The influence of rain on the weathering of bricks may be considered from yet another standpoint. Where the brick is fairly porous, its durability is liable to be materially influenced through the agency of successive frosts. The water finds its way a short distance into the brick and saturates it. During frost the water is turned into ice at and near the surface of the brick. In forming, the ice exerts considerable expansive force, which forces asunder the particles (sand-grains and the like) of which the brick is composed—that is to say, near the surface of the brick. The accumulated effects of successive frosts in this way tends to weather the brick by breaking up its exposed surfaces. To be materially affected, however, the brick would have to be of very poor quality, and it will be seen that the presence of cracks would much facilitate the operation.
The style of a building, the manner of its construction, and especially the class of metals used for exterior decoration, all assist rain in its work. A projecting course will have its upper surface washed clean, whilst the underside remains very dirty—in cities, becoming quite black. The limit of this dark discolouration is often frayed out by the irregular action of the rain dripping from the projecting ledge, assisted by the wind. Where the projection is so designed that the rain is induced to drain to one point, and then to fall over on to the wall, an unsightly streak down the latter is the result. The free use of metal ornaments, railings, for supporting signs, for down-pipes, &c., is unfortunate in not a few instances. At the point of junction between the metallic substance and the brick into which it is inserted, or in the immediate neighbourhood above which it is fastened, the brickwork is sure to be discoloured. This may arise from the dripping of rain-water from the metal, or it may be from the decomposition of the latter, or from both. Iron rust leads to brown streaks, zinc-compo. to dirty red, and so on.
The action of the wind as affecting the durability of bricks is sufficiently important to warrant passing allusion. It drives rain and its deleterious acids farther into the brick than the moisture would soak in the ordinary way. It leads to wet walls interiorly, unless the latter are so constructed as to overcome the effects. On the other hand, a gentle breeze dries moisture on the face of the brickwork. In cities, wind indirectly assists rain and its impurities by blowing organic matter from the streets into niches and corners, where it lodges, and, decomposing, provides powerful acids capable of doing much work. Discolouration is the chief effect produced on the average brick through this medium. In certain countries, wind, by driving dust, sand, &c., acts as a species of sand blast.
Considerable diurnal variations in temperature are known to be peculiarly destructive to certain kinds of brick and terra-cotta work. Very porous bricks are not much affected, but the more compact kinds, and especially terra-cotta blocks, often suffer. These observations do not so much apply to our own country as to warmer climates; though we are not altogether without experience here. On being heated these materials expand; when made loosely, as in rubbers and the like, the effect of the expansion is not very manifest, because the motion is absorbed, so to speak, by the brick itself. On the other hand, increased compactness of the particles leads to a perceptible increase in the size of the bricks, and when the sun has gone down contraction takes place as the bricks are cooling. It often happens in hot climates that the brick or terra-cotta block is unable to part with its heat as rapidly as the surrounding air becomes cooler, although it tries hard to do so, and this leads to corners of the brick being broken off, the physical forces exerted during the struggle doing the damage.
A highly interesting case of the effects of temperature on terra-cotta was detailed by Mr. T. Mellard Reade, C.E., F.G.S., a few years ago.[12] He shews that the cumulative effect of small, but repeated changes of temperature is very striking, and describes the lengthening of a terra-cotta coping in that connexion. The coping in question, which was freely exposed to the direct rays of the sun, consisted of two courses of red Ruabon terra-cotta bricks set in cement upon a fence wall, built with common bricks in mortar, a brick and a half in thickness. The courses were level, but, in consequence of the inclination of the road, the coping stepped down at intervals, so that the undercourse of bricks of one length was just gripped and held in position by the top course of the next length of coping. It will be observed that that form of construction constituted, by liability to lifting, a more delicate test than ordinarily of any increase of length, that might take place in the coping. On subsequent examination of the coping, the end position of one length, abutting against the next length at the drop in the level, was found to be thrown up into an arch-shape bend of about 6 feet span; the coping bricks being lifted in the highest part one inch from their bed. There was a fracture at the crown of the arch, and another at the foot or springing, but for a distance of 30 feet the coping was practically one solid continuous bar. A careful examination shewed that the coping had “grown” about a quarter of an inch longer than when it was first set, and that this lengthening, as shewn by movement on the corbel bricks which occur at intervals, was evenly distributed along a length of 30 feet.
Mr. Mellard Reade tells us that this is by no means an isolated case. In the neighbourhood of Blundellsands inspection of brick copings shewed that it was quite a common feature, and he has noted several instances in which the end brickwork and piers have been badly fractured by the force of expansion. In a case where the coping was of blue Staffordshire bricks, the top course in cement and the under course in mortar, a change in length was clearly shewn by the coping being lifted off the wall at each of the two ramps which exist in its length, and the movement was readily measured on the corbel bricks as in the case previously detailed. In this case the lengthening was also a quarter of an inch, and was evenly distributed over a considerable length of coping.
Whilst speaking of changes of temperature in their effect on bricks, we may allude to the behaviour of the material in severe conflagrations. A general rule cannot be laid down, because it is customary now-a-days to use fire-bricks for ordinary building purposes which will withstand practically any heat to which they may be subjected. Leaving them out of the question, and referring to ordinary bricks, it may be said that those of an inferior class frequently become cracked all over during a fire, or, it may be, by the sudden cooling after the fire has been put out, or by the sudden lowering of the temperature in them by the continuous action of the fireman’s hose. All the same, the average brick withstands heat far better than any kind of granite, or similar igneous holo-crystalline rock; loosely compacted sandstones and limestones crumble up on the surface, or flake, or may be utterly destroyed when subjected to a conflagration that would not have the slightest effect on bricks.
CHAPTER XI.
THE MICRO-STRUCTURE OF BRICKS.
The reader may be tempted to enquire, What is the use of knowing the micro-structure of a brick? We have anticipated the question to some extent in dealing with the structure of brick-earths, but it may be well to enlarge upon it here. In the first place, the study of the minute structure enables the manufacturer to ascertain whether the brick is thoroughly and homogeneously burnt. It tells him whether the materials mixed together in the earlier stages of manufacture were thoroughly incorporated or not, whereby, if need be, he can improve that part of the process. In carefully examining what the average manufacturer would call a well-burnt brick, the microscope assists us in perceiving that it is often anything but well burnt, small local patches—“tears”—of semi-vitrified matter being observed, which should not exist, of course, in a perfectly homogeneous brick. And if the brick is not homogeneous, it suffers in respect of its strength as a whole, and in the majority of cases its colour is not uniform. To arrive at the cause of this lack of uniformity is to indicate the manner in which the manufacture of the brick may be improved, and the microscope often enables us to arrive at a satisfactory solution of the problem.