Of What Material Were the Plates?
Were the Book of Mormon plates pure gold, or were they made from an alloy that looked like gold? The most serious investigation of this question was done 45 years ago by Read H. Putnam of Evanston, Wyoming, a blacksmith and metallurgist.  Working first from the general dimensions of the set of plates as reported by eyewitnesses, he calculated that a block of pure gold of that size would have weighed a little over 200 pounds. A number of witnesses, however, put the weight of the set at about 60 pounds. The discrepancy can be partly accounted for by the fact that the leaves must have been handcrafted, presumably by hammering, and irregularities in flatness would have left air space between the plates. This led Putnam to surmise that the entire set of plates would have weighed probably less than 50 percent of the weight of a solid block of the metal.
Because the weight of a metal depends on its purity, we must also consider whether the plates were of pure gold. The Nephites were aware of purity distinctions and alloys. We know, for example, that the "brass" plates were of an alloy (quite surely bronze, a copper-tin mixture)  and that the plates of Ether were specifically distinguished as being of "pure" gold (Mosiah 8:9). Furthermore, Nephi taught his associates "to work in all manner of" metals and "precious ores" (2 Nephi 5:15). Yet nowhere does the text say that the Nephites' plates were of pure gold.
Joseph Smith's brother William specifically said that the material of the plates was "a mixture of gold and copper."  (Someone must have provided an objective basis for that statement, for the natural assumption would have been that the plates were pure gold.) The cautious statements by other witnesses, including Joseph Smith himself, who spoke of the plates as having "the appearance of gold," suggest that the metal may have been an alloy. 
Putnam observed that the only two colored metals from antiquity were gold and copper. An alloy of those two elements was called "tumbaga" by the Spaniards and was in common use in ancient tropical America for manufacturing precious objects. Putnam put forward the reasonable hypothesis that metal plates made in Mormon's day were of that material (the earliest Mesoamerican archaeological specimen of tumbaga—made from a hammered metal sheet—dates to the same century, the fifth century AD, when Moroni hid up the plates he had in his possession).  If Mormon's Book of Mormon plates were made of tumbaga, their weight would have been much less than had they been made of pure gold. Putnam made that point in mathematical detail and concluded that the total weight of the plates in Joseph Smith's charge would have been near the 60-pound figure reported by several witnesses.
It is of interest that tumbaga was commonly gilded by applying citric acid to the surface. The resulting chemical reaction eliminated copper atoms from the outer .0006 inch of the surface, leaving a microscopic layer of 23-carat gold that made the object look like it was wholly gold.  Plates having "the appearance of gold," then, are exactly what we would expect if they were made of tumbaga. 
 "Were the Plates of Mormon of Tumbaga?" Improvement Era, September 1966, 788–89, 828–31; also in Ross T. Christensen, ed., Papers of the Fifteenth Annual Symposium on the Archaeology of the Scriptures (Provo, Utah: Extension Publications, BYU Division of Continuing Education, 1964), 101–9. Putnam's findings are summarized in "The 'Golden' Plates," in Reexploring the Book of Mormon, ed. John W. Welch (Salt Lake City: Deseret Book and FARMS, 1992), 275–77.
 See John L. Sorenson, An Ancient American Setting for the Book of Mormon (Salt Lake City: Deseret Book and FARMS, 1985), 283–84; and his "Metals and Metallurgy Relating to the Book of Mormon Text" (FARMS, 1992).
 William Smith interview, The Saints' Herald, 4 October 1884, 644.
 "The Testimony of Eight Witnesses," Book of Mormon; and Joseph Smith Jr., "Church History," Times and Seasons, 1 March 1842.
 David M. Pendergast, "Tumbaga Object from the Early Classic Period, Found at Altun Ha, British Honduras (Belize)," Science 168, 3 April 1970, 117.
 Putnam, "Were the Plates of Mormon of Tumbaga?"; and Heather Lechtman, "Pre-Columbian Surface Metallurgy," Scientific American 250 (June 1984): 56–63.
 It is also possible that other metallurgical treatments such as a hammered copper-silver-gold alloy could have furnished a product similar in appearance (see Lechtman, "Pre-Columbian Surface Metallurgy"; and Dorothy Hosler and Guy Stresser-Pean, "The Huastec Region: A Second Locus for the Production of Bronze Alloys in Ancient Mesoamerica," Science 257, 28 August 1992, 1215). Moreover, Nephi's original plates might have been of different composition than Mormon's plates.