suprex clay for comparison besides the water insoluble si69 there was used in these same amounts water soluble al89 the products obtained thereby were again worked into natural and synthetic rubber for comparison si69 in the corresponding concentrations was directly worked into the rubber mixtures table 2 shows the results obtained it can be seen that with the water insoluble si69 there was a modification of the filler which exceeded the separate addition into the rubber mixture in example 3 there is shown the mode of operation with additional fillers three different fillers in aqueous suspension were modified with 0 5 parts by weight of si69 based on 100 parts by weight of filler there was added at room temperature with stirring an emulsion of 80 grams of si69 in 200 ml of a solution of marlowet gfw having a concentration of 40 g l to a suspension of 16 kg of hexafil in 20 kg of water after half an hour the suspension was poured on a metal sheet and dried in a drier the working up was carried out as in example 1 the same process was carried out with hewp in which case a part of the amount added was changed as follows 15 kg of hewp 86 kg of water 75.