A hardening composition comprising a particulate glass, said glass comprising10-35% by weight of silica, 10-35%by weight of alumina, 3-30% by weight of zinc oxide, 4-30% by weight of P205and 3-25% by weight of fluoride, and an acidiccopolymer reactive with the particulate glass under aqueous conditions. Anaqueous polymer solution comprising 10 to 65% byweight of the acidic copolymer of the hardening composition with a meanmolecular weight of from 50,000 to 200,000, obtainableby a process comprising the copolymerization of a mixture containing acrylicacid and an acrylic acid ester (I) and / or acompound of the formula (II) and, optionally, a compound of the formula (III):Formulas (1) (II) (III) wherein k is an integer of from 1to 5； h is an integer from 0 to (5 - k)； n is an integer of from 1 to 3； R1 ishydrogen or a C1-6 alkyl group, and； R2 and R3independently represent hydrogen, a C1-6 alkyl group or a moiety of thefollowing formula (IV): Formula (IV) wherein mindependently is an integer of from 0 to 3, and L is -CH2- or -CH=CH-；provided that at least one of R1, R2 or R3 is not hydrogen.
