Cross-linked polymers are vitally important materials used all the time in modern life. For example, the resins used in composite materials are usually some type of cross-linked polymer. What type of cross-linked polymers could be made by people with access to pre-industrial technology? The only one I can think of would be ebonite, made by heating natural rubber with sulfur.
Keratin is the main protein comprising hair, wool, and fur. It owes many of its structural properties to the crosslinking between polymeric keratin chains.
Disulfide bridges In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking—in much the same way that non-protein sulfur bridges stabilize vulcanized rubber.
One can manipulate these crosslinks with a permanent wave - crosslinks are relaxed with a chemical and heat, then the chemical is neutralized and the crosslinks reform in a new configuration - curling straight hair or uncurling curly hair. The permanent wave is a recent invention, but people have been manipulating the crosslinks in wool this way for millennia. The tremendously useful product: felt.
Felt from wool is considered to be the oldest known textile. Many cultures have legends as to the origins of felt making. Sumerian legend claims that the secret of feltmaking was discovered by Urnamman of Lagash. The story of Saint Clement and Saint Christopher relates that the men packed their sandals with wool to prevent blisters while fleeing from persecution. At the end of their journey, the movement and sweat had turned the wool into felt socks.
Wool felt is dense, durable, warm and useful to humans in many ways. It is not hard to make. You can make clothes, tents and blankets out of it. Your pre-industrial folks will have felt.