Daguerre, a skilled professional artist, was familiar with the "camera obscura" as an aid for establishing correct proportion and perspective, sometimes very useful when planning out the celebrated theatrical scene backdrops he painted and the even larger ultra-realistic panoramas he exhibited in his popular Diorama. The daguerreotype image is formed on a highly polished silver surface. Usually the silver is a thin layer on a copper substrate, but other metals such as brass can be used for the substrate and daguerreotypes can also be made on solid silver sheets. A surface of very pure silver is preferable, but sterling (92.5% pure) or US coin (90% pure) or even lower grades of silver are functional. In 19th century practice, the usual stock material, Sheffield plate, was produced by a process sometimes called plating by fusion. A sheet of sterling silver was heat-fused onto the top of a thick copper ingot. When the ingot was repeatedly rolled under pressure to produce thin sheets, the relative thicknesses of the two layers of metal remained constant. The alternative was to electroplate a layer of pure silver onto a bare copper sheet. The two technologies were sometimes combined, the Sheffield plate being given a finishing coat of pure silver by electroplating. In order that the corners of the plate would not tear the buffing material when the plate was polished, the edges of the plate were bent back using patented devices that could also serve as plate holders to avoid touching the surface of the plate during processing. To optimize the image quality of the end product, the silver side of the plate had to be polished to as nearly perfect a mirror finish as possible. The silver had to be completely free of tarnish or other contamination when it was sensitized, so the daguerreotypist had to perform at least the final portion of the polishing and cleaning operation not too long before use. In the 19th century, the polishing was done with a buff covered with hide or velvet, first using rotten stone, then jeweler's rouge, then lampblack. Originally, the work was entirely manual, but buffing machinery was soon devised to assist. Finally, the surface was swabbed with nitric acid to burn off any residual organic matter. In darkness or by the light of a safelight, the silver surface was exposed to halogen fumes. Originally, only iodine fumes (from iodine crystals at room temperature) were used, producing a surface coating of silver iodide, but it was soon found that a subsequent exposure to bromine fumes greatly increased the sensitivity of the silver halide coating.