This week in objects conservation: These are part of our collection of decorative hair combs. We are trying to find out which combs are made of the early plastic, cellulose nitrate. Cellulose nitrate is one of the first plastics, often called by the trade name Celluloid.  It was invented in the 1860’s and was often used in jewelry to mimic much more expensive materials such as horn, ivory, amber and coral. Unfortunately for museums, cellulose nitrate is a very unstable plastic. As it ages and degrades it gives off extremely flammable gasses and corrosive acids.  These degradation products are harmful for humans and for other objects in the collection. In order to identify cellulose nitrate a simple scratch test can be performed: an edge of each object is rubbed on the surface of a white, unglazed ceramic tile to lightly abrade the object and release a small amount of material without causing noticeable damage. A small drop of clear indicator is added to this sample and in the presence of cellulose nitrate, the solution will turn a deep blue color. If cellulose nitrate is identified the comb will be stored in a safer, separate environment to extend its life and to protect the rest of our collection.