PAINT. A paint may be defined as a fluid suspension of finely divided solids which, when applied to a surface, will "dry" or set to an opaque film, either by oxidation or evaporation.
A paint consists of a mixture of (i) solid pigment (or a mixture of pigments), and (ii) a liquid medium (known as the vehicle).
The vehicle may be linseed or other drying oil; a water solution of casein or glue; a natural or synthetic resin solution in volatile solvents; a cotton ester solution in volatile solvent ; or a natural or synthetic oil varnish base reduced for application with volatile solvent. The oil or other film forming material is added to furnish a binder for the pigment, and the volatile material to give the paint the proper flowing consistency, and to promote ease of application. When an oil paint is applied, the solvent, being volatile, rapidly evaporates from the painted surface, leaving behind the pigment and oil mixture as a wet coating; this mixture gradually dries into an elastic solid skin owing to the oil ab sorbing oxygen from the air.
The solidified oil acts as a binder for the pigments and holds them in their place; a tough paint film is thus formed which adheres firmly to the surface, and serves both to protect and decorate.
A good quality of paint should be easily applied, cover well, and be opaque, or as it is generally described, possess good hiding power. The hiding power of a paint is dependent on the amount and nature of the pigments it contains in suspension; and, as a rule, the finer the state of subdivision of the particles of the pigment the greater the hiding power. The durability of a paint is influenced greatly by the nature of the liquid portion, hence only pure drying oils or compounded vehicles of known per formance should be used in its preparation. It is sometimes stated that the oil is the life of the paint. If this were true, oil alone should be more durable than paint, which is far from being the case, since, in fact, oil films do not wear as well as paint films because the addition of pigments to the oil films tends to reinforce them and to make them dry harder and become more impervious to air and moisture. Ordinary paints should dry
with a glossy surface, but they can be made to dry with a flat finish by decreasing the oil content and increasing the proportion of turpentine or other volatile thinner.
The durability of an exterior paint can only be determined by practical exposure tests over a period of years. While raw or untreated drying oil is generally recommended, there are many special cases where oil processing and the addition of special resins is advantageous.
On the other hand, the use of mineral oils, rosin oils, rosin varnishes or other soft resin varnish mediums, may cause the rapid deterioration of paint films on exposure, resulting in the paint's cracking and chipping off after a comparatively short exposure.
The protective and anti-corrosive properties of paint vary greatly with its pigment as well as its vehicle composition. For example, house paints are most satisfactorily formulated from lead, zinc, and titanium oxide mixtures, properly balanced with lesser amounts of extender or suspending inerts. Where it is de sired to protect iron work, it is advisable to take advantage of the rust inhibitive properties of such pigments as red lead or the lead and zinc chromates.
Paints on exposure over a period of years may chalk (powder off), check, crack, blister or peel ; the colour of the original paint may also change or even completely disappear. A high-class durable paint for outdoor use should wear well over a period of four or five years, retain its colour, and chalk only to a moderate degree. Paints which blister or crack are unsatisfactory inasmuch as it is then necessary to remove completely all the loose and badly adhering paint before any repainting can be done.
In this article pigments are first described, then the mediums or vehicles by which these pigments are converted into paints, and lastly the paints themselves.