Nitro Explosives: A Practical Treatise by P. Gerald Sanford

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[Footnote A: "Cellulose," by Cross and Bevan, ed. by W.R. Hodgkinson, p.
9.]

The nitrate which goes under the name of gun-cotton is generally supposed
to be the hexa-nitrate, and to contain 14.14 per cent. of nitrogen; but a
higher percentage than 13.7 has not been obtained from any sample. It is
almost impossible (at any rate upon the manufacturing scale) to make pure
hexa-nitro-cellulose or gun-cotton; it is certain to contain several per
cents. of the soluble forms, i.e., lower nitrates. It often contains as
much as 15 or 16 per cent., and only from 13.07[A] to 13.6 per cent. of
nitrogen.

[Footnote A: Mr J.J. Sayers, in evidence before the court in the "Cordite
Case," says he found 15.2 and 16.1 per cent. soluble cotton, and 13.07 and
13.08 per cent. nitrogen in two samples of Waltham Abbey gun-cotton.]

A whole series of nitrates of cellulose are supposed to exist, the highest
member being the hexa-nitrate, and the lowest the mono-nitrate. Gun-cotton
was at one time regarded as the tri-nitrate, and collodion-cotton as the
di-nitrate and mono-nitrate, their respective formula being given as
follows:--

Mono-nitro-cellulose C_{6}H_{9}(NO_{2})O_{5} = 6.763 per cent. nitrogen.
Di-nitro-cellulose C_{6}H_{8}(NO_{2})_{2}O_{5} = 11.11 " "
Tri-nitro-cellulose C_{6}H_{7}(NO_{2})_{3}O_{5} = 14.14 " "

But gun-cotton is now regarded as the hexa-nitrate, and collodion-cotton
as a mixture of all the other nitrates. In fact, chemists are now more
inclined to divide nitro-cellulose into the soluble and insoluble forms,
the reason being that it is quite easy to make a nitro-cellulose entirely
soluble in a mixture of ether-alcohol, and yet containing as high a
percentage of nitrogen as 12.6; whereas the di-nitrate[A] should
theoretically only contain 11.11 per cent. On the other hand, it is not
possible to make gun-cotton with a higher percentage of nitrogen than
about 13.7, even when it does not contain any nitro-cotton that is soluble
in ether-alcohol.[B] The fact is that it is not at present possible to
make a nitro-cellulose which shall be either entirely soluble or entirely
insoluble, or which will contain the theoretical content of nitrogen to
suit any of the above formulae for the cellulose nitrates. Prof. G. Lunge
gives the following list of nitration products of cellulose:--

[Footnote A: The penta-nitrate C_{12}H_{15}O_{5}(NO_{3})_{5} = 12.75 per
cent. nitrogen.]

[Footnote B: In the Cordite Trial (1894) Sir F.A. Abel said, "Before 1888
there was a broad distinction between soluble and insoluble nitro-
cellulose, collodion-cotton being soluble (in ether-alcohol) and
gun-cotton insoluble." Sir H.E. Roscoe, "That he had been unable to make a
nitro-cotton with a higher nitrogen content than 13.7." And Professor G.
Lunge said, "Gun-cotton always contained soluble cotton, and _vice
versa_." These opinions were also generally confirmed by Sir E. Frankland,
Sir W. Crookes, Dr Armstrong, and others.]

Dodeca-nitro-cellulose C_{24}H_{28}O_{20}(NO_{2})_{12} = 14.16 per cent.
nitrogen. (= old tri-nitro-cellulose)
Endeca-nitro-cellulose C_{24}H_{29}O_{20}(NO_{2})_{11} = 13.50 per cent.
nitrogen.
Deca-nitro-cellulose C_{24}H_{30}O_{20}(NO_{2})_{10} = 12.78 per cent.
nitrogen.
Ennea-nitro-cellulose C_{24}H_{31}O_{20}(NO_{2})_{9} = 11.98 per cent.
nitrogen.
Octo-nitro-cellulose C_{24}H_{32}O_{20}(NO_{2})_{8} = 11.13 per cent.
nitrogen. (= old di-nitro-cellulose)
Hepta-nitro-cellulose C_{24}H_{33}O_{20}(NO_{2})_{7} = 10.19 per cent.
nitrogen.
Hexa-nitro-cellulose C_{24}H_{34}O_{20}(NO_{2})_{6} = 9.17 per cent.
nitrogen.
Penta-nitro-cellulose C_{24}H_{35}O_{20}(NO_{2})_{5} = 8.04 per cent.
nitrogen.
Tetra-nitro-cellulose C_{24}H_{36}O_{20}(NO_{2})_{4} = 6.77 per cent.
nitrogen. (= old mono-nitro-cellulose)

It is not unlikely that a long series of nitrates exists. It is at any
rate certain that whatever strength of acids may be used, and whatever
temperature or other conditions may be present during the nitration, that
the product formed always consists of a mixture of the soluble and
insoluble nitro-cellulose.

Theoretically 100 parts of cotton by weight should produce 218.4 parts of
gun-cotton, but in practice the yield is a good deal less, both in the
case of gun-cotton or collodion-cotton. In speaking of soluble and
insoluble nitro-cellulose, it is their behaviour, when treated with a
solution consisting of 2 parts ether and 1 of alcohol, that is referred
to. There is, however, another very important difference, and that is
their different solubility in nitro-glycerine. The lower nitrates or
soluble form is soluble in nitro-glycerine under the influence of heat, a
temperature of about 50 deg. C. being required. At lower temperatures the
dissolution is very imperfect indeed; and after the materials have been
left in contact for days, the threads of the cotton can still be
distinguished. The insoluble form or gun-cotton is entirely _insoluble_ in
nitro-glycerine. It can, however, be made to dissolve[A] by the aid of
acetone or acetic ether. Both or rather all the forms of nitro-cellulose
can be dissolved in acetone or acetic ether. They also dissolve in
concentrated sulphuric acid, and the penta-nitrate in nitric acid at about
80 deg. or 90 deg. C.

[Footnote A: Or rather to form a transparent jelly.]

The penta-nitrate may be obtained in a pure state by the following
process, devised by Eder:--The gun-cotton is dissolved in concentrated
nitric acid at 90 deg. C., and reprecipitated by the addition of concentrated
sulphuric acid. After cooling to 0 deg. C., and mixing with a larger volume of
water, the precipitated nitrate is washed with water, then with alcohol,
dissolved in ether-alcohol, and again precipitated with water, when it is
obtained pure. This nitrate is soluble in ether-alcohol, and slightly in
acetic acid, easily in acetone, acetic ether, and methyl-alcohol,
insoluble in alcohol. Strong potash (KOH) solution converts into the
di-nitrate C_{12}H_{18}O_{8}(NO_{3})_{2}. The hexa-nitrate is not soluble
in acetic acid or methyl-alcohol.

The lower nitrates known as the tetra- and tri-nitrates are formed
together when cellulose is treated with a mixture of weak acids, and
allowed to remain in contact with them for a very short time (twenty
minutes). They cannot be separated from one another, as they all dissolve
equally in ether-alcohol, acetic ether, acetic acid, methyl-alcohol,
acetone, amyl acetate, &c.

As far as the manufacture of explosive bodies is concerned, the two forms
of nitro-cellulose used and manufactured are gun-cotton or the hexa-
nitrate (once regarded as tri-nitro-cellulose), which is also known as
insoluble gun-cotton, and the soluble form of gun-cotton, which is also
known as collodion, and consists of a mixture of several of the lower
nitrates. It is probable that it chiefly consists, however, of the next
highest nitrate to gun-cotton, as the theoretical percentage of nitrogen
for this body,. the penta-nitrate, is 12.75 per cent., and analyses of
commercial collodion-cotton, entirely soluble in ether-alcohol, often give
as high a percentage as 12.6.

We shall only describe the manufacture of the two forms known as soluble
and insoluble, and shall refer to them under their better known names of
gun-cotton and collodion-cotton. The following would, however, be the
formulae[A] and percentage of nitrogen of the complete series:--

Hexa-nitro-cellulose C_{12}H_{14}O_{4}(NO_{3})_{6} 14.14 per cent.
nitrogen.
Penta-nitro-cellulose C_{12}H_{15}O_{5}(NO_{3})_{5} 12.75 per cent.
nitrogen.
Tetra-nitro-cellulose C_{12}H_{16}O_{6}(NO_{3})_{4} 11.11 per cent.
nitrogen.
Tri-nitro-cellulose C_{12}H_{17}O_{7}(NO_{3})_{3} 9.13 per cent.
nitrogen.
Di-nitro-cellulose C_{12}H_{18}O_{8}(NO_{3})_{2} 7.65 per cent.
nitrogen.
Mono-nitrocellulose C_{12}H_{19}O_{9}(NO_{3}) 3.80 per cent.
nitrogen.

[Footnote A: Berthelot takes C_{24}H_{40}O_{20} as the formula of
cellulose; and M. Vieille regards the highest nitrate as
(C_{24}H_{18}(NO_{3}H)_{11}O_{9}). _Compt. Rend._, 1882, p. 132.]

~Properties of Gun-Cotton.~--The absolute density of gun-cotton is 1.5.
When in lumps its apparent density is 0.1; if twisted into thread, 0.25;
when subjected, in the form of pulp, to hydraulic pressure, 1.0 to 1.4.
Gun-cotton preserves the appearance of the cotton from which it is made.
It is, however, harsher to the touch; it is only slightly hygroscopic (dry
gun-cotton absorbs 2 per cent. of moisture from the air). It possesses the
property of becoming electrified by friction. It is soluble in acetic
ether, amyl acetate, and acetone, insoluble in water, alcohol, ether,
ether-alcohol, methyl-alcohol, &c. It is very explosive, and is ignited by
contact with an ignited body, or by shock, or when it is raised to a
temperature of 172 deg. C. It burns with a yellowish flame, almost without
smoke, and leaves little or no residue. The volume of the gases formed is
large, and consists of carbonic acid, carbonic oxide, nitrogen, and water
gas. Compressed gun-cotton when ignited often explodes when previously
heated to 100 deg. C.

Gun-cotton kept at 80 deg. to 100 deg. C. decomposes slowly, and sunlight causes
it to undergo a slow decomposition. It can, however, be preserved for
years without undergoing any alteration. It is very susceptible to
explosions by influence. For instance, a torpedo, even placed at a long
distance, may explode a line of torpedoes charged with gun-cotton. The
velocity of the propagation of the explosion in metallic tubes filled with
pulverised gun-cotton has been found to be from 5,000 to 6,000 mms. per
second in tin tubes, and 4,000 in leaden tubes (Sebert).

Gun-cotton loosely exposed in the open air burns eight times as quickly as
powder (Piobert). A thin disc of gun-cotton may be fired into from a rifle
without explosion; but if the thickness of the disc be increased, an
explosion may occur. The effect of gun-cotton in mines is very nearly the
same as that of dynamite for equal weights. It requires, however, a
stronger detonator, and it gives rise to a larger quantity of carbonic
oxide gas. Gun-cotton should be neutral to litmus, and should stand the
Government heat test--temperature of 150 deg. F. for fifteen minutes (see page
249). In the French Navy gun-cotton is submitted to a heat test of 65 deg. C.
(= 149 deg. F.) for eleven minutes. It should contain as small a percentage of
soluble nitro-cotton and of non-nitrated cotton as possible.

The products of perfectly detonated gun-cotton may be expressed by the
following equation:--

2C_{12}H_{14}O_{4}(NO_{3})_{6} = 18CO + 6CO_{2} + 14H_{2}O + 12N.

It does not therefore contain sufficient oxygen for the complete
combustion of its carbon. It is for this reason that when used for mining
purposes a nitrate is generally added to supply this defect (as, for
instance, in tonite). It tends also to prevent the evolution of the
poisonous gas, carbonic oxide. The success of the various gelatine
explosives is due to this fact, viz., that the nitro-glycerine has an
excess of oxygen, and the nitro-cotton too little, and thus the two
explosives help one another.

In practice the gases resulting from the explosion of gun-cotton are--
Carbonic oxide, 28.55; carbonic acid, 19.11; marsh gas (CH_{4}), 11.17;
nitric oxide, 8.83; nitrogen, 8.56; water vapour, 21.93 per cent. The late
Mr E.O. Brown, of Woolwich Arsenal, discovered that perfectly wet and
uninflammable compressed gun-cotton could be easily detonated by the
detonation of a priming charge of the dry material in contact with it.
This rendered the use of gun-cotton very much safer for use as a military
or mining explosive.

As a mining explosive, however, gun-cotton is now chiefly used under the
form of tonite, which is a mixture of half gun-cotton and half barium
nitrate. This material is sometimes spoken of as "nitrated gun-cotton."
The weight of gun-cotton required to produce an equal effect either in
heavy ordnance or in small arms is to the weight of gunpowder in the
proportion of 1 to 3, i.e., an equal weight of gun-cotton would produce
three times the effect of gunpowder. Its rapidity of combustion, however,
requires to be modified for use in firearms. Hence the lower nitrates are
generally used, or such compounds as nitro-lignose, nitrated wood, &c.,
are used.

The initial pressure produced by the explosion of gun-cotton is very
large, equal to 18,135 atmospheres, and 8,740 kilogrammes per square
centimetre for 1 kilo., the heat liberated being 1,075 calories (water
liquid), or 997.7 cals. (water gaseous), but the quantity of heat
liberated changes with the equation of decomposition. According to
Berthelot,[A] the heat of formation of collodion-cotton is 696 cals. for
1,053 grms., or 661 cals. for 1 kilo. The heat liberated in the total
combustion of gun-cotton by free oxygen at constant pressure is 2,633
cals. for 1,143 grms., or for 1 kilo. gun-cotton 2,302 cals. (water
liquid), or 2,177 cals. (water gaseous). The heat of decomposition of gun-
cotton in a closed vessel, found by experiment at a low density of charge
(0.023), amounts to 1,071 cals. for 1 kilo. of the substance, dry and free
from ash. To obtain the maximum effect of gun-cotton it must be used in a
compressed state, for the initial pressures are thereby increased. Wet
gun-cotton s much less sensitive to shock than dry. Paraffin also reduces
its liability to explode, so also does camphor.

[Footnote A: "Explosives and their Power," trans. by Hake and M'Nab.]

The substance known as celluloid, a variety of nitro-cellulose nearly
corresponding to the formula C_{24}H_{24}(NO_{3}H)_{8}O_{12}, to which
camphor and various inert substances are added, so as to render it
non-sensitive to shock, may be worked with tools, and turned in the lathe
in the same manner as ivory, instead of which material celluloid is now
largely used for such articles as knife handles, combs, &c. Celluloid is
very plastic when heated towards 150 deg. C., and tends to become very
sensitive to shock, and in large quantities might become explosive during
a fire, owing to the general heating of the mass, and the consequent
evaporation of the camphor. When kept in the air bath at 135 deg. C.,
celluloid decomposes quickly. In an experiment (made by M. Berthelot) in a
closed vessel at 135 deg. C., and the density of the charge being 0.4, it
ended in exploding, developing a pressure of 3,000 kilos. A large package
of celluloid combs also exploded in the guard's van on one of the German
railways a few years ago. Although it is not an explosive under ordinary
circumstances, or even with a powerful detonator, considerable care should
be exercised in its manufacture.

~The Manufacture of Gun-Cotton.~--The method used for the manufacture of
gun-cotton is that of Abel (Spec. No. 1102, 20. 4. 65). It was worked out
chiefly at Stowmarket[A] and Waltham Abbey,[B] but has in the course of
time undergone several alterations. These modifications have taken place,
however, chiefly upon the Continent, and relate more to the apparatus and
machinery used than to any alteration in the process itself. The form of
cellulose used is cotton-waste,[C] which consists of the clippings and
waste material from cotton mills. After it has been cleaned and purified
from grease, oil, and other fatty substances by treatment with alkaline
solutions, it is carefully picked over, and every piece of coloured cotton
rag or string carefully removed. The next operation to which it is
submitted has for its object the opening up of the material. For this
purpose it is put through a carding machine, and afterwards through a
cutting machine, whereby it is reduced to a state suitable for its
subsequent treatment with acids, that is, it has been cut into short
lengths, and the fibres opened up and separated from one another.

[Footnote A: The New Explosive Co. Works.]

[Footnote B: Royal Gunpowder Factory.]

[Footnote C: Costs from L10 to L25 a ton. In his description of the
"Preparation of Cotton-waste for the Manufacture of Smokeless Powder," A.
Hertzog states that the German military authorities require a cotton which
when thrown into water sinks in two minutes; when nitrated, does not
disintegrate; when treated with ether, yields only 0.9 per cent. of fat;
and containing only traces of chlorine, lime, magnesia, iron, sulphuric
acid, and phosphoric acid. If the cotton is very greasy, it must be first
boiled with soda-lye under pressure, washed, bleached with chlorine,
washed, treated with sulphuric acid or HCl, again washed, centrifugated,
and dried; if very greasy indeed a preliminary treatment with lime-water
is desirable. See also "Inspection of Cotton-Waste for Use in the
Manufacture of Gun-cotton," by C.E. Munro, _Jour. Am. Chem. Soc._, 1895,
17, 783.]

~Drying the Cotton.~--This operation is performed in either of two ways.
The cotton may either be placed upon shelves in a drying house, through
which a current of hot air circulates, or dried in steam-jacketed
cylinders. It is very essential that the cotton should be as dry as
possible before dipping in the acids, especially if a wholly "insoluble"
nitro-cellulose is to be obtained. After drying it should not contain more
than 0.5 per cent. of moisture, and less than this if possible. The more
general method of drying the cotton is in steam-jacketed tubes, i.e.,
double cylinders of iron, some 5 feet long and 1-1/2 foot wide. The cotton
is placed in the central chamber (Fig. 10), while steam is made to
circulate in the surrounding jacket, and keeps the whole cylinder at a
high temperature (steam pipes may be coiled round the outside of an iron
tube, and will answer equally well). By means of a pipe which communicates
with a compressed air reservoir, a current of air enters at the bottom,
and finds its way up through the cotton, and helps to remove the moisture
that it contains. The raw cotton generally contains about 10 per cent. of
moisture and should be dried until it contains only 1/2 per cent. or less.
For this it will generally have to remain in the drying cylinder for about
five hours. At the end of that time a sample should be taken from the
_top_ of the cylinder, and dried in the water oven (100 deg. C.[A]) for an
hour to an hour and a half, and re-weighed, and the moisture then
remaining in it calculated.

[Footnote A: It is dried at 180 deg. C. at Waltham Abbey, in a specially
constructed drying chamber.]

[Illustration: FIG. 10.--COTTON DRIER.]

It is very convenient to have a large copper water oven, containing a lot
of small separate compartments, large enough to hold about a handful of
the cotton, and each compartment numbered, and corresponding to one of the
drying cylinders. The whole apparatus should be fixed against the wall of
the laboratory, and may be heated by bringing a small steam pipe from the
boiler-house. It is useful to have a series of copper trays, about 3
inches by 6 inches, numbered to correspond to the divisions in the steam
oven, and exactly fitting them. These trays can then be taken by a boy to
the drying cylinders, and a handful of the cotton from each placed in
them, and afterwards brought to the laboratory and weighed (a boy can do
this very well), placed in their respective divisions of the oven, and
left for one to one and a half hours, and re-weighed.

When the cotton is found to be dry the bottom of the drying cylinder is
removed, and the cotton pushed out from the top by means of a piece of
flat wood fixed on a broom-handle. It is then packed away in galvanised-
iron air-tight cases, and is ready for the next operation. At some works
the cotton is dried upon shelves in a drying house through which hot air
circulates, the shelves being of canvas or of brass wire netting. The hot
air must pass under the shelves and through the cotton, or the process
will be a very slow one.

~Dipping and Steeping.~--The dry cotton has now to be nitrated. This is
done by dipping it into a mixture of nitric and sulphuric acids. The acids
used must be strong, that is, the nitric acid must be at least of a
gravity of 1.53 to 1.52, and should contain as little nitric oxide as
possible. The sulphuric acid must have a specific gravity of 1.84 at 15 deg.
C., and contain about 97 per cent. of the mono-hydrate (H_{2}SO_{4}). In
fact, the strongest acids obtainable should be used when the product
required is gun-cotton, i.e., the highest nitrate.

The sulphuric acid takes no part in the chemical reaction involved, but is
necessary in order to combine with the water that is liberated in the
reaction, and thus to maintain the strength of the nitric acid. The
reaction which takes place is the following:--

2(C_{6}H_{10}O_{5}) + 6HNO_{3} = C_{12}H_{14}(NO_{3})_{6} + 6 H_{2}O.
324 378 = 594 108.
Cellulose. Gun-Cotton.

Theoretically,[A] therefore, 1 part of cellulose should form 1.8 part of
gun-cotton. Practically, however, this is never obtained, and 1.6 lb. from
1 lb. of cellulose is very good working. The mixture of acids used is
generally 1 to 3, or 25 per cent. nitric acid to 75 per cent. sulphuric
acid.

[Footnote A: (594 x 1)/324= 1.83.]

[Illustration: FIG. 11.--TANK FOR DIPPING COTTON.]

[Illustration: FIG. 12.--THE COOLING PITS.]

The dipping is done in cast-iron tanks (Fig. 11), a series of which is
arranged in a row, and cooled by a stream of cold water flowing round
them. The tanks hold about 12 gallons, and the cotton is dipped in
portions of 1 lb. at a time. It is thrown into the acids, and the workman
moves it about for about three minutes with an iron rabble. At the end of
that time he lifts it up on to an iron grating, just above the acids,
fixed at the back of the tank, where by means of a movable lever he gently
squeezes it, until it contains about ten times its weight of acids (the 1
lb. weighs 10 lbs.). It is then transferred to earthenware pots to steep.

[Illustration: FIG. 13.--COTTON STEEPING POT.]

~Steeping.~--The nitrated cotton, when withdrawn from the dipping tanks,
and still containing an excess of acids, is put into earthenware pots of
the shape shown in Figs. 12 and 13. The lid is put on, and the pots placed
in rows in large cooling pits, about a foot deep, through which a stream
of water is constantly flowing. These pits form the floor of the steeping
house. The cotton remains in these pots for a period of forty-eight hours,
and must be kept cool. Between 18 deg. and 19 deg. C. is the highest temperature
desirable, but the cooler the pots are kept the better. At the end of
forty-eight hours the chemical reaction is complete, and the cotton is or
should be wholly converted into nitro-cellulose; that is, there should be
no unnitrated cotton.

[Illustration: FIG. 14.--HYDRO-EXTRACTOR.]

~Whirling Out the Acid.~--The next operation is to remove the excess of
acid. This is done by placing the contents of two or three or more pots
into a centrifugal hydro-extractor (Fig. 14), making 1,000 to 1,500
revolutions per minute. The hydro-extractor consists of a machine with
both an inner cylinder and an outer one, both revolving in concert and
driving outwardly the liquid to the chamber, from which it runs away by a
discharge pipe. The wet cotton is placed around the inner cone. The
cotton, when dry, is removed, and at once thrown into a large tank of
water, and the waste acids are collected in a tank.[A]

[Footnote A: Care must be taken in hot weather that the gun-cotton does
not fire, as it does sometimes, directly the workman goes to remove it
after the machine is stopped. It occurs more often in damp weather. Dr
Schuepphaus, of Brooklyn, U.S.A., proposes to treat the waste acids from
the nitration of cellulose by adding to them sulphuric anhydride and
nitric acid. The sulphuric anhydride added converts the water liberated
from the cellulose into sulphuric acid.]

~Washing.~--The cotton has now to be carefully washed. This is done in a
large wooden tank filled with water. If, however, a river or canal runs
through the works, a series of wooden tanks, the sides and bottoms of
which are pierced with holes, so as to allow of the free circulation of
water, should be sunk into a wooden platform that overhangs the surface of
the river in such a way that the tanks are immersed in the water, and of
course always full. During the time that the cotton is in the water a
workman turns it over constantly with a wooden paddle. A stream of water,
in the form of a cascade, should be allowed to fall into these tanks. The
cotton may then be thrown on to this stream of water, which, falling some
height, at once carries the cotton beneath the surface of the water. This
proceeding is necessary because the cotton still retains a large excess of
strong acids, and when mixed with water gives rise to considerable heat,
especially if mixed slowly with water. After the cotton has been well
washed, it is again wrung out in a centrifugal machine, and afterwards
allowed to steep in water for some time.

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