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cherry burl darkening

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david johnson

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Sep 9, 2009
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I have been turning cherry burls for the last few years and notice they darken like cherry wood. It seems to me the burls darken faster and to a darker color. Does any one know a method to prevent this darking.

Thanks for any help

Dave Johnson
 
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Cherry is very photoreactive.....

Hi Dave:

Cherry ("Black Cherry" typically Prunus serotina) and other closely related Rosaceae woods are photoreactive, and will darken over time with exposure to light. I don't know if it is is specifically UV light or whether it includes visible light as well. The photoreactivity of cherry is well known - there are many stories of woodworkers finishing pieces of cherry furniture, putting them in a room with a bowl, candlesticks, boxes, ceramic pieces or other opaque things on the top of the piece, and then reporting areas underneath these objects that are obviously less effected by the room light, and show up as pale areas where the cherry did not receive as much light compared to the uncovered areas of wood. I can only speculate that the anomalous, unorganized, and probably denser growth of the burl has a higher concentration of photoreactive chemicals ("extractives") in the burl wood (perhaps due to higher cell densities?) which causes the burls to darken more intensely. BTW - Other photoreactive woods are "true" mahogany (Swietenia macrophylla) and purpleheart (Peltogyne sp.).

I suppose that if you kept the piece in the dark (.....or wrapped aluminum foil around the wood!) you could possibly reduce the amount (or rate) of color change. There are probably a few film finishes that might have UV inhibitors in them that could slow the process (something like marine spar varnish with UV inhibitors), but it is likely that you will never stop it altogether.

I have no idea what wood bleach would do to this photoreactive effect in terms of reducing the level of darkening, but you might consider doing the experiment and seeing if it does (or doesn't) change the color/patina developing properties.

I hope this helps!

Rob Wallace
 
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I'm guessing that burls have more endgrain which might be why they look darker. I've bleached cherry and it works quite well. I can't tell you about long term results of the bleaching because all of those pieces sold many years ago.
 
When I sold cabinetry, that was one thing we mentioned about cherry- it will darken over time. Why? Haven't the foggiest but I'll go with Rob's explanation.
 
It is oxidation, or kind of like rusting. Exposure to air and sun light cause it to darken. If you want to be artistic, it is a 'patina'. A spar varnish has UV inhibitors in it. I have heard some say that Armor All, will do the same thing.

robo hippy
 
david johnson said:
I have been turning cherry burls for the last few years and notice they darken like cherry wood. It seems to me the burls darken faster and to a darker color. Does any one know a method to prevent this darking.
Click to expand...

Bleaching will set the process back a lot, but with time it will eventually begin to darken again. You are correct in assuming that random grain orientation returns less light to the eye. The darkening is pretty superficial with our northern stuff. A darker cherry blank from five years ago will turn fairly bright once the surface 1/8" is removed. We noticed at school that cherry from a kiln which had been steamed took a very long time to develop a patina equal to an air-cured piece.

Cherry is often patinated with alkali. Fuming with ammonia or painting with lye are two methods often used. So chemically, I guess something is being oxidized. UV light, having high energy, must promote the same type of chemistry.

Personally, I'd let the stuff patinate. If you want white, get maple.
 
Use a vinyl sealer on it. I've had success using it on flaming box elder. After 7 years the bowls still have the vibrant red color. Should work the same on the cherry burl.
 
I don't think I tried a vinyl sealer but I did some experiments with lots of finishes and even things like Automotive UV waxes. None of them worked when the Box elder was subjected to direct sunlight. I have box elder pieces that have been in the house for many years that still look good and they just have regular lacquer on them.
Cherry and Purple heart have the same problem. If you lay a quarter on one and lay it outside by tomorrow you will see a strong outline of the quarter. Penny's won't work because they aren't worth anything anymore. 🙂
 
John,

My FBE's are inside but the light shines through the window on them. I'll look when I get home to see which brand I bought. It's what's used in the kitchen cabinet industry so probably ML Campbell.
 
The finish I used was Sherwin Williams Vinyl Sealer with 24% solids and then top coats were Sherwin Williams Cab Acrylic Lacquer.
 
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