[Billman]

Gonzalo, thank you for your long and detailed response. It is always good to have different points of view, especially with reference to historical research. The use of the word machete in the 16th century, in the OED, does not give the context - the full OED would, but mine is the two volume abridged - so I can only give the first reference for the word, and of course it may relate to something other than what we today call a machete...

I know that in the UK molten lead was used to anneal the tangs of files that had been hardened to glass hard - this allowed the tang to be quickly heated without the heat transferring more than a 1/2" (say 15mm) beyond the shoulder of the file.

Most edge tools prior to about 1870 were made by forge welding carbon steel to a wrought iron or mild steel body. This process continued in many small forges well into the 20th century, and is still being carried on in the forge of Bernard Solon, the last traditional taillandier, working in Orléans, France. All-steel tools, usually stamped 'cast steel' or 'solid steel', were not widely available before the late 19th century, and after Bessemer and Siemnens Martins steels became widely available.

It was not until the early 20th century that alloy steels were used for edge tools... Hardening and temering of plain carbon steels involves quenching from red heat and then reheating to a much lower temperature to temper the blade... even with a soft steel or iron body, most makers still tempered the edge, even if only lightly, to remove brittleness. Tempering was often carried out in a bed of hot sand...

Modern steels may be capable of being hardened and tempered in one process, but most that contain over 1% of carbon still require some type of quenching to retain keep the cementite from reverting to pearlite if hardness, as required in a cutting edge, is needed.

I will need to research further into the use of lead or lead alloys for either hardening or tempering.. It may be that Collins were using the same technnique as file makers, and only annealing the tang area of the blade to further soften it to resist shock...