In situ ESEM observations of spruce wood (Picea abies Karst.) during heat treatment

An environmental scanning electron microscope (ESEM) equipped with a hot stage was used to investigate how the anatomical features of spruce wood respond to heating in vacuum conditions. The ESEM allowed for continuously observing the swelling, shrinkage, and other deformations of certain anatomical elements during heating. Observations showed that, as the temperature increases, there is a slight initial swelling due to thermal expansion of wood and steam and gas release from the cell wall, up to a threshold of 90-100 oC. Subsequently, the size of the various anatomical elements remained stable up to 200 oC, after which the cell walls withdrew and deformed. There were few significant differences between earlywood and latewood, while the deformation of cell lumen was strongly influ- enced by the dimension and the arrangement of the walls that surrounded them. None of the observed cracks was directly attributable to the impact of increased heat. Nevertheless, existing intercellular spaces strongly increased in size, arriving to grow up to 300 % of the initial dimension. These results emphasize the impor- tance of sound starting materials to avoid defects in the treated samples.