The present invention is an additive for producing a positive active material for lead-acid storage batteries on the basis of finely divided tetrabasic lead sulfate. The additive contains a tetrabasic lead sulfate of an average particle size less than about 3 .mu.m as well as finely divided hydrophobic silicic acid for preventing agglomeration of the particles of the tetrabasic lead sulfate. During maturation, this additive ensures the formation of the structure of a tetrabasic lead sulfate crystal with a very narrow bandwidth of crystal sizes and a very homogeneous distribution. In the subsequent electrochemical formation to lead oxide, this leads to particularly efficient lead-acid storage batteries. Furthermore, the invention relates to a method of making the additive according to the invention as well as its advantageous use in the positive material for the maturation and drying of plates in the production of lead-acid storage batteries.