AFTE Store - The Effect of Relative Humidity and Direct Contact with Water on the Dimensions of Tool Marks in Wood

Tool marks are valuable evidence recovered at the scene of crimes involving forced entry. In many cases, particularly residential burglaries, the tool marks are left in a wood medium. The hygroscopic properties of wood leave it particularly susceptible to the effects of changes in relative humidity; in fact dimensional change of wood as a result of moisture has long been a concern for the construction industry. This study investigated the effect of differences in relative humidity on the measurements of tool marks in wood. Eight species of wood were used in testing; three tools were used in making five marks in each sample of wood. The wood samples were placed in six separate locations with different relative humidities. After the samples acclimated, marks were made and measured; the wood samples were then collected and placed in a laboratory hood to simulate storage in an air-conditioned evidence storage room. Subsequent measurements were made after one and two weeks. Results obtained showed that all marks changed in size to some degree; some marks actually disappeared, while others became visible by a change in moisture levels. Further experiments on the effect of direct contact with water on the size of tool marks showed a wide range of results depending on the type of wood used. It is evident that moisture can significantly change tool marks made in wood, hinder identification to their source, and even prevent marks made by the same tool from being linked to each other.