Two types of historic hydraulic brick-lime plasters from a selected Ottoman bath were examined in order to characterise their technology and suitability for this application. The first type of plaster is original and structurally sound, while the historic repair plaster is the second type and was found to have deteriorated despite being exposed to the same environment. This difference was investigated by comparing their raw material compositions and the pozzolanic activities of the brick powders used in the plaster. Although these results showed no significant differences, ettringite crystals were detected in the historic repair ones by XRD, FTIR, and SEM-EDS analysis. The repair plaster may have lost its integrity owing to the expansion generated by the growth of ettringite crystals in the plaster. Ettringite formation is discussed in relation to the hydration reaction products of lime-brick plasters, possible sources of gypsum, and the climatic conditions of the historic bath building. 17 refs.