flat-sawn boards or panels are held flat and restrained from attempted cup-
ping, cracking may result along the grain into the concave face.
Aless obvious source of cupping in painting panels is compression
shrinkage. The causal mechanism is typified by a panel painted only on the
face side, its unpainted back therefore exposed to much more rapid mois-
ture sorption. In the case of such a panel originally coated with gesso and
painted when the wood was at a fairly low MC, subsequent exposure to
high humidity causes the wood at the back surface to adsorb moisture
and go into compression set. If the panel were mounted by fastening at
its edges, the expected cupping concave to the painted surface would be
largely restrained. Upon restoration of a normally low humidity condition,
the rear of the panel now manifests its compression shrinkage and short-
ens; the panel then cups concave to the unpainted surface. This mecha-
nism is commonly the real source of cupping that has been attributed to
tangential/radial shrinkage and “drying out.” Uneven compression shrink-
age can overshadow the effects of tangential/radial shrinkage and can also
produce cupping in radially cut panels that would otherwise remain flat
under simple moisture cycling.1 The term greenas applied to wood suggests the moisture condition in the living tree or in
freshly cut timber. However, because many important properties, such as dimension and
strength, are unchanged by loss of free water, green wood is taken as any condition of MC
above the FSP.Chudnoff, Martin
1984 Tropical Timbers of the World. Agriculture handbook no. 607. Washington, D.C.:
U.S. Department of Agriculture, Forest Service.Princes Risborough Laboratory
1972 Handbook of Hardwoods. 2d ed. Rev. R. H. Farmer. London: Her Majesty’s
StationeryOffice.1977 A Handbook ofSoftwoods. London: Her Majesty’s Stationery Office.U.S. Department of Agriculture
1987 Wood Handbook: Wood as an Engineering Material. Agriculture handbook no. 72.
Madison, Wisc.: U.S. Forest Products Laboratory.References
Note
20 Hoadley