boiling vs steaming

[Kevin Jenness]

Some people boil roughouts, particularly of unstable species and large pieces with a lot of work tied up in them, to reduce cracking by "relaxing the fibers". It seems to me that steaming in an insulated chamber would be more fuel efficient and equally effective as long as the blanks are heated to close to boiling temperature. Has anyone tried this approach? There is plenty of information about steam bending and boiling turnings for stability but not on the effectiveness of steaming for stability, and I have looked around a bit including Hoadley's Understanding Wood and the USFPL's Wood Handbook​.

[Richard Coers]

I've understood the boiling was to break up cell walls to allow bound water to escape. Not to relax the fibers.

[Bill Howatt]

My understanding as well.

[Kevin Jenness]

Yes, I've read that as well - I guess that would be another way of understanding the phrase "relaxing the fibers", but I have not seen any technical study or discussion of the process - have you? In any case is there any reason to think steaming wouldn't have the same effect?

[Edward Weber]

As far as efficiency goes;
It takes 1 btu to raise the temp of one pound (roughly a pint) of water 1 degree.
It takes 970 btu's to change that same pound of 212 water to 212 degree steam. (change of state)
Steaming takes more energy.

[Richard Coers]

Some real nice information from Andi Wolfe in this discussion. https://groups.google.com/g/rec.craf.../c/0GK9RGi3Su0

[Kevin Jenness]

Edward, thank you for reminding me of the cost of phase change. I guess I will save steaming for bending wood (although boiling can work too, I believe) and look for a large non-reactive vessel for boiling. That is, if I do it at all. I have had pretty good results with air-drying and am not generally in a hurry, but I have the impression that boiling can reduce checking/cracking with problematic species and particular pieces such as those containing knots or perhaps pith, and heavily spalted stuff. I have not used much material like that, but if/when I do perhaps I will try the boiling route.

Richard, thank you for the link, that is very informative about wood structure and chemistry.

[Bill Howatt]

If you try boiling, remember to have a pot that is larger than your blank so it cannot jam against the sides and form a pressure vessel underneath.

[Kevin Jenness]

On thinking a bit more about the process I see that while it takes a large input of energy to make water into steam, that energy is released into the steam chamber as the steam condenses back into water, so the issue of efficiency between boiling and steaming in heating up the turned vessels is not quite so simple. As well, dealing with 20 gallons or more of boiling water is a bit daunting, so I am still on the fence. I came across some videos by Lou Sauzedde who steams boat parts inside a plastic bag and I may try that with a borrowed propane burner and boiler and a 30 gallon 6 mil poly bag from my dust collector. https://www.youtube.com/watch?v=50uXPPt8-VI&t=43s With this setup I would not need a large non-reactive stockpot for boiling or a permanent steambox taking up space in my shop.

[Kevin Jenness]

I did a test using electrically generated steam that seems promising. Two 1500 watt Earlex steam generators piped into a 30 gallon poly bag registered about 206 F on a $8 meat thermometer - I got the same reading in a pot of boiling water. One steamer alone showed 200F on the gauge. The units put out about 6600 btu/hr each as opposed to the 50-100k btu propane burners typically used to boil vessels in large stockpots. The Earlex steamers hold about 5 qts, run for about 2 hours and shut off automatically when the water gets low. I feel a lot more comfortable with them than working with an open flame and a vat of boiling water, the energy cost is less and I am using solar power rather than dead dinosaurs.

I steamed several pieces from the shelf of shame, rejects for various reasons, to see how they would fare. They all discolored superfically to some degree, which I have read is common to boiling as well. They all were once-turned and air-dried and showed a moisture content of 20-30% after two hours in the bag. The spalted piece changed color the least and it was not objectionable, but I think the process is better suited to twice turning for other types of wood as one would have to sand heavily to get through the discolored areas.

Having a "steambox" I can fold up when not in use is a great advantage in a small shop, and as shown in the video linked above can be adapted to pieces of any size. I see no reason why steaming wouldn't be as effective as boiling for turned work. Next time I will use some type of tub under the bag to catch the condensate, and perhaps use some type of insulating blanket over the affair.

I would be interested in hearing from anyone else who has used steam for this purpose.
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[alan weinberg]

I assume one can only bend the wood right after it is steamed or boiled, before it cools. How do you handle it?

[Peter Blair]

In the beginning I was always in a rush to get wood dry so I could turn it. Now I just turn it no matter what state it is in. I boiled in a large pot so cold get several pieces of wet wood in at the same time and once I loaded it so well that the bottom piece fit in the bottom of the pot and the water under it boiled dry and burnt the piece of wood while under water! I have also made a small steam box but I only made and used it to deform a finish turned pice out of round. I'm just sayin . . . .

[Kevin Jenness]

In the beginning I was always in a rush to get wood dry so I could turn it. Now I just turn it no matter what state it is in. I boiled in a large pot so cold get several pieces of wet wood in at the same time and once I loaded it so well that the bottom piece fit in the bottom of the pot and the water under it boiled dry and burnt the piece of wood while under water! I have also made a small steam box but I only made and used it to deform a finish turned pice out of round. I'm just sayin . . . .

The main reason I would use this process is not to speed up drying (though that would be a welcome knock-on effect) but to prevent or lessen checking/ cracking in certain pieces like heavily spalted wood or pieces containing knots.