With ordinary Gault bricks we find a range in strength from 145 tons to 173 tons per square foot; but Professor Unwin,[20] in his more recent experiments, finds that of four Gault bricks, one reached as high as 197.6 tons per square foot, and he gives 182.2 tons as the average strength.
To shew the absurdity of alluding to the strength of “blue Staffordshire” bricks, without also giving the precise locale of the samples dealt with, the reader is requested to refer to Table III., where the figures indicate a range from 275 tons to 464 tons per square foot, and to compare them with the results on Staffordshire bricks as stated in Table II., where we find a range from 651 tons to 1,064.2 tons per square foot. Of what value can a single formula be which gives the strength of Staffordshire bricks as a whole as based on such widely divergent figures as these? Professor Unwin, in his recent series of experiments alluded to, finds that with four Staffordshire blue bricks, the weakest gave a result of 564.8 tons per square foot, and the strongest 788 tons; the mean of the four being 701.1 tons per square foot.
The results on the Leicester “reds” are no more encouraging; the figures in the foregoing tables are 150.6 tons, 229 tons, 308 tons, and 337 tons per square foot. Similarly, Professor Unwin has more recently found that the Leicester “reds” from Elliston, near Leicester, bear a crushing strain varying from 311.4 tons to 591.4 tons per square foot in four samples.
From the foregoing it will appear to the reader that average results are of very little value to the architect or engineer, unless—(1) the brickyard is mentioned from which the bricks experimented with came; (2) the particular class of brick from that yard; (3) the method of experimenting, as to whether any substance was placed between the dies of the press and the brick to be crushed, and if so, what; (4) if recessed or initialled; (5) whether machine or hand made, and (6) as to whether the surfaces of the bricks were concave, convex, or flat.
Results on bricks not localised are not of much value, and it is absolutely useless for working purposes to give in one figure the strength of “London Stocks,” “Staffordshire blues,” “Leicestershire reds,” and the like. In a general way, of course, it will be admitted that the “Staffordshire blue” is a stronger brick than the “London Stock,” and so forth; but that is as much as can be permitted—it is of no practical use to give relative figures in general terms.
It frequently happens that the capacity of the machine used for testing the strength of bricks is not enough for those bricks having a very high resistance to crushing. In the recent experiments by Professor Unwin, more than once alluded to in this article, it was found necessary to experiment with half-bricks only, and he ascertained that bricks tested as half-bricks shew about 25 per cent. less resistance per square foot than when tested as whole bricks.
Further observations on strength are made under the next heading in connexion with other forms of testing the value and physical properties of bricks.
CHAPTER XV.
ABRASION, SPECIFIC GRAVITY.
Abrasion.—In this country it is not customary to test bricks and stone by means of the abrasion process, though many English materials have been dealt with in this manner on the continent.