The Great Maple Stain of 2002
Here are some examples of the types of stains that were seen around the region. The first, and more commonly seen in previous years, is a bluish-gray stain that becomes a darker gray on drying and planing. This type of stain is sometimes attributed to log end stain, but in many cases this past year, as we see in the first sample to the right, the blue stain is not appearing at the end of the log, but at random regions through the lumber.
This was also the case for the second, and more unusual type of stain, shown in the second sample. This type of stain appeared bright-green upon discovery; in fact, one mill was calling it "frog-green" because of its noticeable coloration. The green color, invisible on the suface of the lumber, would appear in freshly hand-planed green lumber, but then would fade over the course of a week so that it was again barely visible to the naked eye. However, drying and planing would bring out the full extent of the stain, again as a dark gray stain.
Both of these coloration stains appear to be fungal in nature; that is, they are stains related to the sugars and moisture in the sap of the wood. We know that the optimal temperature for the development of these fungal stains is between 70 and 90 degrees farenheit, although they can form at lower temperatures. Also, we know that the sugars in the sap provide the food for the development of the fungus. In both the fall of 2001 and late spring and fall of 2002, we had optimal conditions for the development of these types of stains. The staining was especially rampant in weeks following unusually high temperatures.
We are trying to reconcile this problem with the preponderance of the third type of stain shown. This is a mechanical stain, caused by the "bruising" of the wood surface in processing. Incidents of this type were called in quite a bit this past fall. We know that damaging the surface cells of the wood can have the same effect as in sticker stain, when blocking the moisture flow through the wood in the drying process can leave chemicals in the wood oxidized at the surface. The odd feature of this fall's preponderance of "bruising" is that every one of the incidents was incurred under conditions that prevailed for years prior to the staining, and yet the operators had never experienced the problem before. Often, when we diagnosed the problem as mechanical bruising, the operators did not believe it, because nothing was different about their operation than had ever been before. Yet, mill investigations proved the bruising diagnosis to be accurate.
As we collected anecdotal evidence of the unusual staining problems, we sought to answer the question why 2002 seemed so much worse than previous years. We don't have solid answers yet, but there seem to be a number of circumstantial factors that, when taken together, seem to add up to something.
First, we are at, hopefully, the end of a three-year drought cycle in the Northeast. The droughty conditions may have impacted the physiological functions of our hardwoods, by putting them under stress and therefore weakening, or in some other way, imbalancing the trees' natural defenses to these stains. Therefore, we believe, the same conditions that could promote higher than usual fungal staining may also increase the incidence of the mechanical staining caused by bruising. So, in all cases, processing techniques which yielded little or none of these problems in past years may have resulted in different results this past couple of years.
Second, even though these have been droughty years, there were weeks with each of them that we had really heavy rainfall, creating conditions when naturally stressed trees may wind up in the woods or log piles hot and soaked. It was typically in the weeks following these warm, humid periods that the stains really blossomed.
Third, the demand and premium for really "white" maple has never been higher. Customers are looking harder at the lumber, and mills are sawing lower grade and smaller logs than ever before. Maple has always been a species with a tendency to stain in warmer periods, or when dried too slowly. The prevailing market conditions for maple, we think, may be contributing to the higher incidence of stain that we're experiencing.
The answer to reducing maple stain is well known. The wood must be processed more quickly and efficiently. That means, logs have to be sawn quickly after felling, and green lumber must be shipped or dried as quickly as possible. In addition, moisture removal capability of the dry kilns should be adequate to bring the moisture content of the wood down to below equilibrium moisture content (EMC, around 30%) within the first couple of days in the kiln. Maple lumber should never be pre-dried in the yard if it can be helped. And the use of a "white schedule" should be used in drying.
We're watching this year's staining trends in Pennsylvania. Please contact us at (814)865-0679 , or email us at email@example.com , if you're seeing staining problems beyond normal. We're trying to evaluate whether or not there is a geographic element to the problem, so we're mapping where our calls come from. We'd appreciate your help in this effort.
Hope this has been helpful. We'll see you again next month.
Chuck Ray, Ph.D.
Penn State Wood Products Operations Specialist