Air Dried v Kiln Dried

[OPJ]

I'm not sure I've actually worked with truly air-dried timber yet... I usually buy all my wood from Interesting Timbers near Bath. Only time I look elsewhere is when they don't have it in stock! Generally, the quality of the stuff I've had from them has been very good. Most of their stock is claimed to be air-dried but I think that, like a lot of other yards, it's spent a small time in the kiln towards the end of the drying cycle, if only to bring the moisture content down a few weeks early so they can start to sell it and satisfy demand.

I think that regardless of whether the timber you buy is air or kiln-dried, it's important to understand that when you cut in to a wide board, it's going to have a higher moisture content on the inside and you should be aware of that when working it.

 

[Philly]

I've had good experiences with air-dried stock. It's definitely more pleasurable to work. Mind you, most of the lined stuff I've had has been good.
Sadly, I've also had some horrible kiln dried timber - from pieces that are full of checks and cracks through to pieces that are just plain horrid . I'm sure its down to the speed and skill with which the timber is kilned.

Personally, whatever the timber, I try and store it in my nice dry workshop for at least a year before using it - some of my boards are going on six years old now. Makes a big difference.

Hope this helps
Philly

 

[Calpol]

This is going off on a bit of a tangent, but I once got some black walnut (kiln dried) and it had a really horrible smell to it. I can't really explain it, but something like manure if I remember right. Anybody had any experince like that and could shed some light?

 

[custard]

This is going off on a bit of a tangent, but I once got some black walnut (kiln dried) and it had a really horrible smell to it. I can't really explain it, but something like manure if I remember right. Anybody had any experince like that and could shed some light?

Faced with a complex glue-up, with the Tite-Bond setting rapidly on a hot day, then my workshop often gets that exact same smell.

:shock:

 

[Sgian Dubh]

In the UK you'll never buy wood that has been air dried outside that is below 17% MC, and 17% MC is a rarity, it's more likely that it's going to be closer to 19 or 20% MC, and all dependent on the season.

If it's been air-dried in an open ended or sided shed you might occasionally find some that is down to 14 or 15% MC. If it's been air dried outside and subsequently placed in a dry outbuilding or storage place you'll sometimes get air dried stock at about 12% MC.

In all cases you will almost certainly have to further condition air dried material to reduce the MC prior to using it for internal furniture.

My experience and research tells me that decrying the qualities of either air dried or kiln dried wood and claiming that you will only use one or the other exclusively is a wrong decision. There is a place and use for both types of seasoned wood. For instance, it makes no sense to make garden furniture, gates, external joinery, etc out of kiln dried stock that is still at 7 to 10% MC. It's generally more expensive and you need to acclimatise it to exterior conditions before you make the furniture or joinery. Slainte.

 

[Sgian Dubh]

Can I invent a term for the soft brown / white squishy bits which can be found on occasions in ash, "bananary".

I'll stick to calling it the traditional name of "slightly rotten", as that's probably what it is, ha, ha. Slainte.

 

[Doctor]

Can I invent a term for the soft brown / white squishy bits which can be found on occasions in ash, "bananary".

I'll stick to calling it the traditional name of "slightly rotten", as that's probably what it is, ha, ha. Slainte.

I think bananary has a nicer ring to it

 

[Ironballs]

I don't have enough experience of air dried to comment, I've used some but not much. I will echo the comments about importance of skill in kiln drying though, chap at the college had a board of 3" oak in that had so many checks in it you could have slapped a sticker on it and called it tartan

 

[Sgian Dubh]

I don't have enough experience of air dried to comment, I've used some but not much. I will echo the comments about importance of skill in kiln drying though, chap at the college had a board of 3" oak in that had so many checks in it you could have slapped a sticker on it and called it tartan

Not all checks are the result of improper drying. Some checks are a natural result of stresses in a living tree and occur before the tree is harvested. In other words the weakness and fractures, possibly invisible to the naked eye, are sometimes present in the wood before it is dried, and drying it simply reveals the weakness as visible checks or splits.

It is a quite common, but erroneous belief, that air dried wood doesn't suffer seasoning faults. It does. I have seen all the seasoning faults commonly put forward as being primarily a result of improper kilning show up in air dried wood. These include the usual suspects such as sticker stain, end checking, surface checking, case hardening, reverse case hardening and honeycombing.

In the end poorly seasoned air dried wood is as bad for woodworking as improperly kiln dried wood. There are plenty of people out there that can screw up the drying of a stack of air dried wood, and they don't even need a kiln to make the mother and father of a pig's ear of the job. Slainte.

 

[BradNaylor]

So as I suspected, it it not a case of one method of drying simply being 'better' than the other.

I have obviously been very lucky in my purchases of KD timber over the years. I have very rarely ever encountered any of the defects outlined by Richard as being popularly attributed to poor kilning. I have to confess though, as someone who makes little use of hand tools, that the subtle variances between the respective working properties as described by Rob sail over my head somewhat!

Ultimately, I am not that interested in the 'working properties'. What matters to me is the appearence of the final piece of furniture, the happiness of the client, and most importantly the profit I have made on it.

From the point of view of a jobbing furniture maker like myself, rather than a creative 'artist' like the Mr Savages of this world, kiln dried timber will always have distinct attractions. It is normally ready to use virtually straight from the timber yard; no-one I know has the time, space, or money to be conditioning the air-dried timber required for their next six months worth of work in conditions akin to those in a centrally heated house.

I am sure that top quality air-dried timber is lovely stuff. Like most things in life however, compromises have to be made. Kiln dried timber is a compromise that makes my job possible.

And if you know where to look, some lovely wood comes out of kilns.

Cheers
Brad

 

[Sgian Dubh]

There is an important characteristic of wood movement to take into account for creating furniture parts, and this relates to the choice made between using air dried wood or kiln dried wood. To set the scene consider the following.

A solid wood panel made up of edge jointed narrow planks, e.g., a table top, raised and fielded panel in a frame, cabinet top, etc., will experience tensile stresses across the wood grain and the joints as it shrinks and, conversely, compression forces as it expands.

The cross grain structure of the wood, and the joints, are both better able to survive under compression forces than they are under tensile forces. In this case a table top glued up of planks at about 13%MC during construction will shrink when put into drier RH conditions. A typical house in the UK exhibits about 40%RH during the winter. This equates to wood eventually reaching about 7.5% EMC. The panel is therefore at some risk of splitting as it shrinks.

On the other hand consider the case of a glued up panel made with planks that are 7%MC at the time of making. During the summer, when internal UK house RH values are approximately 60%, it will expand as the wood moves towards 11.5%MC, which represents its EMC at that RH. Here, the joints and grain experience compressive forces, which are less likely to cause grain or joint failure.

Subsequently the panel will experience seasonal change over its entire life and will expand and contract to suit. The tensile forces and compression stresses within a panel will alternate as the seasons pass and it settles into a routine where the joints are usually able to withstand these stresses and survive very well. But, importantly, joint failure during the first year of an edge jointed panel’s life in service tends to happen most commonly when the wood shrinks after construction rather than when it expands. As a general rule of thumb it’s safer to glue up wide panels of very dry wood (6% - 7%MC) than to use damper wood, e.g., 12%- 15%MC. Slainte.

 

[Ironballs]

Worth knowing, really do think I should get a moisture meter

 

[Anonymous]

A new term of reference?

Wood snobery? :lol:

Never had a problem with kiln dried and can't say I noticed any differences or improvements with air dried.

Wood is wood is wood and you get good or bad regardless of the drying mechanism - it is (was) a living, organic and dynamic material and the original tree has more impact on the appearance and 'workability' than the drying!

 

[Paul Chapman]

Never had a problem with kiln dried and can't say I noticed any differences or improvements with air dried.

You've probably just been very lucky....... :wink:

Cheers :wink:

Paul

 

[Sgian Dubh]

A new term of reference?
Wood is wood is wood and you get good or bad regardless of the drying mechanism - it is (was) a living, organic and dynamic material and the original tree has more impact on the appearance and 'workability' than the drying!

Not quite true in all cases Tony. For instance, if you are heavily into steam bent wood incorporated into your furniture using sawn kiln dried wood will always leave you frustrated and disappointed. It's brittler and stiffer, and the grain runs off at inconvenient angles.

You get much better results with riven or cleaved green wood, in this case wood that has never been seasoned; but after the bending there's a lot of shrinkage and distortion likely to occur.

The generally accepted best compromise is to use riven wood air dried to something near or slightly above 20% MC as this gives the best of both worlds, ie, relatively easily bent, and any major drying faults are likely to have showed up by that point of the drying process. Slainte.

 

[promhandicam]

Richard,

Thank you for taking the time to post your thoughts on this subject - your posts are easy to understand yet very informative.

Much appreciated,

Steve

 

[newt]

Yes Richard, thanks for taking the time.

 

[moz]

I've just had to return a batch of kiln-dried oak to the timber merchant and thought I would share. I agree that defects can happen with both air dried and kiln dried timber but kilning seems to me to be a much more complex business.

AIUI differences in drying cause differences in colour. The timber would have been air dried to a degree, with the outer few millimetres developing the darker colour but with the core still at a MC of probably above 40%. Putting it in the kiln at this point and raising the temperature to try and compensate leaves the centre white. Maybe someone else understands the chemistry better than me but I know that the colour reaction happens higher up the moisture content scale and stops happening when there is not enough moisture left for the reaction to proceed.

This is a particular b*****d because, unless you notice the tell-tale stain on the end grain, it suddenly materialises in the form of patches and streaks just as you begin to approach dimension. From the merchant's point of view, the problem is invisible. They replaced without quibble so it's been expensive for both of us.

I don't know about other timbers but I much prefer air-dried oak if I can get it, but you do need a moisture meter. I remember a few years ago planing up some 3 inch 'air-dried' oak which was practically splashing me in the eye and which subsequently distorted beyond recognition.

John

 

[Sgian Dubh]

John, what you show may be something to do with the tannin found naturally in oak which reacts in the presence of water and ferrous compounds (iron) and produces brown staining. This well known reaction may also be a red herring in seeking a cause for what you show because if you take a white oak or European oak and stick a piece in some hot or boiling water in a non-ferrous container you get water that turns brown just like brewing tea in a teapot, and tea has plenty of tannin in it.

What is going on during seasoning is that as the wood dries the wetter core transfers water towards the intermediate zone which then transfers the water to the shell and finally the moisture is relaeased from the shell to the air.

From this point I am speculating, but it is well known that heat commonly speeds up chemical reactions, and if there is brown tannin stain transference from the core towards the shell, the higher heat of a kiln might intensify the residual staining towards the shell.

I don't know if you've ever looked inside a kiln that is used regularly to dry white oaks or European oaks, but I have, and nearly all the exposed surfaces are covered in a brown tannin stain forced out of the oak, as in the snap below. The attached photo was taken about twenty minutes after a batch of oak was removed from the kiln, hence the steam you can see. Slainte.

PS. What does the letterisation AIUI mean? It's irritating me that I can't guess, and I'd like to know!

 

[Chems]

As I understand It.