Of the merely mechanical aspects of the operations of drying and burning bricks, we shall say little or nothing. But there are just a few points of a more or less scientific nature that offer themselves at this juncture to which we desire to allude.

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. And several manufacturers have their particular methods of “doctoring” the raw earths to prevent cracking. These are invariably “trade secrets;” though usually of a very open and transparent character, however, to the student of the subject.

It is most curious to learn the different reasons for adding this or that ingredient to the earths to prevent the brick from cracking. One who in a district has found that the addition of a little sand is beneficial, imparts that information by degrees, either personally or through his workmen, and in time it is laid down as a general axiom that “sand will prevent cracking.” Another has discovered that clay should be mixed in small quantity to produce the desired result, so he and his neighbours do that, and pity the ignorance of the “sand mixers.” A third feels quite certain that crushed brick, or brick dust, is a good thing; while a fourth will add a little lime. Now, each of these ingredients is useful in its way; everything depends upon the class of brick-earth to be dealt with. It may happen that what will, in a measure, prevent cracking, will be a bad thing in the burning, and the art of the brickmaker is to know what to do under the varied conditions.

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; and there is no better method of ascertaining the quantity required than by subjecting the materials to direct practical experiment in the kiln. Where no sand is available, it frequently happens that brick-dust will answer the purpose, though this may be at the expense of homogeneity in the long run. In the semi-dry process of manufacture the initial causes of cracking are not present, the block having to contract so little that it may be taken from the press and stacked in the kiln for burning. Unless the brick-earth be carefully prepared, however, the surfaces of the hard blocks produced by that process are liable to develop minute cracks. And here it may be stated that unless the clay, with brick dust or other foreign substance, be thoroughly incorporated prior to being sent under the press, and the whole ground very fine, it is impossible to prevent cracking during some part of the process.

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.

The commoner kinds of brick-earth, as we have seen, mostly possess gross particles, grit, pebbles, &c.; these act as so many centres from which cracks radiate either during the drying or burning, and apart from their influence in a chemical sense, they are apt to seriously weaken the brick.

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.

Turning to a smaller matter, the hand-barrow coming from the drying stacks to the kiln is unprotected, which often means that a good brick is spoilt. Of course, we are not alluding, in this connexion, to what takes place during clamp stacking; the brick produced by such a process must take its chance. The method of stacking in the kiln or clamp is very often responsible for damage to the bricks. A common method is to build them sloping outwards, and all sorts of strains and stresses are thus set up, which have their effect in producing lines of weakness, if not of actual visible cracks.

The “London stock,” if not a thing of beauty, is usually strong, and that in spite of the “breeze” which forms so many points from whence cracks radiate. We must not forget, however, that a really good London stock is, above all things, thoroughly burnt, and that is a set-off against the numerous and often wide cracks.

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. Even when dealing with one particular kind of brick-earth, the maker must be careful to notice the relative moistness of his charge, and vary the mode of procedure accordingly. Suppose the brick to be as “dry as a bone” before being put in the kiln, we shall notice a considerable amount of moisture coming out of it as soon as the fires are alight; and if the heat is applied too suddenly, the bricks are not improved—they contract unevenly and too quickly, and warp. When well alight, care should be taken to keep the temperature as uniform as possible, and when sufficiently burnt it must be lowered by almost imperceptible degrees. Above all things, there should not be too great a disparity between the temperature in the kiln and the outside air when unloading. Except to those who had minutely studied this matter, such a precaution might seem superfluous; it may be that no damage caused will be visible to the naked eye, but the microscope frequently shows flaws due apparently to this cause. The manufacturer may test this for himself by heating a good, sound medium burnt brick to the temperature usually found in his kiln when unloading, and suddenly plunging it in snow. It is not, perhaps, that any one of these things is especially dangerous to the brick, but it is the combined effect of all of them trending in the same direction. We desire to be clearly understood on this point. The cracks produced may not seriously impair the strength of the brick; they may be merely superficial, and they mostly are. But they materially assist the agents of denudation in “scaling” the brick, and weathering it unevenly. To this we shall return later on.

Let us now say something concerning the superficial changes produced in bricks by burning. The most important of all is the change of colour, upon which the sale of the brick depends in ninety-nine cases out of a hundred. We said a few words on this subject when dealing with the behaviour of individual minerals in the kiln. 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. 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. That is a most absurd notion. In a general sense the principle could be correctly applied to a limited district, and with one class of brick-earth; but it cannot be made to apply all round. There is nothing like experience in regard to a point like this. In a general way, of course, a pink, red, or blue tint may be produced from one brick-earth depending upon the temperature employed; but the bulk of brick-earths would melt and the whole kiln-full be ruined in any attempt to attain such a temperature as is used in burning a sound “Staffordshire blue.” Quite a large number of bricks made in the Southern half of England, may be described as having been dried in the kiln only—they cannot be said to be burnt, except that the heat employed was enough to turn them red, or to make them piebald; the particles are not agglutinated by fusion, and, indeed, there is often no trace of the constituents having been melted. On the other hand, we have red bricks in which the constituents are distinctly agglutinated by fusion, and the whole burnt thoroughly. The brick-earth of which these latter are made, would barely turn tint—would certainly not become red—at so low a temperature as that employed in producing the red in the non-agglutinated bricks alluded to.

It is not always an easy matter in burning a red brick to obtain two kilns full of the same tint, even in the same yard. When the employment of pyrometers becomes more general, that will be considerably simplified; but it is a difficult matter to get a reliable instrument, none of the forms hitherto invented being altogether suitable. That by Professor Roberts-Austen is as good as any. Many manufacturers, we are sorry to say, place colour before everything else; they even sacrifice durability to attain a certain tint. And there is much excuse for them so long as they find a ready sale for the material. When colours are made from artificially introduced mineral matter (which is not so often the case as some appear to think) the mineral introduced is, most commonly, iron; though it will be understood, from what we have previously said, that it must be used very sparingly.

The ultimate tint assumed by the brick cannot always be judged beforehand from the colour of the brick-earth. In brickmakers’ language, a red clay is one that produces a red brick, a blue clay a blue brick, and so on. For the most part, colour depends on the proportion of hydrated oxide of iron in the clay; if iron is present in an earth that contains no lime, or similar mineral substance, the colour produced in the brick at a moderate red heat will be red, and at the same temperature, with the same brick-earth, the more iron present the deeper the tint. In an ordinary brick-earth, when more than 10 per cent. of iron is present, the clay is apt to burn bluish, however, and, in certain cases, almost black. With a smaller proportion of iron, and the application of intense heat, the same tint may result, and the brick become vitrified. A brown colour may frequently be obtained when the brick-earth has from 2.75 to 4 per cent. of magnesia, or a similar proportion may be artificially added to the earth.

To obtain a white brick, so that it shall also be of excellent quality, the pure white clays of Devon and Cornwall are the best, though the so-called “white” is, in the majority of cases, a light cream colour, unless, of course, the brick is glazed. In the neighbourhood of London, a whitish brick results from a mixture of chalk (carbonate of lime) with clay or loam, and is known as a “malm.” In parts of Yorkshire, white pressed bricks are manufactured from common red clay mixed with magnesian lime (made from magnesian limestone) in a slacked condition. The latter ingredient, on introduction, immediately absorbs about 40 per cent. of the moisture present in the clay.

Yellow bricks can easily be manufactured from the more impure kaolins; also from certain clays in Cambridgeshire, Huntingdonshire, Kent, &c. (gault bricks); “malms” are mostly yellow, though called white.

Laboratory experiments, many years old, show that with white clay as a basis the following tints may be obtained. Phosphates of lime of various kinds = very light blue bricks. The phosphates, mixed with a quarter by weight of alum = brighter blue bricks. A mixture of white vitriol (sulphate of lime) three-quarters, with borax one-quarter = light dirty green. Sulphur and tin oxide in equal proportions = yellow. These experiments are interesting, but the ingredients would, as a rule, be too expensive for ordinary brick manufacture. They are more applicable for the production of ornamental tiles.

A time-honoured method of producing black bricks is to make any ordinary bricks red-hot and to dip them in a cauldron of boiling coal-tar for a few seconds. It is essential that the brick should be very hot, or the black staining will rub off. A good test that the operation has been successful is, that the surface shall be dull black, not shining. And there are many other ways of obtaining different tints, the description of which would be beyond the scope of the present work.

Unless a brick is extremely well burnt it is not uniform in colour throughout. A considerable proportion of a “draw” is often ruined in regard to tint by the adoption of an unsuitable form of kiln. Where the brick is actually burned (as distinguished from being baked), the contact of the flame from the fires is almost sure to lead to uncertainty in that respect along the flues. Impurities in the coal, such as iron pyrite, are the chief delinquents, and there is sure to be a certain amount of “flash.” In that, as well as in the baking method, bricks are liable to be discoloured by the bringing out of impurities which they themselves contain.