Steam drying - effects?

[condeesteso]

I'm interested to understand this better and have searched a bit, but there seems little regarding effects on the wood from a working and structural point-of-view. I suspect super-heated steam is being used to stabilise lower grade stocks (Oak Furnitureland, cheap worktops etc)? I think normal steam (100C) has been used for years as a way of darkening and/or controlling colour. I did find a pdf summary of a presentatiojn on the subject which is quite interesting:

http://www.coste53.net/downloads/WG2/WG ... 7-Gard.pdf

One slide says this:

So I expect there are 2 different steaming processes and the results may differ a lot. The idea of boiling off m/c at well above 100c suggest to me that the natural oils in the wood may be altered or to some extent removed.
I'm interested as I feel a difference working beech (say) and in a very non-scientific way it just feels to me that all kilned is dry and brittle, air-dried is oilier and nicer. I can't say I can tell between steamed and unsteamed, but I doubt the steamed I have has been superheated.

Any knowledge of this topic and effects on working properties would be appreciated.

 

[Sgian Dubh]

The process is experimental as far as I am aware. My understanding is that it's an offshoot or developmental process that builds on the experience and knowledge that's been gained, in part, through over one hundred years of use of both the conventional kilning procedure and the low temperature dehumidification kilning techniques, both of which are pretty much the industry standard methods, and both of which use steam in their processes.

Hot, wet steam has been employed, also for a long time, to effect colour changes in wood, the most well known being walnut, beech and pear. In the case of contemporary practice, this steaming occurs prior to the wood going through a normal drying process, usually in conventional kilns or dehumidification kilns, although there's no reason I can think of to prevent air drying - kiln drying is air drying anyway, but with control of conditions switched to the kiln operator rather than letting nature take its course.

Working kiln dried wood is somewhat harder than working air dried stuff. The drier wood becomes, the stiffer and harder it becomes, and the drier the wood the stronger it becomes. As wood dries from FSP to 5% its stiffness increases by about 50% and the modulus of rupture approximately doubles, so you can see there's a significant difference in wood strength even between wood at 20% MC and wood at 8% MC. So, in working drier wood with cutting tools you're having to deal with its additional strength and stiffness. For an extreme comparison, cutting and shaping wet wood is relatively easy compared to dry wood - compare turning wet wood to turning the same species of wood at, say, 8% MC. I wouldn't describe air dried wood at, say 18% MC compared to kiln dried at, say 8% MC as oilier, just less stiff and strong, and that's what you're feeling. Interestingly, once a piece of wood has been dried to a very low MC, e.g., 5 - 7%, it will never quite return to its more supple state that existed in it as it passed through 25% MC, 20% MC, etc during the time it dried, which is a result of one form of hysteresis loop (a bit more involved than I want to go into here I'm afraid, primarily due to time).

As to chemical or oils being boiled off or chemically altered, I suppose that's possible, but you'd probably need a chemist with a sound knowledge of timber technology too (they exist) to discuss things like the boiling point of any oils in the wood, chemical bonding and volatility of those bonds due to changes in temperature. Slainte.

 

[condeesteso]

Thanks Richard. I've just this minute read elsewhere here that steamed is more stable than unsteamed...??!
Re m/c I keep a fair bit of beech in the workshop and several blanks cut to size (around 500 x 100 x 40) so I plan to re-check m/c as I assumed (without checking) that the kilned and aired had both normalised to my shop, typically everything ends up between 10 and 12%. I'm going to get some steamed next also, but I'm not sure I'll sense any difference.

 

[Sgian Dubh]

I've just this minute read elsewhere here that steamed is more stable than unsteamed...??!

That's true, again caused by a hysteresis effect whereby the current behaviour of a material is dependent upon the conditions it experienced in its immediate history. In this case I'm fairly certain this is due to steaming causing chemical changes in the wood, i.e., a decrease in the hydroxyl groups and the effect is to reduce the range of dimensional change, primarily across the grain, as the wood takes on or loses moisture. Slainte.