The basic size of a brick has remained virtually unchanged for many hundreds of years and its popularity as a building material derives from the ease with which it can be handled by the bricklayer who can hold a brick in one hand and pick up a trowel full of mortar with the other. Bricks also allow the workman to follow his plans with a facility which other materials do not allow.
There is a wide variety of bricks and of names to describe them. The colour, texture and qualities of the bricks depend largely on the clay from which they are made and this varies from area to area. Because of this the names of the bricks often include the name of the area, such as Staffordshire Blue, Accrington Red or London Stock.
Apart from clay bricks there are calcium silicate bricks, sand-lime bricks and flint-lime bricks which are made from a mixture of hydrated lime and sand or crushed stone, or a mixture of both.
In addition to solid bricks there is the type with an indentation known as a frog in either one or both sides. Other bricks have perforations through them, the main object of which is to reduce the weight of the bricks. Standard bricks are rectangular and measure nominally 9 in X 4 ½ in X 3 in. They are, in fact, made slightly smaller than these measurements in order to allow for the mortar joints. Metric bricks are 225 mm X 112.5 mm X 75 mm nominal size, actual, or work size, 215 mm X 102.5 mm X 65 mm. There are, of course, a number of standard shapes as well; these include bevelled bricks and bullnose bricks, the latter having a rounded corner at either one or both ends.
Although the finished product from each type of clay will be different in colour and texture, good bricks of any type will have the same characteristics. They will, for instance, be of the same size and be regular in shape, unless they are of the hand-made rustic type whose charm lies in not only their colour and texture, but also their irregularity. The edges of a good brick should be sharp and when two bricks are knocked together, they should ring with a slightly metallic sound. When broken in two, the inner faces should be uniform in appearance and have a granular texture. When choosing bricks, weathering ability is just as important as the colour and texture.
Commons are bricks which have no particular claim to provide an attractive appearance, but are suitable for general building work. The term is not a guide to quality; many common bricks have excellent properties.
Facing bricks are those which are intended to provide an attractive appearance to the building. They are made in a wide range of colours and textures, some called rustics have the texture imposed by machine. Others like sandfaced have, as their name suggests, sand incorporated on the surface during manufacture.
Flettons is the name given to bricks made from the Lower Oxford clay in the Peterborough, Bedford and Buckingham areas. They are both commons and facings.
Wirecut bricks are those that are made by a process of extruding the clay through a die then cutting it with wire.
Handmade bricks are still produced, although the process is not now entirely by hand, but the bricks still retain a certain attractively uneven finish which in some circumstances is considered to be well worth the high cost.
Clay bricks may also be of the perforated type, that is they may have a number of holes through the full thickness of the bricks. These holes vary considerably in number and size and under British Standard definitions they are called perforated or hollow, dependent on the sizes and total volume of the holes. The perforations do not seem to affect the amount of rain penetrating a wall. There is some reduction in the weight of the wall and a slight increase in the thermal insulation value. Because the holes soon fill with water, unfinished work is vulnerable to saturation and the top of a wall should be covered in wet weather.
Calcium silicate, or sand-lime bricks, made from a mixture of sand, crushed flint, pebbles or rock, or a combination of these materials and hydrated lime are moulded under pressure and hardened in autoclaves where they are subjected to saturated steam under pressure.
These bricks are made in the same sizes as the clay bricks and in the same range of standard shapes such as bullnose, bevelled or squints. They are usually white or off-white, cream or pale pink in colour. Compared with clay bricks they are smooth textured but, of course, the texture varies according to the aggregate used. Facing bricks are made in various pastel shades and are also available with textured surfaces.
Care in handling these bricks is important as the arrises are susceptible to damage, which because of the inherent regularity of the bricks, is very noticeable.
Apart from this their main advantage is the low cost and the accuracy of their size and shape. This makes them easy to lay and to plaster. Being free from soluble salts they do not suffer from efflorescence which can cause the breakdown between the bricks and the plaster.
Calcium silicate bricks tend to shrink as they dry out and this can give rise to cracks in the brickwork. It is therefore important to keep the bricks as dry as possible and to avoid wetting them before laying. If the suction is too great it is better to add more water to the mortar than to soak the bricks.
Long lengths of this type of brickwork should be divided up to permit the movement and dry joints should be made at between 7.5 m and 9 m intervals.
The durability of calcium silicate bricks is related to their strength, which is itself variable by adjusting the time the bricks are autoclaved. In general, high compressive strength and low water absorption gives a guide to good frost resistance of clay bricks, but there are some high absorption, low compressive strength bricks which are frost resistant.