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Yellowing of kiln dried oak.

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MikeG.

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I mentioned this issue in another post, so I thought I would see if anyone can help with an explanation. Sometimes with kiln dried oak there can be patches of a really quite distinct yellow stain on the surface. I'm seeing it more recently, but as yet only in American white oak. The explanation I have heard is that it is associated with over-cooked timber (ie a kiln that was too hot and dry, drying the timber too fast), but I am intrigued as to what is actually going on. What is it that turns yellow, and why?
 

Blackswanwood

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Mike, you mentioned on your answer on the other thread a preference for air dried. Is that based on something aesthetic or the way it works?

I have only ever used kiln dried and yours plus a comment from Woodbloke on another thread has made me wonder if I am missing out.
 

MikeG.

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Air dried oak is milder, less splintery, less brittle, somewhat easier to work with. It's marginal, but distinct. Put it this way, kiln dried oak is still oak, and still fantastic. Air dried is just a bit easier.
 

Woody2Shoes

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MikeG.":3tw63br6 said:
I mentioned this issue in another post, so I thought I would see if anyone can help with an explanation. Sometimes with kiln dried oak there can be patches of a really quite distinct yellow stain on the surface. I'm seeing it more recently, but as yet only in American white oak. The explanation I have heard is that it is associated with over-cooked timber (ie a kiln that was too hot and dry, drying the timber too fast), but I am intrigued as to what is actually going on. What is it that turns yellow, and why?
I've seen the same in ash that I dried myself - at the time I decided it was fungal and due to excessive water ie rain ingress during the early stages of drying.
 

Sgian Dubh

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I think you already have your answer, Mike, because you can find it in links already provided by other respondents, so my contribution here is maybe superfluous, but here goes anyway.

Evidence of yellow streaks longitudinally in a board of oak, walnut or chestnut is caused by “metabolization of tannic acid by the fungus Paecilomyces variotii” (Gard, 2010, p6)*. Maintaining sufficient air flow of 0.5 metres per second (~1 MPH) and holding relative humidity in the kiln to below 80- 85 percent are suggestions for preventing the problem. The chances of this fault developing in air dried wood are small. Slainte.

* Gard reference link
 

AJB Temple

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The chances may be small, but to quote the war of the worlds "still they came".

My own limited experience is that some green oak that I have bought from Eastern France, despite bing in open air and sticked for several months, can still exhibit these annoying yellow patches and streaks.

The other issue commonly found, again in my limited experience, is that where oak has been stored in stick ( I am talking about anything up to 8" beams here, which is the largest I have ever bought) can acquire quite deep black staining from the oak offcuts typically used as sticks.

I know yellow stain has been evident for at least the last two decades, but it seems to be getting more common. I always thought it was due to poor kilning practices (rushing basically), but good timber merchants know that discerning customers will reject badly kilned stock and so it surprises me that anyone would take this risk.

All of my home grown oak in store has never been near a kiln and none of the stock I have used or sold has ever shown this phenomenon.

I've got some yellow stained oak at the moment that I bought from a supplier I have not used before (it is machined for floor boards). In present circumstances the supplier would rather write it off (nearly 40% of the order) than collect it as a return in present circumstances. I plan to experiment with bleaching and oxalic asset treatment just to see what happens.

For interest, I also think that Wenge I have bought recently from one source (never used before) is abysmal quality compared with what I used to get or indeed what I bought last year from my favoured supplier in Tonbridge. Same supplier shipped some walnut (not much) that I don't think is English walnut. We are having a little chat about proof and source.
 
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