clay must necessarily result in laminations or lines of demarcation between the different speeds of the clay bar similar to the veins of a glacier.

If the air has been expelled from the clay by the pug-mill, these lines can be largely closed up again by a properly shaped die, and first-class brick will result in which the laminations will be inconspicuous and of no importance. But if the air has not been expelled, or the mill and die are not properly designed, there will be an excessive amount of concentric lines that almost divide the cross-section of the brick into a series of shells or concentric cylinders that greatly weaken the brick for withstanding blows or frost. The character of the clay also greatly increases these laminations, as the softer it is tempered, or the more plastic it is, the more serious is this trouble. The clay should be worked as stiff as possible, not only to make it dense and reduce the shrinkage, but also to reduce the laminations. A very stiff clay requires more power to work it, however, and if too stiff is very apt to break down the machine.

Repressing.

Repressing consists in putting freshly made stiff-mud brick into a die-box and momentarily subjecting it to a heavy vertical pressure, which is usually applied on the flat side. This fills out the angles and edges, making a much more shapely and uniform brick which is slightly denser, but probably also decreases the laminations.

Drying.

The stiff-mud brick are pilled in a sort of checkerwork on cars as high as they will bear their own weight, some six or eight courses high, and dried in long tunnels or drying-chambers, heated by direct fires, steam-pipes, or hot air. On account of the marked difference in the drying properties of clay, the selection and design of the dryer is a very important matter and it must be adapted for the specific clay to be used. Some clays can be readily dried in 18 to 30 hours without checking or injuring, while others need 48 to 60 hours, or longer, to avoid cracking to pieces. This means a great difference in the drying arrangement and expense of operating the drying plant, which too frequently is not appreciated by

the brick-maker or enthusiastic venders of patented dryers, and generally results in an expensive drying department.

Burning.

This is a most important part of the paving-brick business, as, no matter how good the clay, or how well it may have been mixed, without proper burning it cannot make No. 1 paving-brick. The kind of kiln employed in burning paving-brick is the down-draft rather than the round or oblong, as the up-draft type produces too heavy a percentage of soft and overburned brick. A continuous kiln has also been tried on paving-brick, but has not been very successful. The improvements that have been made, however, would seem to indicate that this type might at some time be used.

It is interesting to note the changes that occur in paving-clays. in passing from the condition of mud to a first-class paving-brick. When the moulded brick go into the dryer and the mechanically mixed water is evaporated, the brick shrink from 2 to 11 per cent to a firm earthy mass that admits handling and in which the individual particles of clay are plainly distinguished. On being heated to a red heat, or about 1200° Fahr., the chemically combined water is driven off, which renders the clay non-plastic and it again begins to shrink and to grow harder and stronger. As the heat is raised above redness, the individual particles of clay may be still easily recognized and the brick are very porous. When the heat is still further raised to about a bright cherry heat or from 1500° to 1800° Fahr., depending on the particular clay, it shrinks. an additional 1 to 10 per cent and is very much stronger and much less porous. It has the acquired hardness of tempered steel and the individual particles are no longer recognizable. This is the beginning of vitrification. From this stage to the molten mass there is no longer any sharp line of demarcation, and as the heat is increased the brick finally become viscous and semi-liquid, and when chilled and broken present a thoroughly glassy appearance.

From the point at which the clay particles have so coalesced that they can be no longer recognized to the point of viscous liquidity requires an increase in temperature of 100° to 600° Fahr., according to the kind of clay, and is usually 400° in a clay suitable for paving-brick. Midway between these two points the clay

ceases to be porous and stops shrinking, which is the maximum degree of hardness and toughness, and is the point at which the burning should be stopped in order to produce an ideal pavingbrick.

The burning usually takes from 7 to 10 days, a shale brick requiring from 1500° to 2000° Fahr., and those of fire-clay from 1800° to 2300° Fahr. If shale brick are heated too hrot, they melt into a more or less solid mass, yet it is usually necessary to bring them to a heat which would cause them to stick together if not prevented by sand that is freely sprinkled between them in setting. At the temperature when they border on the condition of a very viscous fluidity, the lower brick become "kiln-marked" by the weight of the upper bricks forcing the lower bricks slightly into one another, and care is required to prevent this pressure from becoming too great by not setting them too high. Paving-brick are set only 22 to 34 courses high, according to the fusibility of the clay. Coal is used throughout in burning pavers, which do not need the preliminary or water-soaking stage. Oil and natural gas however, have been used in some localities and are far superior to coal in reducing labor in burning, and producing a superior quality of brick, from the uniformity of the fire and avoidance of the air-checks that result from chills when cleaning the gratebars.

Annealing.

After the kiln has been maintained long enough at the vitrifying temperature to heat the bricks through the centre, the kiln should be tightly closed and allowed to cool very slowly. Slow cooling is the secret of toughness, and the slower the cooling the tougher the brick. This annealing stage is often curtailed, on account of insufficient kiln capacity, and the kiln cooled down in 3 to 5 days in order to hurry up the brick, often to removing bricks that are so hot as to set fire to trucks. At least 7 to 10 days should be allowed for cooling to secure tough brick, and those who desire the best article can well afford to pay the extra cost of still slower cooling if quality is the first consideration.

Sorting.

If the kiln is properly burned, it will be found to have from 1 to 4 courses, the top brick, that are burned extremely hard, and which are more or less air-checked by being struck by cold air in coaling or cleaning the fires. The top course is also more or less

covered with a film of ashes and dust that has been carried over by the draft. Such bricks are excellent for sewer or foundation work, as they have the maximum resistance to crushing strength and minimum porosity. Beneath the top layer the brick to within 2 to 12 courses of the bottom are No. 1 pavers, or brick that should be perfectly sound, completely vitrified, and have the maximum strength, hardness, and toughness. Beneath these are 2 to 10 courses of brick which have not received sufficient heat to completely vitrify them and which are classed as No. 2 pavers, and used as the foundation or the flat courses in paving. Beneath the No. 2 pavers are from 1 to 6 courses of brick which have not received heat enough to be able to withstand the frost and are called builders, as they are about equivalent in strength, hardness, and porosity to the hard-burned building-brick.

With a fire-clay it is possible to produce 90 per cent of No. 1 pavers, as there is no risk from overfiring them, while 80 per cent is a high average for shale. One frequently sees claims by venders of patented kilns of 90 per cent of No. 1 pavers, but such a very high percentage is rarely attained with careful grading, while 80 per cent is a high yield, and most yards do not get as high as 70 per cent of strictly first-class No. 1 pavers.

CHAPTER V.

CEMENT, CEMENT MORTAR, AND CONCRETE.

WHEN. a pure limestone has been properly burned or calcined the result is lime, that is, the carbonic acid has been driven off by the action of the heat. When water is applied to the lime it slakes with a great increase in volume, and if more be added it can be formed into a paste, which when mixed with sand will harden or set if exposed to the air.

Limestone, however, is very seldom found in a pure state, the principal impurities generally being silica, alumina, iron, and magnesia. When these impurities exceed 10 per cent the resulting lime has the property of setting under water and is said to be "hydraulic." If, however, the rock contains about 40 per cent of silica and alumina, the product of the calcination will not slake upon the application of water, but must be reduced to a powder in mills,. when it is made into a paste as with the lime. This product is known as "cement." It differs from lime physically in that it requires to be reduced to a powder before being used, and does not materially increase its volume in setting.

Cements were known to and largely used by the Romans, and it is said that the workmen excavating in London, England, in 1892 found a natural-cement concrete which was known to have been laid eight hundred years before. During the middle ages there seems to have been little knowledge of limes and cements, as what is known at present dates back to the time when John Smeaton, in seeking for a mortar with which to construct the Eddystone lighthouse, discovered the hydraulic character of certain limestones, and that this property was caused by the presence of clay in the original rock.

Cements are generally spoken of in this country as "natural"