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.
Before use, clean the corks in clear cool water to remove all dust; if they are yet hard, soak them in summer in cold, in winter in warm, water a little while. Hot water should not be used, as the corks would get too soft and lose their bright color. It is recommended to add a little sweet oil (olive oil) to the warm water they are soaked in, so that the corks can be forced easily into the head of the bottle.
If corks are used for bottling ferruginous mineral waters, the tannin, which all corks contain, will gradually darken their color; if they are of a very good quality, the effect will be less marked and but on the exterior surface. Mineral waters, containing much salt, particularly much magnesia salts, cause, after months of storage, an evident dark coloration on the cork, the latter becoming hard and non-elastic. To remove the tannin from the surface of the corks Dr. Hirsch recommends to digest them in a warm one per cent solution of sulphate of iron (one part of the salt in 99 parts of warm water) for several hours and then rinse them in pure water; however, the external appearance is suffering to some extent by this treatment.
Dr. Hager recommends to soak the corks in a solution of 10 parts by weight of sulphate of iron and 2 parts of muriatic acid in 1000 parts of water, at a temperature of 50° C, for 5 hours, occasionally stirring. From this bath the corks should be brought into another one, containing but one part of muriatic acid in 1000 parts of water. After this they should be washed several times with pure water.
By this treatment the tannin is extracted from the exterior surface of the corks.