I am trying to cast some simple medalions out of silver and suddenly
have a very strange problem.
I made several with little problem melting pure silver and 7.5%
copper in a clay graphite crucible which is much too large for the
project so I bought a couple of much smaller graphite ones.
I took some of the scrap and sprues and melted them in the new
crucible with a tablespoon of borax and after 3 pours, I have yet to
get one till fill the sand mold. It just does not pour nicely.
This is the same pattern that worked before.
What is very strange is that the parts have coatings in spots that
appear to be glass and extremely hard glass at that.
I suspect this has something to do with the borax but I thought I
read somewhere that this is the proper flux for silver.
This glass seems to be what is making it difficult to pour.
Any ideas? Could it have anything to do with the graphite crucible
vs the clay graphite?
Is a tablespoon of borax too much for 500 grams of sterling?