Yes,they didn't really know what caused the metal to become hardened until in the 19th. C.,they began to have some understanding of chemistry. I have 3 chemistry books by the same company. The first one is Civil War age. A little over an inch thick. Full of the phrase "This process is not well understood". My 1903 one is THREE inches thick,and obviously full of real info. Shows how far they have come in not that many years apart.
The old people just knew empirically that if you did this and this, this will happen. They packed wood,urine from a red headed boy,or from a wine drinking friar,and a bunch of other worthless stuff(HIGHLY SECRET,no doubt!!) into a crucible,or a large chest of sandstone if a large steel making furnace was being used. They looked like huge funnels sat dide end down on the ground. Men,whose life could not have been too long,would crawl into these furnaces,when cold! They would pack soft wrought iron bars in many layers,adding the secret compound between layers. It was the carbon bearing stuff that case hardened the rods. The rest burned off. Then the poor devils had to load the furnace with a huge quantity if fuel. Coal or better,wood. Coal had sulfur in it,which made the steel "hot short",or inferior in other ways. The English did not understand this process,and bought their best iron bars from Sweden. They only used wood in Sweden. Countless rumors abounded as to WHY Spanish gun barrels were the best(they thought it was the AIR in Spain!)
After the furnace had been kept hot for several days,they'd let it burn till cool. The men would crawl back inside,amidst the powdered ashes that were very deep,and would get breathed. They'd open the chest and take the deeply case hardened steel bars out. BUT,the middles of these bars were always soft wrought iron as they were not fully penetrated by the carbon. Bundles of these bars would be wired together,and welded into a solid bar. This was called SHEAR STEEL. If they folded the big bar over and welded it into one bar again,that was the more expensive "Double Shear Steel. The soft spots in double shear steel had been made smaller,but they were still there.
In the 18th. C.,Benjamin Huntsman,a WATCH MAKER,of all things! Was frustrated with his watch springs breaking because they had those soft spots in them. He tried putting chunks of wrought iron and carbon compound into a crucible,luting it shut,and actually heatinf it so hot that the iron melted,and absorbed carbon evenly all the way through. THIS WAS TRUE TOOL STEEL,about like our W1 water hardening steel. His steel became in high demand,and he ended up making steel.
This was called "crucible steel",or CAST STEEL,and that term can be found,boasted upon chisels and other tools made in the 19th. C.. Butcher was a well known brand. You can still find them in Pennsylvania flea markets.