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1759 The Dowlais Ironworks was established near Merthyr Tydfil by a partnership of nine members: Thomas Lewis, Thomas Price, Richard Jenkins, Thomas Harris, John Curtis, Nathaniel Webb, John Jones, Isaac Wilkinson, and Edward Blakeaway. It had the second coke-fuelled blast furnace in South Wales (after Hirwaun Ironworks)..
1767 John Guest was appointed manager of the works. He discovered coal on Lewis's property and used it to replace charcoal for smelting.
1782 He became a partner in the business in 1782 with Lewis and salesman William Taitt who later became his son-in-law.
1787 Thomas Guest succeeded his father in 1787.
1795 Though there have been claims of steam power at Dowlais as early as 1753, it is more likely that it was Thomas who introduced steam for blowing the furnaces with a Watt steam engine in 1795.
1815 By the time John's grandson, John Josiah Guest took sole charge of the works, the company was the largest iron and steel producer in the world.
1816. 16th October. Disturbances in the area by the workers. 'On Wednesday last about 400 rose from their work at Tredegar Ironworks; they came on to Rumney works and put the blast out from the furnaces, and pressed several men to go along to the Dowlais works; where they did the same; also at Penydarren, they stopped all the furnaces on the hills (their plea was an increase in wages) the mob passed by Pontmorlais to Cyfarthfa Ironworks, and stopped all the blast-furnaces and most of the works; they pressed many of the workmen to go along with them. On Wednesday evening their number were increasing to many hundreds if not thousands. They got to the Plymouth works and stopped them all. In the evening (after stopping all the works in the neighbourhood) the men came in to the town of Merthyr in a large body, perhaps many thousands . . .' 
1821 The works owed much of its success to a contract for iron rail, as rail was needed in ever greater quantities to build the rapidly expanding railroads.
1824 Guest Lewis and Co were producing 15,000 tons per annum
1839 Built the Ifor Works at Dowlais as an expansion to the existing works.
1845 Employed 7,300 people; its 18 furnaces produced 89,000 tonnes of iron each year.
1848 William Price Lewis died (a partner in the firm Sir John Guest and Co); he owned 5 sixteenths of Dowlais and left his estate to his nephew William Wyndham Lewis but it took some years before the precise implications of the will were settled in court.
1851 The partnership between Sir John Josiah Guest and Edwin John Hutchins, ironmasters, miners and merchants of Dowlais, Cardiff, London, Liverpool and Mitcheldean, under the styles or firms of Dowlais Iron Co, Guest, Lewis and Co, and Guest and Co, was dissolved.
1852 John Josiah Guest was assisted in the management of the Dowlais Works by his wife Lady Charlotte Guest. On John Josiah Guest's death, the works was administered by his trustees, G. T. Clark and Henry Austin Bruce, 1st Baron Aberdare.
1850s The first organisation to licence the Bessemer Process for steel production
1850s Some notable engines at Dowlais in the 1850s: A beam blowing engine was erected in 1851 with an air cylinder 144" diameter and a stroke of 12 feet. The steam cylinder had a stroke of 13 ft and a bore of 55". The engine could develop 650 HP with steam at 60 psi. Speed 20 rpm. Weight of engine and bedplate approx 300 tons. A pair of rolling mill engines installed in 1857 developed about 1000 HP at 24 rpm, and weighed about 1000 tons. Cylinders 45" diameter, stroke 10 ft. The entablature supporting the beam was itself supported by eight cast iron columns 24 ft high. The columns and cylinders and bearing pedestals were attached to a massive iron bedplate whose many individual sections were connected by dovetail joints. The cylinders for the blowing engine and the rolling mill engines were cast by Perran Foundry, but all other parts were made at Dowlais Iron Works . Some drawings of these engines may be found in the 1862 book 'The Iron Manufacture of Great Britain Theoretically and Practically Considered' by W Truran 
1857-9 A powerful new mill for producing large rails was constructed (the 'Goat Mill').
1859 the Dowlais Ironworks Company bought the mineral ground of the Penydarren Ironworks.
1865 The first Bessemer steel was rolled at the works. Unlike the Cyfarthfa Ironworks nearby, the Dowlais Ironworks converted to steel production early (becoming the first licensee of the Bessemer process in the 1850's), allowing it to survive into the 1930's.
1869 'Monster Casting.— A casting of enormous proportions — a 70-ton block for a steam hammer — has just been successfully executed at the Dowlais Steel Works. The first charge of iron was tapped shortly after midnight, and successive charges followed from two cupolas throughout the day until 3 p.m., when the full weight of 70 tons had been poured into the mould, the metal keeping in a state of fusion for over 12 hours, and will not be cold enough to fix in position for some 12 or 14 days.'
1888 Construction of new works on this site.
1888 Erection of the new Dowlais-Cardiff Works on Cardiff Moors began.
1891 Two blast-furnaces were blown in February 1891
1895 The steel works and plate mills started work.
1898 After the death of G. T. Clark in 1898, John Josiah's son, Ivor Bertie Guest, 1st Baron Wimborne, became active in the management of the works but he was distracted by other interests. He responded to an approach in 1899 from Arthur Keen.
On 9th July 1900, the Dowlais Iron Company and Guest and Co, both controlled by Lord Wimborne, amalgamated with the Patent Nut and Bolt Co owned by Arthur Keen to form Guest, Keen and Co . This would give Keen's company its own coal and iron supplies and a diversified range of products
In 1912, King George V of the United Kingdom and Queen Mary made an official visit to the ironworks as part of a tour of south Wales. They entered through a specially-constructed arch of coal, and left through an arch of steel.
The 1919 O.S. map here shows part of the works, including the Goat Mill, coke ovens, brickworks, 'Dowlais Great Tip', and an extensive railway network.
1930 A new company British (Guest Keen, Baldwins) Iron and Steel Co was formed to amalgamate the heavy iron and steel businesses of GKN and Baldwins; GKN contributed Dowlais Iron and Steel Works amongst other units
1935 The Dowlais works were redundant.
1936 Iron production ceased; the company built a new iron and steel works at East Moors, adjacent to the docks at Cardiff. The iron foundry and engineering works, still known locally as the "Ivor Works" after Ivor Bertie Guest, son of John Josiah Guest
1937 The one remaining blast furnace at Dowlais was brought back into operation in response to the shortage of pig iron. Dowlais had been making pig iron for 170 years until closure in 1929
Post-WWII New facilities were built after World War 2. It continued for some years under the name of Dowlais Foundry and Engineering Company
1967 Dowlais Foundry and Engineering ordered a new automated iron-making plant from Germany, as part of a comprehensive re-equipment programme
1967 Became part of British Steel
1987 The works closed