Wood Analysis

Contributed by Michael A. Trimpe

A piece of wood as large as a chunk of a tree left on the bumper of a car in a hit and run, or as small as a fragment on the tip of a bullet, can be identified by microscopy. The quality of the sample in question dictates the potential degree of identification. For example, a minute piece of sawdust may only be identified as a softwood versus a hardwood. Some wood samples may be identified to their genus or family, for example Pinus (pine). Still others may be identified to their species like Pinus resinosa (red pine) or Quercus alba (white oak).

The size of the piece of wood in question greatly determines how the sample should be prepared for microscopic identification. If the sample is large enough, the analyst will observe the color and odor. Then its gross features will be observed under the stereomicroscope. This initial observation will include looking at the cross section to determine if it is a hardwood (with vessels) or a softwood (without vessels) (Fig. 5).

Then the sample is prepared for more detailed observation under higher magnification with a comparison microscope. Microscopic thin sections of the wood are viewed from three cut angles. A cross section (Fig. 1) cuts across the diameter of a tree, as would be made to cut the tree down. A radial section (Fig. 2) is cut perpendicular to the cross section and along the rays that run from the center of the tree to its outer edge. A tangential section (Fig. 3) is cut perpendicular to the cross section and perpendicular to the radial section. The cuts are typically made by hand with a sharp blade, but if the sample is already microscopic then a histological microtome could alternatively be used to cut the sample. A red dye (Safranin-O) is often used to stain the wood sample for easier identification under the microscope.

After the wood sample is prepared and mounted on a microscope slide, the three thin sections are observed for identifying characteristics using a wood identification key. Many keys are available in literature which are followed step by step until a species identification can be made.

Quite often wood identification is used in fire debris cases in which it is necessary to know if the wood collected at the scene is a softwood. Softwoods are known to naturally emit terpenes (found in turpentine) which may cause complications with examinations for ignitable liquids. Even charred wood can be identified using these methods(Fig. 4). Wood identification can also be important in assault cases where splinters left in the victim can be compared to a wooden object used as a possible weapon.

Fig 1. Yellow Pine Cross-section
Fig 2. White Pine Radial Cut
Fig 3. Tangential Cut
Fig 4. Charred Yellow Pine
Fig 5. Hardwood vs Softwood Cross-section

Additional References:

1. ASTM Designation: C 856-95, Standard Practice for Petrographic Examination of Hardened Concrete.

2.Bisbing, R. E. and Schneck, W. M., Particle Analysis in Forensic Science, Forensic Science Review, 18(2), July, 2006.

3.Brown, R. S., Boltin, W. R., Bandli, B. R., Millette, J. R., Light and Electron Microscopy of Mineral Wool Fibers, Microscope, Vol 55:1, p. 37-44, 2007.

4. Campbell, D. H., Microscopical Examination and Interpretation of Portland Cement and Clinker, Portland Cement Association, Skokie, IL., USA, 1999.

5. Carr, D. D., Industrial Minerals and Rocks, 6th Edition, Society of Mining, Metallurgy, and Exploration, Littleton, Colorado, 1994.

6. Double, D. D., and Hellawell, A., The Solidification of Concrete, Scientific American, pp. 82-90, July, 1977.