The fiber saturation point: does it mean what you think it means?

Wood–water interactions are central to wood science, technology, and engineering. In the early twentieth century, the term “fiber saturation point” was coined to refer to the point of transition from the domain where wood properties change with moisture content to the domain where wood properties are constant. This conceptual model assumed that the wood cell walls are water saturated at this transition point and that capillary water appears above this point. This has since then been contradicted by multiple studies. Additionally, the fiber saturation point has been associated with techniques that do not necessarily measure the same moisture state. Some techniques characterize either the transition state at which wood properties change, or the state at which the cell walls are water saturated. These are, however, not the same moisture state. The aim of this paper is to clarify which moisture states the various fiber saturation points represent discussed from a conceptual model consistent with current experimental evidence. To avoid confusion, we propose that the transition state at which wood properties change is the only state called “the fiber saturation point”, or, for even more clarity, “the property intersection point”. For other moisture states, we strongly recommend that the term fiber saturation point is avoided. The term “maximum cell wall moisture content” should be used for the state at which the cell walls are water saturated. Finally, we highlight the importance of considering which moisture state is relevant for a specific application and selecting an appropriate method to characterize that state.