In the following lines are laid down some general rules that govern this phenomenon, and from the same draw some practical conclusions, the object of which will be to set the question at rest. The blistering of paint is in a large measure traceable to the position of the surface, it is usually found on work presenting a south aspect, or exposed to the full rays of the sun. As a defect it is associated with the summer season, winters being opposed to its action. The deduction to be drawn from this is that it is the effect of heat. Paint is a body both mineral and metallic, made into a plastic condition by oil, the object of which is to keep out the moisture from exposed surfaces in buildings, and to offer on internal work a uniform and pleasing surface to the eye. The oil used is linseed, which by boiling attains setting or drying qualities and becomes better by keeping, its thick or heavy nature when loaded with mineral and metallic matters being reduced for working purposes by spirits of turpentine, a volatile spirit that is a mere aid to the spreading of paint. Paint so largely composed of oil will never fairly set or assume a dry state.

However dry and brittle it may appear, it is capable of being rendered soft and plastic by the application of heat, and hence the hand stove of the painters is the most ordinary instrument for the removal of old paint. We mention this, for it is clear that, approach the subject as we will, we find heat the prime cause of the blistering of paint. Closing in with the subject, and bringing it into narrower lines, blistering, properly speaking, is wholly confined to wood as a base or groundwork. It is true it is not unknown to iron or plaster, but in these cases it is variant in form, and not blistering in the true sense of the term. The blistering of paint on iron is not traceable to 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 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 adm sion of water from gutters or copings, the lifting being t 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 craeks 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 groundwork 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, 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, 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.