So far in our operations we have expended labor only. The material spoiled can all be sent back and reworked, but we are now to undertake an operation which, if unsuccessful, will entirely ruin it, except the small proportion that might be used as grit It will certainly pay us, then, to see that no brick already imperfect shall enter the kiln. To this end it is well to punish severely any cleaner who passes through an imperfect brick. The brick are right under the cleaner's eyes, and no defect can escape his attention if he keeps his eyes open. After seeing that we have perfect brick to enter the kiln it would be the height of foolishness to set them in anything but a perfect kiln; but here opinions differ. Which is the perfect kiln? For enameled brick economy of fuel cuts less of a figure than in some other branches. Let us suppose our brick have cost us eighteen dollars per thousand, set in the kiln, and we set 20,000 in the kiln. This kiln full of brick has cost, then, $360. We will assume we should get 90 per cent of firsts brick, and that the kiln consumes forty tons of coal at $3 per ton; that the cost of burning and emptying kiln is $40. We would have 18,000 No. 1 brick at a total cost of $520, or $28.88 per thousand. We will take another kiln that will consume only twenty tons of coal, but, we will say, due to its poor burning, we get 15 per cent additional bad brick, or 75 per cent in all of No. 1 brick. We would have 15,000 No. 1 brick at a total cost of $460, or $30.66 per thousand. We will take another kiln and assume it burns five tons of coal, but spoils an additional 15 per cent in the burning. We would then have 12,000 No. 1 brick, at a total cost of $415, or $34.58 per thousand. I have taken extremes in the difference of amount of fuel consumed and have been very mild in the difference in percentage of perfect brick produced, and yet we see the less economical kiln produces a larger output and at less cost per thousand. Now, I do not mean to argue that the best kilns are the least economical; on the contrary, I believe the best kilns are the most economical - that is, the most economical of their style. I merely wish to warn those who contemplate building kilns for the purpose of burning enameled brick not to look so much to the economy of fuel as to the efficiency of the kiln. Put up a kiln that will burn evenly throughout and trust to luck on the coal bill. Having a perfect kiln, don't overcrowd it at a risk, for even if you succeed in getting 25,000 brick in it instead of 20,000, and thereby have an additional 7 per cent spoiled, it does not pay, supposing the kiln to burn with the same fuel. Again, let the setting be perfect for the material employed. If your brick will stand the weight perfectly set eight feet high, set them that way, but don't risk them ten feet high. If they will not stand well over four feet high, make some arrangement for supporting them. If they will not stand open burning, burn in saggers. The difference in cost between saggers and open burning is not so tremendous as some writers try to make out It is probably from $3 to $3.30 per thousand.
The up-draft kiln will probably stand more faulty construction than the down-draft kiln and give fair results, but the down-draft kiln, when properly constructed, will give excellent results. The best kiln, in my opinion, for this class of work, is the down-draft, with under-draft to center. It is under perfect control, and there is no long hold necessary to bring up the bottom; in fact, the bottom can be fired hotter than the top if desired. It is an extremely economical kiln. This is undoubtedly due to its even-burning qualities. No fuel is wasted waiting for cool spots to heat up. As usually built, with annular flue for the waste heat, it is an expensive kiln to build, but this might be obviated without sacrificing its good qualities. It is better not to build the kilns too large; beyond eighteen feet in diameter I believe to be a mistake. The English are probably ahead of us on kiln building for this class of work. It is natural that they should be, not only on this point, but on others also, as they have had years of experience. I understand they use rectangular kilns in many instances, and I have had described to me kilns working in succession; that is, the waste heat from one kiln drying off and watersmoking the next. In this country, so far as I have seen, round kilns are used, of various styles, plain up-draft and plain down-draft, and down-draft with under-draft. For open fire the kiln usually should not exceed eight feet in height to spring of crown, though there may be clays that will stand higher setting. For setting in saggers the kiln may be as high as fourteen feet to spring of crown.
There are two methods of setting open. One is to set in concentric circles, all brick on end, with enameled faces about one inch apart. After setting three high on end, cover and tie the two rows with a tile; then set three high to top, occasionally tying two rows or circles together. There should be occasional openings left through the rings, so that the different flue spaces will not be isolated. Between the double rows a two to two and one-half inch space should be left. Where it is desired to set quoins, the brick stretchers are set five to six inches apart, each course covered and the quoins put between the stretchers. The other method is to set two to three feet high in parallel straight courses, and then to set two or three feet more, the line of the second set being at right angles to the first. Some brick are set on end and some on edge, the headers being used as ties. In this method very few, if any, tile are used. This second method gives a more uniform draft, but it is more difficult to keep the brick straight The setting in saggers is impossible to describe, but it is easy to obtain setters who thoroughly understand it The advantage of saggers is great as there is unimpeded draft in all directions, and a kiln so set will burn more uniformly. Each brick has to bear its own weight only, so that clays may be used that would be unfit for open burning. Also, many clays will burn easily in saggers without cracking that are almost impossible to burn in an open-fire kiln. A convenient size for saggers is 9 1/4 x 10 inches by 9 1/2 inches deep. Such a sagger will hold six English size brick, set across the 9 1/4-inch way, and will have a space of one inch between the faces, or the same sagger will hold eight American size brick, set the other way, and leave a one-inch space between the faces.
The burning of enameled brick may be done with either hard or soft coal or fuel oil. There seems to be no special advantage in any one of these fuels except the local one of ex-pense. The firing should be slow and steady. Especial pains should be taken not to allow the heat to go down at any point from the beginning to finish. About five days is necessary to burn a kiln set open, but the burning can be done in three or four days when the brick are placed in saggers. The same care is necessary from the time the watersmoke commences to leave the brick until it is all out, as with other brick, and between these points the firing must not be pushed. With lime glazes the cooling can be done slowly from the time kiln is finished until cold, but with glazes containing lead the kiln should be cooled as rapidly as possible for three or four hours, or until it has perceptibly cooled, and then it should be closed up and cooled slowly.
Muffle kilns are sometimes used, but they possess no advantages and many disadvantages. They are expensive to burn and keep in repair, and are difficult to burn evenly.
The finished brick are usually assorted into three grades. The No. 1's are the perfect brick; the No. 2's are those with slight defects, but good enough to present a fair appearance; the No. 3's are those which are unfit for use as enameled brick, and include the cracked and chipped brick and those which are underfired or overfired. The packing is usually done in barrels, and sawdust makes the best material in which to pack. Where they can be handled open at the building it is cheaper to pack directly in the car, using straw or sawdust as a packing material. The brick really carry better than in barrels. Some manufacturers are using boxes made of corrugated or indented strawboard, and each brick is placed in a separate box. This makes a very nice way of packing the brick, especially if they are piled in stock in these boxes, as they are not apt to receive any injury, but if a large stock is carried considerable extra capital is locked up in these boxes.
A comparison between the economy of English kilns and kilns on this side of the water is probably in order at this time. I quote the following from an English writer: "The quantity of coal required to complete the burning of a kiln of 18,000 bricks is found to be from twenty-four to twenty-six tons, the coal employed being of first-rate quality and of high calorific value." This is a little better than I have been able to do, burning in saggers in an up-draft kiln, but in down-draft kiln, with under-draft, brick placed in saggers, about thirty pounds of sagger to forty-two pounds of brick, I have burned 7,500 English size brick with seven tons of bituminous coal, and I understand in larger kilns 15,000 are being burned with from ten to twelve tons of bituminous coal. The heat used was certainly not lower than the English, as three brick, from three of the best English makers, were placed in the kilns on this side and the glaze was overfired.