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Here is a pic of a carved back. When I purchased the wood it was from a rough billet. Both pieces appeared to be next to each other in the milling process. After planing it, I discovered the shade discrepancy and decided to use it anyway. I like the figure on it very much and consider it a learning experience. It is light dependent and not always this bad. I know I read about this phenomenon, but have forgotten. I would like to know the correct explanation for the difference, how you might avoid it when purchasing rough stock and most importantly how I can deal with it in the final guitar. Thanks all!

Robert

Book matched set often display this. this is a bit extreme so I suspect there is some oxidation play a role but nothing a fresh sanding will not handle. The cause of this effect is when you book match a top or back on one side you looking up stream with the grain on the other half you are looking down stream of the grain. looking up stream for lack of better term you are looking into the grain and it will appear darker. because less light is reflected back at you.

If you look close enough at every book matched top or back you will notice this effect. it is easier seen in maple than other light woodsfor some reason

Michael gives a good description.

The general angle of the grain not being close to parallel with the surface causes this(runnout if you will, much the same as runnout causes the look of the curly figure). You can see this effect in soundboards that have a lot of runnout. To see how much is oxidation vs face grain runnout, flip the set 180 degrees and take another look. If you notice the side that did appear dark in your picture remains pretty dark you are seeing coloration. If you see the two sides flip(lighter and darker) it is mostly runnout.

One can learn a lot just by regularly reading this forum. I wish I had found it a year ago. I had an engleman top that did the same thing - it was runout.

Steve

Michael thanks for the explanation. It turns out that the color stays the same regardless of orientation looking at it. That is the same side is always redder. Also, the pic is after freshly carving. Therefore, it seems not likely to be runout. That means from the other answers it must be a coloration difference. Interesting how that could have occurred in a split billet if they were side by side. Perhaps they weren't that close, but I don't think they were that far away because other grain aspects appear to match. Any thoughts on treating/dealing with the wood to achieve a uniform finish? If coloration, then I either bleach/lighten the darker side or stain the lighter side. How about subjecting the lighter side to UV for a period of time? Thanks again for the help.

Robert

It is interesting that a billet would have a perfect transition of color that would be night and day with only a small distance(a clean resaw and surface, removing 1/8" or less. More and the bookmatch would suffer). If one face was contaminated, or exposed to light differently than the other, it could make sense. However the effect would have to run deep enough to remain consistant after carving and surfacing. All seem unlikely, at least it would seem you would be aware of these potential causes. There is also the possibility that one side was joined upside down, but again you would know and should be able to detect that even with a really well quartered piece(the figure would be the clue).

There are a couple types of wood bleach that could be used if it is either coloration or if it is contamination, but you need to figure out what the cause was. Also you would want to test anything you do on off cuts from the piece, or at a minimum sample test on the back side. Before I would go that far, I would expose the surface to air and some sunlight(avoiding heating it up much, if you expose it to the sun). Then you can also look at finish coloration, but you are going to have a hard time dealing with the line at the joint. Trying to color only one side is not likely to work well, but a bit of color in the finish will act as an overall distraction from the harsh transition.

All that said, if it is just the direction of the grain and the way light is reflected. These things offer nothing(coloration, may adjust a bit of the look, but again as a distraction). It is not far fetched that the face grain could be angled, even if the billet was squared to avoid for drying damage or adjust for distortion, the billet could have runnout. If the billet is very white, the medium runnout would be notable.

Good luck with it,
Rich