Improvements in the Manufacture or Production of Lead Pigments.
Year: 1889

page_text: Improvements in the Manufacture or Production of Lead Pigments.
I JAMES YATE JOHNSON of 47 Lincolns Inn Fields in the County of Middlesex
Gentleman do hereby declare the nature of this Invention which has been com-
municated to me from abroad by Ambrose George Fell of the City County and State
of New York Chemist and in what manner the same is to be performed to be par-
ticularly described and ascertained in and by the following statement :-
This Invention relates to a novel lead piguient and to a method of preparing the
same from metallic lead and the object of this Invention is to produce a white lead
pigment which, while possessing as good a colour and body as are produced by the
old Dutch method, also possesses the desirable qualities of durability and resistance
to decomposition in a greater degree than the lead carbonate produced by methods
hitherto in use.
The white lead of commerce, which is a carbonate more or less impure, is white in
colour, and its molecular structure is very fine. It is this latter quality which
recommends it most especially to painters, for the reason that the fineness of its par-
ticles or molecules enables it to cover "well and to spread evenly under the brush.
But the carbonate does not always possess this desirable quality, and this is so
especially with that produced by precipitation from lead solutions.
The lead compounds vary in colour from black to white and vary in structure from
a coarse crystal to an exceedingly fine amorphous particle, and the carbonate,
while always white, differs greatly in structure according to the manner in which
it is produced. That produced by corrosion is superior in this respect, and that
produced by precipitation is inferior. But the latter is the cheaper method of
manufacture.
The uncertainty attending the production of lead carbonate by corrosion, and the
practical impossibility of properly governing or controlling the process of manu-
facture so as to give economical and uniform results, has led to various attempts to
produce a satisfactory white lead pigment by other more controllable and economical
means. The present invention is intended to accomplish this desirable result.

page_text: There are two well known compounds of sulphur and lead namely, the sulphide (Pb S)
and the sulphate (P6 SO). The former is black, and extremely fine in structure
when produced by precipitation, and the latter is white, but crystalline in texture.
Therefore the former as a pigment possesses good "body" and the latter possesses
good colour but neither possesses both of these qualities combined.
The new pigment the subject of the present invention is a compound of sulphur
and lead which in its chemical relations stands between the sulphide and sulphate,
containing more oxygen than the sulphide and less oxygen than the sulphate. It
resembles the sulphide in texture and the sulphate in colour, and it is these
characteristics which recommend it as a pigment for use as a substitute for the
carbonate.
"
The new pigment is substantially or practically amorphous in structure; its
molecules are very fine; it possesses "covering" qualities equal to the best carbonate,
and is very white inclining to blue, and when tested in the usual way of examining
samples of dry white lead is readily distinguished from all other white lead by
yielding a Blue Streak" when a small portion of the dry pigment is streaked on a
dark surface by means of the thumb nail, in the usual manner as known to the trade;
other white leads yield a yellowish or grayish streak, according to quality. This
feature not only renders the white load made according to this Invention easily
distinguishable, but at the same time indicates a quality which recommends it to the
dealer and consumer. It is much less soluble than the carbonate, and resists
better the decomposing effects of sulphuretted hydrogen. Therefore it is more
durable than the carbonate when exposed to the action of the elements.
The process which is to be employed in producing the improved pigment is as
follows:-
The metallic lead is either reduced to small pieces or particles, or it may be
reduced to an oxide by any known process. It is next submitted to the action of a
solvent, acetic acid answering as well as any. It is not necessary that a perfect
solution of the metal shall be effected, as a more or less soluble paste will answer the
purpose, such as, for instance, the chloride of lead produced by grinding together
oxide of lead and common salt (sodium chloride); or protoxide of lead may be
acted on by acetic or other acid and brought to a more or less pasty condition. A
small percentage only of acid should be usod. The lead contained in the solution or
mixture is converted first into sulphite (P SO₁) by the action thereon of sulphurous
oxide (SO) or a compound of the same, and the resultant sulphite is next converted
into a sub-sulphite by the action of steam; and the pigment after being freed from
foreign matters, is dried and ground in oil for use.
In practice the proceeding is as follows in carrying out this invention:
The metallic lead is granulated or made spongy by melting and pouring it into
water. Then this granulated metal is placed in a series of suitable vessels (wooden
tanks will answer the purpose), three being sufficient, and the first vessel is filled
with water containing ten or more per cent. of commercial acetic acid. This mixture
would contain about two and one lialf per cent. of the anhydrous acid. The lead is
permitted to digest in this dilute acid for about thirty minutes, when the liquid is
drawn off into the second vessel. After remaining in this vessel for about the same
length of time, it is drawn off into the third vessel and allowed to remain in that for
about the same period. In the meantime the lead in the first two vessels has been
undergoing oxidation from contact with the air, such oxidation being accompanied by
the usual rise in temperature. The liquid is now transferred to the first vessel again,
and it is thus transferred from vessel to vessel, dissolving a portion of the lead oxide at
each period of digestion and increasing in density. This process is continued until
the solution marks from 15° to 20° Baumé. This is a solution of basic acetate of lead.
Where a battery of three vessels is employed in this way, and the period of
digestion is thirty minutes, the period of oxidation of the metal will be sixty
minutes.
The solution of basic acetate is now drawn off into a deep tank and sulphurous acid
gas forced through it. This, owing to the greater affinity of the sulphurous acid for

page_text: the lead, decomposes the acetate and converts the lead into white sulphite. The
acetic acid is liberated taking the form of a solution of acetic acid and basic acetate.
The sulphite hold mainly in suspension in the liquid, is separated preferably by means
of a filter press, then washed, either in the press or after removal therefrom, and
then well agitated with steam, which first drives off the remaining traces of the free
acids, and secondly, as I believe, drives off a portion of the sulphurous acid chemically
combined with the lead, reducing the sulphite to a sub-salt and thus improving the
colour, and body of the pigment. The pigment is then dried, (the steaming facilita-
ting the drying by heating the pigment) ground in oil or other liquid vehicle, but
preferably linseed oil, and packed for the market.
If a lead compound capable of being acted upon by sulphurous acid be used in a
pasty condition the use of shallow troughs would be preferable to the use of deep
tanks when it is subjected to the action of the sulphurous acid.
It is proferred to produce the sulphurous acid gas used in this process by burning
sulphur in compressed air, the tension of the air being utilized to force the resultant
sulphurous acid gas into and through the lead solution. But I do not limit myself to
the production of the gas in this manner as it may be obtained, for instance, by roasting
ores containing sulphur.
I do not claim the treatment of metallic lead, or its oxide, as above described, for
obtaining a solution or soluble paste of lead, in itself, but only as a step in the process.
Nor do I limit myself to the employment of any particular salt or compound of lead.
It is only necessary that the compound used shall be of such a nature that the
sulphurous oxide (SO) or its compounds will decompose it and combine with the lead
it contains. The solution of basic acetate as described is preferred for the roason,
mainly, that there is no appreciable loss of acid as the dilute acid from the filter press
is returned to the digesting vessels for use over and over again.
.
The advantages of the improved product as a white lead pigment over the ordinary
carbonate have been hereinbefore enumerated and the advantages of this process
over that employed in producing the best carbonate, that is by corrosion, are economy
and uniformity.
The theoretical increase in the weight of the product over the weight of metal
consumed is about 25 per cent. by the corrosion process, while by the process according
to this Invention it is about 38 per cent. Owing to the facility with which the
improved process can be controlled as contrasted with the corrosion process, a pigment
of uniformly good quality can be produced and the above stated theoretical increase
in weight can be approached much more nearly than is the case in the preparation of
carbonate by the corrosion process.
In treating the pasty or semi-fluid mass with steam after it comes from the filter
the following procedure is suitable. The mass is put in a tank or vessel and live
steam is admitted in such a manner as to pass through it and heat it. The stoa may
be relied on to agitate the mass or it may be agitated mechanically, the object being
to mechanically and chemically act upon the sulphite as before described which action
removes a portion of the sulphurous acid chemically combined with the lead as I
believe; and such appears to be the case since the sulphite so treated does not so
readily change to sulphate as does the sulphite in its normal condition and it is well
known that all the sulphites are decomposed by heat. Moreover the physical condition
of the improved pigment is materially affected by the action of the steam, the particles
become more opaque and compact, which to the painter constitutes additional
"body," while the colour is also improved as indicated by its general appearance and
by means of the "Blue Streak" trade test before mentioned. Moreover if a sample
of the ordinary sulphite be mixed with oil and dried after exposure to the elements
for a sufficient length of time it assumes a greenish yollowish hue which is probably
due to the combination of a portion of the sulphur of the sulphurous acid with the
hydrogen in the moist air, producing first sulphuretted hydrogen and then lead
sulphide. This objectionable action is not observed when the sulphite has been
steamed as indicated. It is apparent that any other means which in its action is
chemically equivalent to the steam may be substituted therefor. For instance it is

page_text: possible to act upon the normal sulphite with an alkaline reagent which would not only
neutralize any excess of free sulphurous acid that may be present but will also
combine with a portion of that combined with the metal, thus practically bringing
about the same result, although in such a mode of treatment a foreign rosidual
substance would be introduced into the resulting pigment which would be objectionable,
also it would be difficult to control the chemical action of an alkaline reagent and the
decomposition might proceed too far and result in injury to the product. But in any
event whether the steam or its equivalent bo employed, so long as the desired result
is attained the means used are immaterial. I prefer the steam as being both effectual
controllable and convenient.
It is believed that the new pigment may be properly termed an "amorphous" sub-
sulphite of lead.
By the term "amorphous herein employed as descriptive of the improved
pigment I mean practically or substantially amorphous and not necessarily
absolutely so. While it appears to be absolutely amorphous and is sufficiently so
to form a satisfactory covering "body" future advances in science may show that
it contains crystals.
While I believe the new pigment to be a basic sulphite further investigation may
prove to be an allotropic form of sulphite of lead.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
1 claim is
1. The herein described method of producing the new pigment herein described
which consists in forming a solution of a salt of lead forcing sulphurous acid through
the said solution whereby the load is converted into normal sulphite separating the
said sulphite from the liquid steaming it and drying it.
2. The herein described process of treating sulphite of lead for the production of
a lead pigment which process consists in subjecting the sulphite to the action
of steam or its equivalent for the purpose and substantially in the manner herein-
before described.
3. The method of producing the new pigment horein described which consists in
forming a solution of acetate of lead forcing sulphurous acid through the said
solution whereby the lead is converted into normal sulphite separating said
sulphite from the liquid, steaming and drying it substantially as hereinbefore
described.
4. As a new manufacture a lead pigment consisting substantially of a lead sulphite,
white in colour and distinguished by yielding a "blue streak" and by the other
properties and characteristics herein set forth.
Dated this 15th day of January 1889.