Solid Wood Products 2659 



for linear expansion data for fiberboards and section 24-10 for a discussion of 

 linear expansion of flakeboards). In solid wood, linear expansion parallel to the 

 grain is near zero. From green to ovendry, pine-site hardwoods average about 

 5.1 percent shrinkage radially across the grain and about 9.7 percent tangentially 

 across the grain; low-density woods tend to shrink and swell less than those of 

 high density (tables 8-9 and 8-12). 



Lutz et al. (1976) evaluated five fibrous sheet materials for potential value as 

 crossbands. None of the synthetic crossbands was as stable or strong in maxi- 

 mum tensile load as 1/20-inch yellow-poplar veneer in the grain direction. 



SURFACE CHECKING IN VENEERED FURNITURE PANELS 



Development of cracks or checks on the veneered surface of furniture, cabi- 

 nets, and wall panelling is a major problem for producers of these items. Checks 

 perpendicular to the grain or in an irregular or net pattern are usually defects of 

 the finish coating and are not further discussed here. 



Surface checks parallel to the grain are usually attributable to shrinkage of the 

 face veneer in response to changes in its moisture content. Species with uniform 

 anatomical structure, such as birch {Betula sp.) offer the most resistance to 

 checking. Oak, with its large pores and wide rays has least resistance to finish 

 checking; the wood rays and the boundaries between rays and vertical tissue are 

 common avenues for extension of lathe checks to the surface. Large pores, with 

 their maximum diameter in the direction of the thickness of flat-grain veneers, 

 are frequently included in the path of developing checks (Keith 1964). 



Keith (1964) and Jayne (1953) found that veneer should be cut with minimum 

 penetration of lathe checks, since surface checks are traceable to their presence. 

 Keith (1964) concluded that face veneers should be no thicker than about 1/20- 

 inch, and that all panel components should be conditioned to a low moisture 

 content before assembly; face veneers should be below 6 percent moisture 

 content, core moisture can be somewhat greater. Veneer surfaces containing the 

 lathe checks (the loose side) should be placed against the core, excess moisture 

 should not be introduced with the glue, and excessive sanding of the assembled 

 panel should be avoided. After assembly, during machining and finishing, 

 panels should not be allowed to increase in moisture content above about 8 

 percent. During storage, shipment, and use, exposure to atmosphere of high 

 relative humidity should be avoided. 



22-10 STRUCTURAL WOOD 



Softwoods dominate the market for structural wood. There are, however, 

 some applications where hardwoods prevail — most notably for crossties, mine 

 timbers, and highway posts. There are also economic trends developing that 

 encourage consideration of some hardwoods (e.g., yellow-poplar) for structural 

 lumber and a range of hardwoods for structural plywood and reconstituted 

 panels. 



