The advantage of knowing the relative absorptive capacity of bricks has been stated in these pages in divers connexions. The means of arriving at the total capacity for absorption of water, as generally practised by experimenters, are very incomplete and founded on an erroneous principle. It is admitted by all that absorption is one of the very best tests as to the quality of a brick, but such tests are meaningless unless they imitate one or other or several of the influences to which the brick would be subjected on being used in the building, or other structure. A common method is to weigh the brick when dry and then to immerse it in water for periods varying from one to three days, subsequently re-weighing it, the difference in weight between the dry and wet states being termed the brick’s “absorptive capacity.”
Mr. Heinrich Ries remarks16 that the absorption is determined by weighing the thoroughly dry samples, immersing in clean water from 48 to 72 hours, then wiping dry and weighing again. Vitrified bricks should not show a gain in weight of over 2 per cent. There are cases where bricks of apparently good quality shew a greater absorption than this, but they have great toughness and refractory qualities. Bricks made from fire-clays which will not vitrify so easily will, naturally, show higher absorption.
133 Again, Mr. E. S. Fickes, of Steubenville, Ohio, has recently made17 a large series of valuable tests of both paving and building bricks, in which he shews the connexion between the power of absorption and the strength of the materials experimented with. Mr. Fickes’ more important conclusions are:—
1. The strength of the building brick, both transverse and crushing, varies in tolerably close inverse ratio with the quantity of water absorbed in twenty-four hours. The strongest bricks absorb the least water.
2. Good building bricks absorb from 6 to 12 per cent. in 24 hours, and with no greater absorption than 12 per cent. will ordinarily show from 7,000 to 10,000 or more pounds per square inch of ultimate crushing strength.
3. Poor building bricks will absorb one-seventh to one-fourth of their weight of water in 24 hours, and average a little more than one-half the transverse and crushing strength of good bricks.
4. An immersed brick is nearly saturated in the first hour of immersion, and in the remaining 23 hours the absorption is only five-tenths to eight-tenths of 1 per cent. of its weight, as a rule.
These experiments are of much interest and are probably approximately correct; but we venture to think that if the absorption experiments had been carried out in a different manner, the results would have been still more valuable.
Long before the publication of the results of the last mentioned series of experiments, the present writer had discovered the close connexion which subsists between the relative absorptive capacity of bricks and their strength; a slight correction must be applied for134 specific gravity. We are not prepared to enter into this subject at any length, but it may be observed that we should not have arrived at such close results had we experimented in the same way as the American authors just quoted (or others, for the matter of that).
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