This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The bricks of which we have hitherto been speaking have been used for internal work alone, and in almost every locality bricks sufficiently good for this purpose are made. The strength of common bricks of average quality is usually so much in excess of any weight which can be placed upon them in ordinary buildings, that no fear need be entertained of their collapse. And as the internal walls of houses are almost invariably covered with plaster or wainscot or other decoration, no objection can be raised against them on the score of appearance. True, living bacteria have been found in the pores of bricks, and inferior bricks will doubtless provide better accommodation for them than will denser bricks; but as common plaster is much more porous than common bricks, and a better breeding-ground therefore for the organisms, the requirements of sanitary construction will be best satisfied by an improved quality of plaster. Of course, it is not meant that any kind of brick is good enough for internal work, but merely that many bricks may be used for internal work which would be condemned for external work on account of their appearance and inability to withstand for any length of time the attacks of wind, acidulated rain, and frost.
Bricks for external facings, however, require careful selection. There can be no doubt about the durability of good brickwork. On the other hand, there can be no doubt that by far the greater part of the brickwork which has been erected in our towns within the last fifty or a hundred years, already shows unmistakable signs of decay. The railway traveller, as he enters London or any of the large manufacturing towns, has merely to look out of the carriage-window, to be satisfied of the truth of this statement. Wind and rain and frost, and the acids present in the air and rain of these towns, are slowly but surely eating away the arrises and faces of the bricks.
It is strange that no scientific system of tests for bricks has yet been formulated. In the case of Portland cement, engineers, after much disputation and some errors, have laid down certain tests by which the strength and durability can be ascertained, but architects and engineers still judge bricks by rule of thumb and more or less bitter experience. Occasionally the weights required to crack and crush certain bricks are obtained by experiment, but beyond this little has been done.
Here are the characteristics of a good brick, according to the recent utterance of a well-known architect: -
"1. Regularity of shape, so that when built into a wall the pressure is equal over its surface.
"2. Toughness as opposed to brittleness, - i.e. it ought not to snap when broken [sic], but should require two or three hard blows.
"3. Clearness of ring when gently knocked against another brick, and not a dull, heavy thud.
"4. Homogeneity of surface and texture in the interior, and, above all, absence of small stones and pebbles or lumps of chalk.
"5. Non-porosity, - i.e. a slowness in absorbing water".
These five points are undoubtedly well worthy of attention, - far be it from me to detract from them in any way, - but they are not sufficiently definite. For instance, what is meant by a "hard blow"? and what by "slowness" in absorbing water?
Clearly a more definite and less empirical system of judging bricks is desirable, and two tests especially should be carefully defined, namely: -
I. The total absorption of water, coupled with the rate of absorption.
2. The resistance of the brick to acids (and perhaps abrasion), so that the durability of the brick in town-air and rain may be inferred.
The former test would present no practical difficulties whatever, and the latter would apparently be no more difficult than (say) the hot-water method of testing the soundness of Portland cement.
Two of the most important facts to be ascertained about a brick, - especially a facing-brick, - are undoubtedly the amount of water which it will absorb, and the rate of such absorption. The former is important, partly because it affords some indication of the proneness of the brick to produce damp walls, and partly because it shows to some extent the degree to which the brick may be acted upon by rain, frost, etc. The rate of absorption is, however, a surer index of the ultimate dampness of the wall, as a rapid rate means that a wall will become damp with every shower, while a slow rate shows that only long-con-tinued rain will seriously affect it.
Tabic I. gives the weight of certain bricks, together with their absorption and rate of absorption. It is reprinted from The Builder for May 25, 1895, with the addition, however, of a column containing the weights per cubic foot of the several bricks. The results show that, as a general rule, the heavier the brick, the less is the amount of water absorbed. This holds true in almost every and it is quite possible that, had the measurements of the bricks l>een given more exactly, the relation between the weight and absorption would have been even more striking. It appears also that some of the bricks had not absorl>ed their full quantity ot water, - Nos. 1, 3, 4, for instance, - and that if the tests had been continued another week, the ratio between weight and absorption would have held true of these bricks also. Indeed, for bricks whose actual substance has the same specific gravity, the total absorption will vary inversely as the weight, except, of course, in the case of bricks coated with an impervious glaze.
Nothing but an actual test can give the rate of absorption, as this depends largely on the nature of the outer skin of the brick. If this be very smooth and vitrified, and free from cracks, the water cannot find easy entrance, nor can the air within the brick escape without difficulty; hence such a brick will have a slow rate of absorption. On the other hand, coarse, soft, underburnt, and fissured bricks may in a few minutes take up nearly all the water they can possibly absorb; notice that in 30 minutes the bricks numbered 7 to 18 absorbed on the average 85 per cent of the total amount. Comparisons may be odious, but they are certainly often useful, and one cannot help remarking upon the superior resistance to damp displayed by bricks 1 to 4.