This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
To prevent cracking, hardening, etc., and to improve it, dip in melted paraffine of 100° C. (212° F.) and keep therein for some time. In a few hours it will have absorbed a small percentage of paraffine, which protects it against injurious influences, without altering its appearance.
Digest for ten days in a 5 per cent, solution of caustic soda at a temperature of from 100 to 125° F. "Wash them and scrape the outer portion with a dull knife until no more comes off. Wash in warm water.
Rubber rings used for bottle-stopping purposes soon become hard and brittle. If they are soaked in ammonia water (one part strong ammonia, two parts water) for half an hour, softness is restored.
A good substitute for rubber stoppers, and quite practical for laboratory use, is prepared thus: Dissolve enough rubber in bisulphide of carbon, petroleum, ether or benzine without the application of heat, until a mass of the consistency of honey is obtained. With this solution coat the corks, which will become impermeable.
Much difficulty is frequently experienced in removing glass stoppers from bottles. If the glass stopper be dipped in melted paraffine, no trouble will be experienced.