I know that Cherry darkens with age and that it does so via two different processes – exposure to UV and via exposure to air. UV darkening it quite quickly and air exposure darkening it much more slowly.
My question is, while I know that a clear finish won’t stop UV darkening unless it has a UV inhibitor (and even then it only slows it way down), won’t most clear finishes like conversion varnish or poly pretty much prevent Cherry from darkening by exposure to air? Finishes like nitro lacquer which “breath” to a certain extent would probably allow some darkening outside of UV exposure. But I’m less certain about conversion varnish.
The background issue here is whether varnished fixtures in an indoor setting will darken much with age unless they are near enough to a UV source to darken that way?
Replies
Kevin,
Cherry is one of the most reactive woods to light that you will find...be it direct sunlight or even artificial lighting. There is no finish that you will apply that will keep natural cherry from changing with exposure to light. Try this as an experiment. Finish a board of cherry using different finishes every foot (or 6 inches). Try sealing it first on one of the sections, and then apply your final finishes. Take a piece of masking tape and stick it down the middle of the length of the board, covering all of the different finishes, and put the board in a room where it will not receive any direct sunlight, but will be exposed to normal day lighting and night time lighting. Remove the masking tape after a week and you will see that all the finishes have changed, though some may change less than others. It will answer your question once and for all, and you will be able to properly choose the finish to apply to your project. JL
Edited 1/26/2007 3:52 pm ET by jeanlou
Edited 1/26/2007 4:46 pm ET by jeanlou
Thanks Jean.
After reading your reply I went back and re-read something that had caused me to ask the question in the first place. Turns out I'd misunderstood one sentence. I thought it was saying that the artificial light from table lamps wouldn't darken the Cherry as much as ambient light and that moving the lamps periodically would help even things out. Turns out what they were talking about was the base of the lamp blocking any light at all from reaching the Cherry... LOL which makes a lot more sense!
Other woods react in a similar way, Kevin; teak and walnut are two examples. Cherry, however, is the most unforgiving of them all. It is a lovely wood and I enjoy using it, but ....and you know the but. JL
Kevin, at a molecular level almost no finish is impervious to the exchange of air, and the moisture it carries. This explains why panels sealed with a film forming finish expand and contract as they absorb and lose moisture. Naturally, the better the barrier the more the molecular exchange activity is slowed down. This has an effect on oxidation as well as moisture exchange.
Therefore you can conclude that three coats of almost any polish, lacquer, varnish or shellac is a more effective barrier than just one. I'm hesitating a bit over the barrier forming ability of an epoxy finish because these are said to be more effective again than the other polish types named at sealing wood, but even here I think there is still some exchange of air and moisture at a molecular level.
Less effective barriers than film forming polishes tend to be the pure oil and wax finishes. Many coats of linseed oil or tung oil will build up a barrier-- often not as resistant as film forming finishes, but resistant nonetheless.
Linseed oil has developed a reputation for being a very poor sealant, in large part I suspect because of the power of the internet to spread such information. However, in part I think this is because most people don't have the patience to build up a proper linseed oil finish. They apply two or three coats, and that's it, so no wonder it's not very water resistant. A linseed oil finish doesn't really build up much resistance to miosture, etc., until you've got perhaps 20 coats on. Thirty or forty is better again, and most people just can't be bothered with the time required to apply so many coats.
Linseed oil continues to darken over time, even more so than tung oil, so this stuff confuses the issue about oxidation and wood darkening. This is one reason why oil varnishes get darker too as most of them contain either linseed or tung oil, or perhaps both. Slainte.
Richard Jones Furniture
Thanks, both of you!
I built a TV stand for my brother. I was at his place about 2 yrs later and happened to think about the cherry darkening. I asked if he had moved the TV that sets on it. He said he had not, but the TV is quite heavy so I opened both doors of the lower cabinet. To my suprise they were essentially the same color as the exposed wood above. I used Danish oil as the finish. Evidently cherry gets darker with age regardless of exposure to sunlight or perhaps the finish had something to do with the darkening.
Yes, Cherry definitely does darken via oxidation as well as with exposure to UV. Everything I've read says that UV exposure darkens Cherry significantly faster than oxidation does, in days versus months. And as Sqian alluded, oil finishes do the least to inhibit oxidation. So the darkening you noticed was more or less to be expected. Although I would have expected that the interior wouldn't darken as much provided it is usually closed.
As a long-time fan of The Antiques Roadshow I've often noted with interest how the furniture experts will examine interior parts of antique woodwork to see how it has oxidized and it is normal for less exposed surfaces to oxidize less than expose surfaces.
Kevin:Light is tricky stuff. Each source is different interms of UV content and potential damage. In general, daylight is the worst, with direct sun at the top of the list. However, halogen lights and all florescents throw off a great deal of UV. Tungsten lights do not throw much at all(old fashioned light bulbs).That said, haolgens and florescents can be bought wuth UV shields, or shielded with aftermarkst products.Wood is the least of your worries. Paper yellows and disintegrates with UV exposure. Textile and art pigments fade. Anyone who has had books on a shelf for a long time can see the difference between the spine and the cover.Textiles disintegrate. Many polymers depolymerize and just fall apart. This is what happens to the vinyl tops of cars. It is worth your while to keep UV out of your house to as great an extent as possible.
Let me make the point that the UV inhibitors added to clear finishs are there to protect the finish, not the underlying wood. The UV inhibitors lose their effectiveness as they do their job. That's why us boaters need to annually sand our brightwork and put on a couple of more coats. Even 5-6 years we must completely strip the finish and start over. So using a finish that contains UV inhibitors is a temporary solution.
Except for completely encapsulating epoxy or thick polyester coating or a thick coating of wax, no finish is impervious to watervapor or oxygen. At most, finishes just slow oxidation.
I am a little puzzled why you don't want the cherry to darken. If you are using a clear finish the darkened surface is superior to the somewhat pinkish tone of the freshly sanded surface.
I think the natural darkening is a good argument against primarily overstaining woods like mahoghany and cherry. Some folks stain it a reddish/rosewood colour. When it is 75 years old it will be a muddyish dark shade that will largely obscure the grain.
I made a piece for my church that is near a stainglassed window on the North side of the building. Despite very little light it has darkened under its BLO/shelac finish considerably.
My 2 cents and off the main topic- sorry.
Frank
Frank
You bring up an interesting point, and that is allowing the natural wood that is finished to age over time and enjoying the changes that are happening.
Many individuals try to keep a certain look forever. It makes me think of people going under the knife and having their body changed so that they can ''fend off the ravages of aging". Personally, I enjoy natural wood finishes and the changes that happen over time. (and people that age gracefully) JL
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