A Question on Steaming versus Heat Tempering

[Eric Garza]

I am curious: Is it necessary to use dry heat when heat treating or fire hardening a piece of wood? If I steamed a bow for 3 or 4 hours, for example, would that accomplish the same thing that cooking the wood over coals would do?

I am gearing up to steam-straighten a few hophornbeam staves, so might be inspired to explore this, unless someone talks me out of it.

[Digital Caveman]

I think for heat tempering to work it needs to get up to around 400 degrees.  Steam can defiantly get that hot, but you wouldn't be able to heat treat just the belly.  That should be answer enough.

I think the scorching in heat tempering is impossible if there is no oxygen present.  I'm not sure if scorching is a necessary part of the prosses, but it looks great on HHB.

[PatM]

If you steam for too long wood seems to weaken permanently.

 

[Del the cat]

Steam is at about 100C (212F) and unless it's under pressure will probably not get much hotter. That will allow the bwood to bend but will not heat treat or harden it.
Dry heat above about 200C  (~400F) is needed for heat treating, and as it is above 100C it will also allow the wood to be bent.
Thus you can bend and heat treat at the same time if you get it hot enough.
Del

[scp]

Steaming works well for bending wood and even for drying wood. But so far as I know, it does not harden wood, other than as the result of drying wood. IMHO heat treatment is for plasticizing and "compacting" lignin in wood to increase its compression strength.

BTW I just realized, if it is really by "compacting" or at least rearranging, I might need to keep the belly down over a source of heat for better result, to allow the gravity to "compact" lignin inside to the bottom of the belly. I have been heat treating with the belly side up. This might be of little significance or rather difficult to verify. Where are all the tree scientists when we need them?

[bownarra]

Nope, no amount of steam will heat treat wood. Too much steam will weaken the wood anyway.

[Eric Garza]

A couple folks have said that too much steam will weaken wood. I have never heard that before, until reading it on this thread. How much is too much? Is it about temperature, or length of time the wood spends steaming?

And if it is true that too much steam weakens wood, does that length of dry heat weaken wood too? Why would wet heat weaken wood but dry heat not?

[PatM]

Likely length of time since most of us are not using anything higher in temp than regular steam.

 Hard to say why.  Seems like something is being leached out of the wood by steam.  Steam actually slightly increases tensile strength of wood according to tests.

 I've oversteamed elm for recurves and it stayed rubbery and wouldn't hold a curve.

 So I would say both types of heat decrease strength of wood if done too much but in different ways.