Bronze and brass artifacts recovered from long term salt-water exposure develop what is commonly referred to as the "Bronze Disease". The tin in bronze and the lead in brass react in salt water to produce a compound called cuprous chloride. This compound will form acid layers throughout the copper based metal item. When the artifact is recovered and exposed to the oxygen in air, the acid layers become active. Despite intensive polishing, the surface will begin to turn black with a progressive corrosion that eventually leads to the complete self-destruction of the entire unit. When the humidity increases the destruction accelerates. There is no known practical way to remove the cuprous chloride compound from the artifact. Conservation experts advise that one treatment is to forever seal the surface from exposure to air. They say that painting a clear coating over the entire surface will hold the corrosion at bay. Another way would be to keep the artifact submerged in water to prevent air contact, but this would defeat the original recovery idea. Bronze propellers recovered from salt-water exposure also suffer from this disease, but they can be successfully reused for their intended purpose to re-power boats and ships. The copper based items salvaged from vessels that have not spent long periods submerged in salt water do not develop this malady.
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