Silk filament was extremely durable and could even be used as fishing line before the advent of synthetic fibers. It was obtained by soaking silkworms in their fifth instar (when they just began to spin their cocoons) in water and vinegar for about 36 hours. After this immersion, the silkworms were split open and the silk glands inside were extracted. Upon contact with air, the silk solidified, yielding a strong thread 30–50 cm long. It appears it was also used to sew wounds.