AFTE Store - Wood Hardness via the Lowly Steel BB

Wood hardness has traditionally been measured by a method developed by the Austrian scientist, Gabriel Janka. His method and the wood hardness scale which bears his name is primarily of value to companies and individuals associated with manufacture of furniture, wood flooring, and certain musical instruments. But, his method is not particularly useful for forensic scientists with a need to evaluate the hardness of various types and forms of wood encountered in casework. The method described here defines wood hardness on the basis of the impact velocity necessary to embed a standard 5.3-gr. (~0.34 g), 0.173-inch (4.39 mm) steel BB flush with the surface of the wood sample. The technique proposed provides a means of comparing specimens and verifying that any test specimens of wood selected for terminal ballistic testing are equivalent to the evidence wood. Such equivalency would be critical if bullet penetration depth or velocity loss as a consequence of projectile perforation were important in a case. This paper additionally describes application of this method in tests of bullet deflection by a specific species of tree branches. The species does not grow in the author’s home state of Arizona, nor could a Janka hardness value be found for it. It was therefore deemed desirable to find a suitable local substitute of equal or slightly greater hardness for some additional deflection tests. The technique described here allows for an informed selection of a local tree as well as a means of verifying the comparability of evidence wood and test samples.