A practical manufacturer or salesman, who understands his business, would in such case, go over and investigate ; he would take pieces of the longest brick least damaged, and pieces of the small ends where most worn, and examine the fracture of same ; he might then come to the same conclusion as we have done under such circumstances. He would say to the manager, " Look at these two fractures ; they are not the same brick or the same material ; the inferior brick is some other manufac ture." Then take the pieces into the stockhouse and compare. Thus, in place of losing a customer, you increase the popularity of your own brick. This irregular wearing in furnace roofs is very important to the mill men, as it is necessary that the roof be perfectly even that the flame may have a clear flow, as in case of its obstruction by uneven tops or eddies, it is not only destroying the fire-brick, but retarding the manufacture of the iron.
Silicious brick have a tendency to expand under the influence of intense heat. This is true to such an extent that in the steel furnaces where they are used, provision must be made for slack ening the tie-rods when the fire is being raised, and tightening them when it is being cooled.
The crushing weight of an ordinary fire-brick is from 600 to 1000 pounds, but some of the best have been known to resist as high as 300o pounds to the square inch. To insure the safety of the structure, and the success of the process, it should not only retain its power of resistance, but should not undergo any change of form nor soften materially under long-continued heat, and at the highest possible temperature should support more than double the strain required without alteration. In the walls of the fire-place those brick will be best wtich are dense, and contain an excess of silica. In the hearth they should contain an excess of alumina. In the arch they should be nearly pure silica, alumina or magnesia. Brick in a roof give out from shrinkage, cracking or splintering. Splintering may take place when silicate brick are made of impure mixture, but it is usually caused from too much fine material and from imper fect burning. Brick which are liable to splinter are generally cross-grained and dense, with a smooth conchoidal fracture, when made from improper mixtures, and when badly burned they generally sound like a cracked vessel when they are struck together. All good brick wear off evenly.
No matter how good a material may be, if its price is so high as to prevent healthful competition, it might as well not be produced. Hence any effort to furnish a good material should have for its aim to make it at the least possible cost.
In discussing the manufacture of a refractory material which is to be used in a given locality, there is to be taken into ac count, first, the clay and other materials to be had ; second, the ore or metal to be treated ; third, the fuel to be used ; and fourth, the foreign substances in the gangue of the ore or metal. Whether to use a given clay, or a mixture of calcined or raw clay, must be determined by direct experiment, and then the size of the grains of the mixture for the special use must be determined, for each substance is more or less refrac: tOry acdording it is coarse or fine. Thus, in Belgium a porous' material with a large grain is used for blast-furnace brick, but a fine material with a close grain for coke-furnaces, the chemical composition being the same in both cases. It must then be ascertained whether the mixture contracts or expands, for there are clays that contract and expand between one-thirty second and one-eighth of their bulk. The way in which material tempers must then be carefully studied. It is not sufficient to have a good material, for almost as much depends on its manipulation as on the material itself. To temper properly, the clay and the manufactured article should both be dried gradually and uniformly. It must be fired evenly, and the temperature slowly raised to the proper point. If it is to be used in the raw state as ganister, it must be equally moist throughout, so as to dry uniformly, and not so wet as to cause it to crack in drying, or so dry as to prevent its binding. The brick, or other materials, once made, should be kept from dampness; as they are porous, and likely to absorb moisture, they should be heated before being used in the furnace, and put in as hot as it is possible to handle them. If the furnace is in blast, this requires a special furnace and a high heat. If it is to be put in blast at once, especially with silica brick, the temperature should be as high as the hand can bear. If the surface is to be a long time standing, this precaution is not necessary, but in the last two cases the furnace must be dried 'very carefully and slowly. No brick which has been dressed should ever have the dressed face exposed to the flame. Without the observation of these precautions a really good brick may give a very bad result. It is too much the habit of ,this to be in a hurry to get results, and this has led some blast; furnace managers to boast that steam was issuing from the:top of their furnace while cast-iron was being tapped from the :bottom; but under such management we never hear of long campaigns, but very frequently hear of disasters.