Process for preparing aerogels

A process for preparing aerogels, which comprises

• a) bringing an aqueous water glass solution to a pH of .ltoreq.3 using an acid ion exchanger or a mineral acid,
• b) polycondensing the silicic acid thus produced, by addition of a base, to give an SiO.sub.2 gel, and, if a mineral acid was used in step a), washing the gel with water until it is free of electrolytes,
• c) if required, washing the gel obtained in step b) with a suitable alcohol or organic solvent until the water content of the gel is <20% by weight,
• d) aging the gel obtained in step b) or c), using a solution of a condensable alkyl and/or aryl ortho-silicate of the formula R.sup.1.sub.4-n Si(OR.sup.2).sub.n, where n is 3 or 4 and R.sup.1 and R.sup.2, independently of one another, are C.sub.1 -C.sub.6 -alkyl, cyclohexyl or phenyl, or using an aqueous silicic acid solution, in order to strengthen the gel structure,
• e) subcritically drying the aged gel obtained in step d).