In 1799, Thomas Bacon, who had been granted the Plymouth works under his father's will, became of age, and agreed to surrender to Richard Hill I all his interest in the Plymouth works, and this he confirmed in 1803 when he was 24 years of age. Being now in full possession of the Plymouth works, he with his sons, Richard II and JOHN HILL, entered into an agreement with the Dowlais and Penydarren iron companies for the construction of a tram road for their joint use, from their works to join the Glamorganshire canal at Navigation (now Abercynon). In the same year, 1803, Richard Hill I, who was a practical engineer, agreed to construct a tram road for the joint use of the same three companies to convey limestone from the Morlais Castle quarries. It will thus be seen that Richard Hill was on very good terms with the neighbouring ironmasters, which was far from being the case between the Penydarren and Dowlais companies.

Richard Hill and son were anxious to improve their business by adding a forge and mills, but were very short of capital for such extension. Partners were sought, and A. Struttle advanced £15,000 and John Nathaniel Miers (son-in-law of Richard Hill I), £5,000 to form the Plymouth Forge Company with a capital of £20,000. Work now proceeded briskly at the Plymouth iron-works where Richard Hill I was ably assisted by his sons, Richard II and ANTHONY HILL (1784-1862). But on 20 April 1806, Richard Hill I passed away leaving all his estate to his widow, Mary, his three sons, Richard II, John Hill (of London), and Anthony Hill, and his two daughters, Elizabeth, and Mary, the wife of J. N. Miers (of Cadoxton Lodge). By 1813, Messrs. Struttle and Miers seceded, and the three brothers became partners. On account of the withdrawal of capital, the brothers had to obtain a loan on mortgage of £54,000 from Messrs. Wilkins (q.v.) of the Brecon Old Bank. Richard Hill II for a time lived at Llandaff and looked after the sales side of the business, while Anthony ably managed the productive side, but the burden of the huge loan was a great impediment for many years to the successful working of the concern.

In 1806, the three furnaces at Plymouth produced 3,952 tons of pig-iron, while in 1815 the same three turned out 7,800 tons. A fourth furnace was erected at Plymouth and in 1819 the first furnace was erected at Dyffryn and c. 1824 two others were erected; like all the others these were worked by water-power in which Anthony had great faith — he was very slow in adopting steam-power as the other iron-masters were doing. With Anthony Hill as managing-partner the works were carried on with great vigour and ability, and their brand of bar-iron had a special value and was known for its excellence throughout the world. The produce of the blast furnaces continued to increase year by year. In 1820, it was 7,941 tons, in 1830, over 12,000 tons, by 1846, it was over 35,000 tons.

In 1826 John Hill sold out his interest to his brothers, Richard II and Anthony. Later, No. 8 furnace was built, which was said by Mushet, the great authority on iron manufacture, to be the largest in the world. Richard and Anthony continued as sole managers until the death of Richard in 1844, after which Anthony continued as sole managing director until his own death at the age of seventy-eight on 2 Aug. 1862. Anthony Hill was regarded as the most scientific iron-master of his district. He carried out many experiments for the improvement of iron-making, and was the patentee of many new methods. Gradually he succeeded in paying off the loan burden and attained great wealth. His death was keenly felt in the district, as he had been ‘associated with good deeds, with broad and enlightened measures for his people's comfort, for their religious welfare, and their education.’ He established a new church at Pentre-bach, and endowed it with £200 per annum; he also founded the Pentre-bach National School and left a sum of money for the ‘Anthony Hill scholarship’ for secondary schools which is still in existence. After his death the works were sold to Messrs. Fothergill, Hankey and Bateman for £250,000; they came to an end c. 1880.