The Chemistry of Powder and Explosives - Sciencemadness Dot Org

LEAD AZIDE 425
used; the azide is exploded by impact, and the tetryl communicates
the explosion to the booster or perhaps to the main charge
of the shell.
Lead azide is produced as a white precipitate by mixing a solution
of sodium azide with a solution of lead acetate or lead nitrate.
It is absolutely essential that the process should be carried
out in such manner that the precipitate consists of very small
particles. The sensitivity of lead azide to shock and to friction
increases rapidly as the size of the particles increases. Crystal*
1 mm. in length are liable to explode spontaneously because of
the internal stresses within them. The U. S. Ordnance Department
specifications require that the lead azide shall contain no
needle-shaped crystals more than 0.1 mm. in length. Lead azide
is about as sensitive to impact when it is wet as when it is dry.
Dextrinated lead azide can apparently be stored safely under
water for long periods of time. The belief exists, however, that
crystalline "service azide" becomes more sensitive when stored
under water because of an increase in the size of the crystals.
The commercial preparation of lead azide is carried out on
what is practically a laboratory scale, 300 grains of product constituting
an ordinary single batch. There appear to be diverse
opinions as to the best method of precipitating lead azide in a
finely divided condition. According to one, fairly strong solutions
are mixed while a gentle agitation is maintained, and the precipitate
is removed promptly, and washed, and dried. According to
another, dilute solutions ought to be used, with extremely violent
agitation, and a longer time ought to be devoted to the process.
The preparation is sometimes carried out by adding one solution
to the other in a nickel ve&'scl, which has corrugated sides, and is
rotated around an axis which makes a considerable angle with
the vertical, thereby causing turbulence in the liquid. The precipitation
is sometimes carried out in the presence of dissolved
colloidal material, such as gelatin or dextrin, which tends to
prevent the formation of large crystals. Sometimes the lead azide
is precipitated on starch or wood pulp, either of which will take
up about 5 times its own weight of the material, and the impregnated
starch is worked up, say, by tumbling in a sweetie barrel
with a little dextrine, to form a free-flowing granular mass which
can conveniently be loaded into detonators, or the impregnated
wood pulp is converted into pasteboard which is cut into discs