Stickers and sticker shadow

A discussion on synthetic and traditional stickers, and an explanation of reverse sticker shadow. May 30, 2001

Question
What are the pros and cons of synthetic stickers for air drying hardwoods? Would small diameter PVC pipe work?

Forum Responses
Pro--durable, no stain, any length.
Con--initial capital outlay.

PVC
1) How would you keep it from rolling around all over the place?

2) Not sure if it's strong enough for all that weight.

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I have seen half-pipes used--they indent quite a bit on high piles. Fiberglass (Simpson Timber EKS), wood impregnated paper, and even recycled plastic (Durastick--advertisement here or at least in the past?).

Gene Wengert, forum technical advisor

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Just watch out for reverse sticker shadow when playing with different sticks.

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What is reverse sticker shadow? Do synthetic stickers increase the risk of it occurring?

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If you use a very dry sticker, then the sticker will dry the region under the sticker very quickly, while the rest of the piece of lumber dries more slowly. Faster is brighter; slower is darker. So the net effect (in only a few cases involving old logs) is that there is a white sticker mark, instead of the more common dark sticker mark.

Reverse sticker shadow is very rare. I am not sure that synthetic stickers will have any effect on reverse sticker shadow at all. I have not seen any for several years.

Gene Wengert, forum technical advisor

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In my operation, I use a lot of outside dryers. One guy ran out of breeze dry sticks and started to use his old flat softwood and we are getting a white sticker stain. We call it reverse sticker stain. I was trying to explain it to one of the managers and not having luck. Do you have any good analogies?

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Details of how sticker stain and sticker shadow develop: It is the free sugars in the wood that oxidize with the help of enzymes present in the wood, the same as a cut apple left on the counter does. In almost all situations, the sugars in woods like maple oxidize. The trouble develops when the sugars and enzymes become concentrated in a region of the board and show up well. The sugars and enzymes are water soluble and travel with the water as it moves toward the board surface. When the water turns into vapor, the sugars and enzymes are left behind. The point where the water turns into vapor progresses towards the center of the board as drying proceeds. If the loss of water is such that the boundary where the water turns into vapor does not move towards the center of the board quickly at any time, the sugars becomes concentrated at that layer. That layer can be on the surface or deeper in the board.

With wet stickers, the boundary layer does not move away from the sticker quickly and the sugars and enzymes concentrate under the sticker, causing sticker stain. If the sticker is very dry and wicks away the water quickly from under the sticker, the boundary layer moves away from the surface quicker than the surface not under the sticker. This makes the change in color not under the sticker more noticeable (sticker shadow). I have seen samples where a definite layer of dark color was about 1/16 inch below the surface. The drying was such that the boundary layer delayed at that position for a while.

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The discoloration in sticker stain and other chemical stains is not in the lumen of the cell (the opening), but is actually small globules inside the cell wall itself. Microscopic examination will show this to be true. Therefore, I do not believe that the sugars or oxidation products migrate (significantly) to the surface or migrate to a point where evaporation is occurring.

It has always been thought that it is the sugars and starches in the ray parenenchyma cell walls, not the sap or fluid in the lumens of these cells, that oxidizes.

Also, note that with many of the chemical stains, especially gray stain, the entire sapwood can be discolored, and not just a small region. In short, the oxidation takes place within the cell wall, depending on the appropriate temperature, moisture, and perhaps other chemical conditions (pH, for example) that are required for the reaction to proceed. Often, the reaction proceeds in the log before the lumber is cut. In this case, attempts to stop the reaction in lumber using anti-oxidants are not successful. There is no evaporation involved.

Gene Wengert, forum technical advisor

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It is true that gray stain is throughout the sapwood and it is the starches that are involved in gray stain. For sticker stain and shadow there is evidence for migration. I did not include before that time, temperature, moisture content and lengthy chemical reactions are involved in both, as Gene mentions.

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I think sticker stain is starches and not sugars. Check your sources, please.

Gene Wengert, forum technical advisor

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How about using MDF?

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The adhesive used would not stand up to the kiln temperatures. There are other problems related to "springback" in thickness and lack of adhesion throughout the piece (low adhesive content usually).

Gene Wengert, forum technical advisor

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What would be the best and cheapest material? I am stickering mostly birch and also spruce, cedar, mahogany and some rosewood. Do I need different sticker materials for each?

I am not drying from green. All the wood is already around 8%-20% and I'm bringing the MC down to 6% for guitar building. Stickers would be in the kiln for as long as that would take. I wouldn't expect this to be 3 weeks. Does the above info on MDF still apply?

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Yes, on MDF.

If you want to go low budget, find a drying operation in your area and ask them if you can purchase the kiln stickers that they are throwing away because they are too short. Do not buy the ones that are warped or otherwise damaged. Then cut them back to a uniform size and use them. Note: Piles narrower than 48" get very tippy, so do not go too high.

Gene Wengert, forum technical advisor