Air Dried v Kiln Dried

[moz]

Richard, I know what you mean about tannin/iron stains but this is something different. It isn't a chemical reaction to metal nor is it a fungal stain. I don't know what the technical name is for it but it results from differences in drying speed and temperature. Let's say, for example, that this timber was air dried too quickly in warm weather to the extent that the outer fibres dropped to a MC of 35% but with the centre nearer to 60%. Then the lot was stuck into a kiln at a high temperature till an average MC of 10% was achieved. What people refer to as 'enzymatic oxidation reaction' i.e. the reaction which alters the colour of the timber is happening at sharply different rates and degrees in different parts of the boards. I don't know how common this is but I have seen it before from a different supplier and have had the opportunity to observe some examples over a couple of years. The contrast in colour doesn't diminish with age and I think can be regarded as permanent. I would hazard a guess that most people would see that typical streaking and assume that it was either a feature of the living tree or of the fallen log. However, the shots of the end grain I posted show how the 'stained' portion is contained within the centre of each sawn board. My point is that, although there are problems and necessary precautions with air-dried timber, the complexities seem to really pile on when you get into proper kiln drying, as this particular fault touches upon.

John

 

[Sgian Dubh]

... the complexities seem to really pile on when you get into proper kiln drying, as this particular fault touches upon. John

John, you might find the discussion linked to below of interest. The author, Eugene Wengert is a renowned authority o. timber technology and kilning issues. I'm not sure the answer to your original question is here, but it's worth a read in any case; or more precisely perhaps, it's worth a read to me because of my particular interest in the subject of timber technology as a sideline to my main interest in furniture design and making. Slainte.

http://www.woodweb.com/knowledge_base/C ... umber.html

 

[newt]

John, I have seen those effects on Euro KD oak, although not quite as severe. At first you can mistake it for sap wood when it is on the edge.

 

[motownmartin]

I am confused because I found some of the same markings in air dried, unless I was told some porkies.

 

[moz]

Thanks Richard, very interesting. the problem obviously has some chemical origin but the 'stains' don't really match any of the descriptions there or anywhere else I have searched. The outer part has obviously changed colour but only to the extent that it looks like any other dried oak. The centre has an almost bleached appearance, as Pete says almost the same as sapwood. However, it must be somehow related to the section on 'chemical stains' - quote:"Chemical stains develop when naturally occurring chemicals in wood react with air (an enzymatic oxidation reaction) to form a new chemical that is typically dark in color". But presumably the normal darkening one would expect as oak dries is also as a result of this reaction. Once the MC has fallen below 40%, the reaction slows and stops. So, in my example, the centre has remained a lighter colour at the point it entered the kiln and fairly rapid drying seems to have kept it so. I would describe it more as a lack of staining but I could be wrong. The timber merchant was familiar with it and said it was related to incomplete air drying in a warm environment followed by kilning quickly at a relatively high temperature. Both examples I have seen, and also Pete's above concern European oak so perhaps climate is a factor. In any case, reading your link, there seems to be so many inter-related factors, who knows?

John

 

[Sgian Dubh]

As I understand It.

Ta. Slainte.

 

[Sgian Dubh]

In any case, reading your link, there seems to be so many inter-related factors, who knows?
John

I am intrigued by the phenomenom your photographs illustrate. I don't know the exact cause but if time allows it I'm likely to sniff about until I find what it is. If I do find out I might even add another small section to the timber technology manuscript I am working on. Slainte.

 

[OPJ]

Sorry for bringing this thread up again but, I wanted to ask what people thought of what's been happening to some of my 3" beech left overs.

The three lumps pictured here have been sat in my garage, no heating or anything. One on the left had some splits when I bought it. Those larger splits on the right-hand one were also quite visible at the yard. The middle lumps' gone a bit crazy though, with splits running down most of the 3ft length... :?

I assume this is air-dried; what do you think? All I've done so far is to remove the waney-edges and give it a quick belt-sanding for a better look at the surface. My apologies as I know this photo' isn't great.

This kind of thing has happened to most of the 3" beech I bought for my workbench, although these three are possibly the worst of the lot. Actually, I've since put most of this back through my bandsaw to try and cut out most of the splitting.

Next time, I might try sealing the ends before I start machining, even though this stuff was seasoned... What do you think?

 

[motownmartin]

Olly, I had a load of Yew that I resawn after a few weeks three quarters of it was only fit for the fire, this was air dried.

 

[moz]

Hi OPJ. How do you know your beech was seasoned? I'd say it was pretty wet to have lost so much width to the point of splitting when bringing it inside. A moisture meter is the only way to be sure. I bought a Wagner a few years ago after a few disappointments with supposedly air-dried timber (actually hardly dried at all - unscrupulous b*****d). It takes a lot of the guesswork out of it and at least it lets you know how best to proceed. Sealing the ends would have helped but, if it was too green, drying should have continued outside with plenty of ventilation and protected from rain.

John

 

[dicktimber]

In one of the mags this month a guy shows how he made a huge thick table for a customer.
He had the material supplier KD the timber below normal moisture standards.
The reason was to counteract the effect of movement and splitting in a centrally heated house, which is usually the main reason KD timber is preferred.
But never the less you can still experience problems.

Mike

 

[OPJ]

Hi moz,

The only truly green timber my preferred supplier sells if oak for framing; the rest of their hardwood stock is all properly seasoned (usually air-dried; sometimes, there's a little kilning towards the end of the drying cycle). I do have a cheap little moisture meter, which reads (on average) 2-3% higher than the £200+ plus meter we have at college. When I got my beech indoors, it was averaging between 16% and 17% on the meter (which could have meant a maximum of 15%).

I started cutting up the timber as soon as I got it home and, from the big stack of wood that spent two weeks indoors (it's only for a workbench...), there was barely any change in the number of or length of splits. The three lengths in the photos above were all left in the workshop/garage and have spent NO timber indoors. Yet, they appear to be the worst of the lot...

I guess it would pay to buy a "proper" meter in the long run :roll: but, I think I also did myself a favour in roughing out the components early on... Otherwise, it may have all ended up looking like those boards above! :shock:

Next time, I'll repeat the same process but will also try sealing the ends. What do you recommend? I know Chestnut sell an "End Seal" product specifically for 'turners but, I've also heard of people using watered-down PVA?

By comparison, the 2" and 1" boards I bought are absolutely fine. I have a dining table to make soon that requires 3" English Oak for the legs - I've learned a lesson here but, I need to get it right next time!! :shock:

 

[Sgian Dubh]

OPJ, it is hard to say what the cause of the splits is, but it's noticeable that the splits in the middle and right hand piece are radial. The heart of the tree is towards the bottom of each board in the picture.

One possibility is that the boards dried quite fast in the early stage causing radial surface checking as the drying shell shrank around the still wetter and full size intermediate zone and core. The medulla are lines of weakness and these are most likely to be the location of surface checks in tangentially cut boards.

Later as the intermediate zone and core lose moisture and they too shrink the surface checks in the shell can close up so you can't see them.

Later again, and a bit more drying too, you'll sometimes see those original surface checks open up again, and sometimes, once the surface checks get going in this later stage they just keep on opening up.

Of course without knowing all the circumstances with your wood and not having it front of me to examine closely, I am making what I believe are informed guesses. However, I have outlined a set of circumstances, the responses you can get in wood, and the checking that can result if things don't go just right.

One other scenario that comes readily to mind is that the splits really have little or nothing to do with the seasoning. It could be that the tree had developed structural stresses due to the circumstances it experienced in life; cutting it into boards simply allowed the stresses to work themselves out as checks and splits as the wood dried. Slainte.

 

[Ross K]

It depends on the application of the wood...

You want dry wood - it has to be kiln dried, realistically*

You want wood that's "fairly" dry** go for low moisture content air dried.

You want wood for outdoor funiture, go for air dried*** or green.

* Say 10% moisture content. Air dried wood at this level is rare. There is nothing wrong with properly kilned wood.

** For interior applications where the construction method allows for the expected movement, I've used up to 18% or even higher with no issue.

*** 20% plus as a rule of thumb or anything up to green (70%+)

 

[Ross K]

As for that beech...

Some trees produce wood that tends to split. Some produce wood that doesn't split. Sometimes it's because the drying is too fast or unevenly, but not always.

Sometimes it just splits because it just does and there's no analysing that can explain it - trees are different from one another!

 

[OPJ]

Richard, thank you for sharing your knowledge on this subject.

Next time I but air-dried timber this thick, I will try sealing the ends to prevent moisture coming in/escaping... My suspicions are that this could be prohibitive to the drying process in not allowing as much moisture to escape...? What would you suggest? I'd be very interested to hear your thoughts.

 

[Anonymous]

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.
. Slainte.

Fair comment Sgian. I have only steam bent wood once and have no experience of how differently dried samples behave when steamed

 

[Sgian Dubh]

Olly, out there amongst wood drying businesses there are fans of end sealing and conversely there are those that consider the financial gains through additional usable wood are too limited to bother with the expense.

So, without getting into a debate on the commercial factors regarding whether or not to end seal here is what I think. To dry freshly felled green wood in a reasonable amount of time for your personal use, rather than drying wood for sale, I think you should take into account the season when you sticker the wood up. You are looking to create a moisture gradient in the wood that gets the moisture out fast enough so that it becomes usable reasonably quickly. Sticker the wood up in late autumn and leave the ends unsealed because relative humidity (RH) is high. If you sticker it up in spring then use the end sealer because RH is lower during spring and summer.

RH and heat, plus air movement are the keys to how fast wood dries. A low RH, eg 30%, and a high heat, eg 35ºC, and fast air movement sets up a steep moisture gradient and dries wood quickly. The trouble is this could easily result in inducing drying faults in air dried wood. Here in the UK we never see such conditions and typical summer average numbers are 60% RH and about 16ºC. Winter sees 80% RH plus, and about 5ºC.

It doesn't look to me from your question that you plan to dry your own wood from green. It seems you are more interested in acclimatising already air dried wood ready for your own use. Air dried wood in this country seldom gets below about 19% or 20% MC. If it is below that it's because it's been air dried in an open sided shed, or it's been air dried outside and later brought into a shed. In your case I would simply sticker up any wood you buy in a reasonably dry environment for some months before you use it, eg, in a dry garage or shed. You only require to lose about 6 or 7% moisture before the wood reaches the sort of moisture content that is at the top end of the suitable range for building furniture destined for indoor use, ie, around 12% or 13%. 8% to 10% is really the ideal range for that purpose, but you can work it cautiously if it's a little wetter.

I don't think there is much benefit to be gained with end sealing if you're drying wood from about 18% to 20% MC down to 10% or 13% MC. It's at the initial green condition, ie 30% + down to about 18% MC that I think end sealing is likely to be most beneficial. Slainte.

 

[Ian Fraser]

I have recently joined the forum and have yet to explore it thoroughly. I am teaching myself wood bending, steam bending, and hot pipe bending. For this I need air dried timber, ideally. Can forum members suggest suppliers to get air dried wood from, particularly in the north of Enngland?

 

[promhandicam]

Welcome to the forum. Probably best to start a new thread about steam bending rather than resurrect one that is two years old :wink: