Celluloid is, unbeknownst to many, a type of plastic. It’s a class of compounds created from cellulose nitrate and camphor. Celluloid was used frequently as the material for vintage pen caps and barrels because it was more durable than hard rubber and because it was available in a myriad of colors, not just cardinal or black. It’s not used as often as plastics/acrylics today for pens because it’s not as stable as these materials, even though it has a very pleasant feeling in hand.
Celluloid is difficult to manufacture, as well as expensive. The process is long and there are several steps to reach the final stage. Long story short, it involves chemical reactions. After mixing, the product is pressed into blocks at high pressure before it can even be formulated for its specific use. Curing is the longest part of the process and the most important part; if not properly cured, celluloid can be prone to celluloid disease/crystallization.
Unfortunately, celluloid can also deteriorate through thermal, chemical, photochemical, and physical reactions. In the case of pens, if the celluloid was not properly cured, the camphor essentially squeezes out of the mass. As this is the plasticizing component, its reduced presence leaves the material weak. Damage is irreversible.
Celluloid is a beautiful material the manufacturing process is one of the reasons celluloid pens command a high price nowadays. Protect your celluloid vintage pens by keeping their storage area ventilated and away from humidity, heat, and light.