by Professor Gene Wengert
We seem to be having a harder time than ever controlling "sticker stain" when kiln drying light hardwoods, such as: hard maple and white ash. The problem is worse in hot, humid weather. I would like to know what causes these stains to develop, and would be grateful for a cost effective solution.
One of the more common drying problems occurring today is a dark area noted on dried, planed lumber. This discolored or stained area runs across the width of a piece of lumber and is in the same location where a sticker was located during drying. Typically, the stain is not seen in the rough lumber, but is only evident after planing. The defect is called sticker stain or sticker shadow. ("Shadow" is often the preferred term by the person grading the lumber, as the word "stain" to a grader means the early evidences of decay, according to the NHLA definition. However, tradition has resulted in widespread use of the term "sticker stain.") Seldom, if ever, will there be any fungi associated with the sticker marks; the wood that is stained is always (except in a few extreme cases) as strong and as solid as unstained wood.
Sticker stain seems to have been more severe and more common the past few years, than during the 1970s and 80s, although there have been recorded outbreaks of sticker stain from time to time in the 1950 and 60s. The reason for the higher incidence is not known (I'll give a few clues later on.), but I believe the main factor is the unusually wet, warm weather we've had recently and the effects of this unusual weather on the tree's chemical makeup.
Several other reasons for the high incidence of sticker stain include the increased use of light finishes, the elimination of bleaching (oxalic acid) from the finishing process, and the general surplus of the white woods in the market place. Perhaps some species of stickers are also aggravating the stain, but they are most certainly not causing it.
Some very helpful information has recently been published by a researcher from the U of Minnesota. He found that stain was almost inevitable in lumber sawn from old logs! He found that when the tree was first cut down, not all the cells died. Rather some of the cells continued to live, and seemed to create chemicals that would eventually turn color during drying. It is worth repeating: Stain is almost inevitable in lumber sawn from old logs, no matter how good the kiln operator is. Or stated another way, much sticker stain is a result of sawmill management's inventory practices and not the kiln operator!
I wonder, too, if the lack of knowledge on drying maple lumber is sometimes a major reason for stain. It seems to me that many drying practitioners have forgotten the basic principles of drying white woods; they learned how to dry oak, but not the other species. Further, much of the drying equipment today is geared to oak drying (especially with respect to slow air speeds), not white wood drying.
Some Basic Principles of White Wood Drying
The basic cause of sticker stain is slow drying (probably under 5% MC per day) at warm temperatures (50o to 130o F) when the lumber is above 40% MC. These conditions begin the chemical reaction within the wood that will eventually, as the lumber dries, lead to discoloration. In fact, the discoloration itself may not show up until much later in drying; that is, at much lower MCs. The final oxidation and discoloration may actually occur below 15% MC. This final discoloration is accentuated by using temperatures over 150o F during the final steps of the kiln schedule. But it is the initial slow drying before the discoloration occurs that is the fundamental cause. In fact, as mentioned, poor log storage can indeed initiate the reaction.
It is important to recognize that sticker stain is not a fungal stain. However, the slow drying that results in sticker stain can also result in fungal staining, if other conditions for fungal activity are appropriate.
Because sticker stain is actually a result of the oxidation of naturally occurring chemicals within the wood (perhaps being catalyzed by enzymes within the wood), there is little that can be done to the freshly cut lumber in terms of chemicals dips, including the fungicide dips for blue stain control, or chemical treatments to inhibit the reaction. Some people have attempted to treat the lumber with oxidation inhibiting chemicals; others have tried using enzyme inhibiting chemicals. The problem with these chemical treatments has been getting the chemical deep within the wood and not just in the surface layers.
Conclusion: THE ONLY PRACTICAL, 100% CURE FOR STICKER STAIN IS TO ACHIEVE FAST DRYING UNDER THE STICKERS WITH FRESHLY SAWN LUMBER.
Risk Factors for Sticker Stain
There are certain procedures or events that will encourage sticker stain formation (that is, cause or result in slow drying under the stickers or alter the wood itself in some way). Most of these procedures or events are most serious when the MC of the lumber is over 40% MC. They include:
- Using old logs that have been stored during warm weather.
- Storing lumber more than 12 hours in warm weather after sawing,but before stacking.
Stacking & Handling
- Using stickers wetter than 10% MC.
- Using stickers wider than 1-1/2 inches or thinner than 3/4 inches.
- Exposing the lumber to excessive rain, especially at high Mcs and in warm weather, after stacking.
- After stacking, exposing lumber to poor drying conditions.
Kiln Equipment and Procedures
- Using a kiln load that is quite wide (over 16 feet) with velocity under 500 feet per minute (fpm).
- Using a "snow melting" or "thawing" kiln procedure.
- Developing kiln relative humidities, after the first 6 hours of drying, that exceed the recommendations (10o F depression is mandatory for green 4/4 and 5/4).
- Using kiln temperatures over 160o F, especially early in the kiln schedule.
- * Using long (over two hours) fan reversal cycles.
The cures for sticker stain involve drying the lumber under the stickers as quickly as possible by avoiding wet stickers, avoiding high kiln temperatures, and using good schedules and controls. Specifically:
- Use fresh logs that have been stored less than 2 weeks during warm weather.
- After sawing, in warm weather, stack the lumber within 12 hours.
Stacking & Handling
- Use stickers between 8% to 10% MC; check the MC with a moisture meter; get the stickers directly from the unstacker rather than from storage.
- Use stickers 1-1/4 inches wide, by 3/4 or 7/8 inches thick.
- Use a grooved sticker for a little extra advantage.
- After stacking, the lumber must be kept out of the rain, especially at high MCs and in warm weather.
- After stacking, if not loaded directly into the kiln, place the lumber in fast drying locations. Fan-sheds (or blow boxes) would be ideal.
Kiln Equipment and Procedures
- Use a kiln load narrower than 16 feet, with velocity over 500 fpm.
- Do not use a "snow melting" or "thawing" kiln procedure.
- Develop kiln relative humidities, within the first 6 hours of drying, that are at least equivalent to a 10o F depression. Larger depressions may be required, if recommended in the schedule. In humid weather, kiln temperatures may have to be raised slightly to achieve the low humidities required.
- Load the kiln with only two rows of lumber rather than 3 or 4 for the first 12 to 36 hours to help achieve the required humidity immediately. Then, after the RH has been maintained for 12 or more hours, load the remaining lumber; the required RH should be more easily achieved in this manner at all times.
- Use low kiln temperatures (110 or 120 F initially), as recommended in the schedule; never exceed 160 F, except during equalizing and conditioning.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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