A new, sinterable yttria (Y/sub 2/O/sub 3/) plaster was developed. The properties of this plaster depend on: (1) starting-power characteristics, (2) type and concentration of the acid used for the liquid phase, and (3) the liquid-to powder (L/P) ratios. Physical and chemical properties of the yttria plaster were investigated. Plasters formed in the yttria/acid/water (dilute-acids) system were determined to consist mostly of needle (or plate)-shaped hydroxysalt binder phases analogous to precipitates in the YX/sub 3//sodium hydroxide/water system (where X represents either Cl/sup -/ or NO/sub 3//sup -/). The binder phases were shown to be of the form Y/sub 2/(OH)/sub 6-m/X/sub m/ x nH/sub 2/O, where m and n generally equal one. The decomposition behavior on heating yttria plasters was studied by thermal analyses. The nitric acid-produced plasters decomposed to yttria below 600/sup 0/C in argon; the hydrochloric acid-produced plasters decomposed to yttria by 1520/sup 0/C in argon or by 1100/sup 0/C in air. Examination of scandium oxide and the rare-earth oxides showed that several exhibited a plaster-forming behavior similar to yttria.