The commonly used microscopy sample preparation techniques -microtoming and razor blade cutting by hand -can generate surface artefacts such as (micro)cracks. In many cases it is impossible to diminish this problem by softening the samples by wetting, because some valuable information can then disappear and make the risk for artefacts even greater. It is especially important when the purpose is to study (micro)cracks, to ensure that the cracks have not been generated during the preparation. In order to avoid these artefacts, we have developed a new technique of preparing samples for microscopy using a high energy beam from a UV laser -laser ablation. This study shows the differences between samples which have been microtome cut, cut by hand with a razor blade and samples which have been laser ablated. The improvement that can be achieved is demonstrated by ESEM (Environmental Scanning Electron Microscope) images.

The objective of this paper is to demonstrate an ultraviolet (UV) laser ablation technique as a tool for sample preparation in microscopy studies of modified wood. Improved techniques for studying the microstructure of modified wood are crucial for a deeper understanding of many of their physical, mechanical and durability properties. The surface preparation technique is described in this paper. An illustration of micrographs of the micromorphology and polymer distribution in some examples of modified wood is also presented. It is clearly demonstrated that in contrast to conventional surface preparation techniques used for light microscopy and scanning electron microscopy, i.e. razor blade and microtome cutting techniques, UV laser ablation does not introduce any mechanically induced microcracks and redistribution of polymers or other mobile substances in the prepared surface. Results also show that, in particular, this technique seems to be suitable for studying polymer distribution in resin-impregnated wood, as well as detection of microcracks in modified wood cell walls.

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This report describes two attempts to test the effect of removing the layer of damaged cells -the mechanical weak boundary layer. In both trials, the wood has been laser ablated using different types of lasers with different wavelengths. The goal has been to determine whether the glue joint strength is influenced by the mechanical weak boundary layer and to show how the laser wavelength affects the glue joint strength. The statistical evaluation of the results shows however no great differences between glue joints made of ablated and glue joints made of unablated surfaces. Nor are there any real differences between the different lasers and different wavelengths. This may be because there are indeed no differences between ablated and unablated surfaces, in terms of a mechanical weak boundary layer, but it may also be due to cracks which occurred in the wood specimens, and it is also possible that the glue itself was too weak. An ESEM (Environmental Scanning Electron Microscope) -analysis showed that fracture occurred primarily in the glueline and not in the boundary layer. In addition to the ESEM analysis and the statistical evaluation, a theoretical FEM (Finite Element Method) -analysis has been used to explain the crack initiation in the second trial. Holzforschung 53 (1999) 93-103 Holzforschung / Vol. 53 / 1999 / No. 1

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