Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/20

between the paint and the plaster, in which respect it bears a strong resemblance to rust, the result of the oxidization of iron. Large faces of plaster are subject to fractures from expansion under the heat of the sun, or from the lifting the upper members of a building, consequent upon the admission of water from gutters or copings, the lifting being the result of secondary crystallization set up in the joints of mortar. This is an explanation of the fact that the blistering of paint always occurs in the neighborhood of cracks or fractures in the plaster, and is more pronounced in the cornice or upper part than in any other part of a building. In proof of its being the result of crystallization, the face of the plaster is always found to be covered over with powdered lime. The painter, finding this, takes care to saturate the disintegrated face of the framework in effecting repairs, but this, as he finds to his chagrin, is no protection against a recurrence of the evil, for so long as water or moisture is admitted at any point, so long will this abnormal blistering ensue. The blistering of paint upon plaster work, like that upon iron, is not dependent upon heat, it is a chemical action set up by water upon a body of dry lime in a partial state of crystallization, it is caused by the lime dissolving, and its removal—it may be but in an infinitesimal degree—and its recrystallization. Upon the water evaporating, the result is a dry powder that works an effectual separation between the film of paint and the ground-work of plaster, and it does not attach itself to either of the bodies, but remains a powder until the film of paint or blister is removed, when it may be dusted off with a brush. The blistering of paint upon wood is distinct in its order, and is the general blister known in the trade. It occurs on the face of woodwork exposed to the sun, and is traceable to the influence of heat. It is not pronounced in the case of new work, where the body of paint is not great; but it is a great evil and an eyesore on old work, where the coats of paint are layered one on the other. Wood, as a groundwork,