According to Brant Gardner, Alma 37:5 references an apparent tradition among the Nephites that the brass plates would " . . . retain their brightness; yea, and they will retain their brightness . . ." The antecedent that we have to this tradition is found in 1 Nephi 5:19: "Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time. And he prophesied many things concerning his seed."
After referring to this tradition, Alma goes on to state the following: "Now ye may suppose that this is foolishness in me; but behold I say unto you, that by small and simple things are great things brought to pass; and small means in many instances doth confound the wise." The conjunction of these two passages suggests that while there is a tradition that the plates of brass will retain their brightness, it is entirely possible that this brightness requires some human assistance, since Alma says that they are bright "if they are kept." This suggests that some aspect of keeping might include an action to maintain the brightness.
Now this "little thing" of the brightness of the plates is but a sign of the more important function of the plates to "go forth unto all nations, kindreds, tongues, and people who were of Nephi's seed" (1 Nephi 5:18). But what is the foolishness? It is hard to know. What we do know is that Alma linked the brightness of the entire plate tradition to the promise of the value of what was on those plates. For Alma, they were indelibly separated, and perhaps it is this that might have seemed "foolishness," that the brightness or physical appearance might have any impact on the value of the contents of the plates. [Brant Gardner, Book of Mormon Commentary, [http://www.highfiber.com/~nahualli/LDStopics/Alma/Alma37.htm], pp. 3-4]