In a page entitled Problems with the Brass Plates of Laban, Richard Packham writes, in part:
Plates would "never perish" or be "dimmed any more by time." (1 Ne 5:18-19)
. . .
Conclusion
For all the importance which was attached to the brass plates, and in light of the prophecies about how they will never be "dimmed by time" but rather "go forth," containing all the prophets and prophecies not included in our present Bible, it seems that the brass plates have utterly failed in their modern mission. It does not even appear that these scriptures were known by any of Lehi's descendants (supposedly the Native Americans) after the Nephites disappeared about 420 AD. They are gone. Not even the modern prophets of the Mormon church can tell you where they are, or give details about what they contain. Apparently they were not so important, after all.
Packham is guilty of eisegesis of the relevant texts. Here is 1 Nephi 5:17-19:
And now when my father saw all these things, he was filled with the Spirit, and began to prophesy concerning his seed—That these plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed. Wherefore, he said that these plates o brass should never perish; neither should they be dimmed ay more by time. And he prophesied many things concerning his seed.
Commenting on this pericope, LDS scholar, Brant Gardner, noted:
The effect of reading the scriptures had a profound effect on Lehi, prompting him to prophesy. Interestingly, Nephi limits his record of that prophecy to the plates themselves. Surely what Lehi had to say about his seed (v. 19) would be more interesting to Nephi than a general prophecy about the effect of the brass plates. Nevertheless, all his records is that the plates would “go forth unto all nations, kindreds, tongues, and people who were of his seed.” This prophecy is apparently directed at the Nephites, rather than at modern recipients of the Book of Mormon, although that new book would surely continue the prophecy of delivering the words (or at least some of those absent from the biblical canon) to all nations.
The reference that the plates “should [not] be dimmed any more by time” may suggest that Laban and previous keepers had not been diligent. The specific reference to being “dimmed” by time may mean that the words on the plates would be preserved and be brightly present in his descendants’ mind, in contrast to Lehi’s lack of specific knowledge of the plates’ contents. Jeffrey R. Chadwick suggests that this statement may be directly related to the metallurgical expertise Nephi shared with his father:
Lehi predicts that the “plates of brass should never perish; neither should they be dimmed any more by time”—a surprisingly accurate statement that could probably be made only by a person experienced with the properties of copper-based alloys like bronze and brass (bronze is a combination of copper and tin, and brass a combination of copper and zinc). Whereas iron, the hardest metal of Lehi’s day (it could even be hardened into steel by Lehi’s time), will oxidize and rust away over time if neglected, copper alloys such as bronze and brass will not. Even the most damp conditions will not cause plates of copper to “perish.” And while it is possible over time for bronze or brass items to be “dimmed . . . by time” with a greenish or greyish patina, even minimal maintenance on a regular basis would prevent this. (Jeffrey R. Chadwick, “Lehi’s House at Jerusalem and the Land of His Inheritance,” in Glimpses of Lehi’s Jerusalem, edited by John W. Welch, David Rolph Seely, and Jo Ann H. Seely [Provo, Utah: Foundation for Ancient Research and Mormon Studies, 2004], 114).
A reference to this brilliance may have become metaphorical. When Alma2 entrusts the plates to his son Helaman, he notes:
Behold, it has been prophesied by our fathers, that they should be kept and handed down from one generation to another, and be kept and preserved by the hand of the Lord until they should go forth unto every nation, kindred, tongue, and people, that they shall know of the mysteries contained thereon. And now behold, if they are kept they must retain their brightness, yea, and they will retain their brightness; yea, and also shall all the plates which do contain that which is holy writ. (Alma 37:4-5)
Alma provides the same essential information about the brass plates as Nephi: they must be preserved; they are to go to all nations, kindreds, tongues, and peoples; and they “must retain their brightness.” Thus, this “brightness” again suggests that they do not tarnish (are not dimmed by age). The close resemblance of this wording to Lehi’s prophecy confirms that Alma knew the prophecy as a keeper of the plates. Of course, on one level the plates would remain untarnished because their keepers cleaned them regularly, but whether literal or metaphoric, such “brightness” was assured because of the plates’ sacred content. Whether achieved by physical or spiritual means, the plates’ brightness symbolized the eternal value of their contents. (Brant A. Gardner, Second Witness: Analytical and Contextual Commentary on the Book of Mormon, 6 vols. [Salt Lake City: Greg Kofford Books, 2007], 1:135-36)