thus seen that a brick which is good for the cupola -would be worthless for the reverberatory furnace; that which answers well for iron would generally be worthless for zinc, and a crucible which is excellent for steel cannot be used for brass. It is not the way to realize progress to keep analyzing natural substances until we find the right one, or make repeated trials and depend upon them alone. All investigations go to show that we should look for artificial, and not for natural com pounds; and that when we have made a mixture which has stood well, we are then to analyze and examine it in order to reproduce it. Failure in this, as in many other cases, is very often owing to wrong application of good materials, rather than fault in the materials themselves.
When broken, fire-brick of good quality should show a ,corn pact and uniformly grained structure, free from cracks, stones, etc. When struck, they should emit a clear and ringing sound. The expansion of ordinary fire-brick by heat in rising from S2° F. to 212° F. is .00005, according to Rankine. All fire brick forming the lining of chimney-shafts should be set in ground fire-clay mixed with water to the consistency of mor tar. The brick are sometimes, before being laid, dipped into a liquid or creamy fire-clay, and when laid in place hammered together so as to be, when finished, brick and brick. This method is now largely adopted, and answers admirably where the temperature is high.
A properly burned brick, uniform throughout its mass, can be obtained only by very slow progressive firing ; a broken brick which has been too quickly burned, though pale on the surface, presents a darker central patch and concentric rings of various shades of color, due principally to the different states of oxidation of the iron, and partly to the presence of unconsumed carbonaceous matter.
A well-manufactured fire-brick should be of a pale cream' or deal' buff color, uniform throughout its mass, and burned to the full extent of its contractibility.
The chemical changes which take place in the burning con sist, first, of the destruction of the disseminated carbonaceous Matter, the dehydration of the. silicates of alumina, destroying their plastic character, and the decomposition of the dissemi nated carbonate or protoxide of iron, converting it into anhy drous sesquioxide to which the yellow of the burned brick is due.
Should the burning be carried to a very high state of vitrifi cation the yellow tint is replaced by a dull gray, due to the partial reduction of the sesquioxide of iron, and its conversion into silicate of protoxide or minutely disseminated particles of metallic iron. Any alkalies also present form vitreous combin
ations with the silica during the latter stages of the burning.
But the paleness of color of a fire-brick is not at all times a safe indication of the absence of iron, as the presence of a large proportion of carbonaceous matter in the clay tends to bleach ing by the reduction of the coloring sesquioxide to lower oxide preserved as a silicate in a comparatively colorless condition. Then, again, the presence of lime and the other alkaline earths, which are disadvantageous fluxing elements, will check the coloring power of a large percentage of oxide of iron by the formation of a pale double silicate of lime and iron. This is largely taken advantage of in the manufacture of buff-colored building brick, and, we are also very sorry to add, in the pro duction of buff-colored terra cotta, by mixing ground chalk with ferruginous clays which would otherwise burn a dark red color.
If the practical manufacturer wants a brick to yield slowly to corrosive influence, a simple test to apply is, to ascertain the number of times which the brick can be melted with oxide of lead and not be eaten through. In fire-brick constructions the use of joints of clay containing free silicic acid (quartz) should be avoided, which can be done by previously saturating the material with a basic burnt clay.
When good fire-brick are used, it is important that clay equally as good as that from which the brick are made should be employed in which to lay them. When ordering, brick consumers should also send their order to the same fire-brick manufacturer from whom they purchase brick, for clay of the same quality. These clays are usually kept in stock, and fire brick manufacturers can furnish them ground, either raw or calcined, in bags of 100 pounds each, or in barrels or in bulk, as may be desired. From 60o to 8co pounds, according to the way the clay is used, is sufficient to lay one thousand nine-inch brick.
It is recommended that No. t clay be used with No. r brick_ The brick should be " dipped " in a thick " soup " of ground clay, similar in character to that from which the brick used are made. There are few points which will so well repay con sumers of fire-brick as this " dipping" will do, as the compara tive first cost of the fire-clay is only trifling.