Heat Treated Timber

[custard]

Does anyone know anything about heat treated timber, the kind where the wood is kilned at about 200 degrees c?

In the UK I've only ever seen it used for external timbers, to prevent rot. But I seem to recall that in the US it's used much more widely, including on furniture woods, I vaguely recall that the benefit is supposed to be extreme stability?

I've been offered some rippled Sycamore that's been heat treated,

There's a wide colour variation as you can see from these photos. I'd have expected the wood to be very brittle, especially as the ripple figure is so pronounced, but these sample boards came through my planer thicknesser as clean as a whistle with almost no tear out. The wood definitely smells "smokey", in fact after just a couple of boards the workshop smells as if there's been a barbecue in there!

The price isn't too bad, so if it's true that you can treat the timber almost like ply or MDF in terms of its dimensional stability, then why isn't it more widely used? I know chocolate coloured Sycamore comes as a bit of a shock, but I guess you could get used to it!

I'm interested because there's a design I'd like to make, with lots of large textural beading cut into wide, curved drawer fronts, but the stability issue means it's a tricky and expensive lamination job unless something like this is viable so it could be made from the solid.

Could anyone recommend a US woodworking forum where I might find makers more familiar with this material?

 

[PAC1]

Looks very interesting. I like you had only seen it on external cladding.
Try Fine Woodworking's forum

 

[Sgian Dubh]

High heat treatment of wood has a long history of research stretching back to the 1920s. The Scandinavian countries, e.g., Finland, and the North Americans have undertaken significant research into treating wood with high heat to impart desirable characteristics for use in the construction, joinery and furniture industries. Charred wood is more durable when used in exterior locations and in ground contact as evidenced by such old techniques as heating the faces and edges of fence posts prior to inserting them into the ground.

High heat treatment involves heating the wood to temperatures between 150ºC and 250ºC (302- 482ºF) depending on the species and the desired end result. These temperatures are much higher than those which darken white timbers such as birch and maple which can show darkening at temperatures as low as 40ºC where wood moisture content is at approximately 25-35 per cent. High heat treatments alone would burn the wood so to prevent this steam is introduced into the heating chamber, and the air circulated quickly and uniformly through stickered piles of wood. In all it takes up to four days to treat wood with high heat; up to two days are required to bring the wood up to temperature, between half an hour and five hours for the treatment itself, and as long as twenty four hours are required for cooling. Half an hour to an hour at 150ºC (302ºF) or just above is used primarily to change wood colour and these ‘lower’ temperatures don’t cause undue brittleness; the treatments are used primarily on hardwoods. Higher heat treatment at approximately 240ºC (464ºF) for five hours improves external durability of the wood at the expense of a significant increase in brittleness. These latter treatments are mostly used on pine or spruce to increase their durability when used for external non-ground contact structures.

Heating wood in this way causes the following effects:

• It alters the physical and chemical properties of the wood permanently. Degradation of the wood hemicellulose occurs, and hydroxyl groups within the wood decrease. As the wood degrades it produces acetic and formic acids along with phenolic and other aromatic compounds.
• The wood darkens— a relatively light heat treatment causes this effect, but the new colour is rather fugitive when exposed to UV light.
• There is a reduction in changes in the wood’s volume in service with changes in moisture content; this is due to 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. Compared to untreated timber, and depending on the species, the volume changes reduce by between 50 and 90 per cent.
• It reduces the EMC of wood through reduced water uptake into the wood’s cellular structure, again, as above, due to smaller amounts of the wood’s hydroxyl groups. This affects the take up of moisture in water based glues thus requiring longer clamping up time when joints are assembled.
• It makes the wood more brittle, and the higher the temperatures used the more brittle it becomes. This may make the wood less desirable where strength is critical, e.g., when used for building materials in the construction industry. More brittle wood also requires sharper cutting tools to work it effectively, and the dust produced is finer than normal and is therefore more likely to be a respiratory irritant.
• Improved durability of wood (for use externally) due to chemical degradation of the wood’s components. Significant improvement in decay resistance occurs when the wood is subjected to temperatures above 220ºC (428ºF), although the treated wood is not classed as durable for use in ground contact situations.
• 5-15% weight loss which may have an impact on transport costs and the weight of structures built with the material. Reduction in transport costs, and therefore fuel savings, have to be weighed against the cost and environmental impact of the high heat treatment.
• Annual rings visible on wide faces of the wood may delaminate, particularly if they show as an arching pattern due to tangential milling and intersect the face at a low angle. For this reason the preferred cut is rift sawn planks where the annual growth rings intersect the faces at between 30º and 60º with 45º being ideal.
• The resin of pines exudes from the wood which means the equipment used to effect the treatment requires frequent cleaning, but the reduced resin content of the wood is an advantage in situations where resin free wood is desirable, e.g., garden furniture such as chairs, benches and tables.
• Knots, particularly in pine, may loosen, crack, or cause distortion in the surrounding wood
• Birch species are liable to warp significantly during the treatment which may make the treated boards unsellable.

 

[Droogs]

Is this the same process as kebonising?

 

[Max Power]

I saw some Accoya recently at the timber yard next to the hardwoods. I was very impressed at how uniformly straight all the various stacks of it were , must be very little waste . Quite expensive though about the same price as Iroko they said.
Can I also say what a fountain of knowledge Richard is , a huge asset to any forum. I am always impressed by his comments both here and over at Woodwork UK =D>

 

[custard]

Thanks for that Richard. I've been doing some more research and it's looking like the maximum movement in service (tangential, 60 to 90% RH)) from Thermo treated Sycamore would be well under 1% as opposed to the nearly 3% that you'd expect from regular kilned Sycamore. I've also found out that this treatment is sometimes used for hardwood flooring that's to be laid across under-floor heating, so more evidence of extreme stability. Furthermore, that might also give an explanation of why this wood is available in the first place, I've previously found superb Curly Cherry and Fiddleback Maple that was rejected by high volume flooring manufacturers as they want uniform, fairly bland but defect free timber in order to offer a consistent product. So that would fit with Ripple Sycamore being rejected.

But I'm still missing two things.

Firstly information about gluing properties. If moisture take up is so low it might suggest that water based adhesives like PVA or UF won't deliver a secure bond and it would need Epoxy?

Secondly, hands on experiences from a maker who has actually used this stuff for furniture! I'm still trying to find an appropriate US based forum (thanks for the Fine Woodworking suggestion PAC1, but I checked and the site traffic looks really low). I've found references to a specialist US lumber mill that was offering Thermo treated hardwoods for furniture, but it appears they've gone out of business. Which is puzzling, much of the history of furniture development is inextricably linked with finding jointing and constructional solutions to the problem of timber movement. Here's a potential solution that might allow solid timber to be used in innovative ways, but the one identified supplier goes bust and there's almost no references to the material in woodworking journals. Why? Is it just because the market is innately conservative, or people reject the colour changes that the process produces? Or is there a more sinister explanation, like severe gluing problems that renders the material non viable in a furniture context? This is why I'm keen to make contact with someone who has actual, first hand experience.

 

[Sgian Dubh]

custard, we had a project made with high heat treated birch in the workshop a few years ago - the treatment turned the wood chocolate brown. I don't recall there being any issues with using PVA. Birch usually undergoes the process at the lower temperatures, so this may have been be a factor. The information I gave above does mention longer clamp-up times required, fourth bullet down. Sycamore and maples are also usually treated at the lower temperatures, ~150ºC for about half an hour, plus the heat up time and cooling off period, of course.

Try looking around the US based forum, Woodweb for more information. It's peopled primarily by professionals, and there may have been some threads on this topic.

Apart from that, I can't offer much more. This type of material isn't commonly found on timber merchant's racks as far as I am aware. Perhaps the relatively recent Accoya developments and availability are a factor, particularly regards durability issues, and it may be Accoya is easier to work and less brittle than high heat treated wood: I don't recall if that's the case or not, and I'd need to do a bit of research to find out. Slainte.

 

[Sgian Dubh]

Is this the same process as kebonising?

Kebonysing is chemical impregnation of the wood to confer durability. It involves heat and a vacuum to help drive the chemical into the wood and to bond with the cell structure. If I remember it correctly, the chemical is a bi-product of the sugar industry, probably alcohol of some sort. I don't know any more than that, and what I've just said could be all wrong too, ha, ha. Slainte.

 

[Jelly]

I saw some Accoya recently at the timber yard next to the hardwoods. I was very impressed at how uniformly straight all the various stacks of it were , must be very little waste . Quite expensive though about the same price as Iroko they said.

Accoya is a different process, to achieve the same changes. To remove they hygoscopic (water accepting) hydroxyl (alcohol) groups in cellulose, they pressure treat the wood with an acetylating agent to replace the alcohol group with an acetate group, using the same chemical reactiom as was used originally to turn the salicylic acid in found willow bark into asprin.

The net effect of very stable, highly durable timber is the same as cleaving the alcohols off in thermal treatment*, but because the temperature is low, the undesirable increase in hardness and brittleness is minimal; however heat and steam are safer** and less energy intensive.

*Which is analagous to the first stage of destructive distillation of wood for methanol and creosote, or of coal to coke and coal-tar


**Acetylating agents tend to be pretty indescriminate, and will just as happily replace hydroxyl groups in the cells of your skin, eyes or lungs, as they would in wood, so properly handling and removing excess chemicals after treatment is important

 

[custard]

Try looking around the US based forum, Woodweb for more information. It's peopled primarily by professionals, and there may have been some threads on this topic.

Just the ticket, will do!