of 440 psi the resultant product had a ge brightness of 89 3 tappi procedure t 646 os 75 as compared with 87 8 without oxygen treatment the starting mixture was prepared and processed as in example v except that the oxygen overpressure was not applied and the reactor pressure was 120 psi the resulting product had a brightness of 89 0 it can be seen that the present invention provides improvements in the field of hydrothermal treatment of clays in that the clay itself can be used as the source of the sodium silicate solution aggregating agent or glue clays unsuitable for calcination can be hydrothermally treated by such solution to produce aggregated clay products the invention offers versatility in the mode of effecting the treatment and the treatment can be made unitary with bleaching further aspects of the oxidative bleaching will now be set forth two identical reactions were carried out in a 2 liter parr high pressure reaction vessel with a total solids concentration of 6 in this reaction 63 g of hydrous waterwashed grey kaolin the particles of which are 90 by weight lt 0 5 u03bc and having a 75 g e brightness were mixed with 71 grams of commercial sodium silicate with a specific gravity of 1 41 and a sio 2 na 2 o ratio of 3 25 and 1366 grams of water in reaction.