In another thread recently ( /forums/1158233/ShowPost.aspx ), I made reference to a most interesting and valuable article, "The Restoration of the Cutty Sark," by Frank G.G. Carr. Appended to the article in the reprint version that I have are several comments from experts whom Mr. Carr consulted during the last major restoration project, back in the early sixties. One of those commentators was a metallurgist, Professor A.G. Dowson, M.A., Ph.D. His comment regarding the bottom sheathing: "Mr. Carr asked me a few years ago to go and look at the copper bottom of the Cutty Sark, which had then been installed at Greenwich for some time, because it was as he put it in meallurgical layman's language, 'rotting.' Indeed it was; it was very very rotten. It is not strictly a copper bottom, it is of Muntz metal, 60 per cent copper and 40 per cent zinc - what we call an alpha-beta brass. This is cheaper than copper (because zinc is cheaper than copper) but for this purpose, not so good, as can be seen from the subsequent history....
"Replacement of the rotten plates has been done in a material called 'Alumbro' an aluminium-bearing brass which does not contain the high zinc content which led to the failure of the original material, and which precludes any sort of preventive measures."
From George Campbell's China Tea Clippers, pp. 222-224: "The copper sheets [in the early days of metal-sheathed bottoms] were originally nearly pure copper which eroded away quickly even though it kept a good clean bottom, the marine growth being shed along with the eroding copper. This was an expensive process and efforts were made to reduce the rate of erosion, or exfoliation, by adding other metals. Muntz introduced a mixture of 50 parts copper to 50 parts zinc in 1830, and by 1846 had changed it to 60 of copper and 40 of zinc, which was the well known Muntz metal in use up to modern times, sometimes with a proportion of tin added. Lloyd's Registers of the period describe ships' bottoms as being yellow metalled, coppered or brass bottomed, all these being slight variations...."
There seems to be minor disagreement here over just why Muntz metal replaced copper as the preferred sheathing metal, but modern historians are, I think, pretty generally agreed that it happened well before the construction of the Cutty Sark. (Another good source on the subject is William Crothers' The American Clipper Ship.)
The American Heritage Dictionary defines "brass" as "a yellowish alloy of copper and zinc, sometimes including small amounts of other metals, but usually 67 percent copper and 33 percent zinc." Sounds to me like brass and Muntz metal were mighty close to being the same thing.
Just what the stuff looked like after it had been on the ship for a while is an interesting question that modelers have been arguing about for years. My recollection of the few visits I've actually made to her (in 1978, 1987, 1991, 1992, and 1997) is that the sheathing metal was a mottled, uneven mixture of dark greens and browns - but I was looking at the "Alumbro." Some authors claim that sheathing metal was normally bright and shiny unless the ship was taken out of the water. (I'm not sure I buy that one, but I can't disprove it.) There is, to my knowledge, one photo of the Cutty Sark during her active years that shows the sheathing fairly clearly. It's an 1872 shot, taken when she was in drydock shortly after losing her rudder at sea. The sheathing metal appears quite noticeably lighter than the black paint above the waterline.
I don't think I've ever seen a model sheathed with brass sheet, but the truth of the matter seems to be that, in representing the sheathing in its original state, a brass color would be more accurate than copper. Modelers have used all sorts of techniques to replicate the greenish and brownish colors that (they think) the sheathing acquired later.
I'm not sure that helps much in practical terms, but it's about the best I can offer. The bottom line is that this is a phase of ship modeling where there's considerable room for personal opinion and taste. Good luck.