steaming science 101

[i'llbeinthegarageifuneedme]

This is not technically a building/repair question, but I figure it's the right audience to ask.
I am doing a little talk & demo on steam bending for my daughter's seventh-grade class, and I need a little refresher on the science of steambending. I know I have read in the past about exactly what the steam & heat do to the wood cells, but, like most of the stuff I learned in school, I forget the details now that I need 'em. Anyone care to (re?)edukate me?

Thanks in advance

[Pernicious Atavist]

steam fills the wood cells with water, the more water, the softer the cell. soft cells flex (think water balloon) dry ones don't. that's pretty much it....good luck! draw pictures, kids love that

[Donn]

"Wood is made up of long tube-like cells of cellulose connected end to end forming long fibers running the length of the tree. Additional fibers run across the grain, tying everything together. Except for the outermost layer (cambium), wood is dead, serving the plant for structural support and water storage and transport. Each year a new layer of fibers is added under the bark, its thickness determined by growing conditions and stress - more moisture = a thicker ring, and more stress = a thicker ring on the stressed side. Over time, the inner layers may fill with resins, forming heartwood, which is usually denser, and more rigid. The newer, outer layers, sapwood, are still relatively flexible and wet. (Excellent bows are made with the compression resistant heartwood on the face and flexible sapwood on the back.) Heating the wet wood turns the water to steam which dissolves some of the bonds between fibers allowing them to realign, reforming the bonds when they cool. So, steam bending is the process of weakening, stretching and reforming wood fibers to the desired shape. Rawhide acts in a similar and more dramatic way when it is wet, and then dried to shape."

link.. (http://www.primitiveways.com/bending.html)

[Ed Harrow]

And you have to have a demo. Just don't invite anyone associated with Death-by-Draino's experiment. ;)

[Ian McColgin]

Or live dangerously and add amonia. Seriously, it enhances the bendability. A nearly pure saturation with hot amonia vapor will make stout planks into noodles.

Amonia vapor is really dangerous and unless you have a chem demo area with proper hood and fan and do enough test runs on site, this is just a frivolous remark.

[NormMessinger]

Just to be argumentive steam is primarily a conductor of heat. Heat plasticizes the cellulous making it easier to bend. When I was building mountain dulcimers I only used dry heat, a few charcoal brickettes in a coffee can. Works with thin stock but, as a practical matter, steam is required to carry the heat into thicker stock. Ammonia is a wetting agent, among other things.

[Bob Smalser]

Another idea is what I used to do for school classes.

Take the hand tool set, Workmate, a bunch of handscrews and set up on long tables. I take a chunk of firewood and with froe, handplane and trysquare, show them how a board comes from the tree and what makes it strong.

Then it's a simple hands-on session where the kids get to use all the tools shown with scrap clamped to the cafeteria tables under adult supervision.

The brace with auger is usually the favorite....I take two or three of those.

[Scott Rosen]

There was an article in WB Mag a couple of years ago that discussed the use of dry heat to bend wood. It works great, but you have to be careful to avoid setting the wood on fire.

[Paul Scheuer]

. . . Examples of steam bent stuff . .
I'm alway amazed at how the piece resumes its rigidity in its new form when dried/cooled.

I agree with Bob - hands on - the noisier, the smellier, the messier the better.

[George Roberts]

i'llbeinthegarageifuneedme ---

Ammonia needs to be dangerous to be effective. So don't use it. Don't talk about it.

Dry heat works well. A hair dryer works on thin wood.

With steam and 20 minutes to an hour per inch you really want thin stock.

[gapup]

Good timing on this thread. Having successfully cold laminated a partial frame several days ago, I set out to repeat this fine experience yesterday. ................ jest when ya think ya got it all down to a science ................ shazam!

I was considering the set up of a simple steam process for bending but on the strength of comments here, will give it a go with dry heat. I've experimented with it in the past and it certainly does work ........... with some limitations on heated length and thickness.

My concern is for that tight radius at the turn to the bilge. If the laminates can be brought mostly into that shape under heat, they will likely finish the bend without breaking. .............. he said confidently.

BTW, I have used a paint stripper type of heat gun for a heat source. A flat iron and a wet towel might be equally effective if the part shape will allow. ............. or maybe it's jest as simple and quicker to set up a bucket of water on a propane stove and chimney to hold the laminates, eh?

There are many things more interesting and vastly more fun than bending wood into unnatural shapes.