As paints go, aluminium paint is a comparatively recent development, but its decorative and protective properties are being increasingly recognised, and it is finding wider application each year. It consists, of course, of aluminium bronze powder mixed with a suitable vehicle, and it has certain unique properties which distinguish it from other forms of paint.
The powder, for example, unlike most other pigments, is not ground, but is simply stirred into the vehicle. In this condition it exhibits the phenomenon usually described as ‘ leafing,’ 1.e. a proportion of the microscopic flakes, of which the bronze powder is composed, swirls round, and though actually heavier than the liquid, they are held up by surface tension, forming what is in effect a film of metal, to the presence of which the paint owes much of its durability.
Aluminium paint can be purchased ready for use, though in this form it sometimes possesses certain disadvantages. These are a tendency to thicken, and also to lose its ‘ leafing ‘ property on standing, while if certain vehicles are allowed to remain mixed with the powder for any length of time they are liable to darken. This difficulty has been overcome by at least one manufacturer by means of a special container so contrived as to keep the vehicle separate from the powder content so long as the tin remains unopened.
Mixing: Linseed oil cannot be regarded as an altogether satisfactory medium for aluminium paint. Raw oil is generally too low in plasticity to allow the paint to set properly, especially on a vertical surface. Boiled oil is somewhat better in this respect, but the ordinary variety seems often to lack sufficient body, and artificially bodied oils give better results. For general purposes, however, a long oil varnish is probably the most successful medium.
The proportion of aluminium bronze powder to medium is approximately 2 lb. per gallon; if, however, a spirit varnish is used, about two-thirds of this quantity of powder is usually sufficient.
Application: Aluminium paints can be applied either by brush or by spray. They spread smoothly under the brush and the ‘ leafing ‘ property referred to above tends to level out the paint, and, consequently, to eliminate brush marks. It is unnecessary and undesirable to brush the paint well out as when, for example, a white-lead paint is being employed. The container should be frequently stirred during application of the paint. If a spray is used, the container should be fitted with an agitator to ensure that the bronze powder remains in suspension.
Properties: Aluminium paint possesses certain properties which make it particularly useful from the decorator’s point of view. It has exceptional opacity, and, if a good quality be employed, it will cover most surfaces quite satisfactorily in one coat. It has also high reflectivity – a fact which, in the United States, has caused it to be extensively employed for the protection of the vast tanks in the oilfields, which can thus be maintained at a relatively low temperature. It is widely used for furnaces, and, in the home, for hot-water pipes and radiators, though opinions differ as to whether it does not, to a certain extent, impede heat radiation.
Its durability and powers of resistance in severe atmospheric conditions have led to its extensive use for piers and other ironwork at the seaside, while many municipal authorities specify it for lamp-posts and similar structures.
While it can be successfully applied on most surfaces, it makes an especially valuable primer for wood, and is being increasingly employed in this country for this purpose. It has another use which is also insufficiently appreciated: if an ordinary oil paint is applied directly over a ‘ bleeding ‘ red or over a bituminous paint, the red or the bituminous material will almost inevitably work its way through and discolour the new finish. A coat of aluminium paint, however – preferably made with a varnish vehicle – will tend to act as a buffer and will appreciably decrease the risk of the new finish being affected. In the same way, aluminium paint can be used as a sealer for resinous areas in woodwork.
Tinting: Aluminium paint is normally produced only in the natural aluminium colour, and though the powder can be dyed, it is rather an expensive process. The addition of dry pigments, such as red oxide, chrome green, etc., will, however, produce attractive shades which have a distinctive character of their own.
In purchasing aluminium paint or aluminium bronze powder, it should be remembered that, as with practically every other material, there are many different qualities, and the best will generally be found to be the cheapest in the long run.
This is frequently used as a less-expensive substitute for gilding work in genuine gold leaf. The ‘ gold ‘ powder is aluminium or zinc with a proportion of copper or bronze; the finer the grade the less granulated the appearance of the paint film. The medium is as described above for aluminium paint. Unless protected by a coat of varnish or lacquer, the film will not long retain its initial brightness when exposed to the air.
The same remarks apply to the so-called ‘ silver ‘ paints, the powder in this case being aluminium or zinc.