Tarnishing of AuAgCu alloys. Choice of an accelerated test

Three tarnish-testing procedures were applied to three 14 and 18 ct AuAgCu and one 14 ct AuAgCuZn alloys widely used in decorative applications. Two gaseous phase tarnishing tests were investigated: (a)the flower of sulphur test carried out at 50 °C and 75% relative humidity for 5 days, and (B)the thioacetamide test performed at ambient temperature and 75% relative humidity for 5 days. In addition, a liquid based method known as the Tuccillo-Nielsen test where the samples are alternatively immersed in an aqueous sulphide solution for 3 h was also studied. It was found that the general tarnishing aspect encountered in field trials was better reproduced in the liquid medium than in the gas phase tests. The colorations are more heterogeneous in the liquid test. The main advantage of the Tuccillo-Nielsen test is that it requires only 3 h, but it can give rise to galvanic effects and may be difficult to apply to any forms of finished products. The thioacetamide test gave poorly reproducible and reliable results. In addition the colour changes after 5 days' exposure were small; moreover, thioacetamide is a carcinogen. The flower of sulphur test yielded large, reproducible colour changes. It should be preferred to the thioacetamide test for the accelerated testing of silver – and copper-containing alloys.