HARDNESS OF WOOD.
Hardness is the ability of wood to resist indentations, and depends primarily upon the thickness of the cell walls and the smallness of the cell cavities, or, in general, upon the density of the wood structure. Summer wood, as we have seen, is much harder than spring wood, hence it is important in using such wood as yellow pine on floors to use comb-grain boards, so as to present the softer spring wood in as narrow surfaces as possible. See Handwork in Wood, p. 41, and Fig. 55. In slash-grain boards, broad surfaces of both spring and summer wood appear. Maple which is uniformly hard makes the best floors, even better than oak, parts of which are comparatively soft.
The hardness of wood is of much consequence in gluing pieces together. Soft woods, like pine, can be glued easily, because the fibers can be forced close together. As a matter of fact, the joint when dry is stronger than the rest of the board. In gluing hard woods, however, it is necessary to scratch the surfaces to be glued in order to insure a strong joint. It is for the same reason that a joint made with liquid glue is safe on soft wood when it would be weak on hard wood.[9]