A process for obtaining a plant extract containing condensed tannins, with astringent characteristics, and modified by copolymerization, produces a material suitable for the tanning and retanning of skins. The process includes adding water to a reactor, adding a copolymerization agent, adding thioglycolic acid and allowing to react, adjusting pH, mixing a powdered extract of a plant or tannin to a desired concentration, raising the temperature, adding sodium bisulfite, and keeping the reaction heated and at pressure for about 4 hours.