Formation and Growth of Tarnish on Evaporated Silver Films

Abstract

We have studied the growth of the silver sulfide tarnish film which forms on silver deposited on smooth, amorphous substrates and rough and smooth polycrystalline substrates. Both ellipsometric and electron‐microscope techniques were used to measure the average thickness of the tarnish films. The growth rate was found to be variable depending on the concentration of the sulfurous gases. In normal laboratory air (measured concentrations of H₂S and SO₂ less than 0.2 parts per billion) a 1‐Å‐thick tarnish film can be expected to form in 1 h, 3–6 Å in 1 day, 15–30 Å in 1 week, and 60 Å or more in 1 month. The tarnish growth can be completely stopped by placing the silver in a dry‐nitrogen atmosphere. Conversely, in a humid atmosphere containing about 10% H₂S, growth rates of as much as 100 Å/h can be obtained. The silver sulfide formed as small, randomly spaced, approximately round clumps having a site density about one hundred times larger than the patch density reported for thermally etched silver surfaces. All clumps nucleated on initial exposure, the clump density remained constant with time, and the clumps grew much slower than the tarnish patches on thermally etched silver. Coalescence toward a continuous film was slow; even after 41 days, only 53% of one silver surface was covered with tarnish.