"Iron" should mean relatively pure iron as they knew pretty well how to make that. "Mild steel" shold mean non-heat treatable steel - around 0.2% carbon content. I don't think they were very sophisticated in identifying carbon content in heat treatable steels. Springs were most likely in the 0.7% - 0.9% carbon content region. The "bloom" was at welding temperature, so the steel was annealed, but the iron and steel chips were not totally melted. If melted, you would have wound up with simply lower carbon steel due to the inclusion of iron. The chips, wheter iron or steel, retained their identies.
Control of this process would involve the ratio of iron to steel, the grade of both iron and steel, chip sizes, uniformity of welding, and the amount of hot working and cold working of the rods produced from the bloom. Patterning should have been random, but probably got some orientation due to chip shapes and hammering.
I'd agree that terminology, materials, and measurements changed dramatically during the production period. I'd also bet that most patterns were "trade secret" rather than patented. Trade secrets tend to disappear with the last man standing.
