As I understand it (at one point we spoke to the lawyer who had
written the US stamping laws), it goes like this:
Canada and many other countries work with regulations (which is why
you must have a registered trademark before you can quality stamp
anything). The US tends to work with core principles, such as ‘thou
shalt not defraud they customer’, and then lets state regulations
tighten up the interpretations.
What theis means is that you can quality stamp workin the US as long
as you also have an identifying mark on the object (any stamp that is
identified with you, and preferably registered at state and federal
levels-it could be [for example], a registered trade mark stamp), so
you can be found if you were trying to defraud someone.
And in regard to mixed metals, you can use all the stamps for each
metal, just treat it like ingredients in a cereal package, sugar first
(because it is mostly sugar) , then wheat (less of the material), salt
(even less) etc. A large piece of platinum with a small piece of 18k
would then be stamped with an identifying mark (maybe a registered
trade mark), PT, 18k.
I could be wrong but this is my memory of what the stamping laws
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