The Science of Brickmaking

by George Frederick Harris

Between 30 and 40 inches = Practically the whole of the south coast from Kent to Devonshire, the whole of Somerset, Wilts., and the west of England generally, with the exceptions about to be noticed.

Between 40 and 50 inches = A great part of Devon and Cornwall, the western half of Wales, with the exceptions presently to be given, a great part of Lancs., and Cumberland.

Between 50 and 75 inches = A small patch in the centre of Devon, a large strip in West Wales, and an enormous tract of country in Cumberland, Westmorland, with Lancs. and north-west Yorks.

Above 75 inches = The wettest parts of the country. A small part of Dartmoor, a region in Wales in the vicinity and to the south-east of Snowdon, and the Lake District.

With reference to statistics concerning rainfall, it should be borne in mind that those relating to special districts, especially to hilly parts of the country, are often very deceptive, and require careful local study. A slight difference in the physical features of a locality is often sufficient to lead to considerable variation—the proximity of a conical hill rising from the plain, the sudden convergence of the two sides of a valley, or, conversely, the widening of a valley into a flat stretch of land, all materially affect the local distribution of rain. A clump of trees situated in proximity to a house will frequently be the means of a downpour that would otherwise have passed over. With winding valleys great latitude must be allowed. Then, again, the geological structure of the locality is an important factor in determining the amount of moisture delivered at a given spot. Where we find a thick clay cropping out in the bottom of a valley, with more or less porous rocks rising on either side of it, we soon ascertain that the houses on the clay receive more moisture (or the latter is distributed over a longer period) than those edifices on the hill sides in the same district.

Our readers could no doubt give us plenty of instances where in a circumscribed area their bricks have behaved very erratically—the bricks of a house in one part of the district weathering well, and in another badly. That may often be due, not only to the actual distribution of the rain, but to the manner in which the rain or dew has fallen. If an inch of rain falls in the neighbourhood in one day, that would not tend to weather the bricks so vigorously as though the fall had been spread over, say, a week.

A very important aspect of the subject is that which deals with the “efflorescence” on bricks. This appears to be greatly misunderstood, being commonly assumed to be due to one set of circumstances rather than to the conspiracy of several. There are many kinds of efflorescence, and an explanation of one of them obviously will not apply to all. The “scum” that appears on the surface of bricks is, however, to some extent bound up in the composition of the rain in the particular locality where it occurs. Examined attentively, the commoner kinds of efflorescence are seen to be minute white and yellowish-white crystals. The substance of which these are formed has been drawn out of the brick, or the mortar, or both, and rain has been the principal agent in accomplishing this work, though its power in that respect must necessarily vary according to the chemical composition and structure of the brick or mortar, as compared with the nature of impurities in the rain. If some substance were present in the rain that could readily form an alliance with an ingredient of the brick, and the union was capable of crystallising out, the surface of the brick would naturally form a convenient spot for the crystallisation to take place. To prevent it, we ought to know the composition of the air at the spot where the house is to be erected, and also the chemical and physical structure of the brick to be employed. That is rather too much to expect from the manufacturer and architect; but there is a method—we will not say an infallible one—which may be adopted to get rid of that particular kind of scum. That method could not always be adopted, as will be seen. The bricks must be burned more thoroughly, and at a high temperature; that would lead in most cases to the active employment of practically all the ingredients of which the bricks are composed, and the impurities in the rain would, in consequence, stand less chance of successfully inducing some of them to break their allegiance. In practice, however, we believe it would be found that the high temperature requisite to bring about the result just stated would either tend to spoil the colour of the brick or partially melt it. The latter could be prevented with due care, but we are afraid the former could not be so easily dealt with, with the majority of brick-earths. And if the brick is to be permanently discoloured to prevent efflorescence, it is better to permit the latter to manifest itself. The life of the “scum” is very variable; sometimes, after having once appeared and disappeared, it will never come again. The passing shower may wash it off (though it is not always so easily removed), and it may come again and again for years. It behaves very erratically. The amount of the efflorescence may be such as, in course of time, to lead to the surface of the brick “bursting” and peeling off, or, on the other hand, it may be a mere film.

There is one thing in connexion with efflorescence which cannot be overlooked in regarding its practical effects in the building. In ever so many cases we find that the scum, or the major part of it, is only to be found in the neighbourhood of the mortar joints. That is a matter of direct observation, and we have taken some considerable trouble to verify it, as it has always been regarded as a point whereon to hinge a debate. We do not say that in all cases the efflorescence appears only in the position on the brick just indicated; but it unquestionably does so in too many instances to enable us to regard its occurrence as mere accident. Taking a large surface of brickwork just commencing to show efflorescence, we find that the vicinity of the mortar joints are the first places, in very many instances, where the nuisance begins to manifest itself. From thence it spreads over the surface of the brick until the whole is more or less discoloured.

It seems impossible to deny that the mortar is guilty, to some extent, in such cases. At the same time, we must confess that we have never seen the efflorescence spreading over the mortar. It would appear that something in the mortar enters into chemical alliance with certain ingredients of the brick, and that neither without the other could produce the phenomenon alluded to. The remedy suggesting itself most readily is to chemically analyse the efflorescence, the brick, and the mortar; supplementing the experiments with a micro-examination to see how far it is possible to locate the deleterious substances found to exist, so that they may be removed in the manufacture of the materials, if that is possible. But information on that head is of the scantiest description, and much more will have to be done before the question is definitely settled.

Another kind of “efflorescence” that often appears on bricks in damp situations is mere vegetable growth, which bears a superficial resemblance to the crystalline “scum” just described, though it can, of course, be easily differentiated on examination with a lens. The damp atmosphere is no doubt largely responsible for this, though ineffectual damp-courses are contributors. The remedy lies in having a less absorbent brick—one that will not afford ready foothold to the vegetation.