In the reduction of the ingot to sheet metal, annealing has to be resorted to, and acid pickles to remove oxides, etc.

The number of times the plated metal is exposed to heat and acid in its reduction to the required thickness, produces a surface of pure silver.

The most of this surface is, however, so rough as to be with difficulty polished, without in places removing entirely this pellicle of pure metal, and exposing a polished surface of the alloy used in plating.

Whenever such metal was used, very unsightly stains or spots frequently disfigured the portraits.
The portrait, or portion of it, developed upon the pure silver, being much lighter or whiter than that developed upon the alloy; it therefore appeared that the purer the silver, the more sensitive the plate became.

Accordingly, we directed Messrs.