Stocks, hard, sound, well-burnt bricks, used for all ordinary purposes.
Hard Stocks, sound but over-burnt, used in footings to walls and other positions where good appearance is not required.
Shippers, sound, hard-burnt bricks of imperfect shape. Obtain their name from being much used as ballast for ships.
Rubbers or Cutters, sandy in composition and suitable for cutting with a wire saw and rubbing to shape on the stone slab.
Grizzles, sound and of fair shape, but under-burnt; used for inferior work, and in cases where they are not liable to be heavily loaded.
Place-bricks, under-burnt and defective; used for temporary work.
Chuffs, cracked and defective in shape and badly burnt. Burrs, lumps which have vitrified or run together in the burning; used for rough walling, garden work, etc.
Pressed bricks, moulded under hydraulic pressure, and much used for facing work. They usually have a deep frog or hollow on one or both horizontal faces, which reduces the weight of the brick and forms an excellent key for the mortar.
Blue bricks, chiefly made in South Staffordshire and North Wales. They are used in engineering work, and where great compressional resistance is needed, as they are vitrified throughout, hard, heavy, impervious and very durable. Blue bricks of special shape may be had for paving, channelling and coping.
Fire-bricks, withstanding great heat, used in connexion with furnaces. They should always be laid with fire-clay in place of lime or cement mortar.
Glazed bricks, either salt-glazed or enamelled. The former, brown in colour, are glazed by throwing salt on the bricks in the kiln. The latter are dipped into a slip of the required colour before being burnt, and are used for decorative and sanitary purposes, and where reflected light is required.
Moulded bricks, for cornices, string courses, plinths, labels and copings. They are made in the different classes to many patterns; and on account of their greater durability, and the saving of the labour of cutting, are preferable in many cases to rubbers. For sewer work and arches, bricks shaped as voussoirs are supplied.
The strength of brickwork varies very considerably according to the kind of brick used, the position in which it is used, the kind and Strength of brickwork. quality of the lime or cement mortar, and above all the quality of the workmanship. The results of experiments with short walls carried out in 1896-1897 by the Royal Institute of British Architects to determine the average loads per sq. ft. at which crushing took place, may be briefly summarized as follows: Stock brickwork in lime mortar crushed under a pressure of 18.63 tons per sq. ft., and in cement mortar under 39.29 tons per sq. ft. Gault brickwork in lime mortar crushed at 31.14 tons, and in cement mortar at 51.34 tons. Fletton brickwork in lime crushed under a load of 30.68 tons, in cement under 56.25 tons. Leicester red brickwork in lime mortar crushed at 45.36 tons per sq. ft., in cement mortar at 83.36 tons. Staffordshire blue brick work in lime mortar crushed at 114.34 tons, and in cement mortar at 135.43 tons.
The height of a brick pier should not exceed twelve times its least width. The London Building Act in the first schedule prescribes that in buildings not public, or of the warehouse class, in no storey shall any external or party walls exceed in height sixteen times the thickness. In buildings of the warehouse class, the height of these walls shall not exceed fourteen times the thickness.
In exposed situations it is necessary to strengthen the buildings by increasing the thickness of walls and parapets, and to provide heavier copings and flashings. Special precautions, too, must be observed in the fixing of copings, chimney pots, ridges and hips. The greatest wind pressure experienced in England may be taken at 56 lb on a sq. ft., but this is only in the most exposed positions in the country or on a sea front. Forty pounds is a sufficient allowance in most cases, and where there is protection by surrounding trees or buildings 28 lb per sq. ft. is all that needs to be provided against.
In mixing mortar, particular attention must be paid to the sand with which the lime or cement is mixed. The best sand is that Mortar. obtained from the pit, being sharp and angular. It is, however, liable to be mixed with clay or earth, which must be washed away before the sand is used. Gravel found mixed with it must be removed by screening or sifting. River sand is frequently used, but is not so good as pit sand on account of the particles being rubbed smooth by attrition. Sea sand is objectionable for two reasons; it cannot be altogether freed from a saline taint, and if it is used the salt attracts moisture and is liable to keep the brickwork permanently damp. The particles, moreover, are generally rounded by attrition, caused by the movement of the sea, which makes it less efficient for mortar than if they retained their original angular forms. Blue or black mortar, often used for pointing the joints of external brickwork on account of its greater durability, is made by using foundry sand or smith's ashes instead of ordinary sand. There are many other substitutes for the ordinary sand. As an example, fine stone grit may be used with advantage.
Thoroughly burnt clay or ballast, old bricks, clinkers and cinders, ground to a uniform size and screened from dust, also make excellent substitutes.
Fat limes (that is, limes which are pure, as opposed to "hydraulic" limes which are burnt from limestone containing some clay) should not be used for mortar; they are slow-setting, and there is a liability for some of the mortar, where there is not a free access of air to assist the setting, remaining soft for some considerable period, often months, thus causing unequal settlement and possibly failure. Grey stone lime is feebly hydraulic, and makes a good mortar for ordinary work. It, however, decays under the influence of the weather, and it is, therefore, advisable to point the external face of the work in blue ash or cement mortar, in order to obtain greater durability. It should never be used in foundation work, or where exposed to wet. Lias lime is hydraulic, that is, it will set firm under water. It should be used in all good class work, where Portland cement is not desired.