the present invention can be prepared by known methods which are described in u s pat nos 2 558 732 2 404 714 3 135 812 and 3 097 053 incorporated herein by reference to disclose such methods the water soluble basic aluminum salt has the formula al 2 lh 1 a m b n c p cl q and in this formula a b and c represent different acid residues which include inorganic acid residues such as nitric acid nitrous acid sulfuric acid phosphoric acid and phosphorous acid if 1 1 m n p q is smaller than 0 4 the salt is highly soluble in water but it cannot provide the preoxidized fiber with the desired crimpability if that index is greater than 0 9 no stable aqueous solution that can be uniformly deposited on the acrylonitrile fiber is obtained specific examples of the water soluble basic aluminum salt that can be used in the present invention include 2 al 2 oh 5 so 2 0 25 cl 0 5 3 al 2 oh 2 8 so 4 0 8 cl 1 6 4 al 2 oh 2 7 so 4 0 26 cl 2 78 5 al 2 oh 2 7 so 4 0 26 po 4 0 3 cl.