Okay, I got this bit of information from another forum. In a book about finnish culture "Suomen kansanomaista kulttuuria", by UT Sirelius there is a caption that goes like this:

"Selkä eli kaari oli rauasta, wielä kahesta rauasta, mellosta ja teräksestä, ulospuoli mellosta jotta ei katkiaisi, sisäspuoli teräksestä."

And that roughly translates in to this:

"Back ie. the bow is of iron, and of two iron that is, of iron and steel, the outer side of iron for the bow not to break, inner side of steel."

So the traditional way to make a crossbow prod in Finland, would have been lamination. So the guy who read this first, wanted to know if it was for real. So he went to Turku provincial museum. I don't know if he's staff or what but he was able to take a prod from the 17th century and file a bit of rust off one and add some nitric acid on it. Voila!


[ Linked Image ]

You can see where there is iron (light) and where the steel begins (dark). He says it's less than 2 mm of 13 mm total thickness. There's no way of knowing if the iron part is thicker in the middle, but he says he's tried laminating with ratio 1/3 iron and 2/3 steel. He says that after he had hammered the pieces together you could barely tell that there was iron on the back at all.

So here's my question to you smith guys. Does that really improve the prod's quality and efficiency and if, then how? Or could this be just another way of saving precious steel?

Well, adding a tough surface layer to a more brittle base material is a technique that is used for some modern cutting tools in order to increase resistance to cracking.

Thus adding a tough iron layer to the outside of a crossbows bow would seem to make sense (crack formation is, on the whole, only an issue when you pull at a material, the compressive stress put on the inside of the bow would simply close any cracks which formed, instead of pulling them apart).

Some theoretically possible (ie I'm simply guessing here) benefits from this could then be used to allow more stress to be put on the bow (to increase the power), decrease the risk of sudden breakage, or to allow the bow to be made thinner (saving steel), or a combination of these. You simply get a bit more "basic quality" to play with.

[Edit] It might perhaps be best to add that the case I was considering here was a steel bow with a (somewhat) thin layer of highly ductile iron added to it, compared to an all steel design.

steel being 'springyer' and more expensive than iron would improve the throw of the bolt over merely iron. also less likely to take a set.

This technique was also done with breastplates - called "duplex". There is a very interesting article in the RA's journal "Arms and Armour," Vol 2, No 1 (2005) pp.5-26.