The housings we keep specimens in also hold hidden dangers. Spirit collections are those housed within a preserving liquid. These can be dangerous because chemicals can be flammable and toxic. The liquid must be changed relatively frequently; at the Harvard University Herbaria all of the liquid in over 3,000 jars have been changed three times over the past twenty years. A glass jar with liquid inside could crack, and the liquid preserving the specimen could leak. Without its liquid, the specimen will become dry and disfigured. The labels, made of tape and glue will become brittle as they age, and will not adhere to specimens, paper or jars. Acidic paper will become dried and discolored over time, and will not preserve specimens as well as those within non-acidic paper.