Abstract: A casting or splinting material in powder form is a mixture of particles of (a) a water-soluble cross-linkable polymer containing carboxylic acid groups, such as polyacrylic acid and (b) zinc oxide and/or one or more zinc salts, this latter material being at least partly coated to reduce its surface reactivity with an organic acid e.g. an aliphatic fatty acid containing from 6 to 20 carbon atoms per molecule. Such material, that can for example be provided as a loading on a Leno gauze, can be made up as a slurry in a volatile organic liquid for coating purposes.