 Lastly quicker quench time for the flask after casting gave us much smaller grain formation.....
 And in closing we will be changing out the crucible for a clay graphite one...