Inorganic fibers including calcia, alumina, potassia and optionally sodia as the major fiber components are provided. Also provided are methods of preparing the inorganic fibers and of thermally insulating articles using thermal insulation comprising the inorganic fibers. The inorganic fibers are soluble in physiological saline solutions, do not form crystalline silica, and are resistant to temperatures of 1260° C. and greater.