In the preparation of the various mortars the first operation to be performed is the slaking of the lime or cement. In the case of the latter class of materials, which are always ground before being used, the only precaution to be observed is to avoid bringing them into a fluid state ; or, in other words, it is advisable only to add enough water to bring them to the consistency of a tenacious modelling clay ; especial care being also taken to moisten the stones or bricks to be used simultaneously with them, so as to prevent any absorption of moisture from the cements. In the case of the hydraulic limes, when used without being previously ground, the slaking is performed either in pits or by aspersion, or by immersion ; and the object to be obtained is to cause the lime to fall into a fine dry powder, in the state of the hydrate ; which powder, however, would not possess the power of crystallising without the addition of a further quantity of water. This additional quantity of water is added at the same time as the sand, introduced for the purpose of affording nuclei around which the crystals of the lime may arrange themselves ; and it is indispensable that the whole of the lime in a mortar should have been thoroughly slacked before it is placed in the work. At the present day, in all large building operations the mortar is made by means of Puc-mit.ts, or mortar-mills, the latter being formed of large stone or iron wheels revolving in hollow basins; and when those machines are properly used the mortar is more uniform in quality, and more free from core than in the ordinary process of manufacture. The core is, in fact, the under-burnt lime, whieh slakes, when in lump, less quickly than the rest of the lime, and is therefore exposed to take on its action of expansion at a different period ; this action would, perhaps, disturb the crystallisation of the rest of the work, and great precautions are required to prevent its occurrence ; it is known amongst workmen by the phrase of "the lime blowing." The pure rich limes are more exposed to this class of action than even the hydraulic ones; because in the process of calcination setae portions of the lime stone become not only under, but also otners become over burnt, and the periods of slaking of the core, produced by either of those causes, differ materially from that of the normal lime. Great care is therefore required in slaking the rich limes, and the old Romans seem to have adopted the custom of shaking them at least three years before the mortars were made ; a far greater quantity of water may be used with these limes than with the hydraulic varieties, and indeed the London builders, when about to employ the ordinary chalk limo for plastering purposes, actually run it, as they Hay ; or they reduce it to a fluid state and run off the liquid into a tank, where the excess of water is allowed to evaporate. This operation of running would effectually destroy any of the hydraulic limes, and it is important with the latter only to present the quantity of water strictly required for its hydration. Unfortunately working men are so careless that it
is practically impossible to ensure the observance of the precaution of keeping the bricks or stones So wet as that they should not be able to abstract moisture from the mortar ; and it thus almost always happens that, even in the manipulation of hydraulic limes, a larger quantity of water is used than would theoretically be required.
The mixture of the sand with the lime, or cement, is either effected by hand or by means of the pug-mill; and the proportion above indicated for the various descriptions of lime should be strictly adhered to, and ascertained by actual measurement ; the lime itself being slaked, reduced to powder, and measured in that state. In ordinary operations it may not be necessary to observe such extreme accuracy, but for hydraulic works it is essential that all precautions should be taken ; and therefore it is equally essential that the proportions of the lime and of the sand should be ascertained by some safer test than the eye of the labourer charged with making the mortar. Be this as it may, the sand to be used for mortar should be a clean, sharp, angular, silicious sand; free from loam, clay, and•l:uge lumps of stone, or of oxide of iron : and it must be distributed as equally as possible through the mass.
The mortars obtained by the use of tram and puzzuolano do not contain the same quantities of sand which they would ordinarily do, because the trans, &c., replace it to a very great extent. In the north of Europe it is customary to mix 3 parts of lime in powder, 3 parts of tram, and 3 of sand for hydraulic works; and in the south of Europe, 33 parts of lime with 45 of puzzuolano, and 22 of sand; but the value of these mixtures depends, after all, on the care and attention with which the various ingredients are brought ha contact with one another. Occasionally other substances, such as smith's ashes, pounded tiles, the roasted clay nodules of the ironstones, &c., have been mixed with lime and sand in the preparation of mortars ; and they commu nicate to them some unimportant qualities of colour, or even at times some additional powers of setting, which render their use in particular places desirable, as for instance, for pointing old brickwork, or for bedding chimney-pots, or other analogous details of a building which are much exposed to variations of atmospheric conditions. The ordinary pointing mortar is made from the purer rich limes carefully slaked and prepared for the purpose.
In the neighbourhood of Paris the gypsum, or sulphate of lime, is frequently used as a mortar, for walling purposes in positions which are not exposed to much humidity ; and this description of mortar is often used as a temporary protection to the slower setting hydraulic lime mortars in sea-water.