Ultrastructural X-ray scattering studies of tropical and temperate hardwoods used as tonewoods

The structure of hardwoods representing eight tropical and five temperate species was characterized from the atomistic level up to the cellular level using X-ray scattering, X-ray microtomography and light microscopy. The species were chosen for this study based on their popularity as tonewoods. The ultrastructure of wood cell walls, including crystallite size, orientation and close-range order of cellulose microfibrils were determined by small- and wide-angle X-ray scattering (SAXS, WAXS). The SAXS patterns were interpreted by using an analytical model of cylinders packed in a hexagonal close-range order with paracrystalline distortion. The values for the cylinder diameters given by this model were compared to the average crystallite widths obtained by WAXS using the Scherrer equation. In six out of 26 samples, all of these representing tropical species used especially in fretboard parts of electric guitars, large differences between these two sizes were obtained. The WAXS and microscopy results of these samples corresponded to tension wood structures. These comparisons and interpretations of SAXS results have not been previously presented for any tropical hardwoods, especially related to those containing tension wood tissue. The importance of the ultrastructural characterization was highlighted in this study in the case of tropical hardwood samples.