CHAPTER XIX
SIGNAL SMOKES
The success of pyrotechnics in night signalling led, during the World War, to considerable attention being paid to the development of pyrotechnic signals for day use. This was mainly directed to the production of distinctive smokes, which should have the same long range visibility under varying light conditions. Since a gray or white smoke might be confused with the smoke produced accidentally by the explosion of shell, it was necessary to use smoke of definite and unmistakable colors, and red, blue, yellow, green and purple smokes were developed. During the early part of the war, only a yellow smoke was in use, though others were added later.
Production of Colored Smokes
There are three possible ways of obtaining signal smokes.
- I. Mechanically dispersing solids.
- II. Chemical Reaction.
- III. Volatilization of colored materials.
I. The first method, while possible, can never be an efficient method of producing signals. Some success was met with in dispersing certain inorganic materials, as rouge, and ultramarine, in projectiles fired from a 3-inch mortar and exploded by a time fuse arrangement at the height of their flight. Various mixtures were also tried, such as antimony oxysulfide and aluminum powder (red), arsenic and antimony trichlorides with sodium thiosulfate (yellow), etc., but these compositions have the disadvantages of being liable to catch fire if dispersed by a black powder explosion.
II. While colored smokes may be produced by chemical reaction, such as the combination of hydrogen iodide (HI), chlorine and ammonia, the clouds are not satisfactory as signals. In this particular case, the purple cloud (to the operator in the aeroplane) appeared white to the observers on the ground.
High temperature combustion smokes have also been studied. These are used in the so-called smoke torches. The yellow arsenic sulfide smoke is the most widely used. Most formulas call for some sulfide of arsenic (usually the native realgar, known commercially as “Red Saxony Arsenic”), sulfur, potassium nitrate, and in some cases, a diluent like ground glass or sand. A typical mixture consists of:
| Red arsenic sulfide | 55% |
| Sulfur | 15% |
| Potassium nitrate | 30% |
A very similar smoke may be obtained from the following mixture: