Fence Sitter wrote:Given the size dimensions (which offhand I do not know) would it be reasonable to assume that plates made of gold would weigh 60 pounds?
Although the plates are often referred to as 'gold plates' they didn't necessary have to be made of pure gold. The witnesses described them as having the 'appearance of gold'. Although Joseph originally said "there was a book deposited, written upon gold plates" (History of the Church, vol. 1, by Joseph Smith, pp. 11-12), Joseph later wrote to John Wentworth that the plates had "the appearance of gold": "These records were engraven on plates which had the appearance of gold.(History of the Church, vol. 4, p. 537).
Many things could account for this such as an alloy of gold and another metal or even polished copper plates. A gold alloy or polished copper or even painted tin would have the appearance of gold yet weigh much less.
Using calculations to arrive at the plates is unnecessary as many of the witnesses have given their estimates as to the weight of the plates by lifting the container that held the plates which several people claim to have done.
Here are all the statements I could find that indicate the weight of the plates from those that claimed to have lifted the container that reportedly contained the plates:
William Smith, a brother of the Prophet
•William Smith, a brother of the Prophet who had handled and hefted the plates in a pillow-case, claimed on several occasions that the set of plates weighed about sixty pounds. (Robert F. Smith, The 'Golden' Plates, pp 276 Reexploring the Book of Mormon, ed., John W. Welch ( Salt Lake City: Deseret Book and FARMS, 1992).
http://www.mormonfortress.com/gweight.html•Joseph's brother William: "I did not see them uncovered, but I handled them and hefted them while wrapped in a tow frock and judged them to have weighed about sixty pounds. ... Father and my brother Samuel saw them as I did while in the frock. So did Hyrum and others of the family." (Zion's Ensign, p. 6, January 13, 1894).
•"I was permitted to lift them... . They weighed about sixty pounds according to the best of my judgement."[8] -William Smith William Smith, William Smith on Mormonism (Lamoni, Iowa: Herald Steam, 1883), 12.
•"I ... judged them to have weighed about sixty pounds."[9]-William Smith William Smith interview with E. C. Briggs. Originally written by J. W. Peterson for Zions Ensign ( Independence, Mo.); reprinted in Deseret Evening News, 20 January 1894, 11.
•"They were much heavier than a stone, and very much heavier than wood... . As near as I could tell, about sixty pounds."[10] -William Smith William Smith interview, The Saints' Herald, 4 October 1884, 644.
Martin Harris
•"weighing altogether from forty to sixty lbs."[7] -Martin Harris Martin Harris interview, Iowa State Register, August 1870, as quoted in Milton V. Backman Jr., Eyewitness Accounts of the Restoration (Salt Lake City: Deseret Book, 1986), 226.
•"I hefted the plates, and I knew from the heft that they were lead or gold."[11] -Martin Harris "Interview with Martin Harris," Tiffany's Monthly, May 1859, 169.
•"My daughter said, they were about as much as she could lift. They were now in the glass-box, and my wife said they were very heavy. They both lifted them."[12] -Martin Harris ( Ibid., 168.)
Joseph's sister Catherine
•Joseph's sister Catherine, while she was dusting in the room where he had been translating, "hefted those plates [which were covered with a cloth] and found them very heavy."[14] -H. S. Salisbury, paraphrasing Catherine Smith Salisbury I. B. Bell interview with H. S. Salisbury (grandson of Catherine Smith Salisbury), Historical Department Archives, the Church of Jesus Christ of Latter-day Saints.
Willard Chase
Willard Chase reportedly also said the plates weighed 50 or 60 pounds but I haven't yet been able to locate the exact quote. Robert F. Smith, "The 'Golden' Plates," Reexploring the Book of Mormon, ed., John W. Welch (Salt Lake City: Deseret Book and FARMS, 1992).
CONCLUSION:
So using the statements of the witnesses, it seems logical that the plates weighed about 50 pounds give or take 10 pounds.
Note: I do however, wonder why the angel would give Joseph specific instruction that he could not use the plates for personal gain if they were not made of gold.