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Showing content with the highest reputation on 07/24/2015 in all areas
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I has to be chemical. The legend shadowing is an expected result from acid based corrosion. The legend by definition involves a change in relief, i.e. you have metal flow to produce it, which will result in metal stress faults where the crystalline structure has been stretched. This will allow any acid to penetrate deeper and so result in a depression around each letter as the acid works more effectively in the stress faults.1 point
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I concur with the acid and have seen very similar effect when high molar acid is used (e.g. nitric, hydrochloric) and there is a more pronounce "boiling" effect of metal off the surface.1 point