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.” Another95 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, c?teris 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 brick96 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, which97 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 and98 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............