Abstract: A saline soluble fiber is disclosed that is highly refractory. A vacuum cast perform of the fiber has a shrinkage of 3.5% or less when exposed to 1260.degree. C. for 24 hours. The fiber may comprise CaO, SiO.sub.2, MgO, optionally ZrO.sub.2, optionally less than 0.75 mol % Al.sub.2 O.sub.3, any incidental impurities amounting to less than 2 mol % in total, and in which the SiO.sub.2 excess (defined as the amount of SiO.sub.2 calculated as remaining after the above named constituents are crystallized as silicates) exceeds 21.8 mol %, with the proviso that, if the amount of CaO is greater than the sum of the amount of MgO and twice the amount of ZrO.sub.2 the calculated ratio of diopside to wollastonite does not lie in the range 0.1 to 5.22. Such fibers are usable at elevated temperatures where refractoriness is of importance and their solubility in saline solution may make the fibers safer than non-soluble fibers.