Porcelains represent the foundation of the ceramic discipline. The variable-phase assembly within porcelains makes these materials very complex ceramics. Fine porcelains from Buen Retiro were produced between 1760 and 1808 by Spanish court ceramists. The factory and its records were totally destroyed in 1812 during the Peninsular War. Recently, some pieces of porcelain and remains of whiteware belonging to the ancient factory were discovered during an excavation. In the present work, some of the secret formulas that enabled the Spanish ceramists to produce porcelains have been investigated by quantitative full-phase analysis (including amorphous content) using the Rietveld method. Three porcelains belonging to the Sureda period (1803-1808) and another from an earlier time of the factory (1760-1783) have been analyzed. The phase results are discussed and conclusions are derived by using appropriate phase equilibrium diagrams. It has also been found that the Rietveld quantitative amorphous content analysis is effective in determining the glassy content in porcelains.