It is documented right on the 3rd page of The Book of Mormon. Right after the Title Page and the Introduction. You can't miss it.
Before we move on to the issue of witness credibility, let us take a look at the testimony of the eight witnesses:
“Be it known unto all nations, kindreds, tongues, and people, unto whom this work shall come: That Joseph Smith, Jun., the translator of this work, has shown unto us the plates of which hath been spoken, which have the appearance of gold; and as many of the leaves as the said Smith has translated we did handle with our hands; and we also saw the engravings thereon, all of which has the appearance of ancient work, and of curious workmanship. And this we bear record with words of soberness, that the said Smith has shown unto us, for we have seen and hefted, and know of a surety that the said Smith has got the plates of which we have spoken. And we give our names unto the world, to witness unto the world that which we have seen. And we lie not, God bearing witness of it.”
CHRISTIAN WHITMER
HIRAM PAGE
JACOB WHITMER
JOSEPH SMITH, SEN
PETER WHITMER, JUN
HYRUM SMITH
JOHN WHITMER
SAMUEL H. SMITH
Do you notice anything familiar about the surnames of the witnesses? Four of them are Whitmers – directly related to David Whitmer, one of the three witnesses. Actually, a fifth is related to the Whitmer family, as Hiram Page is the husband of David Whitmer’s sister. Of the remaining three, one is Joseph Smith’s father, and the other two are Joseph’s brothers. This isn’t exactly a variety of witnesses, and is actually quite nepotistic!
First, whether the witnesses saw actual plates, or only had visions of the plates, it is apparent that the plates where not necessary in the translation of the Book of Mormon. In Whitmer’s address, he describes the translation process:
“I will now give you a description of the manner in which the Book of Mormon was translated. Joseph Smith would put the seer stone into a hat, and put his face in the hat, drawing it closely around his face to exclude the light; and in the darkness the spiritual light would shine. A piece of something resembling parchment would appear, and on that appeared the writing. One character at a time would appear, and under it was the interpretation in English. Brother Joseph would read off the English to Oliver Cowdery, who was his principal scribe, and when it was written down and repeated to Brother Joseph to see if it was correct, then it would disappear, and another character with the interpretation would appear. Thus the Book of Mormon was translated by the gift and power of God, and not by any power of man.”
So by the accounts of the witnesses, the plates remained covered during the translation. The Book of Mormon came about because Joseph Smith stuck a rock in his hat, followed it with his face, and spoke the translation for Cowdery to write down. How does the existence of plates fit into this scenario? It does not.