Bourne Archive: Brees: Lime mortar

http://boar.org.uk/aaiwxw3BreesLime.htm     Latest edit 7 Mar 2010


 

The Bourne Archive

 


The Architectural and Engineering Glossary of S.C. Brees (1852)


Extracts Concerning Lime Mortar.


pp. 252-4

LIME, a valuable substance much used in building, and for other purposes, being the most essential ingredient in all cements; it forms one of the primitive earths, although never found native or in a state of purity, but it is always combined with acids which exist in prodigious quantities in it; marble, limestone and chalk, are all carbonates of lime, and gypsum is sulphate of lime. Lime may be prepared from any carbonate of lime, as limestone or chalk, calcined or well burnt in kilns for some time to a white heat, by which the carbonic acid and acid contained in those substances are expelled, and the earth left in a fragile mass with very little coherence, having lost nearly 44 per cent. of its weight; it may now be easily reduced to a powder, when it is called quick lime, in which state it shows a great disposition for water, upon applying which it instantly swells and cracks, producing a considerable degree of heat (it will absorb one-fourth of its weight in water, and yet appear dry), it then falls into a fine white powder, when it is called slaked lime, having about double its former bulk.

Stone lime is generally used in extensive buildings, and the quality of the lime is supposed by many to be in proportion to the hardness of the stone from which it is produced. Brown stone lime is said to be the best for all kind of cements, although blue lias lime is considered by some to be superior, as it withstands the action of water exceedingly well; it was used by Mr. Smeaton in building the Eddystone lighthouse, where it succeeded after all other descriptions of lime had failed. Good chalk lime, although said to be inferior to stone, is yet much esteemed. Lime should always be kept under an enclosed shed, particularly chalk lime, as it suffers particularly from exposure to the air: the efficacy of lime also depends on being well burnt, after which process it should be used as soon as possible.

LIME AND HAIR, a mixture of lime and hair, and employed with the plaster in first coats and floating; a greater quantity of hair is used in the latter than in the former. It is also sometimes called coarse stuff.

LIME CORE, the coarse lumps of extraneous matter and inefficiently burnt stone lime, which do not pass through the screen in the operation of screening.

Lime core is unfit for making cement and mortar, but it is very serviceable as dry filling at the backs of walls and under floors and paving.

LIME KILN, a kiln employed in burning limestone, shells, and other calcareous matter.

The most simple kind of lime kiln consists of a hole dug in the ground in the shape of an inverted cone, and situated on the top of a high bank, in order to admit of the materials being put in at the top, and taken out at the bottom. The sides may be built up in bricks or sods, or left naked, according to the nature of the ground. The limestone properly broken is spread in alternate layers with coal of furze, the top being covered with sods, and 1 bushel of coals produces about 3 of lime. The fire is lighted at the bottom, where a door is left to remove the lime and admit a draught to the interior. The better sort of lime kilns are formed in the shape of a hogshead or an egg broken off square at each end, and compressed a little at the base to an oval shape, with an eye, or draft-hole, at each end of it. There is an inside and outside wall, a layer of clay, 2 feet in thickness, being interposed between them. A metal plate below receives the materials, admits the draught, and facilitates the discharging. These are constantly burning, and are distinguished by the name of draw-kilns.

LIME STONE, the stone from which lime is produced; the most crystalline marble, all calcareous stone, as well as chalk, being composed of lime. Every stone, in fact, which will ferment with an acid (as aqua fortis) is capable of being burnt into lime, and the harder the stone the better will be the lime. Limestone is of various colours, as brown, blue, grey, red, yellow, and green. When any of these are combined, it is called marble. The limestones found in this country are mostly either of a yellowish red or blueish cast.

The limestones used for building are classified into three, - 1st, the pure limestone, which contain from 79 to 93 of carbonate of lime; 2ndly, the oolite, containing about the same quantity; and 3rdly, the magnesian limestone. The Balsover, [Anston stone from grid ref. SK 5282] of which the new Houses of Parliament are built, is at the head of this class, and contains 51.1 of carbonate of lime, 40.2 of carbonate of magnesia, and 3.6 of silica. The cohesive powers of the Balsover are upwards of five times as great as the Bath oolite. It requires 296.01 cwt. to crush a cube 2 inches square of the former. An admixture of silica is very advantageous; thus the Chilmark limestone possesses 10 per cent of silica, and consequently rises almost equal to the Balsover in point of cohesion, and is superior to it in non-absorbent properties.

pp. 284-5

MORTAR, a cement used for building purposes, composed of lime, sharp coarse sand, and the hair of cattle, which should be thoroughly mixed together in a pug mill, or well tempered with wooden beaters, with a small portion of water, in the proportion of 1 of lime to 2 of sand, well chafed. The lime should be used as fresh and stiff as possible, and it ought to be kept under an enclosed shed. The bricks or stones should, if possible be well saturated with water, particularly in hot weather.

Twenty-seven cubic feet make 1 load of mortar, which contains a hundred of lime, and a proportionate quantity of sand; and a hod is 9 inches by 9 inches, and 14 inches long; 2 hods of mortar make nearly 7 bushels.

No more than about a bushel of lime should be slaked at one time, and no more water should be employed than is required to reduce it to powder, when it ought to be immediately covered with sand to prevent the gas escaping, which constitutes its indurating quality. The mortar should be beaten three or four times over, so as to incorporate the lime and sand together, and to break any pieces of lime that may have passed through the sieve, the operation being performed with scarcely any water. This improves the strength of the mortar considerably. If the mortar is laid by for any time, it ought to be beaten up again before being used, to save the time of the bricklayer, and is should be used soft in summer, and rather stiff in wintry weather.

The ruins found at Rome being so remarkably tenacious has often led to a belief that ancient architects were acquainted with some peculiar kind of mortar or mode of preparation, and the latter is the most probable.

When a small drop of water is exposed to the air, the calcareous matter contained in it begins to separate from the water, and to reassume its native form of limestone or marble, and upon the calcareous matter being perfectly crystalized, it becomes, in fact, limestone or marble, of the same consistence as before. The perfection of the cement must, therefore, depend entirely upon the perfection of the crystals. That the crystalization may be more perfect, it has been recommended to mix the ingredients well together with a large quantity of water, and allow the drying to be as slow as possible/ The middle of the old Roman walls was composed of pebbles thrown in at random, and some think that the mortar must have been liquid, so as to have been poured in among them. This would have the effect of dissolving a large quantity of the lime, and thus render the crystalization very perfect, so that the mortar would become as hard as the stones themselves.


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