In 1743, Thomas Boulsolver of Sheffield, England, discovered that he could bind a sheet of sterling silver to a copper ingot by pressing and rolling the two metals into a workable sheet. The friction from the process caused enough heat to fuse the metals to each other. This sheet could then be made into a teapot, a tray, or other useful object. During the late 1760’s, the silver was fused to both sides of the copper making a kind of “silver sandwich”. Silver objects made from this sandwiched metal eventually became known as Old Sheffield plate. The English excelled at this even though other countries produced it in the 18th and 19th centuries.
You can tell Old Sheffield plate in several ways. The underside is tinned rather than silverplated for reasons of expense and therefore that surface has a slightly rougher, brighter look. And, many old pieces such as urns and coffeepots were formed by making a cylinder out of a sheet of sandwiched material and rolling it which creates a seam when burnished. After years of polishing, the seam will show if you breathe on the area, forming a frost, exposing the red-orange cast of the copper.
Also, look for rolled edges. Since Sheffield plate is a “silver sandwich” the exposed edges have to be rolled over or covered with silver wire or thread (called a wrapped edge) to prevent people from seeing the copper layer in the middle.
If your object has a coat-of-arms engraved on it, try breathing on the edges. Sheffield plate manufacturers inserted silver sections so the engraving would not cut through to the red-orange layer. The air will condense around the outline, giving you an indication that the piece is not solid silver.
Later on, maker’s marks were used to identify their wares.
Pamela Pierrepont Bardo, ASA, AAA