"Butternut first appeared extensively at South Mountain (14 September 1862), when it was noticed that hundreds of the Confederate dead were wearing the new, coarse uniform. Production was simple and ideally suited to the home economy then being forced by necessity on the South. Wool and cotton were carded together and spun into yarn. This was then dyed with walnut or butternut oil and woven into cloth on homemade loms. The cloth was then dyed again until it became a reddish brown, after which it was cut and sewn into uniforms."
Combat Uniforms of the Civil War by Mark Lloyd, Mallard Press, 1990.