properties of steamed wood

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JimC
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properties of steamed wood

Post by JimC »

I'm just working with some highly bent pieces of ply which I steamed into shape a couple of days ago, and it seems to be rather brittle. Are there known to be changes in mechanical properties post steaming, or am I just being careless?
Last edited by JimC on Tue Nov 18, 2014 1:35 pm, edited 1 time in total.
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Ed
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Re: properties of steamed wood

Post by Ed »

Good question. Not aware of any, but it would make some sense I guess....

what do others think?

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Re: properties of steamed wood

Post by Rupert »

Pretty sure that if it is steamed for too long or too hot there will be changes - I guess the only question is whether that point comes before the ply is bendable to what you want or after. Does more steam for a given temperature stop the brittleness? Dry heat is certainly a problem.
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chris
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Re: properties of steamed wood

Post by chris »

Interesting...
in real wood the bending happens more by compressing the inside of the curve rather than stretching the outside so question 1 Is this true of bent ply? ...answer: "dunno". But that bit of 3mm poplar ply I bent in the microwave to a 1.3 inch radius and has stayed there quite happily up till now. I have just straightened out in my hands and I it hasn't broken or split but the outside of the curve has crumpled a bit whilst the inner part has returned quite normally. This may suggest that the out side was stretched rather than the inside compressed. Quite what that says I have no idea! But I suspect the glue and the cross grain layer will have something to do with that.
I can only think that the reason for brittleness must have something to do with over heating. Steaming softens whatever the natural 'Glue' is between cells and allowing cells to slip a little. Perhaps over steaming just washes away that 'glue' thus leaving little bonding there.
That's all guesswork mind.
Michael Brigg
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Re: properties of steamed wood

Post by Michael Brigg »

Wood is a composite of Cellulose fibre and protein matrix (Lignin.)

I would imagine the cellulose is relatively heat stable as stringy beans are still inedible after cooking unless like a cow you have 5 stomachs and a supersize caecum.

Under heat though the protein component will become pliable, and presumably at a critical temperature will allow the cellulose fibres to "slip," but too much heat causes a complete loss of the protein structure with loss of cross linkage (Di-Sulphide bonds) within the protein molecule. The matrix structure then becomes a brittle crystaline structure rather than a covalently bonded polymer resulting in delamination of the fibre/matrix structure, the tensile strength of the fibre is no longer combined with the hardness of the solid, and the weaknesses of both components are exposed.

A bit like Jam, you need to get the temperature just right. Too little and it wont set, too much and it looses the "set" and turns to slop.
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Re: properties of steamed wood

Post by Obscured by clouds »

Wot he says up there ^^^^^^

Celulose good for tension. Lignin good in compression.
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Re: properties of steamed wood

Post by roger »

Totally off topic. Micheal, a cow has four stomachs Rumen, Reticulum, omasum and abomasum. The fourth is the true stomach. Non ruminant herbivores have the oversized caecum to do what the rumen does, breaking down the cellulose.

just sayin 8)
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chris
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Re: properties of steamed wood

Post by chris »

There we are I knew someone would know the technical stuff!
I seem to remember from college days 130degrees is the right temperature.
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neil
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Re: properties of steamed wood

Post by neil »

now I'm going to have to borrow a kitchen thermometer to measure the temperature in my steaming set up. All I do know is that after a few weeks of steaming most evenings I'm sure my skin makes me look 10 years younger. :mrgreen:
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Re: properties of steamed wood

Post by JimC »

chris wrote:I seem to remember from college days 130degrees is the right temperature.
Fahrenheit I presume, not centigrade? That would explain it, I soaked those pieces in boiling water.
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Re: properties of steamed wood

Post by Michael Brigg »

JimC wrote:
chris wrote:I seem to remember from college days 130degrees is the right temperature.
Fahrenheit I presume, not centigrade? That would explain it, I soaked those pieces in boiling water.
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130'Fahrenheit would be about as hot as your average Steam room in a turkish bath. (Remember the "old" normal body temparature is 98.6' Fahrenheit. People still panic at a fever in excess of 104!)

Water boils at 212'F or 100'c. It stays at 100'c until all the water is boiled beacuase "Latent heat" must be added. This is the amount of extra energy to convert water at 100'c to Steam at 100'c.

Thereafter the steam can go up to any temparature you like. If you want to make the water boil at a higher temperature it will need to be pressurised.

Different proteins will "scramble" or cook, at different temparatures. eg: Eggs will scramble at @65'c (Egg white) and 70'c (Egg yolk.) so, for a perfect "soft" boiled egg, heat it slowly to just over 65'c, and then hold it at that temparature for 5 or more minutes.

With Jam (using Pectin) it is 220'Fahrenheit (104.5'c) which is achieved by adding sugar to the mix so that it increases the boiling point of Water. (More interesting physics there as well. for another time.)

I presume from Chris's comments that for Lignin (wood) this will "cook" at @ 130' centigrade.

Steaming is used to heat the wood because the steam tranfers heat better than (pressurized) water because it carries more energy as latent heat and is safer than super heated water. This is enhanced even more by pressure, which allows the steam to condense into water at higher temparatures. When it cools and condenses on a substance, the latent energy is released into the surface on which it has condensed, thereby heating it up.

This is how "pressure cookers" speed up the cooking time (Although please note that in the unlikely event of heating your steam above 374'c, no amount of pressure will convert it back into water unless you cool it back below its "critical temparature.")

You can "control" the temparature of your steamer if you like with a pressure valve to maintain plenty of heat transfer at just the right temparature. Mind you, if you are using old sections of Proctor boom section this could be hazardous to say the least! Best leave that to the industrial autoclaves!
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Re: properties of steamed wood

Post by chris »

see http://www.dpi.nsw.gov.au/__data/assets ... Timber.pdf

Steam doesn't need superheating but 130 is a maximum I think, Bending needs to take place v. quickly before the temperature drops though. I expect ply, with a large surface area compared to its thickness will cool quickly. In other words it will need to have been bend within seconds of it coming out of the steam chest.

For solid timber 1hour per inch thickness is recommended.
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