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 hot, 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"

artificial." The former, as the name implies, is made from the natural rock, while the latter is an artificial mixture, the ingredients being so proportioned as to bring about the best results. As might be expected, the latter are stronger, more durable, and much more expensive. Artificial cements are also known as "Portlands" from the fact that they were first manufactured in England, and that when set they bore a strong resemblance to the natural stone found in the island of Portland. In a very few localities limestone has been found which when burned has almost the same composition as the artificial Portlands. On account of this similarity these have been called "natural Portlands." A cement of this character was produced in France in 1802. Portland cement as known at the present time was first manufactured in England in about 1824, although patents for "Portland cements" had been issued several years previously.

The following is the description given by the patentee in the first specifications issued:

"I take a specific quantity of limestone and calcine it. I then take a specific quantity of clay and mix it with water to a state approaching impalpability. After this proceeding I put the above mixture into a slip-pan for evaporation till the water is entirely evaporated. Then I break the said mixture into suitable lumps and calcine them in a furnace similar to a lime-kiln until the carbonic acid is entirely expelled. The mixture so calcined is to be ground to a fine powder, and it is then in a fit state for cementing. The powder is to be mixed with a sufficient quantity of water to bring it into the consistency of mortar, and this applied to the purposes wanted."

In 1796 a Mr. Parker of London patented a process of making a "Roman" cement. This was so called, and properly, on account of the similarity to the cement in use by the Romans so many years before.

In this country a cement similar to the above was manufactured at Fayetteville, N. Y., in 1818.

Portland cement was first produced in the United States in 1865. At the present time the principal works are situated in Pennsylvania, Ohio, New York, and one in South Dakota.

In 1828 a natural-cement rock was discovered at Rosendale, New York, and afterwards similar formations in other portions which, on account of the similarity, were also called "Rosendales," being distinguishable from each other by a special name for each brand. As the country was settled and construction work was undertaken in other sections, more deposits were found, a notable one near Louisville, Ky., and now some authorities call all natural cements "Rosendales," to separate them from the "Portlands."

In the new Building Code recently adopted by the city of New York the following is found in relation to Portland and other cements:

"Cements classed as Portland shall be considered to mean such cement as will, when tested neat, after one day set in air be capable of sustaining without rupture a tensile strain of at least 120 pounds per square inch, and after one day in air and six days in water be capable of sustaining without rupture a tensile strain of at least 300 pounds per square inch. Cements other than Portland cement shall be considered to mean such cement as will, when tested neat, after one day set in air be capable of sustaining without rupture a tensile strain of at least 60 pounds per square inch, and after one day in air and six days in water be capable of sustaining without rupture a tensile strain of at least 120 pounds per square inch. Said tests are to be made under the supervision of the Commissioner of Buildings having jurisdiction, at such times as he may determine, and a record of all cements answering the above requirements shall be kept for public information."

It will be seen by the above that the cement is graded by its strength. This standard is, perhaps, as satisfactory as any, if the tests are carried on for a sufficient length of time, but most engineers would hesitate to accept or reject cements of which they knew nothing, from the result of so short a time-test as seven days. Under this clause no cements can be used that develop a strength of less than 60 pounds in one day set in air.

Natural and Portland cements can be readily distinguished, however, by their composition.

Table No. 14 is made up from analyses of well-known cements and taken from Cumming's "American Cements."