It is commonly thought that a foundry existed on the site of Ynysfach Ironworks at the end of the eighteenth century, although reliable evidence for this is not well documented. In 1801 Ynysfach Ironworks was opened under the ownership of Richard Crawshay as an extension of the Cyfarthfa Ironworks to handle smelting and refining – all puddling and rolling was undertaken at Cyfarthfa. It was the first ironworks purposefully built to use the new 'Welsh process' of refining technology. Ynysfach employed steam blowing engines from 1801 and as a result its output far exceeded that at Cyfarthfa. The main function of the works at Ynysfach was to produce 'finers metal' to feed the growing demand at the puddling furnaces at Cyfarthfa.
Increase in demand for the finers metal at Cyfarthfa resulted in modernisation and expansion at Ynysfach. Between 1836 and 1839 a significant building program was undertaken by William Crawshay II which included the re-building of the northern engine house, the construction of two additional blast furnaces, two casting houses, a refinery building constructed to the east of the casting houses, a new engine house at the south of the site with an associated boiler house and chimney stack. A further extension to the refinery was made in c.1850. This increased the productivity at Ynysfach to supply sufficient 'finers metal' to Cyfarthfa.
The reconstruction drawing shows the Ynysfach Ironworks in around 1860 when the works were at their most productive. The expansion had been made some twenty years previously and the four blast furnaces can be seen with their stone chimneys. In the top right of the image smoke can be seem issuing from a row of calcining kilns where iron ore was roasted. The workers on the calcining level can be seen shovelling iron ore into the kilns. On the level below this the workers are barrowing the roasted iron ore out of the ovens to be fed into the top of the blast furnaces along with limestone and coke, which has been stored in rows in the coke yard in the very top right of the image. Each blast furnace is shown with its corresponding casting house with numerous side arches and circular apertures to allow light in and the heat and gases out. Through the arches in the casting house walls, the runs of grey iron pigs can be seen set into the casting sand. To the north of the 1801 blast furnace is the four story engine house with its windows and ground floor arches. It is in here that the massive beam engine would have sat, providing the blast to power the works. This northern engine house provided the blast to the 1801 twin blast furnace and was itself powered by the boiler house adjacent to it. When the works were expanded in c.1836 and a further two blast furnaces were built, an additional engine house was built to the south with an almost identical design to the original. This was required to provide the additional blast needed to power the new furnaces. A new boiler house was also constructed behind the new engine house. Each engine and boiler house complex had its own brick built chimney stack to expel the gaseous emissions high into the atmosphere above the levels of the works. In the bottom right hand corner a small building can be seen which is the forge. This building would have made and repaired all manner of metal tools and fittings required for the works.
The long, single storey building in the bottom left of the reconstruction drawing is the refinery. The northernmost part of the building, picked out with ashlar limestone quoins, shows the refinery as it would have been in c.1836 with two run-out trays protruding through the two large arches and a drain complex running out through the three smaller arches. Two chimneys indicate the positions of the two refining furnaces below. At some time after between 1842 and 1850 the refinery was extended to the south so that the entire refinery complex comprised six refining furnaces and six run out trays. This would have allowed for efficient refining of the grey pigs to white 'finers metal' before being taken to Cyfarthfa for further refining and rolling.
Ynysfach Ironworks is thought to be the first ironworks built to incorporate this additional stage of refining from the beginning. The grey pigs coming from the casting houses would have been put into the refining furnaces along with scraps of iron, some broken sandstone and coke as fuel. Blast was added to the furnace through tuyères (a nozzle through which air is blown into the furnace) directed at the pigs. When the pigs had reached a molten state the metal was run out onto trays that protruded through an arch in the refinery wall so that more than half of the length of the tray was outside the building. The tray would have sat on top of a large cast iron cistern filled with water that served to cool the metal in the tray above. The metal with the slag on the top would have also been cooled from above through the explosive action of throwing water onto the molten iron and slag or by directing water at it by means of a hose. This 'finers metal' was then taken by canal or rail to Cyfarthfa for puddling, further refining and rolling into a final product.