The design of his puddling furnace allowed various types of fuel to be used. Much of the wood produced by intensive coppicing in this area would be on a 20 - 30 year cycle, some of it intended for charcoal making. Charcoal was used for throughout the world as fuel for iron production; its properties made it ideal for manufacture of good metal. Charcoal making was time consuming, expensive and Cort needed lots so he was interested in developing the use of coal, which was plentiful and more readily available.
Even more important than the furnace itself were the trained puddlers who must have been both highly skilled and totally fearless in their task. It was their control of the operation with all of their senses that governed the quality of the iron product.
At a certain point in the process the damper is closed and the door is opened the puddler stirs the iron with an rake or rabble. As it is stirred, the carbon burns out and the iron gets purer - it is turning into wrought.
Mixtures have a lower melting point than pure things; cast iron melts at 1200°C; steel, with far less carbon, at 1600°C - and pure iron takes about 1800°C before it melts. As the carbon burns out, it turns into dust and the iron can no longer stay molten. If the eutectic point is reached, the temperature suddenly takes off the iron becomes pure, far too soft and totally useless.
The iron master watches for a particular state when it balls into huge, malleable lumps which are called loops. The puddler gets a loop of iron out of the furnace, and it is given a few blows of a hammer to squeeze it up. Then it is ready for part two of the process - rolling.
The resultant iron was, Cort declared, as good as the best Swedish product, at a fraction of the cost.