Heat bending kiln dried wood question

[Tetsuaiga]

I have been reading that you should use air dried for bending on a hot pipe.

Yesterday I tried air dried, then I also gave some quartersawn oak and black walnut and it worked just fine.

Is there any reason why you should avoid using it? Are there any structural reasons why you shouldn't use kiln dried, is it less strong? Will it be more likely to crack in the long term?

 

[Peter Sefton]

Kiln dried timber will usually have a much higher failure rate than air dried. The kiln drying process kills the natural elasticity of the timber by damaging the lignin in the wood.
Cheers Peter

 

[MIGNAL]

Walnut is just about the easiest wood to bend on a hot pipe. I've not tried Oak but that is almost certainly going to be a little more difficult. I've always found that once the 'natural' moisture content has been driven away from the wood (whilst bending) it reaches a point where it becomes much more difficult to get it to react to the heat and pressure. Letting it rest (rehydrate) for a day or so and it becomes pliable again.

 

[Tetsuaiga]

I was thinking about trying to use it for some instruments, but unless I could find people who actually do this I would just use air dried as is recommended.

 

[Peter Sefton]

I understand musical instrument makers tend to use bending irons rather than steam bending.
Cheers Peter

 

[Sgian Dubh]

The kiln drying process kills the natural elasticity of the timber by damaging the lignin in the wood.
Cheers Peter

Peter, it's not really that kilning wood "kills" the elasticity of the lignin. It's more to do with a hysteresis effect on the cell walls of the vascular tissue whereby they become less elastic once they have been dried, whether that be through a kilning process or some other drying process. Essentially, in this case, hysteresis means that the elasticity lost in the cell walls during the drying cannot be fully reversed simply by bringing them back up to a wetter state.

However, wood that has become very dry is less amenable to bending (either through steaming or dry bending) than wood that has never been dried, so I've no disagreement with you on that general principle; it's just the specifics of the mechanism where I'm drawing a distinction. Slainte.

 

[Argus]

.

Bending .............. on the infrequent occasions that I've tried it, I've had a disappointing high rate of splittage. Must be me!

Then there's steam bending......

Steam bending is fraught at the best of times, but I'm reminded of a section at the back of John Brown's excellent book on Welsh stick chairs where he describes his method of bending Ash for the continuous chair arms in a home-made steam box connected to a boiler constructed from an old gas bottle.

He suggest that freshly cut, un-dried and riven wood is the best to minimise side splits, reasoning that cut or sawn wood will inevitably contain the severed ends of a wavy grain on the sides, weakening the arms. He maintained that wet timber, already containing natural resin and moisture bent quicker and easier. He made them in batches and left the whole thing trussed up to dry until needed to be used and then did the final shaping of the sides with a spokeshave.

I realise that supplies of dead straight, riven lengths of freshly cut wood is a tall order in this day and age, but there's his technique.

.

 

[Tetsuaiga]

I understand musical instrument makers tend to use bending irons rather than steam bending.
Cheers Peter

Yes this is what i've been using bending iron, I thought this is same as hot pipe bending. No steam is involved.

I have been able to bend the wood easily within the tight radiouses required for many instruments without any splitting.

I do think the kiln dried wood has a more brittle quality to it though. I could do with an experts opinion so untill I can find one i'll continue with purely air dried. It is a bit of a shame though as quality air dried wood is hard to find, a lot of instrument wood is already broken down for you and to me seems more expensive than it could be if you can find whole planks to use.

 

[Sgian Dubh]

I do think the kiln dried wood has a more brittle quality to it though. I could do with an experts opinion so untill I can find one i'll continue with purely air dried.

You are largely right. Wood that has dried to a low MC is stiffer than when it was originally wet (green), though not necessarily more brittle. Hysteresis is the cause again, one definition being "the current behaviour of a material is dependent upon the conditions it experienced in its immediate history" (my own quotation from a text of mine), and relevant here. In this case, wood that has become very dry (through whatever means) will never again be as flexible and supple as it was when green, i.e., at or above fibre saturation point (FSP), or any point between green and its driest condition when the wood gets wetter through taking on moisture again.

Incidentally, Argus made good points about using riven or cleaved wood for bending tasks because the grain is continuous throughout the length of wood, and therefore less likely to experience tensile failure, especially on the outside of the bend due to short grain that is often formed in boarded up wood. Slainte.

 

[Tetsuaiga]

Thank you. Yes i'm sure the closer to perfectly quarter sawn the better.

 

[MIGNAL]

Quarter sawn makes it slightly less prone to corrugate but it won't reduce fracture. It's minimising grain run out (short grain) that makes it less prone to fracture. I've probably bent over 100 sets of ribs on a hot iron over the years. I've only had a few fracture, always in highly figured Maple. The key point is to keep them thin. It's years since I last did one that was over 2 mm's. Most of the time I'm bending at 1.8, sometimes less.
Last week I bent 3 sets - 2 walnut, 1 rosewood. One of the Walnut ribs took me 5 minutes, start to finish. Probably the fastest I've ever done. Unfortunately one of the Rosewood ribs spoilt everything and took 35 minutes but the machine men can keep their bending contraptions.
How much of this stuff is air dried I'm not sure. I buy it all through the specialist suppliers. I suspect that it's really the soundboards that are air dried. It wouldn't surprise me at all if some of the Back/Sides were kiln dried. The vast majority of makers are pushing their boards through thickness sanders, so it probably doesn't really matter to them.

 

[Tetsuaiga]

Is short grain the same thing as non-straight grain then?

Or at least straight grain that is parallel to the edges of the piece of wood in question?

 

[Peter Sefton]

Is short grain the same thing as non-straight grain then?

Or at least straight grain that is parallel to the edges of the piece of wood in question?

Yep, for steam bending you will get the best results by using straight grain, no knots, and green timber or not kiln dried.

Peter

 

[MIGNAL]

Is short grain the same thing as non-straight grain then?

Or at least straight grain that is parallel to the edges of the piece of wood in question?

No. Straight grain and short grain (runout) do not refer to the same thing:

http://www.lutherie.net/frankford.runout.html


The second tree drawing down shows runout. In actual fact that would be considered very little runout. It gets much, much worse than that.
The pictures of the Guitar Top at the bottom of the page shows the visual effect in a bookmatched set. That's why the two halves appear to be two different colours. It's actually the way that the light reflects off those two halves, on one half you are practically peering into end grain. On the other half the grain is running in the opposite direction - because of the bookmatch.

Riving or splitting the wood gives you a much better chance of zero runout. It doesn't eliminate it though because there is always the factor of spiral growth.
I wouldn't worry about Runout on Sides. Figured Maple is effectively full of runout - hence the figure, it's practically end grain. Figured Maple is often used for Sides though. Runout just makes it more prone to cracking when it is bent.

 

[Tetsuaiga]

That link is perfect, I think I understand it fine now.

This kind of stuff is why I like the idea of getting materials in as rough a form as you can as then you get to control and know what's been done to it.