Such being the case, Cort's attention became directed to the subject in connection with the supply of iron to the Navy, and he entered on a series of experiments with the object of improving the manufacture of English iron. What the particular experiments were, and by what steps he arrived at results of so much importance to the British iron trade, no one can now tell. All that is known is, that about the year 1775 he relinquished his business as a navy agent, and took a lease of certain premises at Fontley, near Fareham, at the north-western corner of Portsmouth Harbour, where he erected a forge and an iron mill. He was afterwards joined in partnership by Samuel Jellicoe (son of Adam Jellicoe, then Deputy-Paymaster of Seamen's Wages), which turned out, as will shortly appear, a most unfortunate connection for Cort.

As in the case of other inventions, Cort took up the manufacture of iron at the point to which his predecessors had brought it, carrying it still further, and improving upon their processes. We may here briefly recite the steps by which the manufacture of bar-iron by means of pit-coal had up to this time been advanced. In 1747, Mr. Ford succeeded at Coalbrookdale in smelting iron ore with pit-coal, after which it was refined in the usual way by means of coke and charcoal. In 1762, Dr. Roebuck (hereafter to be referred to) took out a patent for melting the cast or pig iron in a hearth heated with pit-coal by the blast of bellows, and then working the iron until it was reduced to nature, or metallized, as it was termed; after which it was exposed to the action of a hollow pit-coal fire urged by a blast, until it was reduced to a loop and drawn out into bar-iron under a common forge-hammer. Then the brothers Cranege, in 1766, adopted the reverberatory or air furnace, in which they placed the pig or cast iron, and without blast or the addition of anything more than common raw pit-coal, converted the same into good malleable iron, which being taken red hot from the reverberatory furnace to the forge hammer, was drawn into bars according to the will of the workman. Peter Onions of Merthyr Tydvil, in 1783, carried the manufacture a stage further, as described by him in his patent of that year. Having charged his furnace ("bound with iron work and well annealed") with pig or fused cast iron from the smelting furnace, it was closed up and the doors were luted with sand. The fire was urged by a blast admitted underneath, apparently for the purpose of keeping up the combustion of the fuel on the grate. Thus Onions' furnace was of the nature of a puddling furnace, the fire of which was urged by a blast. The fire was to be kept up until the metal became less fluid, and "thickened into a kind of froth, which the workman, by opening the door, must turn and stir with a bar or other iron instrument, and then close the aperture again, applying the blast and fire until there was a ferment in the metal." The patent further describes that "as the workman stirs the metal," the scoriae will separate, "and the particles of iron will adhere, which particles the workman must collect or gather into a mass or lump." This mass or lump was then to be raised to a white heat, and forged into malleable iron at the forge-hammer.