of the potassium hydrogencarbonate predominance area between 6 37 and 10 33 and preferably close to 8 35 this carbonic acid is introduced in the mold with the help of an injection pump and comes in contact with the powders reacts with the metallic carbonates to form the associated metallic hydrogencarbonate according to the reactions described previously r 1 r 2 and r 3 then the compression and removal from the mold take place as described previously r 4 et r 5 the process according to the present invention comprises several possible treatments after the formation and the extraction of the block of material the potassium silicate treatment is the most efficient at the time of the step of removing the material from the mold the block of material contains a significant quantity of very soluble calcium hydrogencarbonate in solution this hydrogencarbonate reacts very strongly with the potassium silicate to form the calcium silicate a stable insoluble molecule with a high hardening power this treatment is carried out by cyclic sprinkling of potassium silicate by flowing or by immersion potassium silicate is available in the trade in a water dissolved form at about 30 in mass it can be used under this form or dissolved in more water sodium silicate gives an analogous reaction but it forms the same way the sodium carbonate which crystallises releasing large white needles known by the persons skilled in the art as being nitre in the past in france the natural stones carved in the quarries were soaked in milk of lime this.