This is a question often asked by our customers when they send in their camera in for a service. Vulcanite is the well known textured skin that was used on Leica cameras from the 1920s right up until the 1980s. Vulcanised rubber includes the stuff used on car tyres but vulcanite is a much harder substance - and was often used to make smoker's pipes and things like pens and razors.
Vulcanised rubber has been modified by adding sulphur, which forms cross links in the polymer chains giving it strength. Unfortunately over time, there are several factors that cause the vulcanite to fail. Firstly, the sulphur bonds first react with oxygen then water and light, gradually breaking down and in doing so, releases sulphuric acid.
Since the vulcanite is bonded to the aluminium shell of the camera, the liberated sulphuric acid causes a reaction on the surface of the aluminium, producing aluminium sulphate, a white powdery substance on the surface shell that in turn, blows off the rest of the vulcanite - just like plaster off a badly prepared wall.
Another mode of breakdown is microbiological. Strains of actinomycetales are known to have an adverse affect on rubber but their actions are slow and not fully understood. A camera is handled regularly and is likely to harbour many organisms and soils and the covering may be many years old, such deterioration must be a consideration.
Contrary to many opinions, there are no effective vulcanite treatments. Once the polymer bonds have broken, they cannot be repaired by any treatment, furthermore, the shell, made from the reactive aluminium may have started to oxidise and the effects of the sulphuric acid on the rest of the camera should not be under estimated.
Modern replacement skins, either of vinyl or even leather offer a far safer and more stable alternative and unless the camera has some particular historical interest, we believe losing the vulcanite is the best thing you can do for your Leica.