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But somehow he hasn't got hold of it quite Or the liquid you pour on it first isn't right So. that's why he works at it night after night 'Til he knows he can do it for certain. (A.A. Milne-"The Alchemist") Abstract: It is pointed out that no completely adequate published accounts of the so-called "tinning test", which is used as an aid to the identification of cassiterite, are known to the writer. All of them lack important details and many contain statements that are wrong. . What is believed to be a correct comprehensive account of the test under review is given, and certain other related qualitative and semi-quantitative tests for tin occurring as cassiterite are described. VARIATIONS OF THE "TINNING" TEST The so-called "tinning test" is one frequently carried out in the field, mill, and laboratory as a rapid and simple aid to the identification of cassiterite, generally by those concerned with the examination of samples of loose grains. The same aid can also be used to identify cassiterite in hand specimens and thin- and polished-sections. The test maybe carried out in a number of different ways, but it may be effected, and often is, by placing the sample in a zinc tray and then covering it with dilute (c. 5N) HCl. After a few minutes any "clean" cassiterite present is usually covered with a grey matt coating which on rubbing with, say, a piece of cloth, assumes a bright silvery appearance. This coating has been generally thought to be metallic tin produced by the reduction of the Sn02 by nascent hydrogen in accordance with the following equations;- Zn+2HCI -+ ZnCI2+2H 4H+Sn02 -+Sn+2H20 However, the reaction is not as simple as this, as "nascent' hydrogen, generated by the reaction between a dilute acid (say HCI) and certain metals other than zinc will not result in the cassiterite being tinned. Data presented elsewhere in this paper serve to confirm this statement. The fact that at least one author (Dana, 1932, p. 497), and contrary to general belief, thought that the coating was zinc, was recently brought to the writer's atten­ tion, and so he decided to carry out a little library research in order to check what others had to say about this test which is in such common use throughout the tin­ fields of the world. The results were most interesting in that none of the writers con­ sulted had described the test and its limitations accurately and completely! Some

Practical aspects on the identification of Cassiterite (SnO2) by the Tinning Test