LETTERS To the Editor: The February article on the "Explosive Hazard of Aluminum-Liquid Oxygen Mixtures" by Austin, et al., certainly was timely. I t serves to point up the fact that aluminum powder-and perhaps other metal powders-may not he the innocent materials they seem to be. The followingexperience illustrates this. About 10 years ago I was engaged in preparing suspensions of aluminum powder (aluminum "bronze") in technicalgrade oleic acid. One such mixture. 20y0 aluminum powder (w/v), was made up to fill a small glass bottle (50 ml). After the powder was added to the oleic acid and the mixture gently shaken, I set the bottle on the lab bench and walked into the other room. The bottle was left unstoppered. In approximately 3 to 5 minutes a low-order detonation occurred, completely shattering the bottle and throwing fragments of glass as far as 15 feet away. A substantial rise in temperature was not detected, nor was there a flash. I have tried, quite unsuccessfully, to repeat this effect since then, using the same bat,ches of aluminum powder and oleic acid as originally employed. The force of the curious explosion was such that a person standing near the bot,tle could have been blinded by flying glass particles.
To the Editor: In the first paragraph of the recent article by Austin, Rohrer, and Seifert, entitled "Explosive Hazard of Aluminum-Liquid Oxygen Mixtures" (J. CHEM.EDUC., 36, 54 (1959)), the authors refer to the original description by Cady (Ibid., 8, 1027 (1931)) of a lecture demonstration consisting of the ignition of such a mixture. I should like to point out that the conditions which are described as having been used in most of the trials a t the Naval Ammunition Depot differed materially from those described by Cady in the 1931 article. Thus, it is stated in the recent article that "the general procedure was to place aluminum powder in an iron crucible contained in a larger iron dish halffull of sand."' This practice may very well have been based upon a misunderstanding of Cady's use of the term "iron sand-bath." I n Cady's article, the reference to the preceding experiment (Experiment 19, page 1039) clearly shows that this term was used merely to designate the dish itself; there is no mention whatever of the use of sand. Furthermore, in most of the trials at the Naval Ammunition Depot, the aluminum was contained in a crucible of such dimensions that the upper surface of the powder was only 1.6 inches (3.8em) in diameter.' Cady, on the other hand, recommended the use of an iron dish 12 cm in diameter. Obviously, even if 50 ml of powdered aluminum were used, in such a dish the layer would not he more than 5 mm in depth. It is evident, therefore, that most of the N.A.D. tests were made under conditions much less favorable to the rapid dissipation of heat than those described in Cady's article.
' Italics mine. ARTHUR W. DAVIDSON
(Letters continued o n page 310)
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