Cell walls shrinkage

One might anticipate that if the water in the cell wall were to be frozen and then the ice were to be subliminated off there should be no liquid capillary tension, no cell wall shrinkage and it should be possible to create a porous cell wall. However, sublimating the water molecules from the cell wall at -20°C does not prevent collapse of the internal pore structure (Merchant, 1957). This implies that the cell wall water is not actually frozen at this temperature the cell wall still shrinks and very little internal surface is created. Indeed there is evidence (Tarkow, 1971) that at least some adsorbed water does not lose the mobility characteristic of the liquid phase until very low temperatures (<-80°C). Of course water in the lumens behaves like bulk water and freezes at a temperature between -0.1°C and -2.0°C, depending on the concentration of dissolved sugars in the sap. 

The types of chemical treatments involved in the conservation of archaeological wood are (i) lumen-filling treatments that fill the spaces within the wood with an inert chemical to provide structural support and prevent collapse, (ii) bulking treatments that enter the cell walls and reduce cell wall shrinkage, and (iii) surface coatings that cover the surface of a dry object. 

The conservation of waterlogged wood involves removal of water, as well as improving mechanical properties. The simplest drying technique is to remove the object from water and allow it to dry. This technique will usually result in unacceptable amounts of dimensional change (i.e., a combination of cell wall shrinkage and cellular collapse). Some type of chemical pretreatment is usually applied before drying to improve the dimensional stability of the waterlogged wood. For damp woods, pretreatments may not be required. 

This method has not, however, worked well. Solvent drying does not stop all cellular collapse. In addition, it does nothing to reduce cell wall shrinkage. Even if it is successful, there is a real possibility that the shape of the object will not be representative of its original dimensions (16). Solvent drying alone has apparently never been applied successfully. 

Water thus virtually adds its volume to that of the cell walls, further indicating that the extent of voids in the dry cell walls must be virtually negligible. Shrinkage due to bulking is reduced merely because there is less moisture to be lost. 

Wood contains water inside its cells and in between cells. When wood is cut (dead), it first loses the free water, and then, more slowly, the cell wall water. This all causes 3-dimensional shrinkage. Ideally, wood should be seasoned, or dried slowly to the point of equilibrium (free water is gone, but there is no loss of cell wall water). If wood is dried too quickly, it may split or crack, because the outer portion is drying and contracting at a different rate from the middle. The outside compresses the more moist inside, which causes the outer part to break. These same reactions can take place if seasoned wood is saturated and then dried too quickly. 

It follows that the gain or loss of water or other liquids or vapors into or out of wood tissue can be influenced greatly by the nature, amount, and distribution of wood polysaccharides. As will be discussed in Chapter 3, these same sorption phenomena (i.e., adsorption and desorption), together with the architectural arrangement of wood cells in the tree, are responsible for particular patterns in wood swelling and shrinkage when wood is subjected to various environments. The arrangement of polysaccharide molecules within the cell wall itself, especially that of the cellulose, also has a profound effect on the physical and mechanical properties of individual cells and wood as a whole. 

As a consequence of its hydrophilicity, wood tissue will seek to maintain, through either gain or loss of moisture, an equilibrium moisture content with the surrounding atmosphere. If the wood takes on water, the cell walls proceed to swell until the cell walls become water-saturated. The latter moisture content is called the wood s fiber saturation point. In contrast, loss of wood water (below the fiber saturation point), due to diffusion and evaporation, results in wood shrinkage. 

Any procedure for wood specific gravity or density based on measuring weight and volume can be considered accurate only if the wood sample has been first extracted with suitable organic solvents to remove extraneous resins, oils, fats, gums, etc. (27). These materials bulk cell walls, block potential sites for water adsorption/ab-sorption, alter potential wood swelling/shrinkage, and thereby interfere with the accurate characterization of wood tissue. 

However, when trees are felled and the cell walls lose moisture, shrinkage occurs in proportion to the extent of loss of this bound water. Because wood in use is generally exposed to cycling relative... 

Conversely it can be shown that the percent shrinkage of the cell wall is given by Equation 7... 

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