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 same 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 faÏence. 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. The 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 faÏence 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. 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 ½-inch objectives being plenty for the purpose. Unless the reader desires to follow the subject 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 passing through the objective passes through that axis. To assist in more accurately centreing than is otherwise possible (depending on the lenses) with this cheap form of instrument, a collar with adjustable screws is ordinarily affixed to the lower part of the barrel of the microscope. The stage, with suitable clips to hold the object to be examined, is graduated so that on its being revolved it is easy to ascertain the number of degrees, at any period of the revolution, through which it has been turned. Thus, it will be observed that the object revolves with the stage. A pointer is placed in a suitable position on the frame of the microscope to facilitate the observation. The Polariscope.—This is an indispensable adjunct, for determinative purposes it is often necessary to observe the object in polarised light. Briefly, the polariscope consists of two parts—the analyser, placed in the barrel of the microscope above the objective, and the polariser, arranged underneath the revolving stage. The analyser is so fitted that it may be shot in and out of the barrel in order that the polariser alone may be used, or the latter may be removed, leaving only the analyser in position, or both may be removed to enable the object to be examined in ordinary light, either reflected or transmitted. The lower nicol14 is made to revolve, and the collar in which it is fixed is broadly graduated and furnished with a pointer.

Reflector.—An ordinary reversible and adjustable reflector is arranged beneath all.

Accessories.—For the more accurate determination of minerals, a quartz wedge, a quartz plate, etc., are used by the petrologist, but the description of these is beyond the scope of the present work. For examination in reflected light it is highly desirable to have a “bull’s-eye” condenser.

An ordinary microscope with a revolving stage may be readily converted to petrological purposes, though it is better to have a special instrument.

The object to be examined may be in the form of (a) a fragment of the brick, or (b) a very thin slice of the same.

The fragment may be securely clipped and held in position on the stage, the “bull’s-eye” condenser being brought into use to throw a strong light on the part immediately under the objective. The polarising apparatus is no use for this, and may be thrown out of gear. A very low power should be employed. The observation may be directed towards ascertaining how far the fragments composing the brick are agglutinated, and their size may be noted. Anything like a discolouration should be specially observed, and a minute description jotted down. In bricks that have not been burnt very hard, and in those that have merely been baked, we shall often be able to detect particles of mineral matter which further investigation, after the manner presently to be described, shows are opaque. Different forms of iron, iron pyrite, fragments of clay that have merely been dried in the process of baking, and minute pieces of chalk (now converted into lime) are amongst the most prominent opaque substances met with in common bricks. These may generally be differentiated and determined at sight, and bricks thus composed are never of good quality, though the ingredients have been ground very fine, and there may be nothing superficially to find fault with. Their bad qualities are usually brought out in the weathering. A great deal may, therefore, be learned from a careful examination of fragments in this manner.

In regard to the examination of very thin slices, that is in the majority of instances the most instructive, and, if we may say so, the most interesting method of investigation, though it must always go hand in hand with the other. The slice of the brick is so thin that the bulk of the constituents is rendered transparent, or semi-transparent. The preparation of such slices15 is not difficult, but demands some experience; those who have neither the time nor patience to make them will find it convenient to send the fragments of brick to Damon, of Weymouth, or some other first-class dealer in geological and mineralogical specimens. The price charged, per slide, is usually 1s. 6d. At the same time, the student will find it eminently to his advantage to prepare the slices himself. In the process he will learn much that escapes attention when the work is done by another.

The thin slice mounted on a slip of glass is placed on the stage of the microscope and firmly clipped, as with the fragment. The reflector is brought into position, and a beam of light thrown through the slice—the thin section is now being examined in transmitted light. At first it will be convenient to study it with the polariser and analyser thrown out of position. A certain proportion of the constituents is found to be opaque, and should be examined in reflected light, as above described. The remainder are more or less transparent, and some of the grains will, possibly, be coloured. We notice the way in which the whole of the fragments are bound together—say, by some opaque mineral such as iron—or whether they seem to be partially or wholly fused together. In the case of a vitrified brick, the latter phenomenon is most usual, and we shall find that although crystalline fragments have been melted, or partially fused, there is commonly a centre or nucleus of each fragment in its original condition remaining, which passes through insensible gradations from the crystalline to the non-crystalline, or amorphous state. This latter circumstance may be ascertained by using the polariscope. Ignoring the opaque matter adverted to, we shall then see that what was transparent in ordinary light appears, for the most part, to be opaque in polarised light. Those portions which still let the light through are truly crystalline, and by revolving the stage we notice that they frequently change tint, becoming alternately light and dark. In that brick where the particles are agglutinated by igneous fusion, we shall observe the light decreasing in intensity from the crystalline portion (forming the nucleus, as it were, of each particle) outwards, and where the crystal fragment has been melted, so as to become fused to its neighbour, the periphery, or rather what was originally the boundary of the fragment, is quite dark. Polarised light cannot pass through non-crystalline matter, and in being melted that portion of the crystal fragment had passed from the crystalline to the non-crystalline stage. It is very easy, therefore, to determine how far the fragments composing a vitrified brick have been melted down and fused together; but to observe the phenomena under the most favourable conditions, the brick must be thoroughly well-burnt, and the section taken, by preference, from near the outside surface of the brick.

In some instances, partial fusion is so well exemplified (especially in bricks from fairly pure china clay), and the brick, after being burnt, has been permitted to cool so slowly, that devitrification has set in, when we are presented with aggregates of crystallites closely resembling the “felspathic matter” of petrologists. That is a circumstance which the maker should note well, for he has burnt the brick to the best advantage, and it is not then so brittle as it might have been had more “glass” made its appearance in the section. Prolonged heat, just above the agglutinating point, has accomplished this, and the microscope here clearly shows the advantage of allowing the kiln to cool slowly, and to permit the lapse of several days in the operation.