I was interested to read in David Keller's article in the 2008 American Period furniture how wood darkens in sunlight (UV). I have observed this on some mahogany parts I had sitting in sunlight in my shop and really like the dark brown oxidized color that resulted. Can anyone tell me if this same effect can be achieved once the finish is applied to the wood. Will a mahogany piece finished in button lac for example, darken when placed in sunlight for a few weeks or does the finish "filter out" UV or prevent the oxidation?
Photo-oxidation of wood using UV
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dkeller_nc
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Chris - The mahogany will still darken, as will the shellac. The mahogany wll darken more slowly than it will if placed in direct sunlight as unfinished wood as the shellac will absorb some of the UV as you mention. One drawback of doing this as a "complete" project is that the UV may also damage the shellac in addition to darkening it. Specifically, it may make it more brittle than it otherwise would be, which might cause it to crack and craze as the wood underneath expands and contracts.
One other note - window glass will also absorb a great deal of the UV in sunlight. Wooden objects placed in front of a window will still change in response to the UV that's left after it passes through the glass, but it will be considerably slower than placing it in direct sunlight.
David
One other note - window glass will also absorb a great deal of the UV in sunlight. Wooden objects placed in front of a window will still change in response to the UV that's left after it passes through the glass, but it will be considerably slower than placing it in direct sunlight.
David
Mark Arnold
Well-known member
David,
I have a question regarding this topic that has always puzzled me. I thought that photo oxidation only occured at the surface, but that wouldn't explain the consistent burgundy hue throughout oldgrowth cherry stock. Was the heartwood of virgin cherry darker than later growth trees to begin with? Or is the color more a function of extractives rather than the presence of O2 and UV? Or does 200 years of seasoning allow oxygen to affect the center of a 3" thick blank? As your article points out, cherry is not the only wood that changes it's hue when exposed to light, but it seems to be the only wood whose most desired hue is the result of oxidation. Do you know if the extractives that impart color can work their magic in the absence of oxygen and light?
I have a question regarding this topic that has always puzzled me. I thought that photo oxidation only occured at the surface, but that wouldn't explain the consistent burgundy hue throughout oldgrowth cherry stock. Was the heartwood of virgin cherry darker than later growth trees to begin with? Or is the color more a function of extractives rather than the presence of O2 and UV? Or does 200 years of seasoning allow oxygen to affect the center of a 3" thick blank? As your article points out, cherry is not the only wood that changes it's hue when exposed to light, but it seems to be the only wood whose most desired hue is the result of oxidation. Do you know if the extractives that impart color can work their magic in the absence of oxygen and light?
dkeller_nc
Well-known member
Guys - Apologies for the late reply, I'd forgotten I'd posted to this thread.
Al - I honestly don't know what the transition point is between darkening of the wood from the effects of oxygen and UV light and bleaching. I have seen this happen to a few pieces that were kept for decades in a sunny window sill, but I didn't have the permission (and probably wouldn't have considered) taking a little off of the surface to see whether this was truly the effects of UV (one would expect that if it's primarily due to light exposure that the effect would be very shallow). One comment here is that of the antiques I've seen pictures of that were "in the rough", I've yet to see a colonial-period antique made of mahogany that was bleached out. They were almost uniformly dark from oxidized finish, age, and in some cases, the effects of linseed oil application. Ditto for cherry pieces, and as one would expect, the opposite for walnut - these almost always seem to have developed a golden-brown type of surface.
Mark - Not all oxidation needs exposure to air, and not all changes to pigments are the result of oxidation. My guess would be that the pigments in cherry change regardless of exposure to light and air - it's just that light and air greatly accelerate the process. From the stanpoint of oxygen getting to the heartwood, I would expect this to occur over many years, as the oxygen can relatively easily diffuse into the wood's pore structure. These pores, after all, must run along the grain lines or the tree wouldn't have grown in the first place since water and nutrients must travel from the roots to the leaves. And a gas like oxygen can difusse through pores much more easily than water can.
However, there's another possibility. So far as I'm aware, cherry with a rich, red color has always been greatly prized. Cherry is a transitional species that forms the first hardwoods in a forest that's been cut over. And the extent of deforestation in the growth range of cherry was truly gigantic towards the end of the 19th century. I would expect that would've resulted in much more cherry being available in the mid 20th century, and it wouldn't surprise me at all to learn that cherry with less than spectacular color was simply turned into architectural parts rather than be cut for lumber or made into furniture. It might well be that what we see when we luck into the sale of a 70 or 80 year old furniture company's leftover "grandpa's stash" simply is the pick of the litter.
Al - I honestly don't know what the transition point is between darkening of the wood from the effects of oxygen and UV light and bleaching. I have seen this happen to a few pieces that were kept for decades in a sunny window sill, but I didn't have the permission (and probably wouldn't have considered) taking a little off of the surface to see whether this was truly the effects of UV (one would expect that if it's primarily due to light exposure that the effect would be very shallow). One comment here is that of the antiques I've seen pictures of that were "in the rough", I've yet to see a colonial-period antique made of mahogany that was bleached out. They were almost uniformly dark from oxidized finish, age, and in some cases, the effects of linseed oil application. Ditto for cherry pieces, and as one would expect, the opposite for walnut - these almost always seem to have developed a golden-brown type of surface.
Mark - Not all oxidation needs exposure to air, and not all changes to pigments are the result of oxidation. My guess would be that the pigments in cherry change regardless of exposure to light and air - it's just that light and air greatly accelerate the process. From the stanpoint of oxygen getting to the heartwood, I would expect this to occur over many years, as the oxygen can relatively easily diffuse into the wood's pore structure. These pores, after all, must run along the grain lines or the tree wouldn't have grown in the first place since water and nutrients must travel from the roots to the leaves. And a gas like oxygen can difusse through pores much more easily than water can.
However, there's another possibility. So far as I'm aware, cherry with a rich, red color has always been greatly prized. Cherry is a transitional species that forms the first hardwoods in a forest that's been cut over. And the extent of deforestation in the growth range of cherry was truly gigantic towards the end of the 19th century. I would expect that would've resulted in much more cherry being available in the mid 20th century, and it wouldn't surprise me at all to learn that cherry with less than spectacular color was simply turned into architectural parts rather than be cut for lumber or made into furniture. It might well be that what we see when we luck into the sale of a 70 or 80 year old furniture company's leftover "grandpa's stash" simply is the pick of the litter.