The manufacture of this important and useful pigment has been very
successfully prosecuted within the past year, by a new process, the
invention of Dr. H. Hannen of this city, and is destined to supersede
the old method, both as regards economy in preparation and purity of
material. The old or Dutch process, requiring some six to eight months
for its completion, fit for painter’s use; while by the Hannen patent
it can be produced in from ten to fifteen days. The quality of the
article is said to be fully equal, if not superior, to that of the lead
made by the old method. The process of manufacture, as far as we can
learn, is as follows:
The best Spanish pig lead is melted in a large iron kettle, holding
from fifteen to eighteen hundred weight, and then drawn off by a
suitable valve, and allowed to run over a cast-iron wheel or drum,
about six inches on the face and three feet in diameter, running at a
high speed, and kept cool by a stream of cold water constantly playing
on it. The lead, in passing over this wheel, is cast into ribbons
about the thickness of paper, it is then taken and placed on lattice
shelving in rooms some eight to ten feet square, made almost airtight
by a double thickness of boards, and capable of holding some three tons
of the metallic lead as it comes from the casting machine in ribbon
form, the temperature of the room is then raised by injecting steam
to about one hundred degrees, and then sprinkled several times a day
with diluted acetic acid, converting it into sub-acetate or sugar of
lead. While this operation is going on, carbonic acid gas is forced
into the room by means of a blower or pump, which decomposes the
acetate and forms a carbonate of lead; this operation of forming an
acetate, and then a carbonate, requires from five to six days, until
a complete corrosion of the lead is effected; the room is now allowed
to cool and the lead to dry, after which it is taken out and sifted
through fine wire sieves, which separates all undecomposed lead or
other impurities. It is then ready for washing and drying. The finely
powdered lead is mixed with water into a thick pasty form and ground in
a mill of similar construction to an ordinary flour mill, from which
it is allowed to run into large tubs filled with water, and thoroughly
washed and allowed to settle. The last or finishing operation is to
place it in large copper pans, heated by steam, when it is dried; from
thence taken to the color grinder, where it is mixed in oil ready for
the painter’s use.