Natural rubber is made from the sap of the rubber tree, which is drained off, heated, and moulded into rubber parts. Rubber has many uses -- it is water-resistant, flexible, strong and elastic. That said, over time natural rubber will dry out, stiffen and crack. It is often cheaper and easier to replace the cracked part, but it is also possible to restore natural rubber's elasticity and softness with some basic household cleaning chemicals and a bit of time.
Restore rubber with ammonia
Use ammonia to restore natural rubber. Ammonia is often used to wash bathroom fixtures and can be used for washing clothes as well. Ensure the ammonia is pure, and doesn't contain any detergents or other chemicals.
Clean the rubber item you are restoring. Scrub it thoroughly with a wire brush and wash it with water until the water dripping from it is perfectly clear.
Pour some ammonia into a pan of warm water and immerse the rubber article in the water for one to two hours. This will be enough time for the ammonia to evaporate. Follow the directions on the ammonia bottle for guidelines as to how much ammonia to add to the water.
Restore thicker pieces of tubing by filling them with ammonia water, plugging their ends, and boiling them in a pan of water set to a low boil for one to two hours.
Restore rubber with glycerine
Wash and scrub the rubber item until it is clean.
Wrap a wad of cotton in stiff wire that can serve as a handle. A bent coat hanger works well.
Heat up the rubber piece in a pan of boiling water.
Pull the rubber out of the boiling water, then dip the cotton wad into some glycerine, and wipe down the rubber item with it. Let it dry for 24 hours, then heat the rubber again and wipe it down with more glycerine.
Restore rubber with lye
Mix up a solution of 95 per cent water and five per cent lye.
Heat the lye solution to between 30 and 40 degrees C (86F to 104f).
Immerse the rubber piece in the lye solution for 10 days. You will need to replace the lye regularly, as it will dissolve away.
Rinse off the caustic soda that forms on the rubber with water, then scrape the rubber piece with a knife to remove any crusty outer layer. The remaining rubber will be soft and pliable again.
Always wear gloves when working with cleaning chemicals. Many of them are quite caustic and will damage your skin. Work in a well-ventilated area where there are no children around.