Scrim is unbleached calico, or at least true scrim is. These days all sorts of cloths get sold under the name scrim.
You need to boil scrim to prepare it but using it initially as a sill cloth and putting it through the wash with the other cloths a few times works as well.
The thing is, and this is important, in the old days scrim was NOT a drying cloth. It was a POLISHING cloth. You dried the window with a shammy (still the best IMO) and then polished off the window with the scrim to remove any drying marks. The result was absolutely perfect, much better than wiping the windows with a damp microfibre and leaving it.
I still keep a piece of scrim in the van and regularly polish off windows after they have dried.
Thats why window cleaners were called shiners or diamond polishers