Abstract: Certain articles required to function in air at temperatures of 250.degree. C. or higher comprise very fine particles of vermiculite from which the interstitial magnesium ions have been at least partially removed and which has been swollen in water. These articles are not ordinarily exposed to water when in use, but if they are they are likely to disintegrate quickly because of the ready dispersibility of the vermiculite particles. By the invention, water-resistance is given to such articles by treating them with a solution of an aluminate salt e.g. NaAlO.sub.2 or a zirconyl salt e.g. ZrOCl.sub.2.