McConkie and Millet explain that among Lehi's joyous prophecies was the full assurance that "these plates of brass should never perish; neither should they be dimmed any more by time" (1 Nephi 5:19). From a very temporal perspective, perhaps Lehi was indicating here a neglect by Laban of these brass treasures, a neglect which would have allowed the plates to become tarnished or corroded. [Joseph Fielding McConkie and Robert L. Millet, Doctrinal Commentary on the Book of Mormon, Vol. 1, p. 50]
“Neither Should They Be Dimmed Any More by Time”
According to Brant Gardner, Lehi's prophecy that the plates would not "be dimmed any more by time" (1 Nephi 5:19) has two possible references. One is that the words of the plates would be preserved and be present in the minds of his descendants. The second is a near magical contention that the effects of time would not diminish their sheen. To be read in this latter sense, we would have to accept some tarnishing of the plates already ("neither should they be dimmed any more by time"). Perhaps they had required cleaning from during the years of storage, and from then on were miraculously kept free of age. As Brant Gardner knows of no further reference to their miraculous state of preservation, he prefers the former reading. [Brant Gardner, "Brant Gardner's Page, "[http://www.highfiber.com/] ~nahualli/LDStopics/1 Nephi/1 Nephi5.htm, pp. 12-13]
Note* Accepting the first supposition also requires that through the years, the message contained on the brass plates had been "dimmed," either from neglectful non-use, or from intentional misrepresentation or intentional silence. It is worth noting that for whatever the cause, this "dimming" of the message on the brass plates was significant enough to cost Laban, their caretaker, his life. One wonders if the "dimmed" message was Christ. [Alan C. Miner, Personal Notes]
Through the Wilderness to the Promised Land
(1 Nephi )