A hardening composition comprising a particulate glass, said glass comprising 10-35% by weight of silica, 10-35% by weight of alumina, 3-30% by weight of zinc oxide, 4-30% by weight of P 2 O 5 and 3-25% by weight of fluoride, and an acidic copolymer reactive with the particulate glass under aqueous conditions. An aqueous polymer solution comprising 10 to 65% by weight of the acidic copolymer of the hardening composition with a mean molecular weight of from 50,000 to 200,000, obtainable by a process comprising the copolymerization of a mixture containing acrylic acid and an acrylic acid ester (I) and / or a compound of the formula (II) and, optionally, a compound of the formula (III): wherein k is an integer of from 1 to 5； h is an integer from 0 to (5 - k)； n is an integer of from 1 to 3； R1 is hydrogen or a C1-6 alkyl group, and； R2 and R3 independently represent hydrogen, a C1-6 alkyl group or a moiety of the following formula (IV): wherein m independently is an integer of from 0 to 3, and L is -CH 2 - or -CH=CH-； provided that at least one of R1, R2 or R3 is not hydrogen.
