The combs sold at a penny each are usually made of celluloid, a composition produced by treating collodion cotton with camphor and methylated spirit. The camphorated spirit dissolves the collodion cotton sufficiently to convert it into a gelatinous mass which can be pressed to any desired shape, and after evaporation of the alcohol this material becomes quite hard. To cheapen the material, large quantities of starch, zinc oxide, whiting, or barytea are mixed with the above material, yielding the ivory or bone-like products usually seen. The coloured varieties are made by incorporating pigments with the celluloid, and tortoiseshell and other forms are made by special treatment. To soften celluloid, break it small, add a small quantity of camphor, and then add sufficient spirit to cover the mass. After standing a few days it will be soft enough to work. Horn can be softened, but not dissolved, by treating it with caustic soda for a short time, while prolonged action of the alkali will convert it into glue.