Rubber cement is a type of strong household glue especially useful for bonding flexible or clear items. If you leave the cap off of a rubber cement container, the contents will dry out and harden. There's no need to waste the product, however, because you can restore dried-out rubber cement with solvents stocked at art supply, hardware and beauty supply stores.
Read the ingredients list on your rubber cement label to find the solvent used to break down the other ingredients. Solvents typically have an –ol, –one, -ane or –ene suffix. Since art supplies aren't required to list ingredients, your product label may not have this list. In that case, call the glue's manufacturer to determine what solvent was used in the product.
Purchase the indicated solvent at an art supply or hardware store. Acetone, a common ingredient in nail polish remover, is available at beauty supply stores.
Put on a pair of goggles, a respirator mask and latex gloves, then take the rubber cement and solvent outside or to a well-ventilated area away from children and pets.
Add the solvent to the dried rubber cement, a few drops at a time. Stir the mixture until it achieves the desired consistency.
Place a lid on the rubber cement, close it tight and store it out of reach of children and pets.