The use of alum and dry plaster as a filling for fire-proof safes, is based upon sound chemical and philosophical principles. The two essentials in a fire-proof safe are, that in ordinary use, it shall be perfectly dry, and that, when heated, it shall become wet. So long as it is wet the temperature in the interior of the safe can never exceed 212 Fah., the boiling point of water, at which temperature everything within it is safe, no matter how excessive the external heat may be. In order that the first requisite (dryness in ordinary use), may be attained, the filling should contain no deliquescent salts. A train of serious evils will result from the use of such salts, as swelling of the filling, and consequent bulging of the plates; corrosion of the metal until it becomes so rotten that a pocket knife may be thrust through its walls ; and dampness of the walls, producing mildew and destruction of papers and books. Potash amm contains f f of its weight, of water, or nearly one-half. All of this water, with the exception of -Jjj-r of the weight of alum, is liberated by a temperature of 856 . At ordinary temperatures it is a perfectly dry substance. It gives off water gradually as the temperature is maintained, and commences to liberate it at 140 . Some other alums contain 55 per cent, of water, A safe, having alum in lumps as an ingredient in its filling, will, when heated, be immediately filled with steam, and, as long as it remains so, must preserve its contents. The dry plaster absorbs the water as it is liberated, and holds it until the heat converts it into steam. Nothing could be more simple than this action, and its efficiency has been often corroborated by the severest tests. Having deemed it necessary to obtain a new safe for tho security of our valuable correspondence, in addition to a number already in use for our books and more valuable papers, we have been supplied with one with alum and dry plaster filling, made to order, at the manufactory of Marvin Co., of 265 Broadway, this city, which is, in every way, so satisfactory both in elegance of design and finish, that we are constrained to bear testimony to the superior workmanship of the safes made by this firm. The safe in question has a feature not before used, which is very convenient for filing correspondence. Two doors are provided on opposite sides of the safe, and a double row of tills, of the right capacity for folded letters, built within the walls ; access being had to the files through the doors from one side or the other, without the trouble of lifting out one case to get access to another set of pigeon holes "behind it. The doors are secured with Sargent's celebrated magnetic combination lock, and the whole safe is a remarkable specimen of good workmanship, both for convenience and in ornamental design. Any one desiring a double safe for their correspondence, or other purposes, will be likely to get some good hints by examining the one at our office before ordering.
This article was originally published with the title "The Philosophy of Alum and Dry Plaster Filling for Fire-Proof Safes" in Scientific American 20, 18, 279 (May 1869)