Low temperature production of high purity fused silica

A method for producing a solid, homogeneous, transparent fused silica article having an alkali metal content less than about 100 PPM which comprises:

• a. preparing solutions having a pH between about 11-15 and containing about 1-12 moles SiO.sub.2 /liter in solution from silicate solutions selected from the group consisting of potassium silicate, sodium silicate, quaternary ammonium silicate, lithium polysilicate, and colloidal silica;
• b. combining those solutions in one of the indicated proper proportions in weight percent equivalent to:
• A. 50-90% potassium silicate solution and 10-50% colloidal silica solution wherein said potassium silicate solution consists essentially, by weight, of 8.3% K.sub.2 O, 20.8% SiO.sub.2, balance H.sub.2 O and said colloidal silica solution consists essentially, by weight, of 40% SiO.sub.2, balance H.sub.2 O;
• B. 50-90% potassium silicate solution and/or sodium silicate solution and 10-50% quaternary ammonium silicate wherein said potassium silicate is that defined in (A), said sodium silicate consists essentially, by weight, of 6.8% Na.sub.2 O, 25.3% SiO.sub.2, balance h.sub.2 O, and said quaternary ammonium silicate solution consists essentially, by weight, of 9.9% quaternary ammonium ions, 45% SiO.sub.2, balance H.sub.2 O;
• C. 50-90% potassium silicate solution and/or sodium silicate solution and 10-50% lithium polysilicate solution wherein said potassium silicate solution is that defined in (A), said sodium silicate solution is that defined in (B), and said lithium polysilicate solution consists essentially, by weight, of 2.1% Li.sub.2 O, 20% SiO.sub.2, balance H.sub.2 O;
• c. reacting an organic compound therewith selected from the group consisting of formaldehyde, paraformaldehyde, formamide, glyoxal, methyl formate, methyl acetate, ethyl formate, ethyl acetate, and mixtures thereof at a temperature between the freezing point and the boiling point of the solution for a sufficient length of time to reduce the pH of the silicate solution below about 11 and to polymerize the silica into a coherent, porous gelled body;
• d. leaching said gelled body with a weak acid solution having a pH greater than about 4 to reduce the pH to at least below 6 to thereby yield a porosity of at least 50% with meon pore diameters ranging between 400A to 4000A and wherein at least 80% of the pores fall within .+-. 30% of the average pore diameter;
• e. consolidating said porous body into a solid, transparent body by firing at temperatures between about 1350.degree.-1500.degree. C.