The invention is a heavy sodium carbonate -dense soda- in the form of briquettes, having great density, and includes also the commercial forms of heavy sodium carbonate in the form of powder, grains and briquettes, which products are obtained by grinding - or not as required - the briquettes. The invention also comprises a novel process which produces these briquettes by briquetting of the dry sodium bicarbonate from the SOLVAY or TRONA soda ash fabrication process, which after filtering or centrifuging and drying is briquetted at a pressure between 500 to 1000 kg/cm.sup.2 ; the briquettes with a density of 1.8 kg/dm.sup.3, are then introduced into a heated static space at 700.degree. to 750.degree. C., and maintained there until the bicarbonate briquettes turn into dense sodium carbonate briquettes, with a density of 2.2 - 2.4 kg/dm.sup.3.