These can be given a clean, bright surface by the use of nitric acid. The desired surface is thus obtained quickly and with little trouble. But there is the objection that a considerable quantity of nitrous fumes are given off, and these red vapors are at once extremely disagreeable, and very prejudicial to health. Their production may be prevented by adding a little solution of bichromate of potash to the dilute nitric acid. This is found to answer perfectly, the copper surface being made clean and bright, without disengagement of vapors.