Oil paints are pigment mixed and mulled with oil, and especially designed to preserve, to protect, and to better the appearance of sur faces to which they are applied. In oil paints of value, the end aimed at is a close union of solids (pigment) and liquids (binder), i.e., inorganic or solid matter in a finely divided state is taken and mixed with organic or liquid matter, and then these are linked together either mechanically or chemically. Ordinary me chanical mixtures in which the solids and liquids have little or no affinity for each other, or in which the powders are feebly suspended in the liquid, scarcely deserve the name of paint. The essentials of a preservative and protective covering for structural steel may be stated as follows: 1. Mechanical Properties.— That it must work properly, that is, offer but a slight re sistance to the stroke of the brush, and be of such fluid nature as to flow together after the brush so that the resulting covering or skin is one of even thickness.
2. Chemical Properties.— That it must not only dry, or oxidize, fast enough, but dry simul taneously throughout; not harden on the sur face and remain soft underneath, or in painters' parlance, °skin over..
3. Physical Properties.—That it must be of such nature that when it has formed a skin upon the surface of the metal it can take other coats of paint without softening under them (hardness) ; that it must wear well, provided a sufficient number of coatings of it is applied (durability) ; that it must exclude moisture from the metal covered with it (imperviosity).
Linseed oil, after centuries of use, still holds its own as the best oil for painters' use where durability is the main consideration.
Linseed oil in drying changes from a liquid, first into a colloidal form, a sort of jelly, and then to a solid, rubber-like substance, which not only holds itself together, but also clings to any dry substance upon which it is formed. Lin seed oil in drying takes something from the air, namely, oxygen, and gives off something to the air, namely, carbon dioxide and water. Mulder beautifully describes the process and calls it °the breathing of the drying oils?' In his book, (Die Chemie der Austrocknenden Ode,' he named the solid, rubber-like substance into which a layer of linseed oil finally hardens (linoxyn.* Linoxyn is a solid, not a liquid. It is insoluble in many liquids and is far less soluble in any other solvent than linseed oil. Specimens of it kept for months in dilute acid and weak alkaline solutions, also in spirits of turpentine, petroleum, naphtha, linseed oil, alcohol, chloroform, acetone, carbon bisulphide, and water, show but slight decomposition or solution, thus indicating its power of resistance to atmospheric influences.
A layer of dried linseed oil (linoxyn) is not strictly waterproof, although no compound is probably chemically more resistant to atmos pheric influences (not mechanical wear). For example, it is claimed that a gallon of oil spread upon 100 square feet of surface will outwear a gallon of any paint spread upon the same area of a similar surface; but it may be noted that it will require about three times as many coatings of the oil to use up the gallon as it will of the paint to use it up. From this it is concluded that a layer of paint is about three times as thick as a layer of the oil. Experi ments of this kind demonstrate that one of the functions of the pigment is to increase the thick ness of the layer of dried paint, and that this increase of thickness is in direct proportion to the volume or fineness of the pigment. They also determine that, given the same volume of oil and the same weight of pigment, the greater the volume of the latter, i.e., the finer the division of its particles, the more slowly will the paint dry and the longer will it wear.
Pigments fit for use in structural oil paints are of two general classes, viz., those that re act more or less with the oil, notably the car bonates and all of the lead pigments, and those that have no chemical action on the oil or binder ,i.e., that are called inert, such as the carbons, silicates, probably ferric oxides. etc. The former class produce the quicker drying and the less durable paints; the latter, the slower drying and the more durable coatings; e.g., a paint made from white lead and linseed oil when properly applied to a sound surface cannot be expected to protect it, under fair conditions, more than five or six years, while a paint made from Venetian red and the same oil, applied to a like surface under similar con ditions, will protect it easily twice as long. Another feature pertaining to inert pigments is, that where the same liquids are used with them for the paint for each coating applied, the ob server finds that the dried paint seldom cracks, peels or blisters, and if it does the inference should be that his paint contained an inade quate amount of pigment. Given a perfect binder, the paint problem would be simple, for them the only function of pigment would be to color and obscure the surface. Until, however, a binder is obtained that will dry fast enough, that when dry is impervious to moisture and gases, and that will not wear out, pigment will be necessary to shield and to protect the dried oil from the agencies that destroy it, the chief of which are rainwater and sunshine.