Luminous Paints.

Manufacturer and Builder 3, 1893

By J. E. Smith.

Nearly every one has heard of luminous paint — the sulphide of calcium — but it is probable that comparatively few persons know much about the behavior of this interesting compound.

When of good quality, it is quite white as seen by reflected light, but the light that is emitted by it in the dark immediately after exposure to the direct rays of the sun is quite blue, and the emitted light is of a lavender hue directly after subjection to the action of ordinary diffused daylight. Both of these colors, however, in a dark room, rapidly fade into a white light that is more luminous. A greater luminosity is produced by a short and near exposure to an ordinary artificial light, or by being placed near a window about sunset on a rainy day. The direct rays of a bright full moon falling on, it for several minutes has very little effect, making it barely visible in a dark room.

After ten seconds' exposure to good diffused daylight, which is as effective as an exposure of ten hours, this substance will give out a practical light for ten or twelve hours, and its luminosity will not entirely disappear in less than thirty hours. This great difference in the times required for the absorption and the emission of light is quite remarkable, and makes it seem as if the light emitted were many times greater than that absorbed.

When luminous paint of poor quality is removed from light to darkness, the light emitted by it fades rapidly and in a few minutes becomes of a dull reddish or smoky color, much like that of the moon during its total eclipse.

A temperature of 300° or 400°' will not put calcium sulphide into a luminous condition, though after exposure to light an increase in temperature of 25° will make it much more luminous. That this is not a conversion of heat into light is shown by the fact that if kept at a high temperature it will become non-luminous in a shorter time. As might be expected, a lowering of temperature by ether or other volatile liquid will diminish the luminosity. The effect of temperature may be strikingly shown by partly filling with hot water a tin pail that has a coat of luminous paint on the outside. The heat of the water will make the lower part of the pail glow much more brightly than the upper part, and the waterline can be easily traced on the outside of the vessel when it is tipped in any direction.

This luminous condition is not conveyed from particle to particle like heat. If a quantity of the dry powder be exposed to the light all day, on breaking through the surface the interior will be found to be nonluminous, the light having affected the outer portion to a depth of perhaps a sixty-fourth of an inch. If a bottle partly filled with the dry powder be revolved in the light until the whole mass has become luminous, and then be set away in the dark, the interior loses its light as rapidly as the surface, but in doing so does not help the surface to glow any longer or more brightly. What becomes of the light of the interior? Does it change into heat? Perhaps some physicist with facilities for delicate measurements can answer these questions. This non-conductivity of light admits of the production of some impressive effects. If the hand, with fingers spread, be held against a flat surface of luminous paint while exposed to the light, a black hand on a luminous field will be seen. If, however, the painted surface, while acted on by light, be well covered with a card having an opening of the size and form of a hand, and then moved about in a dark room, nothing will be seen but a white, floating specter hand. Forms of various articles may be thus shown; but perhaps the most pleasing effect is produced by a piece of lace drawn tightly over the paint while in the light. After the removal of the lace in the dark, all the designs will appear in black on a white ground.

Some text books on chemistry say that calcium sulphide is luminous when recently prepared. These books were probably written before the manufacture of this compound was well understood.

As a matter of fact, the luminous property of this substance is known to have remained unimpaired for more than five years.