This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
The hardening of gum articles is generally referable to these having been kept for a long time in some warm, dry place, though keeping them in the cold will produce the same effect. Hardness and brittleness, under any reasonable care and conditions, are usually signs of an inferior article of goods. Articles of Para rubber, of good workmanship, usually maintain their elasticity for a very long time. Before attempting to soften hollow rubber ware, such as flasks, water bags, or bottles, etc., they should be well scrubbed with a wire ' brush (bottle cleaner) and warm water, so as to remove ail dirt and dust. This scrubbing should be continued until the wash water comes away clean and bright. For softening, the best agent is dilute water of ammonia, prepared by mixing pharmacopceial ammonia water, 1 part, and water, 2 parts. There should be enough of this to cover the articles, inside and out. Let them remain in the mixture until the ammonia has evaporated. Warm water works better than cold. From 1 to 2 hours will be long enough, as a usual thing. Thick and massive articles such as large rubber tubing, require more energetic treatment, and the journal recommends for the treatment of these that they be filled nearly full with the ammonia mixture, corked at both ends, and coiled up in a kettle, or other vessel, of sufficient size, warm water poured in sufficient to cover the coil completely, and lightly boiled for from 1 to 2 hours. The water lost by evaporationshould be replaced from time to time, and the vessel should never be allowed to boil violently. When the proper time has arrived (and this must be learned, it appears, by experience, as the article quoted gives no directions save those translated), remove from the fire, and allow to cool gradually.
Glycerine has been also recommended, and it may be used with advantage in certain cases. The articles must first be cleaned with the brush and warm water, as above detailed. Heat them in water and rub them with a wad of cotton soaked in glycerine, drawing the wad over them, backwards and forwards. This wad should be wrapped with good stout wire, the ends of which are prolonged, to serve as a handle. Where possible the articles should be stricken with the glycerine inside and out, the article being, naturally, held out of the boiling water, sufficiently, at least, to make bare the part being rubbed at the time. Let rest for 24 hours, and repeat this process. With goods kept in stock, that show a tendency to grow brittle, this treatment should be repeated every 6 months or oftener. Never put away tubing, etc., treated in this manner until every particle of moisture has drained off or evaporated.
Another authority, Zeigler, has the following on this subject: Tubing, bands, and other articles of vulcanized caoutchouc that have become brittle and useless, may be restored to usefulness, indeed, to their pristine elasticity, by treating them as follows: First, put them in a hot aqueous solution of tannic acid and tartar emetic. Next, transfer them to a cold aqueous solution of tannic acid and calcium sulphate. Mix the two solutions and heat to about the boiling point, and transfer the articles to the hot solution. This treatment should bemaintained from 1 day to 3 or 4, according to the nature and condition of the articles.
To restore rubber stoppers that have become too hard for usefulness, digest them in 5 per cent soda lye for about 10 days at 86° to 104° F., replacing the lye repeatedly. Next, wash the stoppers in water and scrape off the softened outer layer with a knife, until no more can be removed. The stoppers (which have become quite soft and elastic again) are next rinsed in warm water to remove the caustic soda. If it is desired to trim them it should be done with a knife moistened with soap spirit.