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Black castings


#1

G’day; I’ll begin by saying that I know nothing about casting.

         I get a black color, sometimes in metal, instead of a
clean sterling cast........ Four came out a nice color of silver cast
and four came out black.  The black can be removed by soaking in
pickle and a short dip in silver tarnish remover.  

I can only think of one form of blackening that would produce the
effect you describe, which is that of silver sulphide. Are you sure
that you didn’t have any sulphur compounds in the investment or
somewhere close? Place in some mix a bit of freshly sanded (not
polished silver; it will bear grease from the polish), preferably
sterling, leave it for a while, remove it and if there is the
slightest trace of darkening on it, then your problem has to do with
sulphur. Otherwise someone else will doubtless come up with a better
answer. Cheers, – John Burgess


#2
             I get a black color, sometimes in metal, instead of a
clean sterling cast....... 

I usually get this when I’ve had a cold cast…flask temp is right,
but the metal was not hot enough…Char


#3
             I get a black color, sometimes in metal, instead of a
clean sterling cast........ Four came out a nice color of silver
cast and four came out black.  The black can be removed by soaking
in pickle and a short dip in silver tarnish remover. 

Your investment is breaking down and releasing sulfur dioxide which
is bonding with the silver as silver sulfide. This can be caused by
several things and without seeing your process I cannot tell you
which is causing the problem. But here are the main culprits.

1: too hot a burnout
2: casting temp too high
3: incomplete burnout leaving carbon in the mold which lowers the 
temp for the reaction that forms the sulfur dioxide

Jim


@jbin
James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
510-436-3552


#4

Thanks for the response on my casting problem. Both John and Jim
Binnon suggest that sulfur dioxide is the culprit. I did some testing
this weekend, casting three small flasks. I did the normal burnout
(6 hours) that I usually use and let the remaining flasks continue as
I casted the others. Each successive flask produced a darker color on
the castings as they remained in the oven longer. I have to assume
that I’m running too hot on the burnout. I’ll try adjusting the high
end temp and see what happens. Thanks again. Bud Cravener