"Blister Steel" was the most commonly available heat-treatable steel of the 18th & 19th centuries. Used for cutting tools, springs, anything requiring decent hardness and strength.
Metallurgy 101 - the difference between elemental iron (close to nails, common mild steel) and STEEL that can get hard is carbon. Nails & such might have about 0.1% carbon, or at least something less than 0.2% carbon. Steel, from which one can make stone chisles, springs, swords files &c has anywhere from 0.6% carbon up to about 1% carbon.
Wrought iron has almost no carbon, say less than 0.1%
Ya want it to be hard, gotta caseharden it (pack in charcoal & heat red a few hours) Surface then picks up 0.6% or more carbon, and hardens when you quench it from the pack.
Want a solid bar of fairly high carbon steel?
Take several bars, usually flat skelps of wrought, iron, pack them in a lot of charcoal & heat the whole mess very red hot (say 1750F) for about 24 hours. This case hardens the $#*! out of them, maybe half way through the bar. Take them bars & forge them together to get a more even mix of carbon through the metal & you have blister steel.
Huh? Why blister?
Oh, that wrought iron is filled with slag stringers. That slag is a mix of some silicates and iron oxide. When carbon diffuses in to those iron oxdes, that carbon reduces them to plain old iron, and carbon monoxide gas.
The gas raises blisters on the surface of the iron. So before you use the stuff you must hammer down those blisters, close them up.
The best steel at that time was so-called "cast steel". Uniform in carbon throughout, no blisters. It was not a casting. Just called that because you start out with blister steel and melt it, CAST it into an ingot, and forge down to bar.
Bonus round: All metal is a mix of crystalls, which us metallurgists calls "grains" Normally they are too small to see with the eye. If you seen bright shiny crystals on a broken piece of steel, they are way, way to big. Eiher it is a forging, as-forged, or you heated it too hot in heat treatment.
You can refine these grains, make new smaller grains, by heating the thing a nice red & coolng it in air or justy lay it on the ground. Then when you heat it again to harden it, it will have reasonably fine grains & make a tougher Widget.
Aren't you glad you asked?
