Page:Cyclopedia of Painting-Armstrong, George D (1908).djvu/19

Rh the softening of the paint, and the shelling up of the same, but to water making its way to the naked iron through some crack or defect in the paint, and becoming an active agent in oxidization. The blister thus forced is clearly the separation of the film of paint from the iron by the formation of rust upon the face, which, as a foreign material, forms an effectual separation of the two bodies. The extension of these blisters is dependent upon the supply of water, and, unlike the true blister, is not dependent upon heat or a south or sunny aspect. The blistering of paint on iron occurs in any aspect or position, in the full light or in the dark,

in the summer or the winter, the destructive agent being water, it is dependent upon no other conditions. The blistering of paint upon plaster is in a large degree analogous to that of iron, inasmuch as it is formed by the disintegration of the base by the action of water. Painted plaster-work, so long as water can be kept from percolating through the cracks or faults, or gaining entrance from above by filtration, or from below by capillary action, is a highly durable material, but the moment water gains a footing the lime in some degree is dissolved, and, upon being removed and redeposited, undergoes the process of recrystallization, a powdery substance is thus formed that comes as a stranger