Method of preparing leather from unhaired hides

The method of preparing leather from unhaired hides which comprises contacting said hides with a tanning agent comprising as the active tanning agent an .omega.,.omega.'-dialdehyde of the formula

• O.dbd.CH--(CH).sub.r --HC.dbd.O,
• wherein r is an integer from 2 to 8, and a polymer containing hydroxyl groups which is made up of
• a') 0 to 10 percent by weight of at least one carboxylic monomer of the formula
• R.sub.2 HC.dbd.C(R.sub.1)--COOH,
• wherein R.sub.1 is hydrogen, methyl, or a --CH.sub.2 COOH group and R.sub.2 is hydrogen, methyl, phenyl, or a --COOH group but where there are not more than two carboxyl groups in the molecule, or of at least one anhydride monomer of the formula ##STR12## wherein A is the group ##STR13## or the group ##STR14## and b') 0 to 10 percent by weight of at least one hydroxylic monomer of the formula ##STR15## wherein R.sub.3 is hydrogen or methyl and R.sub.4 is an alkyl or cycloalkyl group having 1 to 12 carbon atoms substituted with at least one hydroxyl group, and
• c') 90 to 100 percent by weight of at least one allyloxy ester monomer of the formula ##STR16## wherein R.sub.5 is an alkyl group having 1 or 2 carbon atoms, and d') 0 to 10 percent by weight of at least one (meth)acrylic acid ester monomer of the formula ##STR17## wherein R'.sub.3 is hydrogen or methyl and R.sub.6 is an alkyl or cycloalkyl group having 1 to 12 carbon atoms.