–by Dana Spessert, NHLA Chief Inspector
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Over the years, there have been numerous studies on soft maple and other species that show signs of impact staining, often times referred to as “bruising”. This impact staining is a form of oxidative stain, which in many cases is not easily identifiable when the board is rough and only reveals itself upon surfacing.
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Studies show, that in the case of soft maple, bruising can be caused by impacts from a hammer. If this is true, then one can assume that any extra force that is exerted onto the log or sawn boards would cause the wood to become discolored in the areas of impact.
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There are two studies on this subject, that may be of interest.
- Exploring Methods for Prevention of Oxidative Stain in Soft Maple (https://www.fpl.fs.fed.us/documnts/fplrp/fpl_rp654.pdf)
- Factors Affecting Oxidative Stain in Soft Maple (Acer rubrum L.) (https://www.fpl.fs.fed.us/documnts/fplrn/fpl_rn311.pdf)
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It has been my experience that for certain species like soft maple and birch, the use of knurled or screw rolls with excessive hydraulic or air pressure can cause this same type of bruising on the boards after drying. One could also assume that the use of modern-day hydraulic log loaders can also exert extreme pressure on the logs and create the same type of damage to the wood.
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I would suggest that when sawing soft maple or birch, you reduce the pressure of the press rolls on the edger, gang saw, re-saw and any other area of the sawmill with press rolls, to reduce the bruising effect that is downgrading the lumber through the process. I realize that there may be safety concerns with the decrease of pressure, but I believe that with some simple studies, there is a pressure that will fill both requirements.
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I would also like to address the effects on grading lumber that has been “bruised” by a process such as described above. When grading hardwoods, the Inspector shall grade the lumber as he/she finds it as described on page 4, paragraph 4 of the NHLA Rules Book:
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“Lumber shall be inspected and measured as the inspector finds it, of full length, width and thickness. No allowance shall be made for the purpose of raising the grade, except that in rough stock, wane, and other defects which can be removed by surfacing to standard rough thickness shall not be considered. Nothing herein shall be construed as prohibiting the shipper from improving the grade or appearance of the lumber at time of or prior to shipment.”
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With the Rule as listed above, the shipper is not allowed to overlook stain. But if it does not show on the surface and only appears after surfacing, then the Inspector would have no other option than to inadvertently cut over it in a clear face cutting. I would caution Lumber Inspectors who have had samples of bruising returned to them, to closely watch the rough lumber. I believe, in many cases, there were signs of the bruised areas that can and should be identified on the surface during the grading process.
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