THE LIME

To make mortar with which masonry is held in place it is customary to use lime. In a sort of trough lined with sand are placed lumps of stone having a calcined appearance, and on these stones water is poured. In a few moments the pile becomes heated to high temperature, cracks and splits and finally crumbles into dust, at the same time absorbing the water, which disappears little by little as it is taken up by the solid matter or vaporized by the heat. More water is added to reduce it all to paste, which is finally mixed with sand. The product of the mixture is mortar. Such is the process often witnessed by Emile and Jules, who are always surprised, that stone, by having water poured on to it, should become hot and turn the water into jets of steam. “Lime,” Uncle Paul explained to them, “is obtained from a widely diffused stone called limestone or, in more learned language, carbonate of lime. The process is of the simplest sort. It consists of heating the stone in kilns built in the open air in the vicinity of both limestone and fuel, so as to avoid the expense of transportation in the manufacture of a product that it is desirable to furnish at a low price.[48]

“A lime-kiln is about three meters high, and is lined with fire-proof brick. An opening at the bottom serves for taking out the lime when the firing has continued long enough. In filling the kiln it is the usual practice to begin by laying large pieces of limestone so as to form a sort of rude vault over the fireplace, and on this vault are piled smaller fragments until the entire cavity is filled. The fuel used may be fagots, brushwood, turf, or coal. After the firing has gone on long enough, operations are suspended and the lime is withdrawn by breaking down the vault supporting the entire mass, which crumbles and comes crowding out at the lower opening, whence it is usually removed.

“Another method still followed in some localities and of more ancient origin consists of filling the kiln with alternate layers of fuel and limestone. The whole rests on a bed of fagots that serves for starting the fire. As soon as the fire has spread throughout the mass, the opening at the top is closed with pieces of sod in order to make the combustion slower and more even.”

“Nothing could be simpler,” said Jules, “than lime-making. Now I should like to know what effect the heat of the kiln has on the limestone. How does it happen that stone turns into lime by passing through fire?”

“Limestone,” answered his uncle, “contains two different substances: first, lime, and then an invisible substance, impalpable as air itself, in fact, a gas, carbonic acid gas. The name of carbonate of lime [49]given to the limestone denotes precisely this combination. As it is when taken from the ground, the stone contains the two substances closely united, so incorporated indeed as no longer to have the qualities characterizing them when apart. Heat destroys this union: the lime stays in the kiln, and the carbonic acid gas is dissipated in the atmosphere with the smoke from the burnt fuel. After this liberation of the gas the lime is left in its pure state, no longer masked by the presence of another substance, but just as it is needed by the mason for making mortar.”

“Then all that the fire does,” queried Jules, “is just to break apart the limestone and drive out the carbonic acid gas that it contained?”

“What takes place in the lime-kiln,” replied his uncle, “is nothing but the separation of the lime and the gas. Now let us turn our attention to the mortar. When lime is watered, it gets very hot, swells, cracks open, and crumbles into a fine powder like flour. The heat that is generated comes from the violence with which the two substances rush together. Before absorbing water lime is called quicklime; after this absorption, which has reduced it to powder, it is called slaked lime. This slaked lime is reduced to a paste with water, and then well mixed and kneaded with sand. The result is the mortar used in laying stone and brick in order to hold the courses firmly together and give solidity to the building.

“There is one thing I advise you to note, if you have not already done so, since it will explain to you [50]the part played by mortar in masonry. Look at the water that for several days has covered a bed of lime slaked by the masons. You will see floating on the surface small transparent particles resembling ice. Well, these tiny fragments of crust are nothing but stone like that from which the lime was obtained; in a word, they are limestone or carbonate of lime. To make stone of that kind two substances are necessary, as I have just told you: lime and carbonic acid gas. The lime is furnished by the water, in which it must be present in solution, since the water covers a thick bed of this material; and as to the carbonic acid gas, it is furnished by the air, where it is always to be found, though in small quantities. Lime, then, has this peculiarity, that it slowly incorporates the small amount of carbonic acid gas present in the atmosphere, and so once more becomes the limestone that it was before.

“A similar process goes on in mortar: the lime takes back from the atmosphere the gas that it had lost in the heat of the lime-kiln, and little by little becomes stone again. The sand mixed with it serves to disintegrate the lime, which thus more easily absorbs the air necessary for its conversion into limestone. When the mortar has fully resumed the form of limestone the courses of masonry are so strongly bound one to another that the stones themselves sometimes break rather than give way.

“What is known as fat lime is lime that develops great heat when brought into contact with water, and also increases considerably in volume, forming with [51]the water a thick, cohesive paste. On the other hand, poor lime develops but little heat, disintegrates slowly, and increases scarcely any in volume. The first kind comes from nearly pure limestone and can be mixed with a large proportion of sand, thus making a great quantity of mortar. The second kind is obtained from limestone having various foreign substances and will admit of but a small admixture of sand, thus yielding less mortar than the other. Both have the property of hardening in the air by the absorption of carbonic acid gas which converts them into limestone.

“There is a third variety of lime called hydraulic lime, which has the peculiar merit of being able to harden under water. It is made from a limestone containing a certain proportion of clay. Hydraulic mortar is used for the masonry of bridges, canals, cisterns, foundations, vaults, in fact for all stone and brick work under water or in damp soil.”