Invented ca. 1870, a tough man-made thermoplastic material, composed of cellulose nitrate and camphor. It can be made transparent and in endless varieties of colors and patterns and has been used to imitate genuine ivory, tortoise shell, and mother of pearl. However, it is highly flammable and poor quality-control in its manufacture can cause it to ramdomly begin to decompose releasing nitric acid vapor. (Therefore, celluloid handled knives should be inspected periodically and never stored in the same area with other knives which can be rusted by the vapor.) It was largely replaced by more stable materials by the mid-20th century but was revived in the 1970s due to collector demand. It is still made today and still has the same problems.