Copper roof [Was Green Patina]

    copper roof was given its lovely green patina by applying many
gallons of horse urine - 

OK, how did they get all those horses up on the roof???

Actually, I need some help. I’ve been casting for years, but mostly
with gold. For the last year it’s been mostly sterling. I keep
having this reoccurring problem and just can’t seem to find the cause.
I get a black color, sometimes in metal, instead of a clean sterling
cast. This weekend, for example, I did a series of eight casts. All
the same investment, burnout and casting procedures. 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. Can anyone tell me what I’m doing that is causing this
discolration? Bud Cravener