Compositional data determined for excavated porcelain sherds suggest that both silicious and phosphatic wares were produced at the Nantgarw and Swansea manufactories. The silicious Swansea sherds are virtually phosphate-free, and have lower MgO (<0·5%) and higher Al 2 O 3 (21–25%) contents than magnesian (“soapstone”) porcelains that characterize part of the production history of this manufactory. Together with the silicious Nantgarw samples, they appear to represent a new class of Welsh porcelains. Although visually similar, the silicious porcelains can easily be distinguished from phosphatic samples by their appearance under a black (UV) light. The silicious paste might represent a technological improvement on the phosphatic recipes used at both manufactories. This hypothesis can be tested using phase diagrams.Ternary phase diagrams suggest that the phosphatic Nantgarw wares have more refractory compositions than expected from the reputation that this manufactory has for having suffered extensive (perhaps 90%) kiln losses. Although adulteration by silica-polymorph (and/or mullite) crystallites precludes the reliable determination of melt compositions, the volume and compositional trajectories of the “glassy” phase in these wares suggest that the phosphatic Nantgarw samples were fired at near-eutectic conditions (approx. 1290°C) in the CaAl 2 Si 2 O 8 -Ca 3 (PO 4 ) 2 -SiO 2 system, yielding a relatively silica-poor melt. In contrast, the silicious Swansea sherds were fired at temperatures approximately 600°C above eutectics (approx. 700°C) in the SiO 2 -K 2 O-Al 2 O 3 system, yielding a higher proportion (approx. 52 vol.%vsapprox. 28 vol.% in representative samples) of melt material, but with relatively SiO 2 -rich compositions. The extreme viscosity of the SiO 2 -rich melt in the silicious wares is inferred to have mitigated some of the firing problems (e.g. distortion or sagging) that apparently plagued the phosphatic porcelains. The use of a recipe with a higher clay/calcite ratio (as represented by Swansea's “duck-egg” porcelain) would have improved the plasticity of the phosphatic Nantgarw paste and reduced the likelihood that it would sag during firing.Two of the Swansea sherds are decorated (i.e. were successfully fired in the biscuit kiln). That the silicious Nantgarw paste also was successfully fired could be confirmed by examining with a black light fugitive specimens that may reside, as yet unidentified, in museum collections.