I was born in Bedford in 1962, and growing up there in the 1970s the presence of the London Brick Company [LBC] was all around. From the big red cabbed brick lorries driving through the town to the massed ranks of towering chimneys just to the south of Bedford and all the way to what was then just Bletchley, to the dirty sulphurous yellow smoke that blew over the town on southerly winds, to the vast pits and monstrous diggers and their clattering conveyor belts we saw from the train or on the regular Sunday walks my parents dragged me and sisters on [ with hindsight the walks were great! ], and sometimes even walking down into the part flooded clay pits, looking for the fossils that were commonly found, you couldn’t miss the brick industry in Bedfordshire!
And of course 1000s of people worked in the various brickworks, and through that the town had become, as it was famously known, a ‘melting pot’, with a large population of hard working immigrants from the centre and south of Italy, recruited to work at LBC in the 1950s when the brick works was booming as Britain rebuilt after the war, as well as Poles, Ukrainians, West Indians and later recruits from Pakistan and India.
Check out these links for a series of interviews with LBC workers. https://www.bbc.co.uk/legacies/work/england/beds_herts_bucks/article_1.shtml
I moved away in 1981 and only occasionally saw LBCs decline but after years winding down by the mid 2000s it was pretty well all closed and all the sites have been built on or planned to be so, though several large pits remain. While as a specific brick making industry it didn’t last much more of a century but it’s effects have been far reaching for London.
The London Brick Company (LBC)   became the biggest brick company in Britain making a specific brick called a Fletton, named after the village near Peterborough where the bricks were first made. What was special about them was the clay they were made from. The brick pit where they were first made had previously only used the surface clays but one year one of the brickmakers tried a lower, deeper clay, the then called Lower Oxford Clay, and found that it was perfect for making bricks as it needed no additives and crucially it needed much less fuel to burn/bake as the clay itself was quite organic or oily, and this became to be known as ‘the clay that burns’. This property, among others, quickly put the Fletton brickmakers at a massive advantage to others.
What is this ‘clay that burns’ though? Wikipedia states “The Oxford Clay Formation dates to the Jurassic, specifically, the Callovian and Oxfordian ages, and comprises two main facies. The lower facies comprises the Peterborough Member, a fossiliferous organic-rich mudstone. This facies and its rocks are commonly known as lower Oxford Clay. The upper facies comprises the middle Oxford Clay, the Stewartby Member, and the upper Oxford Clay, the Weymouth Member.” 
The clay that burns is what used to be called the Lower Oxford Clay but is now called ‘the Peterborough Member’ and as the British Geological Survey states, it is bituminous! “Mainly brownish-grey, fissile, organic-rich (bituminous) mudstones” 
Richard Hillier in his seminal history of LBC documents the start of the Fletton brickmaking industry, from the purchase of 4 lots of land in Fletton and the then change in brickmaking from the upper Oxford Clays to the Lower Oxford Clay: “.. a revolution had taken place in the brickmaking industry. .. one or other of the brickmakers .. had, for the first time, tried to make his bricks not from the surface clay, but from the Lower Oxford Clay beneath… it’s unique and valuable property remained undiscovered until these brickmakers’ trials: the clay itself would burn.” 
Alan Cox in his encyclopaedic survey of brickmaking in Bedfordshire in 1979 also called this a revolution “ … which outdated the traditional brickworks overnight, and which was to transform the landscape of Bedfordshire’s central vale, while turning the county into a major brickmaking centre. This revolution began at Fletton, near Peterborough, where in 1881 it was first discovered that below the top callow of the Oxford Clay was a shale-like, grey-green clay deposit, known as “The Knotts”, which provided ideal for what have since become known as Fletton bricks.” 
Hillier did a mass of research into old documents and believes that the Hempstead Brothers were the first to use this lower clay, noting complaints taken out against them from oily ‘noxious fumes’ saying “this action clearly implies it was Hempsteds who had experimented with, if not indeed discovered, ‘the clay that burns’”.
The advantages of this clay were numerous. It being oily or bituminous burnt with very little additional heat, clearly saving significant costs compared to traditional brick making, though London Stock Bricks, using ash, with bits of charcoal in, were able to self-fire to an extent [ see https://buildinglondon.blog/2022/07/12/41-londons-canary-yellow-stock-brick/ ] . The clay also had a perfect moisture content that was dry enough to grind but wet enough to then be pressed immediately and stacked and then needed little drying and the beds of the clay were deep and uniform.
Cox states “The four notable characteristics of these knotts are : first, the moisture content is constant, allowing the clay to be crushed into a granular form – this can then be pressed into a brick shape which can be fired immediately without having to wait for the green bricks to be cure; second, the knotts contain sufficient organic combustible material to assist during firing, to the extent that the bricks are completely burnt and fuel consumption is reduced by about two-thirds; third, the lime content is also constant and provides precisely the correct amount required to prevent the bricks cracking during firing; finally the clay contains few impurities…”
Arthur Perceval in a British Brick Society article in 2009 about the production of London Stock Bricks in Kent was affected by the arrival of Flettons, notes “Fletton is a village near Peterborough and in 1881 it was discovered that the lower Oxford clay, found underneath a layer of ordinary brickearth there, was oil-bearing. This meant that a brick could be made largely self-firing without having to have ash added to it. The Fletton, pink and machine-made, became ubiquitous in London and elsewhere in the home counties. One by one, most o f the Faversham brickfields closed in the early twentieth century.” 
The London brick Company – LBC
There had been brickmaking before in mid-Bedfordshire, see Cox and ‘Brickmaking’ from the Bedfordshire Geology Group  but within a few years most of these small brickworks near Peterbrough and south of Bedford were making Fletton bricks, LBC one of many. A Forterra, who now own LBC, document from 2007 states the LBC origin as being from 1877 “It all really started in 1877 when James McCallum Craig bought a property at auction near Peterborough, known as Fletton Lodge”  but the London Brick Company itself started, also at Fletton after “… John Cathles Hill, a developer-architect who built houses in London and Peterborough .. bought [in 1889] the small T.W. Hardy & Sons brickyard at Fletton in Peterborough, and the business was incorporated as the London Brick Company in 1900.” 
In Bedfordshire the brick works in the Marston Vale that used the Lower Oxford Clay were at Elstow, Wotton Pillinge, Marston Moretaine, Brogborough, Ridgemont and Lidlington. As the LBC grew it started to acquire these and most importantly in 1923 merged with a company called BJ Forder based at Wotton Pillinge owned by the Stewart family. 
The Wotton Pillinge brickworks then expanded rapidly and soon became “…the largest brickworks in the world … “ with 32 chimneys each standing 70 metres tall…” and “over 2,000 people were employed” In the wider Marston Vale there were 167 chimneys “… linking 8 villages including Stewartby” and “Peak production was reached in 1936 when some 500 million bricks were made. This equates to the bricks required to build 61,154 houses in a year (based on an average 8,176 bricks per house).” 
A year later in 1937 a model workers village that had been developed by the Stewart family at Wotton Pillinge, built with decent housing and sports and community facilities, even if it did sit under a cloud of smoke for much of the time, which was re-named Stewartby in the honour of the family.   
By 1971 London Brick Company owned all the brick works in the Marston Vale and in 1973 were producing vast amounts of bricks “ … Stewartby .. 738 million bricks, Ridgemont 406 million, Kempston Hardwick 16 million, Coronation 121 million and Elstow 26 million” and even though by 1974 they had been consolidated into only “… 3 Fletton brickworks in production. Stewartby with it’s 30 chimneys and Ridgemont which now has 25 chimneys are said to be the largest and second largest brickworks is in the world …[ and in] … 1976 these works manufactured a total of 1,191 million bricks, which represents about 45-50 per cent of the national Fletton production and about 20% of all bricks made in this country” 
The other 50% of Fletton production was from Peterbrough, Fletton and Whittlesey and in Buckinghamshire at Calverts and Bletchley, which further Building London blog posts will cover.
Brick site Brocross states “It has been estimated that a third of all the brick houses in England are built from London Brick Company bricks. The London Brick Company started production just over a century ago and usage peaked in the Post-War rebuilding period up to the Nineteen Sixties. Maximum production rose, at one point, to an amazing 16,000,000 bricks per day. In other words – quite common bricks!” 
The Bedfordshire brickworks gradually closed except at Stewartby which itself finally stopped production in 2008 due to the cost of adhering to pollution standards. The chimneys, even though having been listed,  were blown up for safety reasons in 2021.    though the massive Hoffman kilns  remain awaiting a major Chinese backed housing development, though this plan now appears to have collapsed.   See this gallery for a series of historic photographs of the Stewartby works –https://www.bbc.co.uk/legacies/work/england/beds_herts_bucks/gallery_1.shtml
The Flettons / bricks LBC produced are not, in my opinion the nicest or prettiest in the world. They are a mix of pink and red but do not look soft like most southern red bricks, nor have the rough character, imo, of London Stock bricks, and they were often consequently used for side and back and internal walls with other bricks used for facing.
North London artist and architect Gordon Shrigley concurs: “The pinkish toned Fletton brick is certainly however not beautiful, especially when compared to the tonal splendour of the world of bricks. The Fletton does nevertheless have ‘kiss’ marks on the stretcher face of the brick, caused by stacking the bricks on top of each other within the kiln, which gives each Fletton a unique, if somewhat accidental decorative pattern. … Flettons have been used mainly for buildings that are simply required to house an activity and not to please the eye, such as the more spartan forms of social housing, factories and barracks.” 
The most famous Fletton and LBC brick was and is The Rustic a rough brick of various shades of pink with a herringbone design pressed into it and the ‘kiss mark’ providing the switch of colours. See pics here
https://www.forterra.co.uk/wp-content/uploads/2021/02/London_brick_Guide_10_2018.pdf and here  
Some Flettons were called Phorpres and this simply refers to that the bricks were not pressed twice, but 4 times!
The London Connection
The Building London blog is of course about the materials that built London and London Brick and Fletton bricks had a direct connection with London from the very beginning! Not only were the brickworks directly linked to London by rail via the East Coast Mainline at Fletton, and the LMS in Bedfordshire, but John Cathles Hill, who founded LBC, did so specifically as he needed bricks in London, hence it’s name!
“Hill became a prolific developer-architect in North London. Records indicate that he was responsible for building 2,397 houses in or near London and in the city of Peterborough. … In the 1890s he built up a good part of Harringay, North London. In addition to a few streets of terraced houses to the east of Green Lanes, the development also included a magnificent terrace of shops, Grand Parade, and a vast ornate public house, The Salisbury. He is also credited with large developments in Crouch End including the Grade II* listed Queens pub in Crouch … the Broadway Parade on the western side of Crouch End Broadway and the Rathcole estate [in Hornsey] … As a developer Hill experienced a shortage of bricks. He resolved this problem by acquiring a brickfield at Fletton in 1889. This eventually became the London Brick Company.”
North Londoners may well know Hill developments on ‘The Ladder’ in Haringey and his twin pubs, the Salisbury and The Queens Head.
Flettons destroyed the London Stock Brick industry in Kent and Essex being so much cheaper, though some specialist makers still operate 
The Building London blog likes to direct people to the sources of what built London but there’s not much left to see of the Bedfordshire brickworks now, especially now the chimneys are all gone, and most of the sites built on. Most of the pits have been filled with London landfill, and though some remain those are generally off limits, but some are visible and can be visited.
But the massive and beautiful Stewartby Lake can be walked all the way around, from either Stewartby or probably better from the Marston Vale Millennium Park/Forest Centre, where also one of the pits has been transformed into a bird reserve. Strangely there is little about the brickworks in display, and there is no other LBC museum.
At Stewartby itself the old kilns can be seen through the fence and the unusual village walked around, though the older people’s cottages area is off limits.   
By train, to get to Stewartby Lake, it’s less than 2 hours from London, either from St Pancras to Bedford or Euston to Bletchley and then taking the Bedford to Bletchley shuttle to Millbrook or Stewartby Stations. The Forest Centre is nearer to Millbrook.
Cycling, take the train to Bedford and then it’s a 40 minute cycle to the Forest Centre and all the sites in Bedfordshire can be fairly easily walked and easily cycled.
Driving, it’s just up the M1 and off the A421.
5] Clay That Burns – A History of the Fletton Brick Industry Richard Hillier 1981 LBC
6] Brickmaking A History and Gazetteer – Survey of Bedfordshire, Bedfordshire County Council / Royal Commission on Historical Monuments (England) Alan Cox 1979