I posted my gleeful discovery on Facebook, where a goodly number of you asked, "what IS osnaburg?" So I shall answer that question here.
Osnaburg is a cheap, coarse fabric originally named for the city of Osnabrück, Germany. In the 18th century, it was made from flax (linen) fibers. In the 19th century it was made from cotton.
18th century osnaburgs were used for working class and slave clothing, as the textile was cheap and plentiful. It also makes excellent linings and inerlinings, particularly if glazed or stiffened with sizing or glue.
In the 19th century, the definition of osnaburg changed to a cotton textile, which it remains today. These osnaburgs are made from waste cotton mixed with a low-grade cotton. They were still used for lower and working-class clothing, as well as items like feed sacks, mattress covers and sheeting, shoe linings, backings of various items, etc.
How do you use osnaburg in historical costuming?
For 18th century lower, working, and slave class representations where historical accuracy is key, use coarse, unbleached linens - W.M. Booth Draper has a wide variety here for around $13/yard. Burnley & Trowbridge also carry osnaburg for $11/yard.
For 19th century lower, working, and slave class representations, use the cotton stuff (thought linen is still just fine, of course). I got mine at Walmart, but you can find it at any fabric store that sells quilting cotton, or online. I paid less than $3/yard; JoAnns has it for around $4/yard.
For interlining and stiffening items of any period, that choice is yours. I spend my money on the fabric that shows, so a cotton osnaburg interlining used on an 18th century item doesn't bother me at all, but to each her own.
Have fun!