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Janito

UV effect on wood colour when kept in a UV light box

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I am interested in what happens to wood over time when an instrument is exposed in a UV box. 

How much time does it take for the wood darkening to change to 'bleaching'?

I realise that there are many variables that can affect the answer to this question, but even rough estimates and some commentary on the conditions, wood type etc. would be helpful.  

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lots of this gets into the base color of the wood itself. In my experience woods that are a darker base color bleach out quicker than lighter base color wood, walnut comes to mind as an example. Based on experience with wood floors I'd say that sometime after about 1 1/2 years of on off over exposure to uv on raw wood . Just a rough estimate 

Also I don;t think a uv box, unless full spectrum, would mimic the suns a,b,c bands of uv, so over exposure to a tanning light vs the sun are probably two different things

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15 minutes ago, jezzupe said:

I'd say that sometime after about 1 1/2 years of on off over exposure to uv on raw wood .

So you would not expect to see 'bleaching' of maple or spruce with UV light box exposures that would be considered practicable (say <3 months).

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4 hours ago, Janito said:

So you would not expect to see 'bleaching' of maple or spruce with UV light box exposures that would be considered practicable (say <3 months).

At times I've seen it develop in days.  Colour (photo yellowing) begins developing and then a washed out appearance starts to emerge.  Other pieces of maple and spruce that I have left sitting in the bottom of the cabinet for years have developed a deep golden brown colour with no hint of photo bleaching.

I suspect that the spectral irradiance curves of whatever UV bulbs you are using, the number and age of bulbs, the proximity of the bulbs to the wood surface, the wood source, humidity within the box and more may be factors that contribute to whatever you end up achieving. 

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11 hours ago, Janito said:

So you would not expect to see 'bleaching' of maple or spruce with UV light box exposures that would be considered practicable (say <3 months).

When I've exposed varnish samples to UVA/UVB bulbs, or daylight for many months during varnish fading and accelerated aging tests, the bare portions of the wood didn't show any signs of bleaching. A UVC source like a germicidal lamp may do something different though. Haven't tested that.

7 hours ago, John Harte said:

I suspect that the spectral irradiance curves of whatever UV bulbs you are using, the number and age of bulbs, the proximity of the bulbs to the wood surface, the wood source, humidity within the box and more may be factors that contribute to whatever you end up achieving. 

Agreed.

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The only "bleaching" I have seen was with a strongly torrefied sample of spruce, which started out as a dark brown.  It lightened into a more orange-brown after a couple of weeks in So. Cal. sunshine.

At the same time, I had a sample of fresh, blazing white spruce in the sun, and it darkened into an orange-brown.  

The torrefied wood was still darker, but they were obviously moving toward some mutual endpoint.  When the sun comes back again, I'd like to run this test for a longer period.

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