Sanding Wood (Pine)

[Stanleymonkey]

I'm lucky enough to be in a job where I get to teach woodwork to primary age kids. Quite an interesting thing to do .

We were making some wooden boxes last week and sanding them down to round off the edges and corners. A lot of them were amazed at the colour change from almost yellow to off-white. When they asked why - I couldn't give a decent answer.

Is removing a layer that has reacted to sunlight and darkened? Is it a reaction to the air or is it caused by the machining of the wood (it was planed timber).

I feel like I owe to them to a least give a reasonable explanation!! #-o

 

[Sgian Dubh]

The effects of either or both oxidation and UV light, prior to exposing a new surface through either planing or sanding are your answer. Slainte.

 

[rafezetter]

The effects of either or both oxidation and UV light, prior to exposing a new surface through either planing or sanding are your answer. Slainte.

What he said

It's also why it's recommended to sand back the glue areas on wood that's been standing around for a while, to ensure a good bond.

 

[Stanleymonkey]

Thanks guys and the oxidation side links nicely with their science topics in other lessons.

 

[thick_mike]

The ma8n effect on old wood is oxidation, but there is also a physical surface scattering effect that causes lightening of the surface. When you sand a surface, it becomes rougher on a microscopic scale. I know that is counterintuitive for your students . Light is scattered in all directions from the surface and makes it appear lighter in the same way that snow looks white even though it is transparent.

You can see this easier on some woods than others, if you sand the surface with progressively finer paper and finally burnish it you will see it get darker.

 

[ED65]

With pine and similar softwoods specifically I think it's largely UV light that's responsible for the typical colour change. It's quite easy to see examples where the wood is partially shielded from light but open to the air, and it is noticeably darker where the light reaches and not all over at least in a shorter timeframe.

Another aspect of this if you're interested in going a bit deeper is what are called extractives, which rarely get a mention here. These are compounds that rise out of wood and deposit on the surface. Old wood that has gone through many many cycles of expansion and contraction has had plenty of time for these to build up on the surface and can account for colour changes in wood that has been protected from light. But you'll invariably see far more pronounced (and much faster) colour changes with light exposure, with some shifts in colour never seen when they're kept in the dark.

 

[thick_mike]

With pine and similar softwoods specifically I think it's largely UV light that's responsible for the typical colour change. It's quite easy to see examples where the wood is partially shielded from light but open to the air, and it is noticeably darker where the light reaches and not all over at least in a shorter timeframe.

Yes, doing a bit of research the process seems to be photoinitiated oxidation of lignin, in other words, light kicks off the reaction with oxygen in the atmosphere. Both are necessary for the yellowing process. Hence why it happens only at the surface and is removed by sanding.

 

[Stanleymonkey]

Wow - this is getting quite complex!

Hadn't thought of the light scattering as you break up the surface - I doubt mine will have the patience to work through the grades though and see the change. But I do feel a science experiment involving laser pointers is on the horizon.

I think UV and oxidisation will be about the right level for most. Slow release of chemicals from within might be understood by some. They'll probably think it's starting to rot or decompose - so I'll choose my words very carefully :shock:

Thanks for all the replies. I'll be testing some water based safe waxes in the morning for after all this sanding.