Lignin skeleton wood structure

Although colored extracts may be separated from most woods by extraction with suitably neutral solvents, it is generally impossible to remove all the coloring matter in this way. This fact means that a considerable part of the coloring matter in wood may be present in macromolecular (insoluble) form, or so firmly bound to the skeleton components - lignin or polysaccharides - that it is not extract-able by means of neutral solvents without destroying the wood structure. Their chemistry is frequently very complex, and few of them have been investigated or elucidated in detail. 

The wood structure after long immersion—which, in the case of archaeological wood, may be hundreds or thousands of years—is swollen. The lignin and ash content of the wood increases relative to the cellulosic portions as immersion time increases. The skeleton of waterlogged, swollen lignin and any remaining cellulosics has free water within the cells. This water is strengthening and prevents collapse. 

In the end, all that is left of the former wood is the frail system of middle lamellae containing small amounts of still-crystalline cellulose. Disintegrating lignin residues are enclosed in this cellular and brittle wood skeleton. As long as it stays waterlogged—that is, bulked with and supported by water—this structure has a certain physical strength. It still carries the anatomical characteristics of the wood it once was. 

Lignins. Lignins are amorphous, cross-linked phenolic polymers that occur uniquely in vascular plants and comprise 20-30% of most wood. Lignins isolated from wood are polydisperse, with molecular weights in the range of thousands to hundreds of thousands (4). Lignins are produced almost exclusively from three cinnamyl alcohols, whose structures are shown in Chart I. These structural units have propylphenyl carbon skeletons and differ... 

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