There is hardly any material used by the architect or builder upon which so much depends as upon mortar in its different forms, and it is important that the architect should be sufficiently familiar with the different kinds of limes and cements to know their properties and in what kind of work each should be used. He should also be able to judge of the quality of the materials with sufficient accuracy to prevent any that is actually worthless from being used, and should have some knowledge of mortar mixing.
Common lime, sometimes called quicklime or caustic lime, is produced by the calcination (or heating to redness) of limestones of varying composition. This is done by burning the stone in a kiln with an oviod vertical section and circular horizontal section. The broken stone and fuel (generally coal) are put in in layers, the fire lighted at the bottom, and as the lime drops to the bottom new layers of stone and coal are put in at the top, so that the kiln may be kept burning for weeks at a time. The limestones from which limes and cements are produced differ greatly in their composition, ranging from pure carbonate of lime, such as white chalk or marble, to stones containing 10 per cent, or more of impurities, such as silica, alumina (clay), magnesia, oxide of manganese and traces of the alkalies. The quality of the lime will consequently depend much upon the percentage of impurities contained in the stone from which it is made. Lime is manufactured in nearly every State in the Union, each locality generally producing its own supply.
There is considerable difference, however, in the limes of different localities, and before using a new lime the architect should make careful inquiries regarding its quality, and if it has not been much used it would be better to procure a lime of known quality, at least for plastering purposes; for common mortar it is not necessary to be so particular.
In most parts of New England lime is sold by the barrel, but in many parts of the country it is sold in bulk, either by the bushel or by weight
Good lime should possess the following characteristics: 1. Freedom from cinders and clinkers, with not more than 10 per cent, of other impurities. 2. It should be in hard lumps, with but little dust. 3. It should slake readily in water, forming a very fine, smooth paste, without any residue. 4. It should dissolve in soft water.
There are some limes which leave a residue consisting of small stones and silica and alumina in the mortar box, after the lime is drained off. Such limes may answer for making mortar for building purposes, but should not be used for plastering if a better quality of lime can be procured.
The first step in the manufacture of lime mortar consists in the slaking of the lime. This is generally done by putting the lime in a water-tight box and adding water either through a hose or by pails, the amount of water depending upon the quality of the lime. Lime such as is sold in New England requires a volume of water equal to two and one-half to three times the volume of the lime. The water is rapidly absorbed by the lime, causing a great elevation of temperature, the evolution of hot and slightly caustic vapor, and the bursting of the lime into pieces, and finally the lime is reduced to a powder, the volume of which is from two and a half to three and a half times the volume of the original lime. In this condition the lime is said, to be slaked and is ready for making into mortar. The Thomaston and Rockland (Maine) lime, as also most other limes sold in New England, slake without leaving a residue, and the mortar is made by mixing clean, sharp sand with the slaked lime in the proportion of 1 part of lime to about 5 of sand by volume. Practically the proportion of sand is seldom, if ever, measured, but the sand is added till the person mixing the mortar thinks it is of the proper proportion. For brickwork over a certain proportion of sand cannot well be added, for if there is too much sand in the mortar it will stick to the trowel and will not work easily. With stonework the temptation is always to add too much sand, as sand is generally cheaper than lime. The architect or superintendent should take pains to make himself familiar with the appearance of good mortar, so that he can readily tell at a glance if it has too much sand. Mortar that contains a large proportion of lime is said to be rich; if it has a large proportion of sand and works hard it is said to be stiff, and to make it work more readily it is tempered by the addition of water. Tempered mortar looks much richer than stiff mortar, though it may not be so. If the mortar slides readily from the trowel it is of good quality, but if the mortar sticks to the trowel there is too much sand in proportion to the lime. The color of the mortar depends much upon the kind and color of the sand used.
Many of the limes used in the Western States when slaked leave a residue of stones, lumps and gravel, so that instead of mixing the mortar in the same box in which the lime is slaked, a larger proportion of water is added, and the slaked lime and water (about as thick as cream) is run off through a fine sieve into another box, in which the mortar is mixed. Such lime does not make as good mortar as that which leaves no impurities, but it does very well for use in brick and stone work.
The general custom in making lime mortar is to mix the sand with the lime as soon as the latter is slaked and letting it stand until required for use. Much stronger and better mortar would be obtained, however, if the sand were not mixed with the slaked lime until the mortar was needed.