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. From a chemical standpoint we know that a high percentage of iron in the average brick-earth is not conducive to the production of a good brick. In the same119 manner by “rule of thumb” we learn that a high percentage of lime prevents the manufacture of the raw material into a fire-brick, unless, indeed, we are making basic bricks. The chemist tells us also of the respective values of potash and soda. Too much iron will cause the brick to “run”; salt has a similar effect; but beyond this the chemist cannot go, except that in the broad sense he explains what unions take place to produce such results.
The microscope, on the other hand, enables one to see exactly what has taken place; the deleterious constituents are detected at their work, and careful chemical investigation teaches us what to add to the brick-earth to neutralise the effects observed; for it is only from its effects that the artificial constitution of the brick-earth can be properly regulated.
The same instrument is extremely useful in all questions concerning the relations subsisting between a brick and the glaze upon it, the cause and prevention of the cracking of the latter, and its general quality from a physical aspect. And, speaking of cracks, we may again draw attention to the influence these have on the strength and durability of the brick: many of these minute fissures cannot be seen by the naked eye. In a similar way can the microscope be made use of in the manufacture of terra-cotta and fa?ence. The cracking of glazes is one of the most troublesome features the high-class brick and tile manufacturer has to deal with. If the character of the surface of the brick is not suitable for “taking” the glaze, the maker knows in a moment; the trouble is where the glaze takes readily and then, some time after the operation is finished, it becomes covered with “spider-web” cracks, unsightly and considerably detracting from the value of the brick. The120 cause of the cracking is commonly attributed to the composition of the glaze, and the manner in which the latter is allowed to cool, and no doubt a great deal is due on both those heads. At the same time, we know of many instances where the same glaze being used under similar conditions on two different surfaces of bricks made from one and the same brick-earth, the glaze cracks in the one case, and hardly ever in the other. The direction of the cracks points to their origin, and the character of the surface is brought in guilty. And yet the average manufacturer would not detect any difference in the quality of the surface—he could not, without a good lens or low power objective, perceive the slightest discrepancy.
The ordinary glaze behaves very much like Canada balsam with reference to surfaces on which it is laid, and something akin to what petrologists call “perlitic” cracks is produced in the glaze. We can make these cracks, and imitate the structure artificially, by suitably distributing the Canada balsam over the surface of a piece of ground glass, and in other ways. That direct relationship exists between the cracks and the grain of the surface on which the preparation is laid, is certain, for we may vary the distribution of the cracks by varying the grain of the surface. An intelligent appreciation of the disposition of cracks in glazes should be the means of preventing them altogether, and not only with bricks, but with fa?ence and vitrified work generally, the study may be best carried on by aid of the microscope.
The microscope, also, may be made use of in identifying bricks in case of dispute, though its applications in this respect are not so important as in dealing with building stones.
121 Questions of durability may frequently be decided on appeal to that instrument. Take a case in which a brick is known to contain a rather high percentage of lime: if the lime were in a combined state, the quality of the brick would not be materially affected; but assuming it were not so employed, it is possible that in a short space of time the brick would be thoroughly decomposed by atmospheric agencies. The microscope tells us at a glance the state in which that and other ingredients exist, in a well-burnt brick. We draw the line at bricks intended for the “jerry” builder; they may well be left to take care of themselves; we allude only to high-class productions in which science may be some aid to the manufacturer.
And now as to the microscope—for we do not use an ordinary one in such investigations. The best kinds of microscope are those used by petrologists in the study of the minute structure of rocks and minerals. The reader will find these fully described in works specially devoted to the subject,13 but we may say a few words thereon.
A common form of “Student’s” petrological microscope, as manufactured by Swift of London, may be described as follows:—
Eye Pieces and Objectives.—These need not be expensive, clear definition being the principal object to aim at; the objectives should be of low power, 2-inch, 1-inch and ?-inch objectives being plenty for the purpose. Unless the reader desires to follow the subject122 from a purely petrological point of view, to study the development of trichites, globulites, skeleton crystals, etc., in vitrified bricks, in such places as these latter have cooled from igneous fusion, there is no occasion to resort to higher powers. We are far from saying that the brickmaker of the present day would not derive any advantage from studying this subject in its higher aspects, for the origin of crystallization appeals strongly to the imaginative mind, and is one of the most remarkable problems that Nature offers for our investigation. But in an elementary treatise of this kind we cannot go into the matter; and, as previously remarked, low power objectives are sufficient for our present purpose. The eye-pieces should be fitted with cross-wires, the use of which will presently be explained.
The Stage.—In the instrument we are now describing this is circular with a hole in the middle, and is so arranged as to revolve horizontally on a collar about an axis, the centre of which comes exactly underneath the centre of the objective. In other words, a straight line drawn through the eye-piece down the centre of the barrel of the microscope, and pa............