Luminous paints, as used by the decorator, are of two kinds, ‘ Phosphorescent ‘ (more commonly referred to as ‘ Luminous ‘) and ‘ Fluorescent.’ Phosphorescent coatings are those which absorb light and continue to glow for an appreciable period after the light source which excites them has been extinguished. Fluorescent coatings, on the other hand, will emit some useful light only during activation by ‘ black ‘ light, 1.e. ultra-violet light. In discussing them it should be noted that we are not concerned with luminous preparations containing radio-active pigments, such as are used on the figures and hands of clocks and watches; the very high cost of such preparations makes them of no interest to the decorating trade.
Phosphorescent or luminous paints may themselves be sub-divided into two classes – those which give a relatively bright afterglow of short duration and those which give a longer but less bright afterglow. For the first class, zinc sulphide or zinc and cadmium sulphide are the pigments most employed; for the second, calcium sulphide, strontium sulphide, or combinations of the two, are commonly used. The medium may be oil or cellulose varnish.
All pigments for phosphorescent paints are comparatively coarse in particle size, since the property of phosphorescence depends mainly on a coarse, crystalline structure; consequently they cannot be ground in a tightly-set roller mill. Either a very loosely-set mill must be used or the pigments must be stirred or mixed in the medium. The range of colours in which they are available is limited to a greenish-yellow phosphorescence for the first class, and a bluish green for the second; in daylight, the colour is a dull white.
Owing to their coarseness, phosphorescent paints have poor gloss, do not flow well, and tend to settle hard and quickly. Best results are obtained if they are applied over a white-base coat, such as a good zinc white; on no account must the undercoat contain any white lead, either as a pigment or in the form of a drier. For outside exposure, the luminous paint must be protected by a coat of clear varnish or lacquer, or it will soon disintegrate. The protective coating will involve some loss of luminescence.
Fluorescent paints are made chiefly on a zinc sulphide or combined zinc and cadmium sulphide base. A range of colours extending from green, through yellow to deep red is available, but the colours in visible light do not correspond to those which appear when the coating is activated by ‘ black ‘ light. They are applied in a similar manner to phosphorescent paints. Special lamps are manufactured for activating surfaces painted with fluorescent paints; they are obtainable with low wattage – sufficiently so to be connected to most lighting circuits.
Fluorescent paints have been fairly extensively employed in the theatre, to obtain unusual staging effects and for advertising signs and similar purposes wherever black light activation is possible. It is highly probable that they will be increasingly used to procure striking decorative effects in public buildings and private houses. Phosphorescent paints, because of their more limited range of colours and more subdued glow, are less spectacular but, as wartime conditions showed, they have their uses in providing a certain amount of illumination at low cost, where the more ordinary means of lighting are undesirable or cannot be employed.