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Chemical answers to the sorting of commercial woods in Canada.

Anatomical microscopic identification can support chemical sorting to provide a more accurate method

The most commonly cut mixed species in Canada is called SPF. In the east, the S stands for black spruce, the P for jackpine, and the F for balsam fir. In the interior of BC, the SPF cut is composed of S for white spruce, P for lodgepole pine, and F for sub-alpine fir. Species from coastal BC are amabilis fir, Douglas fir, and western hemlock. Also, a Douglas fir/western larch mix is cut in the interior, but it is not a large one.

There are several commercial reasons for sorting SPF. Some have higher values if they are sorted as furniture stock. The F are generally wetter than the P and S. So, F lumber is best kiln dried correctly apart from them. The S are difficult to treat with preservatives, so it helps to have P and F poles sorted in the woodyard. The following SPF reagents were tried in a variety of lumber mills across Canada. The most useful of the three reagents is the FBBS for the P, but the low temperatures in winter cause the rate of reaction to be very slow.

Extractives highest in the heartwood

The seminal research for this work was the paradigm of Erdtman. For example, he and his co-workers |1~ studied the pines in a Swedish forest to show variations in extractives contents between trees. Other early research was done by Barton |2~ and Swan |3~ in the Western Forest Products Laboratories of the federal government.

Recent research conducted by Forintek Canada Corp. shows that there are several useful reagents which react with these woods. The success of the reactions depends on the presence of specific compounds in each species and are called extractives. Some extractives are unique to each species. The amount of extractives varies with their locations in the boles of the trees. Generally, extractives contents are highest in the heartwood, followed by juvenile wood and sapwood.

In the SPF sort, the sorting of the P is best with stable diazo salt reagents. The most useful one is fast blue B salt (FBBS), which is a bis-diazo amine stabilized with zinc chloride |4~. The sorting of the F is best performed with Ehrlic's reagent |5~, p-dimethylamino-bezaldehyde and hydrochloric acid.

A useful reagent for sorting the S are the indicators which measure pH differences between species. The best one is bromophenol blue |6~.

The mill production speeds are about one stud per second. A single piece must therefore be sprayed every second, and only a few seconds are available before sorting by color development in the mill. The F sorting is even slower and takes several minutes to develop even at ambient temperatures. Also, the F extractives are not evenly distributed across the board so that the color occurs in uneven streaks. The reagents do not react with the sapwoods so the pieces are all sorted as S; the S reagent varies with wood surface pH across the species.

In any experiment, all the pieces of all the species must be sorted by anatomical identification techniques. This allows you to calculate the percent sorting efficiency (PSE) using the formula:

PSE = 100 (no. chemically identified) (no. anatomically identified)

Mill sorters generally perform the sort visually, and by smell. These people have a PSE of about 80% and it varies during the day. The PSE calculation is only possible with the laborious cross check of anatomical research.

The PSE value for P heartwood is generally about 99%. The F sort is not as accurate (about 90%) and the S sort is about 80%. The reason for the S failure is because the BPB reagent does not react equally with west S as it does with east S. However, this BPB reagent was used successfully to separate Douglas fir from SPF in a mill trial.

The PSE values for this unusual sort was about 92%. Another successful separation in the laboratory was Douglas fir from western larch. The commercially important B.C. coast product mixed amabilis fir -- western hemlock -- could not be successfully separated. Reasons for this failure is the similarity of the extractives of these species, and the poorly defined heartwood/sapwood boundary in them.

Also, the industrial applications of these reagents is troubled by environmental considerations. The workers attempting the SPF separation may suffer from unknown health effects from their handling of the mill's products, although some of them are readily available, e.g. BPB is sold in pet stores for pH determinations in aquariums. And inspection of the structure of FBBS and the P phenolic extractives show that their product must be a polymer.

The SPF sort is best performed in the laboratory where fume cupboards and other safety equipment is available. Actually, the SPF is first sorted into P and SF with FBBS reagent, and then the SF into F and hence S with Ehrlich's reagent. The SPF samples must be freshly cut and contain heartwood.

Research on the sorting of these species is continuing using techniques other than chemical sorting. The problems with the chemical sort are the slow speed of reactions in winter and the variety of tissues in the woods. Sapwoods generally have little or no extractives present.

These problems may be circumvented by using physical methods of sorting. The two most promising techniques are Fourier transform infrared spectroscopy |7~ and ion mobility spectroscopy |8~. The reader is directed to the introductions in |7, 8~ for further study.


Chemical answers to the sorting of commercial woods have been developed. The most useful sort is for SPF which is also the most widely cut species. These have sorting efficiency values ranging from a high of over 98% for P to 90% for S. Physical methods of sorting these species are being developed presently.

The sorting efficiency for the physical only ranges from 80 to 95%, and not enough samples are studied in this sort. The chemical sort will continue to give answers to accurate sorting when supported with anatomical microscopic identification.


I thank Ms. Josephina Gonzales for the anatomical identification of the hundreds of samples used in this research.


1. Erdtman, H., Frank, A. and Lindstedt, G. Svensk Papperstidn. 54: 275-279 (1951).

2. Barton, G.M. Can. For. Ind. 2: 1-5 (1973).

3. Swan, E.P. For Prod. J. 16(1): 51-54 (1966).

4. Nault, J., Daniels, C. and Swan, E.P. Forintek Canada Corp. Report 5543K006, Aug. 1987.

5. Fraser, H.F. and Swan, E.P. BiMon. Res. N. 28 (5): 82 (1972).

6. Kutscha, N.P., Lomerson, J.T. and Dyer, M.V. Wood Sci. Tech. 12: 293-308 (1978).

7. Nault, J.L. Ph.D. Thesis, Faculty of Forestry, University of BC, 1989.

8. Lawrence, A.H., Barbour, R.J. and Sutcliffe, R. Anal Chem. 63 (13): 1217-21 (1991).
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Author:Swan, Eric P.
Publication:Canadian Chemical News
Date:Jul 1, 1993
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