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Solder contamination and casting

Hi folks,

On gold casting and metal contamination.

After retiring with apparently too much time on my hands, and some
sleepless nights, I am wondering why, I was taught and have read in
several books… why is it important not to have a smidgen of
solder from say, a ring resize, in the casting crucible with your
fresh metal. (keeping well within the 50% new rule) Was told by my
teacher that it will create soft spots or other problems in the new
casting. I followed this rule (blindly I must admit) for 40 years,
never “contaminating” the new casting metal, always cutting out
welds, etc… But since alloys are made up of the same materials as
the solders, I don’t see why there wouldn’t be a homogenous melt
with just a hundredth (if even that much) part addition of solder to
the crucible. Especially medium or hard solders. I stopped casting
with “used” gold 35 years ago except for some sentimental exceptions
for obvious reasons of not wanting to mix different alloys, but a
smidgen of solder?

Whats your take on this?