Homopolymers or copolymers of acrylic or methacrylic acid or their salts, and in particular the products sold under the names VERSICOL E or K by ALLIED COLLOID, ULTRAHOLD 8 by CIBA GEIGY, DARVAN No. 7 by Van der BILT, VINAPOL 1640 by SHEBY and CARBOSET 514 by GOODRICH; the acrylic acid/acrylamide copolymers sold in the form of their sodium salts under the names RETEN 421, 423 or 425 or HERCOFLOC 1