Raw Materials for Smoke Clouds
It is obvious that only gases or substances capable of being brought into the vapor state or into a very fine state of subdivision can be used for producing smoke clouds. The reaction product, of which the smoke particles consist, should preferably be:
(a) Solid. Otherwise the particles will tend to grow in size by condensation of the liquid particles present in the cloud.
(b) Non-volatile. If volatile, the particles will disappear by evaporation as the cloud is diluted by air currents. Larger particles will also form at the expense of the smaller ones.
(c) Non-deliquescent. If the particles are deliquescent, they will tend to grow by condensation of water vapor upon them.
(d) Stable towards the usual components of the atmosphere, especially moisture.
While it might seem that it would be difficult to fulfill these conditions, there are several chemical compounds which have been successfully used as smoke producers. This does not mean that they fulfill all the conditions, but they represent a compromise between the various requirements.
Phosphorus. One of the earliest materials to be used in smoke clouds was phosphorus. This is prepared on a commercial scale by heating phosphate rock (which contains calcium phosphate) with sand and coke in an electric furnace. Phosphorus occurs in two forms, white and red. White phosphorus, which is formed when the vapor of the substance is quickly cooled, is, in the pure state, almost colorless, melts at 44° C., boils at 287° C., is readily soluble in various solvents, and is luminous in the air, at the same time emitting fumes (the oxidation product, phosphorus pentoxide). On gentle warming in the air, it takes fire and burns with a brightly luminous flame. Red phosphorus is obtained by heating white phosphorus out of contact with the air, to a temperature of 250° to 300° C. Red crusts then separate out from the colorless liquid phosphorus, and almost the entire amount is gradually converted into a red, solid mass. If this is freed by suitable solvents from the small amounts of unchanged white phosphorus, a dark red powder is obtained, which remains unchanged for a long time in the air, does not appreciably dissolve in the solvents for white phosphorus, does not become luminous, and can be heated to a fairly high temperature without igniting. Further, red phosphorus is not poisonous, while white phosphorus is highly so.
Either form burns to phosphorus pentoxide, which is converted by the moisture of the air to phosphoric acid,
4P + 5O₂ = 2 P₂O₅
2P₂O₅ + 6H₂O = 4H₃PO₄