Galena, also known as PbS, was widely used in the production of lead glazes from the beginning of the 18th century to the second half of the 20th century. Although the PbO-SiO2 system has been studied for years, the PbS-SiO2 phase diagram, involved in the formation of a glaze with galena, has not yet been investigated. Temperature transformations for the system 75 wt% PbS-25 wt% SiO2 are investigated in a high-temperature resolved X-ray diffraction experiment with synchrotron radiation and compared to those of the equivalent system 70 wt% PbO-30 wt% SiO2. Lanarkite, PbOPbSO4, is the phase predominantly formed as soon as galena decomposes during the heating. The results show that the system melts at a temperature higher than the PbO-SiO2 system, but far lower than those expected for the PbO-PbSO4-PbS system. A historical misfired lead glaze produced with galena is also studied. The presence of galena, lanarkite, and mattheddleite, Pb-10(SiO4)(3.5)(SO4)(2)Cl-2,Cl- is determined and discussed in terms of the composition of the galena mineral used and the firing conditions in light of the high-temperature transformations previously obtained.