One of the fascinating materials that was used in building London is/are the melted, vitrified, burnt London Stock bricks that were a by products of the old inefficient London brick clamps and kilns. They are known as burrs, clinkers, wasters, crozzles or just burnt bricks. The bricks stacked nearest the heat source in the clamps and kilns simply got burnt and indeed often melted together into glassy, vitrified, surfaced lumps.
While many were crushed down into road stone or for concrete there was a fashion to use them for garden walls in the late 19th to early 20thC, and they can be seen in many parts of London often indicating where there would have been the short lived bricks works thrown up to build houses in the locality, but also brought in as a decorative material.
This is not the same as the burnt vitrified dark headed decorative bricks that were used to create the diaper diamond features in Tudor and mock Tudor brickwork which may have been produced deliberately. See the post on Sutton House. https://buildinglondon.blog/2021/08/23/13-16thc-bricks-in-hackney-the-arcadia-beyond-moorfields/%5B1%5D 
And note the clinker is also a word sometimes used for a paving brick and again that’s different.
They were talked about as far back as 1836, here in the Penny Cyclopedia of the Society of the Diffusion of Useful Knowledge 1836 edition which described “black looking masses of vitrified brick that are worth 10s a load… burrs or clinker brick are those which are much vitrified in the fire: sometimes 100,000 of them have run together in one mass. Brick having a smoothed or glazed surface are sometimes made: this is done in the burning.” 
The National Encyclopedia of 1864 notes “… burrs, or clinkers, which are black-looking masses of vitrified brick, of very inferior value.” 
Dobson writing in 1899 ( though it’s word for word the same as the 1850 version ) in his ‘Rudimentary Treatise on the Manufacture of Bricks and Tiles containing an outline of the Principles of Brickmaking’ …… states “The bricks near the live holes are generally partially melted and run together in masses called clinkers or burrs…The clinkers are sold by the cartload, for rockwork in gardens and similar purposes.” “These are only used for making artificial rockwork for cascades or gardens, etc” and “Those bricks which are in contact with the live holes or flues melt into a greenish black slag. If too much fuel be used, there is danger of the bricks fusing and running into a blackish slag Burrs or Clinkers.—These are only used for making artificial rockwork for cascades or gardens”. He also implies they are a London thing.
The unusually named Alfred Broadhead Searle states in his “Modern Brickmaking” of 1911 “Over-burned bricks will melt and run together forming burrs, which are useless except to be broken up for road metal or concrete.” and then plagiarises Dobson word for word! 
M.J. Crute in The British Brick Society in his ‘ Brick Making Terms mainly from South-East England’ in British Brick Society Information 72 October 1997 gives definitions that appear pretty much the same for Burrs and Clinkers!
“Burr – Brick fused with another through serious overfiring, sometimes used for garden walls and rockeries
Clinker – Partly melted brick or fused material caused by serious overfiring” 
And the main use for these burrs in England was for garden walls, though in the USA they became an architectural feature in the early 20thC.   Sometimes these walls are just of burnt London stock brick and sometimes other re-cycled earthenware items, gas retorts and slag and broken fire bricks e.g. at Millfields in Clapton and Chester Road in Tottenham, which will be posted about separately.
Burr walls are found across London and anywhere in the South-East where there were brickworks and in fact can often be used to indicate where brickworks had been and a quick look through old maps at e.g. The National Library of Scotland / NLS  will sometimes confirm this though sometimes these burnt bricks did end up going further afield.
Some good examples around London are:
– Quixley Street, Blackwall where the bricks appear to be imported.
– Roads south of Pole Hill in Chingford e.g Eglinton Road, associated with the 2 Chingford brickworks.
– In South West London roads in the estate around the John Innes brickworks now a park called Mostyn Gardens 
– Holmbury View in Stamford Hill maybe associated with the Millfields brickworks in Clapton but by this point may have come from either Chingford or the many brickworks in Edmonton by rail.
– Forest Road, Walthamstow maybe associated with the brickfields at nearby Chapel End. 
– Earlsthorpe Road in Sydenham ( Thanks Ju! )
– Wanstead, though the bricks may be from Ilford 
There are also numerous examples around Sittingbourne and Faversham related to the massive brick making heritage of that area and equally in south Essex for the same reason.
 The National Encyclopædia, William Mackenzie, London, circa 1864 http://www.hertfordshire-genealogy.co.uk/data/projects/brickmakers-talk/brick-15-hand-made-bricks.htm
 10th Edition 1899 https://www.google.co.uk/books/edition/A_Rudimentary_Treatise_on_the_Manufactur/0RYLAAAAIAAJ
 One Man’s Trash Is Another Man’s Treasure: The Transition Of Clinker Brick From Disposable To Decorative Alafia Akhtar Columbia University 2013 https://academiccommons.columbia.edu/doi/10.7916/D8B85GDZ/download
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