Examples of anionic protective colloids are homo- and copolymers of monoethylenically unsaturated carboxylic acids and salts thereof, in particular sodium and ammonium salts thereof, e.g. homo- and copolymers of acrylic acid, of methacrylic acid or of maleic acid, optionally with neutral comonomers chosen from the abovementioned monomers A and the monomers D2, e.g. from styrene, esters of (meth)ac