Abstract
A LETTER has been received from Mr. T. G. Booking, Princes Chambers, 6 Corporation Street, Birmingham, 2, giving an account of some observations on the magnetic properties of bricks. Bricks were selected from a number of South Staffordshire kilns, the direction in which the bricks were lying when baked being noted. The polarity was most clearly marked when the bricks had been lying in a north-south direction, and it was found that the bricks were magnetised approximately along the line of magnetic dip. Among the bricks examined were some made from Etruria marl, containing about 11 per cent iron oxide. The kiln temperature was 1150° C. The content of iron oxide (mainly Fe2O3), to which such ferromagnetic properties may be attributed, varies considerably in the materials from which bricks are made. It is usually well below 2 per cent in the fire-clays giving white and cream bricks, about 7 per cent in the clays giving red bricks, and 10 per cent or more in those giving blue and black bricks. Among recent relevant investigations are those of Koenigsberger (Phys. Z., 33, 468; 1932) on haematite (Fe2O3), magnetite (Fe3O4) and other ferromagnetic compounds. He finds that haematite, when cooled down in the earth's vertical field (0*4 gauss) from above the Curie poini (about 670° C.), shows a residual magnetisation which approaches the saturation remanence, and may be a considerable fraction of the saturation magnetisation. It is, of course, not possible to generalise about bricks. Each set of bricks presents a special problem, and precise discussion of the magnetic properties would require a detailed knowledge of the chemical composition of the clays, and of the conditions of baking and cooling. It is, however, probably not widely realised that most bricks are magnets-though feeble ones; and Mr. Booking's observations are of interest in indicating that the phenomenon of thermo-remanence may be demonstrated with such a common object as an ordinary brick.
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Thermo-remanence of Bricks. Nature 135, 61–62 (1935). https://doi.org/10.1038/135061d0
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DOI: https://doi.org/10.1038/135061d0