MESSRS. EDITORS:--I have read, with much interest, Dr. Chandler's
colorimetric test of the purity of white lead, as published in the
SCIENTIFIC AMERICAN sometime ago. I enclose another test, which,
though not new, is of value to all using white lead on account of its
simplicity and effectiveness. It has been in use here for nearly two
years, and has been found reliable. Having never seen it in print, I
have tried to put it in as simple words as possible.
FELIX MCARDLE, Analytical Chemist.
St. Louis, Mo.
Take a piece of firm, close grained charcoal, and, near one end of it,
scoop out a cavity about half an inch in diameter and a quarter of an
inch in depth. Place in the cavity a sample, of the lead to be tested,
about the size of a small pea, and apply to it continuously the
blue or hottest part of the flame of the blow pipe; if the sample be
strictly pure, it will in a very short time, say in two minutes, be
reduced to metallic lead, leaving no residue; but if it be adulterated
to the extent of ten per cent. only, with oxide of zinc, sulphate of
baryta, whiting or any other carbonate of lime, (which substances are
now the only adulterations used), or if it be composed entirely of
these materials, as is sometimes the case with cheap lead, it cannot
be reduced, but will remain on the charcoal an infusible mass.
Dry white lead, (carbonate of lead) is composed of metallic lead,
oxygen and carbonic acid, and, when ground with linseed oil, forms the
white lead of commerce. When it is subjected to the above treatment,
the oil is first burned off, and then at a certain degree of heat, the
oxygen and carbonic acid are set free, leaving only the metallic lead
from which it was manufactured. If, however, there be present in the
sample any of the above mentioned adulterations, they cannot of course
be reduced to metallic lead, and cannot be reduced, by any heat of
the blow pipe flame, to their own metallic bases; and being intimately
incorporated and ground with the carbonate of lead, they prevent it
from being reduced.
It is well, after blowing upon the sample, say for half a minute, by
which time the oil will be burned off, to loosen the sample from the
charcoal, with a knife blade or spatula, in order that the flame may
pass under as well as over and against it. With proper care the lead
will run into one button, instead of scattering over the charcoal,
and this is the reason why the cavity above mentioned is necessary. A
common star candle or a lard oil lamp furnishes the best flame for use
of the blow pipe; a coal oil lamp should not be used.
By the above test, after a little practice, so small an adulteration
as one or two per cent. can be detected; it is, however, only a test
of the purity or impurity of a lead, and if found adulterated, the
degree or percentage of adulteration cannot be well ascertained by it.
Jewellers usually have all the necessary apparatus for making the
test, and any one of them can readily make it by observing the above
directions, and from them can be obtained a blow pipe at small cost.
If you have no open package of the lead to be tested, a sample can
most easily be obtained by boring into the side or top of a keg with
a gimlet, and with it taking out the required quantity; care should be
used to free it entirely from the borings or particles of wood, and it
should not be larger than the size mentioned; a larger quantity can be
reduced, but of course more time will be required, and the experiment
cannot be so neatly performed.
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