These early furnaces only operated for a few months of the year, and the amount of iron they produced was very small. One of the most important areas for Welsh iron in the 1500s was the area around the source of the River Taff. The rocks of the valley sides provided a good quality ironstone and pebbles or nodules of ironstone could even be found in large quantities in the beds of the rivers and streams. The area around the small village of Merthyr became known as a good place to build a furnace and a number of small businesses were producing pig iron by the 1650s. The pig iron was taken by horse cart to Cardiff where it was shipped to Bristol, and even London where it was bought by ironworkers who forged the basic iron into useful products.

The difficulties in transporting heavy pig iron down the valley to Cardiff meant that these early businesses were not very profitable. However, the iron ore in the hillsides was of such good quality that various businessmen from all over England would keep trying to make out of Welsh iron. The big change in Welsh fortunes came with the discovery of using coal (or coke) to fire the furnaces instead of charcoal. Throughout the 1720s, technical developments in Coalbrookdale in Shropshire resulted in more understanding on how to use coal instead of charcoal. However it was a slow process and it was not until 1757 that John Wilkins of Brecon and Thomas Mayberry of Staffordshire took the risk of building a coal fired blast furnace at Hirwaun. The 'revolution' had begun…

The map below shows the main centres where iron (and in many cases in later years, steel) was made across South Wales. There was a very strong concentration of iron making in the northern part of what would eventually be known as the South Wales coalfield. This is the area between Hirwaun, Merthyr, Tredegar, Ebbw Vale and Blaenavon. This map only shows the main areas of iron making, there were many others throughout this area.