THE MANUFACTURE OF FIRE-BRICK AT MOUNT SAVAGE, MARYLAND.
BY ROBERT ANDERSON COOK, A.M., MOUNT SAVAGE, MARYLAND.
THE subject of refractory materials occupies such an important position in all metallurgical works, and particularly in those of iron and steel, that any data concerning it must be of interest to the metallurgist. No apology is needed, therefore, for this attempt to describe the mining of fire-clay and the manufacture of fire-brick at one of the largest establishments in this country.
Mount Savage is a small village situated in the northwestern part of the Cumberland coal basin, on the Cumberland & Pennsylvania Railroad, and at the foot of Savage Mountain, from which the village takes its name. Fire-brick have been manufactured here almost, if not quite, as long as in any place in America ; and as the brick are still shipped to nearly half the States in the Union every month, the manufactory is probably the most generally known. In the year 1837, a company was formed called the Maryland & New York Coal & Iron Company. It built two blast-furnaces, the ruins of which still remain. It was in the construction of these furnaces that the first fire-brick made here were used ; and though the iron works ran but spasmodically, the brick works have been in constant operation ever since. From 1837 until 1846, the Maryland & New York Coal & Iron Company, from then until 1848, the Lochiel Iron Co., and from 1848 until 1864, the Mount Savage Iron Co., ran the brick-works, and in connection with them the blast-furnaces and extensive rolling-mills.
In 1864 the Consolidation Coal Co. ran the entire works; and it was not until 1868 that the iron manufactory was given up, with the exception of the foundry. In 1870 The Union Mining Co. of Allegheny county was formed, and is now engaged in the manufacture of bricks from the mine which was opened about 1841.
The Clay Mine.
The mine is situated on the south side of Savage Mountain, three miles from the works by the tram-road. The bed of clay crops out along the summit of the mountain, and runs nearly northeast and southwest. The only other mine on this bed is a very small one, two miles southwest from that of the Union Mining Co. The clay from this mine is brought, to Frostburg, where it is manufactured into brick. The large bed was first opened on the outcrop, and for a number of years all the clay was dug from open pits, and hauled at great expense down the mountain in wagons to the works. Finally, when this method of mining had been carried on as long as was economical, the mine began to be worked systematically, and levels were driven on the outcrop, on one side, wherever it could be reached by reason of the formation of the hill. From this level, galleries were driven at an angle up on the bed, clear through to the old workings. Chambers were driven out from these galleries, connecting the galleries as often as the nature of the ground would permit. When these chambers are all driven through, that part of the mine is robbed of as many of the pillars between the chambers as it is practicable and safe to remove. There are several of these levels driven, the last one about 100 feet below the next above, and as the bed dips about one foot in every four on an average, one can calculate on the amount of clay each level will yield. From the present outlook there is enough to run the works for a great many years. At the time this more systematic mining was begun, some cheaper mode of transportation was also sought. First, a wire tram on the English system was tried, consisting of an endless wire rope, with buckets of the capacity of fifty pounds, and a stationary engine of eight-horse power at the bottom. This plan involved much trouble, and never could supply the requisite amount of clay; and when winter came, with its extreme cold and snow, the plant was practically useless.
Then the regular three-rail incline was
adopted, which is in common use in this coal region, and which has worked well ever since.
The only peculiarity of this incline is its great length. It is a mile and a quarter long,
and the rise from the
bottom to the top is 1240 feet. Six cars run upon it at a time; three loaded ones coming
down haul up the three empty ones. The rope is of steel, five-eighths of an inch in
diameter, and runs over two shrive wheels twelve feet in diameter, on each of which is a
band brake. One man to run these brakes, two men to load, one man to unhook at the bottom,
and one to look after the rollers on the incline, are all that are necessary to run 100
tons of clay per day. The cars when empty weighting 1800 pounds, and two tons of clay are
loaded on each car. It takes seven minutes, on an average, to rim one trip. This is said
to be the longest gravity road of its kind in the world. From the bottom of this incline,
the loaded curs run down by gravity on a tram-road to the brick-yard, and the empty cars
are hauled back by mules to the foot of the incline. The bed of clay lies at the very
bottom of the coal-measures of this hasiu. On top of the clay lies an 8-inch bed of coal;
beneath it lies from three to four feet of shale; and then comes the conglomerate rock
which marks the boundary of this basin. The bed of clay varies from eight to twenty feet
in thickness. The clay is divided into two varieties, the hard and the soft; and these are
distinguished by their physical properties. One of these varieties is of a medium gray
color, shading almost to black. This clay is very hard, and rattles like crockery when
thrown into the chutes. It has a distinct, though not regular, conchoidal fracture; it is
non-plastic unless ground to an impalpable powder, and does not crumble much when exposed
to the weather in heaps, being affected for only about three or four inches from the
surface though exposed for years. In parts of the mine this clay, when finely broken, is
sharp enough to cut one's hands. The other variety is a very plastic clay, of much lighter
color, weathering very rapidly, and in one season's exposure crumbling to powder.
The peculiarity of this deposit is, that the two clays are so intermixed in the same bed, and in such a way, that in the present development of the mine there is no accounting for the difference in structure of the clay. In one place the bed will be full from roof to floor of hard clay, and in another place, within a few feet of the former, the clay will all be soft. These sudden changes cannot be accounted for. Usually, the soft clay lies on top of the hard, and acts as a sort of protector for it, keeping off the coal-water. In some places, again, there is a gradual change from one to the other, from hard to soft and back again; and often the hard clay lies between layers of the soft. This is what causes the difficulty in the mining work and makes it seem irregular; for where the hard clay is struck small pillars and large chambers are made, and vice versa. The impurities in this day are much the same as in all other clays, except that they are fewer and smaller in amount. There are some balls of iron-ore found in the bottom of the bed, but these can readily be seen. The most objectionable impurity is iron pyrites, which is found in the slips of the soft clay, and particularly in the casts of roots in that variety. The detection of this iron pyrites is impracticable until after the bricks have been subjected to the intense heat of the kiln, when discoloration is shown in spots on their surfaces.
The coal which is used at the works is obtained on the property from the coal-measures above the clay. It is mined from a vein twenty-two inches thick, and is brought down to the head of the tram-road by a short incline, and there it is run in with the clay, and trains made up of both are run down to the brickyard. A good many analyses of this clay have been made, at various times and by different chemists, but it would not be safe to take any one of the various results as a test, for the difference in them is probably due as much to the chemists as to the samples. The following is an average of several results, which will probably give as accurate an analysis as one could obtain:
Impurities 2.06 "
Water, . 10.37
The Brick Manufacture.
The plant for carrying on the manufacture of brick here is as complete as can well be
found. There is a foundry and a machine-shop where every machine used in the manufacture
of the brick is made. The rollers and pans for crushing the clay, the tempering pans,
presses (hand and steam), are all made entirely at the works, so that all varieties of
shapes of brick can be made and pressed. The works are complete for turning out all brick
from the smallest nine-inch shape, weighing three pounds, to the largest glass-house
shapes, weighing three thousand pounds. The kilns for calcining the clay are built of.
brick with a boiler-iron shell. They are fifteen feet high and eight feet in diameter,
with fire-holes a few feet from the bottom. The top is dome-shaped with a chimney from the
center having a damper on top.
The clay is charged in through a hole near the top of the
dome, and is drawn out at the bottom of the kiln on iron plates, through two drawing-
doors, one on each side of the kiln, twenty tons being the daily product of one kiln. The
most important constituent is the calcined clay or chamotte. This will not shrink, and
possesses the power of union in the greatest possible degree. These two important
qualities have more to do with the production of a brick, regular both in size and in
quality, than any other features in the material or process employed. Another advantage in
calcining clay is that it enables one to throw aside any clay in which there are
impurities that may have
been previously overlooked, since these are much more easily seen when the clay has been
burned. The proportion used must, of course, vary with the size of the brick or tile, and
the particular use for which it is intended. From the crushing-pan, which consists of two
heavy rollers on a revolving grate, the clay goes by an elevator, through the screens,
into a hopper above the tempering-pan. This pan is on the same principle as that used in
crushing, except that, instead of having a grate- bottom, it is of solid iron. The clay is
carried to the tables of the various moulders by an endless belt. The brick are all
moulded by hand. They are dried on a brick floor, heated by flues running the whole length
of the yard. When dry enough, they are pressed in hand-presses unless they are too large.
In that case they are made in a steam-press at first, and smoothed up by hand when dry
enough. No machine for moulding is used here, though it was tried at one time with more or
less success, but the bricks were
never as even and regular as they are now. After the bricks are moulded, pressed, and thoroughly dried, they are ready for the kilns. The kilns used here are of two varieties. One is the regular down-draught kiln, rectangular in shape, with fire holes on two sides, and a flue in the bottom, running the whole length of the kiln, having a direct connection with a stack at one end, which gives the draught. Breasts of bricks are built in front of the fire-holes to protect the brick nearest the fire from direct contact with the flame. The other is a gas-kiln, having some of the peculiarities of the Hoffman, in that some solid fuel is used, and also the heat from the burning bricks dries the brick gradually. In these two points it resembles the Hoffman kiln, but it differs from that in not being a continuous kiln, being rectangular in shape, instead of elliptical, and using gas for its principal fuel.
The kiln is two hundred feet long, forming a tunnel with rows of holes through the roof for the admission of the gas and coal. These holes are provided with iron caps which can easily be removed. Along one side of the kiln is a flue leading to the stack, and flues leading out at right angles to this run across the kiln, so that the gas comes in at the top and goes out at the bottom. At one end of the kiln are fire-holes, and the brick are set at this end first, and the kiln set full for a certain distance. Care is taken to leave a clear space under each hopper in the roof, so that the fuel can go to the bottom. Iron plates are used to shut off the brick already in the kilns from the men setting the bricks, and these plates are made tight by smearing them with clay, making of each section a chamber by itself. The fires are lighted, and burn very slowly for a time until the first brick are gradually but thoroughly heated, when some fine coal is dropped in through the roof. The gas is then admitted through the holes in the roof, and the fire-holes are tightly closed, only enough air being admitted through the red hot bricks for the combustion of the gas. The gas-producer is on a truck, which runs on a track the full length of the kiln, and as fast as the brick are burned it is moved along the kiln. As it progresses the iron plates are drawn out and placed further up the kiln, adding new chambers. The brick are very gradually dried in this way, and a greater heat is developed with less cost in this kiln than in any of the down-draught kilns, which are heated in the ordinary direct way with coal. This kiln will hold three hundred thousand brick, and it takes thirty days to burn it from end to end. One man and a boy are all that are necessary to run the producer, except when it must be moved.
The combined product of the works amounts to the equivalent of something over twenty thousand nine-inch bricks per day.
The writer has been employed by the Union Mining Company in making such tests as seemed to be desirable to keep the brick up to the best form for any change which might take place in the market. It was not intended that other clays should be bought to mix with those found here; and from tests made here o'f brick from other places, and calculations from analyses of other clays, it is doubtful if any could be procured which would be of any advantage in the general run of fire-brick work. For the calculations in getting at the value of a fire-clay from its analysis, the formula used was one given by a German chemist, Dr.Carl Bischof, who is a recognized authority on the subject of refractory materials, and whose investigations on the subject have been carried out and verified to some extent in this country. He divides the clay into two parts, the silica and alumina constituting the refractory part, and the impurities the fluxing part. Dr. Bischof, in this formula, uses the impurities as a whole, but in another he divides them according to their relative strength as fluxing agents. Taking the alumina divided by the total impurities as a dividend and the result of the silica divided by the alumina as a divisor, the quotient will be a measure of the refractoriness of the clay as compared with that of another clay treated in the same way. Calling RO the impurities, the formula will be as follows :
As small a difference as 0.05 between the quotients thus obtained for two different clays indicates a difference in refractory quality which, other things being equal, will show itself in a furnace-test. These calculations are of great use in comparison of different clays, but the result one might expect from them may be entirely changed when the clays are made into brick. The physical qualities of all clays must be tested before an absolutely perfect comparison can be made. A sample of the same clay being used by two different brick-makers, yet the one brick made from it may not be as refractory as the other, though the sample may have been thoroughly mixed ; for if in one case the clay be coarsely, and in the other finely, ground, the coarse one will stand a great deal more heat than the other before it vitrifies to a homogeneous mass. This has been often observed before ; and the writer has found it perfectly true as regards this clay, that the more finely it is ground, the less refractory it becomes. At the same time, the more finely it is ground, the stronger and harder the brick becomes, the more abrasion it will stand, and the less likelihood there is of its being broken in handling. Though refractoriness is an essential of firebrick, yet it is not the only one. For the various positions in which the brick are placed, and the duties they are expected to perform from the upper part of a blast furnace where the heat is low, and the abrasion of stock is the greatest clement in the destruction of the brick, to the ports of an open- hearth steel furnace, where intense heat is the most destructive element, particular mixtures of clays should be made, to get the best results from raw materials. The greatest trouble of a brick manufacturer is that he cannot be sure for what purpose the brick will be used, or in what position in the furnace they will be placed. Another trouble is to find out where the fault lies, when complaint is made. This is almost impossible. It may be in the construction of the furnace, or in bad bricklaying, or in the grade of the brick, or that the brick were not hard-burned. And if a sample lot of brick is sent to a mill to be tested the chances are that when the superintendent is asked how the brick stood the test, he will have forgotten all about them. The only way for a manufacturer to test the brick, is to build a furnace and test them himself, and to do this under as nearly, as possible, the same conditions as those under which they will be used in practice. The furnace used by the writer for making such tests had nearly the form of a puddling-furnace. One-half of it was built of one mixture and the other half of another, running through the furnace from end to end. Bridge-wall, roof, side-wall and neck would show how the brick stood in each position. From the results of the tests a fair comparison could be obtained of the value of the brick. The draught was a direct one to the foot of a large chimney, and the coal used was a mixture of the best Cumberland coal and our own. A brick of the mixtures used in building the furnace was taken as a standard. One of these bricks, with another, either of some other mixture, or some brick which we svished to test, were placed side by side in the neck of the furnace, which was then fired as hard as possible for a certain length of time. When the furnace had cooled off, the bricks were removed, and the effect carefully noted, particularly as regards shrinkage and vitrification, and the effect of the heat on the furnace was also noticed. The heat in thirty-six hours was intense enough to vitrify any brick, but not enough to destroy them.
As the demand now is for a hard-burned brick, the difficulty of spotted bricks arises. These are bricks which appear to be of poor quality, for though a clay may not contain more than one per cent, of oxide of iron, yet, if it is exposed to a great heat, these spots will show and, at present, buyers, with the exception of a few who have learned their value, will not take spotted bricks. All the brick from other places, which the writer has tested, will, when exposed to our greatest heat, show some spots, although as they come out of an ordinary kiln, they are free from spots.
The two peculiarities which have made this fire-clay so famous, are its freedom from impurities, and the fact that it contains such a proportion of silica to the alumina that the brick, after they have been hard-burned, will swell a little instead of shrinking, no matter how much they are heated. The care which the company has taken in preparing its product for market, has borne profit in the gradual increase of its sales from year to year.
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