Chemical Agents as Screening Smokes FIRST LIEUTENANT DOUGLAS G. NICHOLSON Chemical Warfare Service, United States Army
T.
HE SIGHT of smoke signifies a waste of fuel due to Improper combustion when viewed by the industrial-minded individual. To the forest ranger a smoke cloud signifies the presence of a forest fire. The military-minded individual views smoke as a material of definite value when properly used under specific tactical situations. From a physical point of view, smoke consists of a dispersion of minute particles in the atmosphere. Normally we think of these dispersed particles as being solid material. We should also remember that finely divided liquid particles, such as fog, have a screening effect and a "smoky" appearance. The use of smoke in warfare is not particularly recent in origin. A river crossing by the forces of Charles XI1 of Sweden was successfully effected in 1701 by the aid of a smoke cloud produced by burning damp straw. In more recent times both the Allied as well as the Axis powers have used screening smoke as an important aid in specific operations. Smoke was used by the Allied forces a t the battle of Loos in World War I, in an effort to "fill in" a time interval encountered in releasing the gas in a cylinder gas attack. Shortly before this attack was to "come off," i t was noted that the entire gas cloud wonld be released in a shorter time interval than that initially intended. Accordingly, gas and smoke were alternately released so as to utilize the entire planned time interval. The effect of this combination of gas and smoke on the enemy was very favorable. It was a new idea in tactics and caused considerable confusion in the enemy ranks. Since that time smoke has been given more and more consideration as a military material, in defensive as well as offensive situations. It is generally believed that an attacking force will suffer casualties about three times as numerous as those of a defending unit of equal strength. Data obtained in field studies have tended to indicate that a group of riflemen firing a t silhouette targets in the field obtain 55 per cent hits. When the same men fire a t the same targets blanketed in smoke they obtain but 12 per cent hits. On the other hand if the riflemen are covered with a smoke cloud and fire out a t the same targets they obtain only 3 per cent hits. These data tend to indicate that smoke placed on the enemy positions will make equal fire-power in the hands of the attacking troops about four times as effective as that of the enemy. Thus the use of smoke on the enemy positions more than offsetsthe increase in casualties normally expected by the attacking force. Figure 1 shpws these figures from a diagrammatic sketch.
n'3oaLl
N DEFENSIVE
3% HITS
SMOKE ON TARGETS
SMOKE ON FIRERS
T
HESE TROOPS MAKE 3% HlTS OFFENSNE FIRE IS FWR TIMES MORE EFFECTIVE THAN DEFENSIVE FIRE.
CHEMICAL MORTARS A-ENEMY LINE , 8-OUR L I N E , C-OUR TROOPS, 0-OUR SMOKE FIRING LINE.
Smoke is generally used: (a) to hinder enemy observation, (b) to reduce the effectiveness of hostile fire, and (c) to hamper hostile operations by causing confusion in the ranks. On the offensive, smoke may be used: (a) to screen the advance of a unit in attack, (b) to protect the flank of an attacking force, (c) to blind hostile observation and defense areas, (d) to screen movements of troops within a friendly position, (e) to deceive the enemy as to the place and direction of the attack, and to screen a river crossing or a landing force. Defensively, smoke may be used: (a) to permit the withdrawal of troops exposed to enemy observation and fire, (b) to cover changes in the disposition of friendly troops within our own lines, (c) to support counterattacks, (d) to screen enemy observation posts, and (e) to screen rear area installations from aerial attack. There are several materials which have been used or are capable of being used for the production of smoke. These materials mav be divided into two classes.. (a) .. those which f o m a smoke (opaque or light reflecting
499
material) by means of a thermal reaction, and (b) those which consist .of substances which react with atmospheric moisture forming a hydrate or addition compound. In some cases the final dispersed particles are formed as a result of both of these reactions. The most common oi our smoke-produciug munitiuns are: (1) , hexachlorethane-nowdcred zinc mixture, ( 2 ) white phosphorus, (3) chl&sulfonic acid-sulfur trioxide mixture, (4) titanium tetrachloride, and (5) oil (partial combustion) or vaporization (fog-producing) units. The hexachlorethane-zinc smoke-producing units (Chemical Warfare Service symbol, HC) contain a mixture of finely divided zinc and hexachlorethane. The hexachlorethane (CzCls) is a solid that melts a t 184°C. and sublimes a t 185OC. The smoke has no physiological effect in normal concentrations; however, when encountered in heavy concentrations (by unmasked personnel), it may cause discomfort. I n addition to the two above-mentioned materials, the HC units contain some ammonium perchlorate to oxidize the carbon released in the basic chemical reaction [see equation (I)]. Ammonium chloride is also present to control the rate of burning and to aid in keeping down the temperature of the unit. A small amount of calcium carbonate is also present to prevent the decomposition of the C2Cls,as well as to stabilize the mixture. The essential reaction which takes place in these units is represented by the following equation:
.
3Zn
+ C.CIa
-
3ZnCL
+ 2C
(1)
Various-sized containers are available for this smokeproducing munition. The time of burning depends on the size of the container and the mission for which i t was intended. The smoke pot shown in Figure 2 produces a dense cloud of white smoke for 12 to 16 minutes.
Fv;um? :l-IfC
SMUKKP w . >I I .
I N ACTIOX
Since the essential reaction taking place in the HC units is exothermic, a slight vertical rise in the smoke cloud is produced by the convection currents present. With a steady breeze of 3 to 12 miles per hour, the initial vertical rise is not objectionable. The smoke forms a white cloud which drifts downwind. While HC smoke is easily controlled and is of particular value for training purposes, i t should not be considered as the most opaque (smoke of greatest obscuring power per unit weight) screening tnaterial. White phosphorus has an obscuring power slightly less than twice (actually 'I4) that of the HC mixture. White phosphorus (Chemical Warfare Service symbol, WP) is a solid which melts a t 44.2"C. It reacts with atmospheric oxygen, forming phosphorus pentoxide a t ordinary as well as elevated temperatures. It bursts into flame a t a comparatively low temperature. This burning material produces a dense white smoke, and if thrown from exploding shells or bombs will cause severe burns on personnel present. The essential chemical reaction which takes place in the burning of WP and in the production of W P smoke is indicated below: 4P
-
+ 5 0 ~ ZP30r
(3)
Since the particles of P206are very hygroscopic, they react with atmospheric moisture according to either or both of the following equations: P20a H1O 2HPOs (4)
-
+
F E ~ 2.-DIAGRAM R
OP THE
HPOI
HC SMOKE-PRODUCING UNIT
HC smoke has found extensive use in training programs. It is often used to simulate toxic gases. It is quite possible that the individual particles of the HC smoke are hydrated forms of ZnCL and not the anhydrous material as indicated in equation (1). Hydration would take place by union of the particles of the anhydrous material with atmospheric moisture according to equation (2). ZnCL
+ xH20
-
ZnCl,sH10
(2)
+ HsO
+ HIPO,
(5)
Thus a cloud .of W P smoke consists of particles of P20sas well as meta- and ortho-phosphoric acids. Since there is considerable heat evolved by the burning phosphorus in the formation of pentoxide, the resulting heated air tends to cause a considerable portion of the cloud of smoke to rise. Thus, bursting WP bombs and shells are accompanied by a rapidly rising smoke cloud as well as a brilliant ~vrotechnic disalay. . - (Each of the flying particles of phosphorus produces streamer of smoke.)
..
a
Burns on personnel caused by WP are very painful, slow in healing, and rather easily infected. Because of these effects, WP is often classified as an incendiary and casualty munition as well as a smoke-producing material. Figure 4 shows a photograph of a bursting WP shell. A solution of 55 per cent of sulfur trioxide in 45 per cent of chlorsulfonic acid is used as a liquid smokeproducing material. It has the Chemical Warfare Service symbol FS. Its obscuring power is about '/I, greater than that of HC smoke mixture. While the chlorsulfonic acid (HClSOa) has fair smoke-producing qualities, the sulfur trioxide is much more effective in this respect. The essential reactions involved in the production of smoke by FS are:
-
so8+ HSO HClS08
+ HnO
H~O,
H.80,
+ HC1
of our large rear-area installations, we have two types of smoke materials available. One of these produces a gray-colored smoke by incomplete combustion of a fuel oil. This smoke is most effective in protecting and concealing an area or installation from a night bombardment. An extremely white fog consisting of oil and water is capable of being produced by a special type of reararea generator. This unit can produce ap effective fog screen from 2 to 4 miles downwind from the point a t which it was produced. Specific wind and terrain conditions alter the effective range of this cloud. While this latter type of rear-area smoke production has been designed primarily for noncombat locations, there is no real objection to its use in the protection of specific units on or near the front.
(6)
.(7)
The moisture utilized in the two above equations is obtained from atmospheric water vapor. I t is readily seen that this material should normally produce a heavier cloud if laid on a damp day, or if laid over water. I t is quite obvious that this smoke consists of minute droplets of sulfuric acid, and thus should have some corrosive effect on some of our more active metals. Likewise, while it is considered a harmless agent, a high concentration tends to cause a tingling-burning sensation when it touches the skit], and extremely high concentrations have a definite corrosive action on clothing, skin, and metals. FS, like HC, has been used extensively as a trainiug smoke to simulate some of the more toxic agents. Since i t is a liquid, it can be sprayed from aircraft as well as loaded into shells and bombs. In contrast, WP is utilized only as a filling for shells or bombs. FM is the Chemical Warfare Service symbol which is associated with the liquid screening-smoke material titanium tetrachloride (Ticla). This liquid melts a t -30°C. and boils a t 135°C. When pure i t is a waterclear liquid, while the commercial product has an amber color. This material reacts vigorously with either water or water vapor, forming a white smoke of orthotitanic acid [titanium hydroxide Ti(OH)r]. FM smoke has an obscuring power slightly lower than that of the HC mixture. Thus i t has the lowest value of any of the materials discussed up to this point. Since the reaction given in equation (8) indicates the necessity of large amounts of water for the production of this smoke, it follows that FM is a very satisfactory smoke-producingmaterial on bodies of water or in areas adjacent to large bodies of water. The smokes described above are primarily designed for combat screening missions. For the concealment
All of the above-described smokes are classified as screening agents which are nontoxic. (WP is also classified as an incendiary and casualty agent.) Colored smoke grenades have been recently developed for use in recognition of ground units by friendly aircraft. These materials have been developed by the Chemical Warfare Service and are available in seven colors: red, orange, blue, green, violet, yellow, and black. Brigadier General Waitt has described the construction and use of these materials in a recent publication.' The above discussion is intended only as a brief summary of the essential composition of the materials used, and of the reactions encountered in the production of screening smoke clouds. There is much more information required as well as a great deal of data involved in the successful laying of either area or screen smoke clouds. This material is not available for release a t the present time. -
i ~ ~ ~ T "Cdored T , smokes for identification." The Infnntry Jnurnol. LII, 6, 41 (1943).
GRAPHIC REPRESENTATION OF THE - .. Q U A L I T A T I V E SCHEME OF ANALYSIS FOR TUF MORE COMMON AN I O N S
pro. 1 (a)
502
I
I
\
\
. ., GRAPHIC REPRESENTATION OF THE QUALITATIVE SCHEME OF ANALYSIS FOR THE MORE COMMON CATIONS
PIC. 2 (a)
504
Fm. 2 (b)
505
