"I know Union sack coats had gold button-hole stitching,
Nein, mein lieber Kindertroll... Where are you "researching?"
The Quartermaster Manual called for dark blue linen thread to be used throughout the blouse. (Although cotton was snuck in.) The prefered dye for dark blue was indigo because of its color fastness. But both arsenals and contractors sometimes got away with substituting inferior blue dyes such as mordanted logwood. Logwood degrades or fades ot brown or tan over time and exposure. (The seeing of brown button-hole thread on original garments once upon a time led some reenactors to believe that the thread was originally brown so makers and vendors made repro garments with incorect brown or tan thread.)
and mostly came lined (but sometimes not).'
The Official Records (Series III, Vol. 5, p. 285) reports Federal total purchases of 3,685,755 lined sack coats, 1,809,270 unlined, and 530,144 knit.
In gleichem Schritt und Tritt, Curt Schmidt
Not a real Civil War reenactor, I only portray one on boards and fora.
I do not portray a Civil War soldier, I merely interpret one.