US silver coinage was typically 90% (.900 fine) silver, 10% (.100 fine) copper. Less than sterling (.925 silver/.075 copper), but greater than many European compositions of the day (.800 silver).
It's important to remember that in the late 19th century, the US was glutted with silver from the western mines, and loads of it was propped up by the US (hence all the Morgan Dollars that never circulated, among others). But that's another story (and for another board, no doubt!).
John