The decarbonating process was performed by enclosing the plate of cast steel properly shaped, in a cast iron box, or case, filled about the plate to the thickness of about an inch with oxide of iron or rusty iron filings; in this state the box is luted close, and placed on a regular fire, where it is kept at a red heat during from three to twelve days. Generally about nine days is sufficient to decarbonize a plate five-eighths of an inch in thickness; when the engraving or etching has been executed, the plate is superficially converted into steel, by placing it in a box as before, and surrounding it on all sides by a powder made of equal parts of burned bones, and the cinders of burned animal matter, such as old shoes or leather. In this state the box, with its contents, closely luted, must be exposed to a blood-red heat for three hours; after which, it is taken out of the fire, and plunged perpendicularly edgeways into cold water, (which has been previously boiled) to throw off the air. By this means the plate becomes hardened without the danger of warping or cracking. It is then tempered or let down by brightening the under surface of the plate with a bit of stone; after which it is heated by being placed upon a piece of hot iron, or melted lead, until the rubbed portion acquire a pale straw-colour. For this purpose, however, the patentee expressed himself in favour of a bath of oil heated to the temperature of 460 degrees, or thereabouts of Fahrenheit’s scale. The plate being cooled in water, and polished on the surface, was ready for use.