Redactive analysis: It is important to remember that the current chapter/verse arrangement of the Book of Mormon does not reflect the original printing, nor (of course) the text on the plates. While Chapter 19 verse 1 does begin a new idea with the writing of the plates, separating verse 19:1 from the preceding 18:25 loses some important context. That previous verse is:
"1 Nephi 18:25 And it came to pass that we did find upon the land of promise, as we journeyed in the wilderness, that there were beasts in the forests of every kind, both the cow and the ox, and the ass and the horse, and the goat and the wild goat, and all manner of wild animals, which were for the use of men. And we did find all manner of ore, both of gold, and of silver, and of copper."
There is a direct connection between the finding of the or, specifically gold, silver, and copper, with the next verse indicating the creation of the plates. Nephi is writing years after their arrival, and in his description he finds a way to connect the important sequences. In this case, the finding of the ore directly leads to the information about the creation of the plates, as the two are intimately tied together.
Historical analysis: The indication of the discovery of both gold and copper in verse 18:25 may be even more significant than might appear on the surface, as the combination of those two metals may be the best explanation for a definable physical problem with the plates Joseph Smith received:
"A surprising amount of consistent information can be gleaned from eyewitnesses: Joseph himself gave us the length, width, and thickness of the whole set of plates as 6" x 8" x 6" in his famous Wentworth Letter. On separate occasions, David Whitmer gave larger dimensions of 7" x 8", and 6" x 9", and 8" x 10"; Martin Harris claimed a smaller set at 7" x 8" x 4". Following Joseph's dimensions would amount to .1666 cubic foot (.005 cubic meter). . .
As shown many years ago by metallurgist/blacksmith Reed H. Putnam, hammered plates of pure twenty-four karat gold would probably not weigh more than about 50 percent of the solid dimensions, i.e., 100.4 pounds (45.2 kilograms)" ("The Golden Plates" in REEXPLORING THE BOOK OF MORMON, Deseret Book Company and FARMS, 1992, Page 276 ).
A calculated weight of 100 pounds would not only make the plates extremely difficult to carry, but contradicts the historical estimates of their weight:
"William Smith, a brother of the Prophet who had handled and hefted the plates in a pillowcase, claimed on several occasions that the set of plates weighed about sixty pounds, as did Willard Chase, while Martin Harris said that they weighed forty to fifty pounds. William Smith added that the plates were "a mixture of gold and copper." ("The Golden Plates" in REEXPLORING THE BOOK OF MORMON, Deseret Book Company and FARMS, 1992, Page 276 ).
This information presents an intriguing solution to the dilemma, as suggested by Reed H. Putnam:
" . . . if the plates were made of the more practical Central American tumbaga alloy of eight karat gold with copper, they would weigh around 53.4 pounds (24 kilograms). ("The Golden Plates" in REEXPLORING THE BOOK OF MORMON, Deseret Book Company and FARMS, 1992, Page 276 ).
This explanation fits quite nicely with both the estimated "hefted" weight of the plates, and with the specific mention of gold and copper ores in the same clause in 1 Nephi 18:25 (though admittedly not conclusive because silver is also mentioned, and is not part of the tumbaga alloy. The proposition that tumbaga might be a better description of the plates that pure gold is strengthened by the discovery of a tumbaga sample in Honduras dating to no later than the fifth century AD. (Sorenson, John L. AN ANCIENT AMERICAN SETTING FOR THE BOOK OF MORMON. Deseret Book 1985, p. 283). While not at precisely the correct time period, it is nevertheless the correct alloy in the correct location, and it is not a far reach to suppose that the alloy was known earlier.
The tumbaga argument also covers both the color (which is "golden"when treated with any simple acid) and is a better writing surface than pure gold would have been. ("The Golden Plates" in REEXPLORING THE BOOK OF MORMON, Deseret Book Company and FARMS, 1992, Page 276 ).
The composition of the plates received by Joseph Smith allows us one further bit of speculation. We know that those who handled the plates describe them as a uniform set, yet we know that the plates must have been composed of two different types of records, those abridged by Mormon and the holographic small plates of Nephi. It should also be clear that there was no direct link between the plates created and used by Mormon and those created and used by Nephi - yet they appear to be the same size. Why?
With a world full of standard paper sizes, we might forget that there was no such ancient standard. There was no particular reason why the custom created plates of any of the Book of Mormon authors should be a particular size. As pure speculation, I suggest that the size of the Book of Mormon plates in later years was a tradition based upon the original plates of Nephi (and indicating that the physical dimensions of the large and small plates were probably very close to the same). I would also suggest, that while there is no direct evidence for it, that the size of the brass plates was the model Nephi originally used when he created his plates in the first place.
Scriptural analysis: It is virtually certain that Nephi's selection of a recording medium was directly influence by the brass plates. With the possibility that the brass plates might have actually been of bronze (see Sorenson, AN ANCIENT AMERICAN SETTING FOR THE BOOK OF MORMON 1985, pp. 283-4) which is a composite of copper and tin, the correlation between the color and the alloys may be even greater. From a spiritual standpoint, however, the connection between the sacred writing on the brass plates and the sacred writings on Nephi's golden plates had to be a conscious and vibrant connection for Nephi, given his personal connection to the obtaining of the brass plates.