The invention relates to a method for increasing strength and/or reducing
strength variations in multi-layer wood, plywood, and the like sandwich
material. The method comprises measuring wood veneers (10) for dry
substance density (.rho.) with high-frequency electromagnetic resonance
(TEM) and additionally analyzing the homogeneity and/or grain structure of
wood veneers from a wood veneer surface in the way of its darkness (R). If
the wood veneer comprises a number of local first regions, which are
darker than a predominant darkness of the veneer surface, the wood veneer
in question has its calculated dry substance density (.rho..sub.C)
established to be lower than its initially measured value (.rho..sub.M).
On the other hand, if the wood veneer has a substantially uniform
darkness, the wood veneer in question has the longitudinal and lateral
distribution of its dry substance density evaluated and, if those are
substantially uniform, the calculated dry substance density (.rho..sub.C)
of the wood veneer in question is established for a sorting process to be
higher than its initially measured value (.rho..sub.M). Wood veneers are
sorted on the basis of said calculated dry densities for at least two
different density categories. In a first density category (A), the dry
substance density is higher than in a second density category (B). Wood
veneers are laid on top of each other for multi-layer wood, plywood, or
some other sandwich material (20), the veneers included in the first
density category serving as surface veneers (13) and the veneers included
in the second density category serving as middle veneers (14).
