By the 1770s he was a leading London iron merchant, dealing mainly in
Swedish and
Russian iron. The firm was Crawshay and Moser in 1774, and then Crawshay, Cornwell and Moser in 1784. The business still existed, as R & W Crawshay, in 1816. By 1775 he was acting as
Anthony Bacon's agent for supplying iron
cannon to the
Board of Ordnance and was from 1777 a partner in that business, casting cannon at
Cyfarthfa Ironworks in
Merthyr Tydfil. This continued until Bacon had to give up government contracts in 1782, because he was a
Member of Parliament. In 1786, following the death of Anthony Bacon, he took over the whole Cyfarthfa Ironworks, in partnership with William Stevens (a London merchant) and
James Cockshutt, who had previously managed the forge and boring mill for
David Tanner. In May 1787 he took out a licence from
Henry Cort for his
puddling process, but the
rolling mill needed was not completed until 1789. He solved the problems of the puddling process by using an iron plate for the furnace ceiling and sea-washed sand for the floor. In 1791 he terminated the partnership, which had made little profit. He continued the business alone, and had two
blast furnaces, eight
puddling furnaces, three
melting fineries, three
balling furnaces, and a rolling mill in 1794. A blast furnace was built by 1796, and a fourth the same year. There were six furnaces by 1810. He thus developed Cyfarthfa into one of the most important ironworks in South Wales. Crawshay was very ambitious and imperious in manner, being called 'The Tyrant' by some, but was without social pretension. He was active in protecting the interests of the iron trade and was a major promoter of the
Glamorganshire Canal which immensely improved transport of iron to
Cardiff Docks. In 1799 he was the sixth wealthiest man or family in Britain (with some forerunners counted for completeness as wider family concerns), owning £2M (). He was one of ten millionaires known that year. At his death in 1810 his undivested estate was sworn at £1.5 million. By his last will he left of his ironworks to son
William Crawshay I, to his son-in-law,
Benjamin Hall and to nephew
Joseph Bailey. He was buried at
Llandaff Cathedral. ==See also==