3. _For shape, size and proportion of the grains._--The first is
judged by eye, and grains of the size required are obtained by the use
of sieves of different sizes.
4. _Density._--The density is generally obtained in some form of
mercury densimeter, the powder being weighed in air and then under
mercury. In some forms of the instrument the air can be pumped out so
that the weighing takes place _in vacuo_.
5. _Moisture and absorption of moisture._--The moisture and
hygroscopic test consists in weighing a sample, drying at 100 deg. C.
for a certain time, weighing again, &c., until constant. The dried
weighed sample can then be exposed to an artificial atmosphere of
known moisture and temperature, and the gain in weight per hour
similarly ascertained by periodic weighings.
6. _Firing proof._--The nature of this depends upon the purpose for
which the powder is intended. For sporting powders it consists in the
"pattern" given by the shot upon a target at a given distance, or, if
fired with a bullet, upon the "figure of merit," or mean radial
deviation of a certain number of rounds; also upon the penetrative
power. For military purposes the "muzzle" velocity produced by a
powder is ascertained by a chronograph which measures the exact time
the bullet or other projectile takes to traverse a known distance
between two wire screens. By means of "crusher gauges" the exact
pressure per square inch upon certain points in the interior of the
bore can be found.
In the chemical examination of gunpowder the points to be ascertained
are, in addition to moisture, freedom from chlorides or sulphates, and
correct proportion of nitre and sulphur to charcoal.
_Products of Fired Powder and Changes taking place on
Explosion._--With a mixture of the complexity of gunpowder it is quite
impossible to say beforehand what will be the relative amounts of
products. The desired products are nitrogen and carbon dioxide as
gases, and potassium sulphate and carbonate as solids. But the
ingredients of the mixture are not in any simple chemical proportion.
Burning in contact with air under one atmosphere pressure, and burning
in a closed or partially closed vessel under a considerable number of
atmospheres pressure, may produce quite different results. The
temperature of a reaction always rises with increased pressure.
Although the main function of the nitre is to give up oxygen and
nitrogen, of the charcoal to produce carbon dioxide and most of the
heat, and of the sulphur by vaporizing to accelerate the rate of
burning, it is quite impossible to represent the actions taking place
on explosion by any simple or single chemical equation. Roughly
speaking, the gases from black powder burnt in a closed vessel have a
volume at 0 deg. C. and 760 mm. pressure of about 280 times that of
the original powder. The temperature produced under one atmosphere is
above 2000 deg. C., and under greater pressures considerably higher.
Experiments have been made by Benjamin Robins (1743), Charles Hutton
(1778), Count Rumford (1797), Gay-Lussac (1823), R. Bunsen and L.
Schiskoff (1857), T. J. Rodman (1861), C. Karolyi (1863), and later
many researches by Sir Andrew Noble and Sir F. A. Abel, and by H.
Debus and others, all with the idea of getting at the precise
mechanism of the explosion. Debus (_Ann._, 1882, vols. 212, 213; 1891,
vol. 265) discussed at great length the results of researches by
Bunsen, Karolyi, Noble and Abel, and others on the combustion of
powder in closed vessels in such manner that all the products could be
collected and examined and the pressures registered. A Waltham Abbey
powder, according to an experiment by Noble and Abel, gave when fired
in a closed vessel the following quantities of products calculated
from one gram of powder:--
Fractions of Fractions of a
a gram. molecule or atom.
Potassium carbonate .2615 .00189 molecule
Potassium sulphate .1268 .00072 "
" thiosulphate .1666 .00087 "
" sulphide .0252 .00017 "
Sulphur .0012 .00004 atom
Carbon dioxide .2678 .00608 molecule
Carbon monoxide .0339 .00121 "
Nitrogen .1071 .00765 atom
Hydrogen .0008 .0008 "
Hydrogen sulphide .0080 .00023 molecule
Potassium thiocyanate .0004
Nitre .0005
Ammonium carbonate .0002
From this, and other results, Debus concluded that Waltham Abbey
powder could be represented by the formula 16KNO3+21.18C+6.63S and
that on combustion in a closed vessel the end results could be fairly
expressed (rounding off fractions) by 16KNO3+21C+5S = 5K2CO3 +
K2SO4+2K2S2+13CO2+3CO+8N2. Some of the sulphur is lost, part combining
with the metal of the apparatus and part with hydrogen in the
charcoal. The military powders of most nations can be represented by
the formula 16KNO3+21.2C+6.6S, proportions which are reasonably near
to a theoretical mixture, that is one giving most complete combustion,
greatest gas volume and temperature. The combustion of powder consists
of two processes: (i.) oxidation, during which potassium carbonate and
sulphate, carbon dioxide and nitrogen are mainly formed, and (ii.) a
reduction process in which free carbon acts on the potassium sulphate
and free sulphur on the potassium carbonate, producing potassium
sulphide and carbon monoxide respectively. Most powders contain more
carbon and sulphur than necessary, hence the second stage. In this
second stage heat is lost. The potassium sulphide is also the most
objectionable constituent as regards fouling.
The energy of a powder is given, according to Berthelot, by
multiplying the gas volume by the heat (in calories) produced during
burning; Debus shows that a powder composed of 16KNO3 to 8C and 8S
would have the least, and one of composition 16KNO3+24C+16S the
greatest, when completely burnt. The greatest capability with the
lowest proportion of carbon and sulphur to nitre would be obtained
from the mixture /16KNO3+22C+8S.
Smokeless and even noiseless powders seem to have been sought for
during the whole gunpowder period. In 1756 one was experimented with
in France, but was abandoned owing to difficulties in manufacture.
Modern smokeless powders are certainly less noisy than the black
powders, mainly because of the absence of metallic salts which
although they may be gaseous whilst in the gun are certainly ejected
as solids or become solids at the moment of contact with air.
_Brown Powders._--About the middle of the 19th century guns and
projectiles were made much larger and heavier than previously, and it
was soon found that the ordinary black powders of the most dense form
burnt much too rapidly, straining or bursting the pieces. Powders were
introduced containing about 3% sulphur and 17-19% of a special form of
charcoal made from slightly charred straw, or similar material. This
"brown charcoal" contains a considerable amount of the hydrogen and
oxygen of the original plant substance. The mechanical processes of
manufacture of these brown powders is the same as for black. They,
however, differ from black by burning very slowly, even under
considerable pressure. This comparative slowness is caused by (1) the
presence of a small amount of water even when air-dry; (2) the fact
that the brown charcoal is practically very slightly altered
cellulosic material, which before it can burn completely must undergo
a little further resolution or charring at the expense of some heat
from the portion of charge first ignited; and (3) the lower content of
sulphur. An increase of a few per cent in the sulphur of black powder
accelerates its rate of burning, and it may become almost a blasting
powder. A decrease in sulphur has the reverse effect. It is really the
sulphur vapour that in the early period of combustion spreads the
flame through the charge.
Many other powders have been made or proposed in which nitrates or
chlorates of the alkalis or of barium, &c., are the oxygen providers
and substances as sugar, starch, and many other organic compounds as
the combustible elements. Some of these compositions have found
employment for blasting or even as sporting powders, but in most cases
their objectionable properties of fouling, smoke and mode of exploding
have prevented their use for military purposes. The adoption by the
French government of the comparatively smokeless nitrocellulose
explosive of Paul Vieille in 1887 practically put an end to the old
forms of gunpowders. The first smokeless powder was made in 1865 by
Colonel E. Schultze (_Ding. Pol. Jour._ 174, p. 323; 175, p. 453) by
nitrating wood meal and adding potassium and barium nitrates. It is
somewhat similar in composition to the E. C. sporting powder. F.
Uchatius, in Austria, proposed a smokeless powder made from nitrated
starch, but it was not adopted owing to its hygroscopic nature and
also its tendency to detonate.
BIBLIOGRAPHY.--Vanucchio Biringuccio, _De la pirotechnia_ (Venice,
1540); Tartaglia, _Quesiti e invenzioni diversi_ (lib. iii.) (Venice,
1546); Peter Whitehorne, _How to make Saltpetre, Gunpowder, &c._
(London, 1573); Nic. Macchiavelli, _The Arte of Warre_, trans. by
Whitehorne (London, 1588); Hanzelet, _Recueil de plusiers machines
militaires_ (Paris, 1620); Boillet Langrois, _Modelles artifices de
feu_ (1620); Kruger, _Chemical Meditations on the Explosion of
Gunpowder_ (in Latin) (1636); Collado, _On the Invention of Gunpowder_
(Spanish) (1641); _The True Way to make all Sorts of Gunpowder and
Matches_ (1647); Hawksbee, _On Gunpowder_ (1686); Winter, _On
Gunpowder_ (in Latin); Robins, _New Principles of Gunnery_ (London,
1742) (new ed. by Hutton, 1805); D'Antoni, _Essame della polvere_
(Turin, 1765) (trans. by Captain Thomson, R. A., London, 1787); Count
Rumford, "Experiments on Fired Gunpowder," _Phil. Trans. Roy. Soc._
(1797); Charles Hutton, _Mathematical Tracts_, vol. iii. (1812); Sir
W. Congreve, _A Short Account of Improvements in Gunpowder made by_
(London, 1818); Bunsen and Schiskoff, "On the Chemical Theory of
Gunpowder," _Pogg. Ann._, 1857, vol. cii.; General Rodman,
_Experiments on Metal for Cannon, and Qualities of Cannon Powder_
(Boston, 1861); Napoleon III., _Etudes sur le passe et l'avenir de
l'artillerie_, vol. iii. (Paris, 1862); Von Karolyi, "On the Products
of the Combustion of Gun Cotton and Gunpowder," _Phil. Mag._ (October
1863); Captain F. M. Smith, _Handbook of the Manufacture and Proof of
Gunpowder at Waltham Abbey_ (London, 1870); Noble and Abel, _Fired
Gunpowder_ (London, 1875, 1880); Noble, _Artillery and Explosives_
(1906); H. W. L. Hime, _Gunpowder and Ammunition, their Origin and
Progress_ (1904); O. Guttmann, _The Manufacture of Explosives_ (1895),
_Monumenta pulveris pyrii_ (1906); _Notes on Gunpowder and Gun
Cotton_, published by order of the secretary of state for war (London,
1907). (See also EXPLOSIVES.) (W. R. E. H.)