From Academic Kids
Sterling silver alone would have been far too expensive for the emerging middle classes to afford, so there was a continual search in mid-18th century England for a suitable replacement. Thomas Boulsover, of Sheffield's Cutlers Company, provided the solution in 1743. While trying to repair the handle of a customer's decorative knife, he heated it too much and the silver started to melt. When he examined the damaged handle, he noticed that the silver and copper had fused together very strongly. Experiments showed that the two metals behaved as one when he tried to reshape them, even though he could clearly see two different layers.
Boulsover carried out further experiments in which he put a thin sheet of silver on a thick ingot of copper and heated the two together to fuse them. When the composite block was hammered or rolled to make it thinner, the two metals were reduced in thickness at similar rates. Using this method, Boulsover was able to make sheets of metal which had a thin layer of silver on the top surface and a thick layer of copper underneath. When this new material was used to make buttons, they looked and behaved like silver buttons but were a fraction of the cost.
The "double sandwich" form of Sheffield plate was developed around 1770. Used for pieces such as bowls and mugs that had a visible interior, it consisted of a sheet of silver each side of a piece of copper; early manufacturers applied a film of solder over the bare edge of copper although such pieces are very rare. Later on, borders were applied with a U-shaped section of silver wire to conceal the copper which can be felt as a lip on the underside.
Sheffield plate is fairly rare today, as it has generally been replaced with electroplating processes. Electroplating tends to produce a "brillant" surface with a hard color, so Sheffield plate is still used in cases where a softer color more like natural sterling silver is needed.