My guess would be...
that the bags were treated with the same substance that the old-fashion blue-gray flannel dust cloths were treated with so they would grab and hold dust. They were sometimes referred to as "oiled" dusters and had a chemical smell to them. It had to be a very, very thin liquid, almost like water. It would be of the same era. But I am not certain if the painting/silk screening was done first or after. The oil, however thin would not evaporate like water, but instead would cause the fibers to swell and remain tightly together, while allowing air to pass through. Would the screening process adhere with the oil in the fiber? Anything changing the character of the fabric would have to be done first, I would think. The instructions always warned against washing the outer cloth bags. I wonder if the lettering could be done with stencils and a Tjanting needle. The needle has a small reservoir of hot liquid wax, and is used to outline or draw a design. It can be removed with a hot iron and paper towel. If the wax was used on the back to keep the paint from going all the way through the fabric, and then outline the lettering on front, then treat the fabric, then paint the lettering, then remove the wax. The lettering would have clean fabric to grip, but not soak through. Due to wicking, the small outline of wax on the front would fill in with the dust proofing chemical after the wax was removed. I think something like lemon oil (which does not really contain lemons) or lamp oil (deodorized kerosene, which can be purchased or made. Purchased is made commercially with sulfuric acid, but it can be made at home by dissolving 1 part parafin to two parts kerosene, making it nearly odorless. I am pretty certain this is how the oiled dust cloths were made.
