An article comprising a substrate which carries a material of formula (I) M1aM2bWcOd(P(O)nRm)e   (I) wherein each of M1 and M2 is independently ammonium or a metal cation; a is 0.01 to 0.5; b is 0 to 0.5; c is 1; d is 2.5 to 3; e is 0.01 to 0.75; n is 1, 2 or 3; m is 1, 2 or 3; and R is an optionally substituted hydrocarbyl group.