Bricks, the chief material of the bricklayer, are hard rectangular blocks, of an originally clayey substance, which has been tempered and moulded into the shapes required, and then burnt in a clamp or kiln until it is quite hard.
Bricks are an ancient building material, having been made and used by the Romans for arches, facings, pavings, etc. Though they were, at that period, of larger and smaller sizes than those used generally in the present day, they were always made with half-bricks or double bricks - double their width in length - to allow of bond, as will hereinafter be explained. They thus varied in length from 7 1/2 to 22 inches, and were generally very thin (about 1 1/2 inch), the smaller sizes being used for facing rubble walls, and the larger sizes for bonding every fourth course or thereabouts. The latter were also used for arches.
In more modern times bricks have been used for walling, facing, arches, and paving; and generally their dimensions are now about 9 inches long by 4 1/2 inches (or half their length) in breadth, so that two laid crosswise will cover two laid lengthwise. They are made from 2 1/2 to 3 1/2 inches in height, according to local custom or the requirements of construction.
They are of numerous different qualities, kinds, and colours, each from its peculiar nature adapted to a particular purpose or use. A concise classification of the various kinds, their peculiarities, qualities, and uses, will be given hereinafter.
It is to be noted that the quality of a brick depends on the clay from which it is made, and on the different manipulation which the clay will allow of in its manufacture.
A good brick-earth or clay, such as is used for common red bricks, is generally composed of silica and alumina, accompanied by a slight percentage of lime or other flux (to fuse the two into a hard mass), and a still smaller percentage of oxide of iron, manganese, magnesia, or an alkali to give it its colour. The properties of the different constituents of clay are as follows : -
Silica, the greatest in bulk, is practically sand: that is, in an uncom-bined state, it is infusible, whether alone or with alumina, without a small quantity of flux in the form of lime or oxide of iron; and it acts as a preventative to cracking, shrinking, or warping. Up to a certain proportion the more silica there is the better the shape and more even the texture of the resulting brick. An excess of silica (or sand) renders a brick too brittle. The silica should be in chemical combination with the alumina, as opposed to a mere mechanical admixture.
Alumina is the principal and most important constituent of a good clay, as it imparts the plastic qualities, though it shrinks, cracks, and warps very considerably under the influence of heat, which renders it very hard.
Lime may be called a flux, though its presence in the bulk has a double effect: it both diminishes the contraction in the process of drying the raw material, and it blends the silica and alumina together in the burning. This carbonate of lime must be present in very small quantities, comminuted and equally distributed throughout the mass; for if it exists in lumps it will be slaked by moisture, and cause the disintegration of the brick, whether laid or not in the finished work.
Iron is also a flux, when in the form of an oxide, and in the presence of nearly equal quantities of silica and alumina. It is the colouring matter of most kinds of bricks, the intensity of the colour (from a light yellow to a dark red) being in proportion to the quantity of iron oxide present. With 8 or 10 per cent of oxide of iron, the colour is a dark blue or purple, and the addition of a small proportion of manganese gives almost a black colour to the brick; and with lime the two impart a cream colour, the one darkening and the other lightening the shade. Magnesia and iron oxide make a yellow brick. The presence of alkalies is generally a source of trouble, as they act too strongly as a flux when in any quantity, though that may he only a small percentage. They melt the clay, as it were, and render the resulting brick unsymmetrical, while giving it a greenish-blue tint.
A good brick-earth contains silica and alumina in due proportions, together with such a percentage of a flux (in whatever form) as will fuse the silica and alumina without running the bricks together and rendering them vitrified.
There are three different kinds of brick-earth, as follows: -
I. Pure clay, also called Plastic or Strong clay, which contains silica and alumina, with such a small proportion of lime, iron oxide, or other flux that the brickmaker calls it"foul clay," as it is of very little use by itself, a soft, uncombined brick being the result of what little burning the clay will stand. The addition of more lime or other flux improves the clay greatly for brick-making purposes.
2. Sandy, Mild, or Loamy clays are so loose that they are useless for the manufacture of bricks unless a flux is added in sufficient quantity to make the fusion perfect.
3. Marls or Limy clays are the best of all, as they contain the necessary •constituents in the first instance, without further addition, except to regulate the burning.
An artificial brick-making clay of this quality, called malm, can be -obtained by washing the clay and mixing it with the necessary lime in a mill.
Having dealt with the qualities and peculiarities of the different clays and their constituents, a short resumk of the process used in the preparation of the clay and manufacture of the bricks may with advantage be given before proceeding to a detailed classification of the different kinds of bricks and their characteristics.