A good resource on the manufacture of silver plates for the daguerreotype is the book
Daguerreotypes, 19th Century Practice and Modern Science by Barger and White.
The majority of 19th century plates are silver clad over copper by rolling (American made) or hammering (French made).
Later in the daguerreian era one company (Christofle of France) provided electroplated plates but by far the most common was clad silver plates.
The silver is as pure as possible .999 fine. The silver thicknesses ranged from about 10 – 20 microns before polishing.
Polishing was necessary to remove the rolling mill or planishing hammer marks.
Regardless of the make, French. American, clad or plated, about 20% of daguerreotypists re-silvered their plates with a galvanic deposit of silver after first creating a fine polish. The usual method of determining if a plate has been re-silvered will be to look for a poorly adhered deposit of silver on the back of the plate.
I sell NEW clad plates to those that want them. The silver is .999 fine approximately 25 microns thick. The total thickness including copper is 0.020 inches thick.
They require polishing to remove the rolling mill surface.
best
Mike Robinson