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.
“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enough, operations are suspended and the lime is withdrawn by breaking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 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 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 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.”