Crepoline

This is strengthened by a layer of crepoline, a transparent cloth, on both sides of the piece of paper.

Source: wiktionary

In garments a wool or half-wool fabric is frequently trimmed with silk, or a wool and silk fabric (gloria) has a cotton lining or cotton stitching. Such wool and silk mixtures find very wide use in dress fabrics, usually with a silk warp and a wool weft (poplin, bengaline, santoy, crepolines, etc.).

Source: wiktionary

The crepoline sandwich was sewn onto a silk mounting, dyed to match the original silk, so the tattered edge is hardly noticeable.

Source: wiktionary