Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/21

Rh is a porous body highly charged with moisture in a natural state, and never free from it in a so-called dry state when used in exposed situations. It may be taken that wood, during the winter season, or one-half of the year, is absorbing moisture. This is seen in outer doors, gates, sliding sashes, and shutters, as the carpenter is constantly being called into requisition to ease the same. This moisture, so largely present in the atmosphere, cannot be kept out of the wood by the most careful painting. In store fronts it has ready access to the back of the woodwork, the face sides being the only ones which are painted, in doors and gates it is absorbed from the sills or the ground, from the fact that the lower edges are unpainted. There is always some portion of the woodwork hidden from the eye which is unpainted, and there the system of absorption is active during the winter or rainy season. Wood in this state during the hottest days in summer will make efforts to throw off this moisture. Then the heat of the sun is applied with great force to the painted face, and the unpainted face is in the cold shade. The effect of this powerful heat is to draw the moisture to the face of the wood, where its course is arrested by the sundry impervious coats of paint, it is here generated into steam, the expansive power of which forces away the paint, and the familiar blister is formed. Paint, as a mineral or metallic body, does not incorporate with the wood, it simply adheres thereto, forcing its fronds, so to speak, in the pores of the wood, and filling up the interstices formed by the bundles of fibers. Hence we find that paint fails to adhere to highly resinous or greasy woods, and the knots themselves, from being hard and compact, must be faced with knotting composition as a ground for the paint.