FIRE-RESISTING DOORS AND SHUTTERS.
A simple and effectual fire-resisting door and shutter is made of two thicknesses of matched pine nailed across each other at right angles of forty-five degrees. If the doorway is more than seven feet by four feet, it would be better to use three thicknesses of stuff; in other words, the door should be a thickness proportional to its area. Such a door should always be made to shut into a rebate, or made flush with the wall when practicable; or, if it is a sliding door, then it should be made to shut into or behind a jam, which should press it closely to the wall. The door and its jams, if of wood, should then be sheathed with tin, the plates being locked at the joints and securely nailed under the locking with nails at least one inch long. No air spaces should be left in a door of this kind by paneling cr otherwise, as the door will resist fire best that has the most solid material in it In most situations it is much better to fit the door upon metal slides rather than upon hinges. This sort of door may be fitted with automatic appliances, so that it will close of itself when subjected to the heat of a fire; but these appliances need not intcrfeic with the ordinary methods of opening and shutting the door. They only constitute a safeguard against negligence. The construction of shutters varies from that of doors only in the use of thinner wood. All the other conditions after the manner described resist fire a dozen times better than the ordinary iron door, whether sheet, plate, cast or rolled, single, double or hollow, plain or corrugated. The wooden door covered with tin only serves its purpose when the wood is fully incased in tin put on in such a way that no air, or a minimum of air can reach the wood when it is exposed to the heat ol a fire. Under these conditions the surface of the wood is converted into charcoal; and charcoal being a nonconductor of heat,itself tends to retard the further combustion of the wood. But if air penetrates the tin casing in any manner, the charcoal first made, and then the wood itself, are both consumed, and the door is destroyed. In ‘ike manner, if a door is tinned only on one side—and there are such—as soon as the heat suffices to convert the surface of the wood under the tin and next to the fire into charcoal, the oxygen reaches it from the outside, and the door is of little more value than z thin door of iron or a plain wooden door.