It is of importance to observe the action of a drying oil, of which linseed oil is the most used, and can be taken as the best example.

It is seldom used alone, but mixed with resins as varnish, or with pigments to form paint. These mixtures, varnish and paint, are applied to a variety of surfaces, such as wood, metal, plaster and stone, substances differing materially in their physical characteristics.

While, of course, the decorative effect of varnish and paint is of great moment, the protective effect, in most cases, is the important feature, as the action of rain, moisture, heat and air, cause wood to discolor and become fibrous, or rot and mould. Unpainted metal, exposed to atmospheric conditions will rapidly rust or oxidize, particularly iron. The surface rust holds the moisture and the iron is rapidly corroded until it may be, in great measure, destroyed.

Plaster is very absorbent and quickly takes up moisture and though it will dry again, the numerous changes causes it to scale or disintegrate and thus be destroyed.

Stone surfaces, in some cases, through similar agencies, will scale or discolor and become unsightly. A layer of oil paint applied to these surfaces, by protecting them from external influences, will prevent all this in great measure, while the coating remains in good condition as a protective covering. With absorbent substances, such as wood and plaster, added protection is given in that some of the paint is absorbed by the pores, and becomes a part of the substance to which the paint is applied.

Now, what is the influence of atmospheric and other external agents on the protective coating of drying oil itself? In the first place, the drying oil, whether in varnish or paint, gradually hardens by the absorbtion of oxygen from the air and the action of heat, whether from the sun's rays or other sources, until at length, the coating loses its binding qualities, and the varnish or paint disintegrates and perishes.

The duration of the life of the coating will depend on the quality of the drying oil and the action or non-action of the resins or pigments with which it is combined, also by the proportion of oil used.

The greater the proportion of oil, the more lasting the coating will be.

Metals, being non-porous, and having the paint entirely on the surface, the coating is shorter lived than on more porous substances.

Of the other agencies which work destruction to the oil may be mentioned rain, fog, and snow, air currents, dust, dirt, or sand, smoke gases and the nature of the pigment used. The practical life of paint on metal work exposed to the elements is scarcely more than two years, while on wood or plaster, five years is the most that can be reasonably expected. The action of rain, snow, and fog, aided by wind currents, can be readily understood by observing the side of a house which is exposed to the quarter from which prevailing storms come.

Wind will also drive dust and sand with more or less force against the protected surfaces and the mechanical action of such substances is much the same as that of the sand blast, though, of course, far less severe.

The action of smoke gases, as has been mentioned before, is of a chemical nature, due to the formation of sulphurous acid from the chemical action of the gases, and steam, which rapidly attacks the oil binder. The action of certain pigments on the oil is very marked and has been previously referred to.

Red lead forms a hard solid coating when combined with drying oil, due to chemical action of the pigment in causing a more rapid oxidation, or drying of the oil and the added fact, that in such paint, less oil is used than for most other pigments. Oil is the life of the varnish or paint.

The pigments in ordinary use require the following proportion of oil to form a substantial paste of about equal consistency before thinning:

White lead,.........8 to 10 per cent.

Red lead,........... 6 to 8 per cent.

Oxide of zinc,..... 15 to 20 per cent.

Baiytes, ............... 8 per cent.

Chalk, ................ 20 per cent.

Chrome green,.....10 to 12 per cent.

Oxide of iron,......10 to 12 per cent.

Umbers, ....................20 to 30 per cent.

Ochres,.......... 15 to 25 per cent.

Siennas,........ . 30 to 40 per cent.

Chrome Yellows,... 12 to 18 per cent.

These proportions vary some, however, as pigments of the same nature differ considerably. Crystalline pigments in general, take less oil than those of a clayey nature. Dried paint will be quickly destroyed by all alkaline solutions, such as potash, soda, or ammonia; acids will also destroy paint, especially those which fume or give off vapor. Heat is a destroyer of paint or varnish. The higher the temperature, the more rapid the action. The protective film becomes rapidly hard and brittle, loses all elasticity and pliability and when changes in temperature occur, causing expansion, or contraction of the substance coated, the covering, no longer elastic, cracks and scales. Thus, we account for much cracking on old painted woodwork and iron.

Heat also yellows white paint and darkens light shades, as it causes the oil to yellow or darken.

We have noted, in general, the destructive agencies to which varnish and paint is subjected. Let us observe the specific troubles met with, first in varnish, and then paint in its order; considering the necessary requirements to form varnish and paint of a satisfactory nature.

Varnish should be pale in color. It should be clear and transparent, and of such consistency as to be easily applied with a brush. The varnish when dry should have the appearance of glass. It should be flexible and elastic, and show no defects when dry. In applying varnish, many defects are observable, and their origin is often hard to trace. These defects may arise from faulty manufacture, defective surfaces on which the varnish is spread, changes in temperature, dampness, or by being improperly applied.