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
Since one pound of phosphorus takes up 1.33 pounds of oxygen and 0.9 pound of water, it is not surprising that phosphorus is one of the best smoke producers per pound of material. Comparison of the value of the two forms for shell purposes have invariably pointed to the superiority of the white variety.
In addition to its use as a smoke producer, it is used in incendiary shell and in tracer bullets. For incendiary purposes a mixture of red and white phosphorus is superior.
Chlorosulfonic Acid. Chlorosulfonic acid, ClSO₂OH, was first employed by the Germans to produce white clouds, both on land and on sea. For this purpose, they sprayed or dropped it onto quicklime, the reaction between it and the lime furnishing the heat necessary for volatilization, though in this way about 30 per cent of the acid is wasted.
Chlorosulfonic acid is obtained from sulfur trioxide and hydrogen chloride, which combine when gently heated:
SO₃ + HCl = ClSO₂OH
Fig. 86.—75 mm. White Phosphorus Shell.
2 seconds after bursting.
On a commercial scale, hydrogen chloride is passed into 20 per cent oleum, until saturation is reached. This is heated in a nitric acid still, when the chlorosulfonic acid distills over between 150°-160° C. With 30 per cent oleum, the conversion factor is about 42 per cent. The residue in the still is about 98 per cent sulfuric acid.
It forms a colorless liquid, boiling at 152° C., and having a density of 1.7.
Chlorosulfonic acid fumes in the air, because reaction with water forms sulfuric acid and hydrochloric acid.
ClSO₂OH + H₂O = H₂SO₄ + HCl
This material was not used by the United States since oleum was found superior.
Oleum. Oleum is a solution of 20 to 30 per cent sulfur trioxide (SO₃) in concentrated sulfuric acid. It has been used by the Germans to produce clouds on land and sea, by its contact with quicklime, and by the Americans for screening tanks and aeroplanes. Sulfur trioxide has been found to be superior as a shell filling. It is believed that the smoke producing power of oleum is due solely to its sulfur trioxide content, the sulfuric acid itself acting only as a solvent. The rather high freezing point of the oleum containing high percentages of sulfur trioxide is a disadvantage.
Sulfur Trioxide. Sulfur trioxide, SO₃, is a colorless mobile liquid, which boils at 46° C. and solidifies to a transparent ice-like mass, melting at 15° C. It is prepared by passing a mixture of sulfur dioxide and oxygen over finely divided platinum or other catalysts at a temperature between 400 and 450° C. Sulfur trioxide can only be used as a filler for shell and bombs, and is probably the best substitute for phosphorus.
Tin Tetrachloride. Tin tetrachloride, SnCl₄, is obtained by the action of chlorine on metallic tin. It is a liquid, boiling at 114° C., and having a density of 2.2. It fumes in the air, because it hydrolyzes to stannic hydroxide:
SnCl₄ + H₂O = Sn(OH)₄ + 4 HCl
It makes a better and more irritating smoke for shell and hand grenades, than either silicon or titanium tetrachlorides. Since there is practically no tin in this country, the other tetrachlorides were developed as substitutes.
Silicon Tetrachloride. Silicon tetrachloride, SiCl₄, is prepared from silicon or from impure silicon carbide by heating it with chlorine in an electric furnace. The raw material (silicon carbide) is a by-product in the manufacture of carborundum. It is a colorless liquid, boiling at about 58° C., and fumes in moist air, owing to hydrolysis:
SiCl₄ + 4 H₂O = Si(OH)₄ + 4 HCl
It is not very valuable in shell, though it is more effective on moist, cool days than on warm, dry ones. Its greatest use is found in the smoke cylinder, combined with ammonia. By the action of the moisture of the air, the following reaction takes place:
SiCl₄ + 4 NH₃ + 4 H₂O = Si(OH)₄ + 4NH₄Cl
The addition of a lachrymator gives a mixture which works well in hand grenades for mopping up trenches.
Titanium Tetrachloride. Titanium tetrachloride, TiCl₄, is made from rutile, TiO₂, by mixing with 30 per cent carbon and heating in an electric furnace. A carbonitride is formed, which is said to have the composition Ti₅C₄N₄, but the actual composition may vary from this to the carbide TiC. This product is heated to 600-650° C., and chlorine passed through, giving the tetrachloride. It is a colorless, highly refractive liquid, which boils at about 136° C., is stable in dry air and fumes in moist air. The best smoke is produced by using 5 parts of water to one of the tetrachloride, instead of the theoretical 4 parts [which would form Ti(OH)₄.] Since it is more expensive to manufacture and not as effective as silicon or tin tetrachloride, it is used only as an emergency material.
Berger Mixture. One of the most important smoke materials was the zinc-containing mixture, which was used in the smoke box, the smoke candle, certain of the smoke grenades and in various forms of colored smokes. The basis of this was the Berger Mixture, which had the composition:
| Zinc | 25 |
| Carbon tetrachloride | 50 |
| Zinc oxide | 20 |
| Kieselguhr | 5 |
This formula produced a light gray carbon smoke, with much carbon in the residue. In this mixture the zinc and carbon tetrachloride react to form zinc chloride and carbon; the kieselguhr keeps the mixture solid by absorbing the tetrachloride, while the zinc oxide is practically useless, as its absorbing power is small.
In order to accelerate the reaction and to oxidize the carbon, thereby changing the color of the smoke from gray to white, an oxidizing agent was added. Sodium chlorate was chosen for economic reasons. The reaction now proved to be too violent, and the zinc oxide was replaced by ammonium chloride. This cooled the smoke, retarded the rate of burning and added to the density of the smoke, since the obscuring power of the ammonium chloride is high. The kieselguhr was replaced by precipitated magnesium carbonate, which is as good an absorbent, gives a much smoother burning mixture, and also adds somewhat to the density of the smoke by virtue of the magnesium mechanically expelled. The mixture then had the composition:
| Zinc | 34.6 |
| Carbon tetrachloride | 40.8 |
| Sodium chlorate | 9.3 |
| Ammonium chloride | 7.0 |
| Magnesium carbonate | 8.3 |