In this category are included all the surfaces that show occasional pin knots, burly spots, and waves from spike knots. All of these marks of natural growth should be considered beauty marks and not defects, except in places where they might impair structural strength.

VENEER MATCHING

Owing to the wide diversity of figure types and to the fact that adjacent veneers from the same flitch can be matched to make symmetrical designs, there is hardly any limitation to the variety of patterns which can be produced. The art of matching veneers has been developed to such a high degree of perfection that the wellinformed designer may obtain almost any desired effect and may realize the full value of every wood. Thus, the exquisite wood panels of modern homes, radio cabinets, or fine furniture, which we so enjoy, are achieved. Three general methods of matching veneers can be considered as basic-book-match, end-match, and slide-match. To obtain additional designs, combinations or variations of these basic methods are resorted to by ingenious matchers. Such variations include diamond match, reverse diamond, herringbone, checkerboard, marginal block, and other effects conceived by the designer.

In book-matching, two adjacent sheets of veneer, as they were cut from the flitch, are opened like a book and then taped side by side. End-matching is similar to book-matching except that the ends instead of the sides of the two sheets of veneer are used as a hinge, and they are taped end to end.

Book-match and end-match combined form a four-way match. Slide matching is accomplished by sliding the top sheet of two sheets of veneer, arranged in the order in which they were cut from the flitch, into a side-by-side position.

In the process of matching, the veneers may have to be squared, or clipped at an angle, or cut into a number of small segments, or treated in some other manner, depending upon the result desired. Since each sheet carries a pattern almost identical to that of the sheet next to it, they must all be cut in exactly the same place. Sometimes a third, a half, or in rare cases only a fourth of each sheet of veneer can be used, merely because only a small portion of its pattern would help form the perfect design needed for a beautifully matched table top or dresser front.

CORE AND CROSS-BANDING

Wood is not a homogeneous material, but has physical and strength properties that vary in direction within itself. With changes in its moisture content, wood shrinks or swells, mostly across the grain; shrinkage and swelling parallel to the grain is practically negligible for most species. Similarly, the strength properties of wood also vary in relation to the direction of the growth rings in a species. In building up plywood with core and cross-bandings, as described in an earlier section, a balanced construction is attained, producing almost perfect equality of these strength and physical properties in two directions-parallel with and perpendicular to the edge of the panel. The greater the number of plies used for a given panel thickness, the more homogeneous in properties is the finished panel.

Plywood cores may be of veneer or of lumber, the choice depending primarily on the use to which the panel is to be put. Veneer cores

Figure 23.-Diamond-match quarter-cut African mahogany, with border of quarter-cut

satinwood.

may be specified for panels to be constructed on sharp curves, or when structural requirements call for a fine balance of strength in all directions along the face of the panel. In plywood for such purposes as desk tops, lumber cores are usually employed.

Lumber cores are ordinarily built up of narrow pieces of lumber, edge-glued together and planed after drying, to insure uniform thickness. Cross-bandings are practically always of rotary-cut veneer.

HARDWOOD PLYWOOD ADHESIVES

The use of adhesives to bind together pieces of wood dates back to ancient times. The tomb of King Tutankhamen revealed numerous articles bearing evidence of the use of glue for both veneering and joint work. Little is known of the character of these glues; and, although they may have served their purpose exceedingly well, their efficiency cannot be compared with the adhesives science has made available today. Recently developed glues are stronger than the wood itself, producing a bond between the glue and the wood of greater affinity than even that which the fibers of the wood have for each other.

The ease or difficulty with which satisfactory joints are made is dependent on the density and the structure of the wood, the presence of extractives or infiltrated materials, and the kind of glue. In general, heavy woods are more difficult to glue than lightweight woods, hardwoods are more difficult than softwoods, and heartwood more difficult than sapwood. Regardless of these differences, a satisfactory bond can be secured through proper preparation of the elements making up the plywood and through the selection of the right binding agent. Endless tests and controls are made by plant technicians before the kind of glue is selected for use.

In modern plywood practice, consideration is given to four types of glue-vegetable glue, animal glue, casein glue, and synthetic resins. In these four classifications there are various subdivisions of quality; and while for certain purposes glue requirements are standard, the experienced plywood manufacturer has wide latitude in his selection. A further division among glues classifies them as hot-pot, cold-press, and hot-press. In the fabrication of plywood, the hot-pot glue has almost disappeared. Among the cold glues, fine vegetable and casein glues have general acceptance.

Vegetable and animal glues.-The fine vegetable glues are produced from carefully graded and scientifically controlled cassava flour and are processed in the United States. The casein glues are made from the solid precipitate of skimmed milk, mixed with lime and other chemicals. The use of animal glues, including blood albumen, in the hardwood plywood field is now restricted to special-purpose panels; and the place of these adhesives has largely been taken by synthetic resins, which are among the latest developments in chemical engineering.

Synthetic resins.-These resins are the strongest adhesives ever developed for the woodworking trade. They are divided into two general classifications-phenol formaldehydes and ureas. Phenol formaldehydes are formed by subjecting phenol (crude carbolic acid) to the fumes of formaldehyde. They are known as hot-press resins, owing to the fact that they require combined heat and pressure to

Figure 28.-Hot plate press.

Resin glues rely for their adhesive quality on the chemical

change effected by simultaneous application of heat and pressure. Once these glues have set, they are waterproof and rot proof. The introduction of phenol and urea resins has greatly expanded the uses for hardwood plywood,