Bricks are generally of a uniform size, of 9 inches in length by 4½ in width and 2¾ inches in thickness. Those bricks which are heaviest and hardest are generally the most durable; bricks of good quality when knocked together give a clear ringing sound.
The relative conductivity for heat of brick as compared with other materials, is shown in the following table, from Galton, which gives the units of heat transmitted per square foot per hour by a plate 1 inch thick, the two surfaces differing in temperature 1° Fahr.:—
| Stone—ordinary free stone | 13·68 |
| Glass | 6·6 |
| Brickwork | 4·83 |
| Plaster | 3·86 |
| Fir planks | 1·37 |
| Brick dust | 1·33 |
It is evident that in this respect, brick walls compare very favourably with stone walls, and are much more economical of heat. Increased conductivity of a material may be counteracted by increased thickness.
Brick is very porous, as shewn by its power to absorb moisture. A good brick can absorb from 10 to 20 per cent. of its weight of water; while good granite only takes up ½ per cent., sandstone usually from 8 to 10 per cent., marble only a trace, and Portland limestone 13½ per cent.
Being porous, brick allows the passage of a considerable amount of air, unless its pores are occupied by moisture. The following table, from Galton, shews the number of cubic feet of air which every hour pass through a square yard of wall-surface of equal thicknesses, built of the following materials, there being a temperature of 72° Fahr. on one side the wall, and of 40° on the other:—
| Wall built of | brick | 7·9 | cubic feet. |
| „ | quarried limestone | 6·5 | „ |
| „ | sandstone | 4·7 | „ |
| „ | limestone | 10·1 | „ |
| „ | mud | 14·4 | „ |
Mortar should consist of clean sharp sand and slaked lime, usually in the proportion of three of the former to one of the latter. Grouting, or liquid mortar, is merely ordinary mortar to which a larger quantity of water has been added. It is used for filling up the crevices between the brickwork about every fourth course, and is required to a greater extent in stone work, owing to the difficulty in filling up spaces left by inequalities in the stone.
The sand used in mortar should be free from small stones. It should not contain any earthy or clayey matters, as these greatly diminish the adhesive quality of the mortar, which depends on the combination of the sand and lime. All the sand used in a building should be washed, unless it is perfectly clean, in order to remove impurities. Many builders use an inferior mortar, in which other materials, such as “road scrapings,” are substituted for sand. Sand taken from the sea-shore is unfit for making mortar, as the salt contained in it is apt to deliquesce and weaken the mortar.
Lime is obtained by burning chalk or limestone in a kiln. Thus CaCO₃ = CaO + CO₂. There are three kinds of lime: (1) Fat or quicklime, used for internal plastering, (2) stone lime, used for ordinary building work, and (3) hydraulic lime, used for building in damp situations. The last named contains a quantity of silicates, and sets under water.