The Science of Brickmaking

by George Frederick Harris

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 nicol[14] 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 slices[15] 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.