PROOF AGAINST FIRE

Press, Volume CIII, Issue 30563, 5 October 1964, Page 21

PROOF AGAINST FIRE

Gypsum plaster not only refuses to burn or to support combustion, but it also refuses to allow high temperatures to pass through it. This superiority is due in the main, to its content of bound water (or water of crystallisation) amounting to approximately 20 per cent of its weight In the presence of heat, if the temperature exceeds that

of boiling water (212 deg F.) this water of crystallisation is gradually driven off in the form of water vapour. If the temperature is raised sufficiently and retained over a long period, all the water of crystallisation will be finally expelled. This is known as calcination. Calcination begins at the surface exposed to the fire and continues gradually into the gypsum plaster. The covering of calcined gypsum formed on the exposed surface adheres tenaciously to the remaining material and serves to retard the progress of calcination, which be-

comes increasingly slow as the depth of thickness of the calcined gypsum increases. A very efficient barrier is thus raised against the passage of heat, and combustible materials protected by gypsum are kept well below the temperature at which there is any danger of them taking Thus the temperature of materials adjacent to or in contact with the unexposed surface of gypsum cannot exceed 212 deg. F. until calcination is complete. This temperature is well below that at which materials used in building construction and practically all materials stored in buildings will ignite.