colloidal silica solids a concentration of 50 90 is preferred 70 90 is even more preferred the colloidal silica used in the composition is generally in the form of an aqueous sol this silica is stabilized with sodium ions has a particle size of 7 25 millimicrons a specific surface area of 125 420 square meters per gram a silica content calculated at sio 2 of 30 50 by weight and a ph of 8 4 9 9 at 25 u00b0 c typical of such a colloidal silica are those sold by e i du pont de nemours and company as 34 ludox hs 40 34 34 ludox hs 34 34 ludox ls 34 34 ludox sm 30 34 34 ludox tm 34 and 34 ludox am 34 mixtures of silicas can be used 34 ludox am 34 is preferred this product is a sodium stabilized colloidal silica whose particles are surface modified with aluminum having a particle size of 13 14 millimicrons a specific surface area of 210 230 square meters per gram a silica content calculated as sio 2 of 30 0 and a ph at 25 u00b0 c of 9 0 the colloidal silica is present in the composition at a concentration of from about 5 through 35 by weight of the total solids instead of colloidal silica a mixture of colloidal silica and a water soluble alkali metal silicate can be used as described in u s pat no 2 825 664 the disclosure of which is hereby incorporated by reference as described in u s pat no 2 825 664 any aqueous solution of an alkali metal silicate or mixtures thereof may be used in this invention such silicate solutions are available commercially in a wide variety of.