pressurized water leads to the formation of many microbubbles which deform the material and generate an expansed silica structure in order to avoid foaming during the fourth heat treatment the xerogels preferably have to be treated at 1 100 u00b0 c for several hours at least one hour by using a ramp of about 1 u00b0 c min between 850 u00b0 c and 1 100 u00b0 c the obtained glasses are then transparent in a preferred embodiment of the invention the porous xerogel dried between about 850 u00b0 c and 1 000 u00b0 c is then dehydrated by means of a fourth heat treatment first under oxygen and then under a o 2 cl 2 mixture and again under o 2 in order to discharge the residual cl 2 it is then densified at a temperature of about 1 300 u00b0 c for several hours at the end of this treatment the monoliths are transparent and free of bubbles in this case the u2014 oh group concentration as revealed by fourier transform infrared spectroscopy is strongly reduced and typically less than 1 ppm a sol is prepared on the basis of tmos methanol h 2 o dmf and nh 4 oh in a ratio of 1 4 2 1 0 005 the obtained solution is placed in closed cylindrical pyrex u2122 receptacles these containers are placed in an oven at 50 u00b0 c for several weeks the dimension of the monoliths produced in this way is limited to about 10 cm beyond this limit the monolith cracks during aging the formed gels with a centimetric size of the order of 1 to 3 cms are subsequently treated at 90 u00b0 c for 24 hours this treatment is followed by a treatment.