London stock brick is the type of hand-made brick

Brick

A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

 which was used for the majority of building work in London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 and South East England until the growth in the use of Flettons and other machine-made bricks in the early 20th century. Its distinctive yellow colour and soft appearance come from the yellow local clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 from which the bricks were made. London Stocks are still made in comparatively small quantities in traditional brickworks, mainly in Kent and Sussex, for heritage work, and machine-made versions are available for use where a cheaper approximation to the traditional product is acceptable.

Red stock bricks are also fairly common, but only the yellow or brown coloured bricks are usually known as London stocks.

During the 19th century the fields around London were built up with new housing. Commonly, a field would be excavated to expose the brickearth or London clay subsoil which was then turned into bricks on the site by moulding and firing them. The bricks would then be used to build houses adjacent to the brick field - transport was expensive. Once the building work was nearing completion the brick field would be levelled and built upon while a new brick field further out would supply the bricks.

Bricks were also made in clay areas surrounding London. During the 19th century and early 20th century 5 million yellowish stock bricks a year were supplied from the brickfields of Yiewsley and Starveall Middlesex for the building of the `new` London. Bricks were also made in Kent, Essex and other areas where they could be imported to London by rail. In Stock, Essex

Stock, Essex

Stock is a village and civil parish in south Essex, England. It is located approximately south of the county town of Chelmsford. The village is in the borough of Chelmsford and in the parliamentary constituency of Maldon.-Facilities:...

 there is a common belief that "Stock Bricks" originated there; bricks were certainly made there, but the name is a coincidence, stock being a common English word with many meanings and also a common place-name element.

Air pollution

Air pollution

Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

 in London during the 19th century and early 20th century commonly caused the bricks to receive a sooty deposition over time, turning the bricks greyish or even black, but the removal of contaminants from the air following the passing of the Clean Air Act

Clean Air Act 1956

The Clean Air Act 1956 was an Act of the Parliament of the United Kingdom passed in response to London's Great Smog of 1952. It was in effect until 1964, and sponsored by the Ministry of Housing and Local Government in England and the Department of Health for Scotland.The Act introduced a number of...

 in 1956, has enabled older buildings to be cleaned and new buildings to retain their natural colour.

The term 'stock brick' can either indicate the common type of brick stocked in a locality, or a hand-made brick made using a stock. A stock or stock board is an iron-faced block of wood fixed to the surface of the moulder's bench. The brick mould fits over the stock; the brick maker fills the mould with prepared clay and cuts it off with a wire level with the top of the mould, before turning out the 'green' brick onto a wooden board called a pallet for drying and firing.

Reclaimed London stock bricks are sought after for decorative and conservation use. The mortar usually used with them in original construction was lime mortar which is much softer and weaker than modern cement based mortar and can be cleaned off second hand bricks easily leaving them ready for re-use. Nevertheless the supply of second-hand stocks cannot always meet the demand, and 'new' second-hand stocks can be obtained from builders' merchants. Unfortunately, some of the new products are sometimes painted white or black to simulate the whitewashed or soot blackened surfaces often encountered in the real second hand bricks - making them unsuitable for face-work.

In the 19th century, London stock bricks were available in a variety of grades priced according to their consistency and their regularity of shape and colour. High grade bricks were used for face work and lower grades were bought for use as internal bricks. Unfortunately it seems to have been common practice for a high grade brick to be broken in half so that it could be used twice, each end appearing as a header in the wall. The result of this parsimony was that the wall was deficient in bonding bricks, i.e. bricks tying the outer skin of brickwork back to the inner part of the wall, often resulting in the outer skin peeling away from the inner and bulging out. This issue, known as snapped or snap headers, leads to walls which need to be repaired either by rebuilding or by fitting various types of proprietary tie.

Most London stock bricks are more or less porous, as is the lime mortar in which they have traditionally been laid. The pointing

Repointing

Repointing is the process of renewing the pointing in masonry construction. Over time, weathering and decay cause voids in the joints between masonry units , allowing the undesirable entrance of water. Water entering through these voids can cause significant damage through frost weathering and...

should be flush pointing so that rain water can run down off the surface and not be encouraged to soak in to the wall as is the case with recessed or struck pointing. When used in this way the brickwork does not get wet all the way through and is thus effectively waterproof.

Lime mortar tends to weaken in London's acidic rainwater and needs repointing several times a century. It has been common since the widespread availability of Portland cement to see London stock brickwork repointed using much stronger cement mortar. As repointing consists of replacing the outer 20 - 40mm of mortar, the effect of this is to make the outer 20 - 40mm of the brickwork harder and stronger than the interior of the wall. This can lead to spalling of the brick surface, and can also encourage the bulging associated with snapped headers.