A study was made of 21 Renaissance plasters from Jacobean halls within a radius of 60 km around Sheffield (England). Using x-ray diffraction, differential thermal analysis spot tests for calcium hydroxide, electron probe micro analysis and scanning electron microscopy. The composition of the plasters fell into 2 groups. Those prepared from carboniferous limestone contained calcium carbonate, m quartz and animal hair but no free calcium hydroxide, despite claims in the literature that it might be found. The plasters which had been prepared from dolomitic limestone contained in addition magnesium carbonate and magnesium hydroxide. Graphs tables photographs. Appendices describing the plasters and details of experimental procedures used. References.