As noted in the previous post on Building London on the beautifully named burrs, wasters, clinkers and crozzles, https://buildinglondon.blog/2022/11/06/51-burrs-and-wasters-clinkers-and-crozzles/ it turns out that many walls initially thought to be built from burnt bricks are built from materials even more unexpected including, as this post will cover, what appears to be the discarded ‘gas retorts’, used to make ‘coal gas’ and associated fire bricks, and some, see the next post, of ‘glass house pots’, used to make glass in!
The first clue was, well, obvious! The lumps of material were clearly not bricks! Were they then discarded land drains, or parts of sewer pipes? Maybe! But then looking closely at the walls it was noted that the word Stourbridge would appear regularly stamped and a number of company names appear e.g. Mobberleys and Harrison and Perry.
And after some research into these companies, and into the Stourbridge fireclay industry, it soon became clear that these names are of companies that were mass producers of heat resistant pipes, bricks, pots and retorts, based in an area renown for it’s fireclay, and looking again at these pieces in the walls the resemblance to gas retorts seems clear. It is also then probable that they are scrapped gas retorts, from Stourbridge on the edge of the Black Country, from local London gas works, and used for visual interest!
But what is a ‘gas retort’ you ask? It was a pipe that coal was heated in to give off gas, ‘coal gas’,  which till the mid 1960s was the gas people used for their heating/cooking, stored in the vast gasometers that stood in every town, a number famously overlooking the Oval cricket ground.  This process became defunct in the UK with the discovery of and switch to ‘Natural gas’, ‘North Sea gas’, in the 1970s. 
Every town, and even some large mansions and buildings like hospitals, had these gasworks but only one now exists to see, at Fakenham in Norfolk. “The Fakenham Museum of Gas and Local History is the only surviving town gasworks in England and Wales, complete with all equipment used for the manufacture of gas from coal: retorts, condenser, purifiers, meter and gasholder.” 
There are still quite a few gasometers surviving though, often at the centre of contested re-developments, e,g, at Bethnal Green and sometimes successful re-purposing as at Kings Cross. 
Coal Gas played a vital role in the change in society and economy in the 19thC. “The advent of incandescent gas lighting in factories, homes and in the streets, replacing oil lamps and candles with steady clear light, almost matching daylight in its colour, turned night into day for many—making night shift work possible in industries where light was all important—in spinning, weaving and making up garments etc. The social significance of this change is difficult for generations brought up with lighting after dark available at the touch of a switch to appreciate. Not only was industrial production accelerated, but streets were made safe, social intercourse facilitated and reading and writing made more widespread. Gasplants were built in almost every town, main streets were brightly illuminated and gas was piped in the streets to the majority of urban households.” 
The Gas Retorts that were a key part of the process needed to be capable of withstanding high temperatures and for that a particular type of clay, a ‘fireclay’, was needed. Stourbridge, on the edge of The Black Country had a particular local ‘carboniferous refractory clay’ sitting below the coal which was mined in the area and became the centre of UK production. These clays are also known as Seatearths as they sit underneath the coal measures.  And “Old Stourbridge Clay”
Fireclays should not burn or fuse in burning or when in use as a firebrick or retort, as do the brickearth of the London Stock Bricks, and so could be used at very high temperatures for gas retorts or making glass etc.  
Fireclay is also called ‘refractory’ which means “… refers to a material that does not melt at normal kiln temperatures or the capacity of a material to withstand heat without deforming or melting” . 
These garden walls lumps seen in London are vitrified though, but that seems either deliberate due to either being baked at very high temperatures so the surface vitrifies or they are maybe actually glazed to increase their toughness. Glazing was often used for pipes to be used to carry liquids but these ‘lumps’ do not look like pipes. 
The Stourbridge firebricks and gas retort industry
In the 19th and 20thC this industry was massive, providing for both the domestic market and globally. This quote illustrates the size of the industry at one time but it’s unclear when it is from, though most likely the early 20thC. “Stourbridge fireclay has a world-wide reputation, and its importance in the manufacture of fire-bricks, glasshouse pots, refractories, and a variety of other purposes in connection with the Industrial Arts need not be enlarged upon. Its chief value consists in its refractory character, which enables it to resist the highest temperatures without melting. As evidencing the importance of this industry, it may be stated that the quantity of clay raised in the Stourbridge district amounts to some 160,000 tons per annum, and the number of fire-bricks produced in a twelve month period cannot be less than 40,000,000.” The article goes on to describe the processes of the Harrison & Pearson company in detail noting that “As there are literally thousands of tons of fireclay goods from small bricks to huge unwieldy gas retorts the stock required to be kept becomes immense. There are battalions of gas retorts of all shapes and lengths at their works, and a further large stock at Ellesmere Port, in the Potteries, Liverpool, and elsewhere” 
And yet all from a remarkably small area around Stourbridge e.g. “Home to high quality deposits of fireclay, Brettell Lane subsequently became a centre of maybe the most essential industry of the Industrial Revolution, the manufacture and trade of firebricks, technically refractories. A refractory is a shaped article made of certain clays and minerals that can withstand variations in temperature for long periods of time without distorting or cracking. The Black Country led the world in the manufacture of refractories and John Cooksey, author of “Brickyards of the Black Country, A Forgotten Industry” claimed that firebricks, were the most important industry in the world, “It is worth considering that without refractories no other industry could have existed. Somewhere along the manufacturing process of every product refractories would be involved, …The refractories industry shaped the way the Industrial Revolution progressed, from bloomeries to iron and steel works, from Cylinder Glass to Pilkington’s float glass furnaces, from the Rocket to the main line steam locomotives, from disease to Henry Doulton’s salt glazed earthenware pipes, from oil lamps to William Murdoch’s gas from coal experiments, and so it goes on…” 
The finished goods were exported globally, initially via the Severn from Bewdley, but with the advent of the railways to different ports accessible from Stourbridge.
The gas retort companies
Gas retorts were produced by a relatively small number of companies. The names seen on London walls so far, and of course there may well be more, are related to Mobberley and Perry and George King Harrison.
Mobberley aka Mobberly and Perry had a massive works at “The Hayes, Lye, Stourbridge, West Midlands. The brickworks for building bricks was on the main Stourbridge Road in Wollacote and the Firebrick works was in Hayes Lane. George Attwood worked the brickworks untill 1840, then Fisher Bros. worked the brickworks until Mobberley & Perry took over the brickworks, the works closed in the late 1960s.”  
See the Mobberley works here in Britain from Above https://www.britainfromabove.org.uk/en/image/EPW035954
Geo King Harrison  were north of Lye, at Brettell Lane, Brierey Hill, a centre of the firebrick and gas retort industry, as were ‘Harris and Pearson’, not a name seen yet on any garden walls as yet, but there is a lot written of interest in this matter about them.  
These walls can be found / seen regularly around London e.g. Wattisfield Rd, Casimir Road and Cleveleys Rd in Clapton and Quixley Street in Blackwall, in front of houses or flats built in the early 20thC.
Another street of unusual garden walls, Abbey Road in South Wimbledon, was also identified to be of broken gas retorts a few years ago by some great detective work by the Faded London Blog! Faded London suggested the broken retorts were from the long gone Eyre Smelting Works rather than a local gas works, which remains more likely but who knows! http://faded-london.blogspot.com/2009/12/just-another-brick-in-wall-fire-brick.html
The Harris and Pearson office building is all that is left of the industry in Stourbridge. The works and factories are all gone and the mines and quarries that produced the fireclay are all invisible now, though deep under the ground will be riddled with abandoned workings.
This You Tube video “Bricks, Coal & Pottery on Brettell Lane”, from The Black Country Discovered channel gives a good visual tour of the area and the basis of a decent walk / explore and covers the Harris and Pearson building. https://www.youtube.com/watch?v=UebdPt0GVAE
See also https://www.blackcountrydiscovered.biz/brettell-lane-another-brick-in-the-wall/ which goes into a lot more detail.