The main objective of this research work was to study the influence of glassy additives on the formation of crystalline phases in sintered red ceramic bodies used for the fabrication of ceramic floor tiles, whose composition is a mixture of quartz (SiO2), kaolinite [Al2Si2O5(OH)4], albite (NaAlSi3O8), muscovite [KAl2(Si3Al)O10(OH)2] and calcite (CaCO3). The additives used were: fly ash, soda-lime glass, borosilicate glass, glass frit, Na2P3O10 and cryolite (Na3AlF6). These were added in variable proportions to the nominal composition of the red ceramic bodies, either as single or as combined additions, aiming to accelerate the densification of the materials during their sintering process. For all the additive types used, the crystalline phases formed in the samples sintered using a peak temperature ranging from 950 to 1030 °C were: quartz (SiO2), anorthite (CaAl2Si2O8), and hematite (Fe2O3). It is known that the presence of anorthite is particularly beneficial for the mechanical properties, chemical stability and densification of the sintered red ceramic bodies. However, some of the considered additions tended to be detrimental for the formation of this phase in the studied materials, especially at the lowest peak sintering temperature employed. This was verified by means of XRD and SEM analysis.