I can understand the use of cotton quite well. And indeed there were some wool bunting US flags early in the war as well as the larger post, storm and garrison flags. Silk unit flags were soon torn as it was from normal use and combat. But I would have hated to have been the color bearer carrying a 6 by 6 1/2 foot wool bunting battle flag - unless I could bench press 400 pounds anyway! This is why they were always made of silk.
You will love this one. While researching in the National Archives in June I was going through stuff in RG92 - Union quartermasters files (which are beyond huge) and in one box of correspondence I found a series of letters from a contractor who was proposing flags made of hammered sheet metal!!!! I am not kidding!
He proposed to make these flags of a specific size and could paint the red, white & blue plus the stars on each side. He even enclosed drawings of what he could do for star patterns. I copied the whole file as it was really fascinating - if completely unworkable. His main argument is that unlike silk, his metal flags would not wear out.
Ain't American ingenuity amazing???