(1) In the opinion of Hempel, the hardening of vulcanized indiarubber is caused by the gradual evaporation of the solvent liquids contained in the indiarubber, and introduced during the process of vulcanization. Guided by this notion, he has made experiments for a number of years in order to find a method for preserving the indiarubber. He now finds that keeping in an atmosphere saturated with the vapours of the solvents answers the purpose. Indiarubber stoppers, tubing, etc, which still possess their elasticity, are to be kept in vessels containing a dish filled with common petroleum. Keeping in wooden boxes is objectionable, while keeping in airtight glass vessels alone is sufficient to preserve indiarubber for a long time. Exposure to light should be avoided as much as possible. Old hard indiarubber may be softened again by letting the vapour of carbon bisulphide act upon it. As soon as it has become soft, it must be removed from the carbon bisulphide atmosphere and kept in the above way.
Hard stoppers are easily made fit for use again in this manner, but the elastic properties of tubing cannot well be restored. (Ber. Chem. Ges.) (2) In order to prevent indiarubber materials from hardening and cracking, they are steeped in a bath of melted paraffin for a few seconds, or several minutes, in accordance with the size of the articles, and then dried in a room heated to about 212° F. (100° C).