Wood for forms must be of a kind that is easily worked and that will
retain its shape when exposed to the weather. White pine is the best
wood, but is seldom used because of its cost. Spruce, yellow pine, and
fir are satisfactory woods for forms and are best, used partially green
or unseasoned.
The edges of boards should be surfaced, tongued and, grooved, or beveled
in order to obtain a tight form, so that the soft mortar will not ooze
out. A better surface* is secured if the boards are dressed on one side
and are free of loose knots or other imperfections.
As forms must be removed, they should be so planned that they can be
taken down without destroying the lumber, especially if the boards are
used for sheathing or again for forms. Therefore the nailing of the
boards to the support should be only sufficient to keep them in place
until the concrete has hardened. Greasing the surface next to the
concrete with crude oil, soap solution, or linseed oil will prevent the
concrete from adhering and facilitate removal.