George Frederick Harris

“ We will assume that the brick has been either naturally or artificially dried, that no cracks have made their appearance, and that it is properly stacked in the kiln ready for burning. Now comes a most important part of the process. It is possible that any microscopic cracks will be closed by fusion or agglutination; but it more frequently happens that in unskilled hands the kiln is responsible for many cracked and “starred” bricks. To know exactly how to introduce the heat so gradually that the bricks shall not be impaired, is an art begotten only of considerable experience. ”

“ THE CHEMISTRY OF BRICK-EARTHS (Continued) .
In this chapter we shall fulfil our promise (ante p. 58) to explain in an elementary manner the precise meaning of ordinary commercial chemical analyses of some typical earths used in brickmaking, etc. We may commence by explaining a few terms used by the chemist.
An atom is the smallest imaginable portion of matter, and all matter is said to consist of atoms. A molecule is the smallest conceivable combination of atoms, and every compound substance is ultimately built up of molecules. ”

“ In spite of that, however, we find that the “crushing strength” is much more popular than the others. The reason, so far as brick manufacturers are concerned, is not far to seek. Architects demand that especial quality. “What is the ‘crushing strength’ of your bricks?” enquires the architect. And if the maker does not know, he stands a good chance of losing the order. Figures are demanded, and if the maker cannot produce a higher figure than his neighbour, woe betide him. But statistics are ever deceptive, and as applied to bricks in regard to their strength especially so. ”

“ Again, a brick cracked all over, not with superficial cracks only, but with those which go far into the interior, will not yield its quality by mere immersion in water. The water, it is true, finds its way right into the brick, but, as often as not, the sides of the cracks are perfectly vitrified and almost damp proof, so that on lifting the brick out of the water the latter rolls off as though it were on “a duck’s back.” Yet such a brick, yielding but the merest fraction as a result of the immersion, may be utterly worthless when put into a building, because it would not be strong enough. ”

“ Turning to brick-earths of marine origin, we may say that these constitute by far the largest class of deposits from which bricks are made in this country, and it will be useful to deal with their origin in some detail. If we attentively watch the action of the weather on a friable sea-cliff we notice that large pieces tumble at intervals on to the beach, and in due time these are washed away by the waves, thus encouraging more to fall when the time is ripe. This process of denudation each year takes tens of thousands of tons of sandy clays and the like from the beaches around our islands. ”

“ Chemical affinity, or chemical attraction, is the force which is exerted between molecules not of the same kind. Thus, in water, which, as we have seen, is composed of hydrogen and oxygen, it is affinity which unites these elements, but it is cohesion which binds together two molecules of water. In compound bodies, cohesion and affinity operate simultaneously; whilst in simple bodies, or elements, cohesion alone has to be considered. To affinity are due all the phenomena of combustion and of chemical combination and decomposition. ”

“ As a general rule, where care is exercised in the drying, the cracks arise from the brick-earth being too wet or plastic in the first place, and it cannot be too well understood that, cæteris paribus, the wetter the earth the more liable it is to crack during drying. The contraction, even when the unburnt brick is shielded, and in the open air, often proves too much for the material. Then we have that class of brick-earth composed of too much clay, and that would be improved by the addition of sand—just how much depends on the particular earth ”

“ The influence of a salt-bearing bed is, naturally, not confined to the immediate vicinity of the formation; salt being so readily soluble in water, it comes forth from the rocks in springs, which, flowing over loams and other similar absorbent earths, impart a saline character to them. In this manner otherwise useful earths for brickmaking are rendered absolutely unfit for the purpose. Salt is one of the most powerful fluxes known; when mixed even in very small quantities with clay it becomes impossible to make a good brick of the substance. ”

“ The elements are classified into metals and non-metals; and it is convenient to give each of them a symbol to save trouble in writing, and to render clearer to the reader the chemical nature of a compound body. Thus, the symbol for the element aluminium is Al; for silicon Si; for carbon C; for calcium Ca; for oxygen O; for iron Fe; for hydrogen H; for chlorine Cl; and so on.
We are taught by chemistry that elements are capable of combining only in definite proportions, and that each substance possesses a definite proportion peculiar to itself. ”

“ Enough has now been said to show that stupendous quantities of mineral matter derived from the destruction of the land are sent down to the sea by natural agencies, and we may at once state that a very large proportion of this, which finds a resting-place in and about the mouths of the rivers and their backwaters, is material suitable for brickmaking at places where it is obtainable. Enormous quantities of muddy sediment, sand and gravel, however, never reach as far as the sea with great rivers. This material is arrested at sundry convenient spots, and, as a rule, forms excellent brick-earth. ”

“ The brickmaker hardly needs to be told that if he places his bricks in the sun to dry, they, or a large percentage of them, will crack, and become practically worthless from a commercial standpoint. To dry a brick properly in the open air is a lengthy operation—too lengthy for many manufacturers, who, in consequence, have had recourse to artificial drying. Many a brickyard has had to be abandoned from the inability of the worker to produce bricks that did not crack at some period of the operation, either in the drying, or burning, or both. ”

“ The results of a chemical analysis of a compound earth, as ordinarily used by the brickmaker, widely differ from those obtained by a mineralogical or petrological examination. The petrologist views the earth as a mineral aggregate, the constituents of which may be ascertained on appeal to a properly-constructed microscope—that is, in the majority of instances. By noting the relative proportions of the different minerals, he is enabled to state, with approximate accuracy, what is the ultimate chemical composition of the whole. ”

“ It is truly marvellous to see how little attention many large makers pay to the initial drying; often the long rows of drying blocks are left unprotected except for a rude kind of roof placed over them. The passing shower of rain drives in underneath, and wets the exposed surfaces, causing the clay to swell. These surfaces, being moister than the remaining portions of the brick, contract at a different rate, the centre occasionally being drier than the outside. The unequal contraction produces minute cracks even in most excellent earths. ”

“ 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. ”

“ The man whose duty it is to determine the persistence, or otherwise, of valuable marine brick-earths has thus a much easier task than when called upon to decide the value of a large tract of land for brickmaking purposes, of fluviatile origin. Finally, brick-earths do not, except in extremely rare instances, vary materially in character when dug into horizontally, thus every opportunity is afforded to the manufacturer for making an unvariable quality brick, tile, or drain pipe. ”

“ The brick would nearest resemble a form of slag. All we now insist upon is that in brickmaking a chemical analysis is only useful up to a certain point, beyond which we must appeal to the microscope to aid us, and this in conjunction with as perfect a knowledge as possible as to the behaviour of earths of certain mineral composition when under the influence of high temperatures. ”

“ Apart from the fierce and variable drying action of the open air, we have a fruitful source of cracks in the indentations made by stamping the makers’ name or trade-mark upon the blocks. With bricks burnt very hard this does not so much matter, but on the commoner kind of materials one may often perceive minute, hair-like cracks radiating from the indentations. We presume that in this age of advertising it is impossible to convince many makers of that fact, yet if full justice is to be done to the material, it will be better not to make any sharp or deep marks on the brick. ”

“ When, at a subsequent period, bricks containing these substances are exposed to the action of the weather, oxidation takes place, sulphate of iron and sometimes of lime are formed, which on crystallizing expand with considerable force and split or crack the brick. From this it is evident that sulphide of iron in any form is not to be tolerated in brick manufacture, and if the earth used in the first place contains much, it must be removed in the preparing process. If permitted to remain, it is impossible to obtain either a durable, or a good coloured brick. ”

“ But what we more particularly desire to draw the reader’s attention to at the present stage, is not the employment of lime in making fire-bricks so much as its mixture with ordinary brick-earth, as in the manufacture of malm bricks. Sometimes the mixture has been effected by Nature, as is the case with true marls; but the brickmaker does not care so much for these, as without considerable and expensive artificial assistance they do not often make readily saleable bricks. ”

“ The production of an uniform tint is the main point aimed at; and it may be at once remarked that unless the brick-earth employed is very homogeneous, or has been most carefully prepared and thoroughly incorporated, the production of an uniform colour is impossible.99 In regard to the tint to be produced, it should be remembered that the temperature employed in burning is a most potent factor. It is frequently laid down that such and such a temperature will form a red brick, and another and higher temperature, a blue one. ”

“ The best plan, without doubt, is to remove them altogether from the raw earth. They are commonly met with in what the geologist calls “boulder clay”—a deposit owing its origin to glaciers and icebergs. Very often the pebbles alluded to are not crystalline, but of an earthy character, as is the case when made of chalk. In the semi-dry process of manufacture, it is next to impossible to incorporate the ground-up particles of carbonate lime sufficiently well to result in the production of such a homogeneous earth as is desirable for making a first-class brick. ”

“ We allude to the fact that some experimenters take the specific gravity of a porous brick, without stating whether the amount of water absorbed, during the process, was taken into account in arriving at the specific gravity or not. Theoretically, of course, the substance to be dealt with is non-porous, and experimenters, worthy the name, either render the brick waterproof, or, ascertaining the amount of water the brick has absorbed, take that into consideration in calculating results. ”

“ Iron, in large proportion, acts as a flux to other constituents when the brick-earth is subjected to great heat in the kiln, and on that account must be carefully watched. But, to the average brickmaker, the ferruginous constituent is far more interesting as a colouring medium. At a later stage we shall have something to say concerning the colouring of bricks, &c., but it may now be remarked that red bricks, in practically all cases, owe their colour to the effects of firing on iron. ”

“ 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. ”

“ It is no uncommon thing for a marine clay—say, 300 feet in thickness—to continue across country for hundreds of miles, stretching from the North of England to the South, and over into the Continent, save for the slight break occasioned by the scooping out of the English Channel. The composition of the Oxford Clay, from which the well-known bricks at Peterborough are made, does not differ in the slightest degree, so far as suitability for brickmaking is concerned, from the Oxford Clay of Bourges or Chateauroux, in the centre of France, or indeed at almost any other point en route. ”

“ That may exist in a brick-earth in a variety of ways, both in a free and combined state; but its behaviour in the kiln is largely dependent on the particular form assumed, not only whether it is free or combined, but as to how it is combined. In a certain sense, it is very doubtful whether even in the best-burnt brick much of the raw material29 becomes chemically combined; a sort of agglutination takes place locally, as is clearly shown by the microscope; at such points true fusion undoubtedly takes place, and there may be actual chemical combination. ”

“ In drying, the greatest care would have to be exercised to prevent undue shrinking, and, in any case, the earth would have to be very thoroughly incorporated to make a really serviceable brick. It is with earths of this character that the majority of brickmakers in embryo come to grief; they know not how to handle them successfully, and twisting, warping, cracking, and “bursting” follow as a natural consequence. It is a common and treacherous material, that could only be made to succeed by perseverance and wide experience. ”

“ 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. ”