IMPROVEMENTS IN THE MANUFACTURE OF A WHITE PIGMENT "
Year: 1883

page_text: LETTERS PATENT to Thomas Griffiths of Oxton in the County of Chester,
for an Invention of "IMPROVEMENTS IN THE MANUFACTURE OF A WHITE
PIGMENT
"
PROVISIONAL SPECIFICATION left by the said Thomas Griffiths at the
Office of the Commissioners of Patents on the 10th February 1883.
THOMAS GRIFFITHS of Oxton in the County of Chester "IMPROVEMENTS IN
THE MANUFACTURE OF A WHITE PIGMENT
"
In manufacturing my sulphate of zinc pigments I find great difficulty in
preventing it discolorizing or rather colorizing when undergoing the calcination
process. The cause of this calcination appears to be oxydisation. To avoid this I
adopted the plan of calcining in crucibles or in closed furnaces without contact of
air. I find however that unless I form an artificial atmosphere round the material
it gets oxydised more or less. I therefore add an ammoniacal salt that will not
easily evaporate such as the sulphide or chloride to the sulphate before calcining
this slowly evaporating forms an atmosphere that effectually excludes the oxygen
Other non oxygenated difficultly volatile compounds will answer but these are the
most convenient. I have hitherto used as a precipitant the sulphide of soda potash
lime or baryta, now I find that a double sulphide or mixed sulphide of barium and
ammonium gives a better result. The barium remains as heretofore the ammonium
remains in solution but can be evaporated as a sulphate or chloride and thus &
valuable bye-product may be formed I however prefer to evaporate it in whole
or in part with a precipitate as by this means it forins the mixture best for
calcination The sulphide of ammonium can be formed in any ordinary manner.
In some cases the ammonia liquor from gas works may be made available by first
purifying it from hydrocarbons and then treating it with a sulphide of an alkaline
earth to remove the carbonic acid.
'

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed
by the said Thomas Griffiths in the Great Seal Patent Office on the
10th August 1883.
THOMAS GRIFFITHS of Oxton in the county of Chester
THE MANUFACTURE OF A WHITE PIGMENT
"
"IMPROVEMENTS IN
In manufacturing sulphide of zinc pigment, great difficulty is found in preventing
it discolorizing or rather colorizing when undergoing the calcination process. To
avoid this I adopted the plan of calcining in crucibles, or in closed furnaces without
contact of air. I find however that unless I form an artificial atmosphere round
the material, it is colorized more or less; I therefore add an-ammoniacal salt that
will not easily volatilize, such as the sulphate or chloride to the sulphide of zinc
before calcining, this slowly vaporizing forms an atmosphere that effectually
excludes the oxygen.
The exact quantities required differ with the furnace or crucible used. If a well-
luted crucible be employed, a very small amount of ammonia salt suffices; if a
muffle furnace be used more leakage and space for gas or air will be found, and
more ammonia salt will be required. An amount of N H, equal to from / to 21,
-per-cent of the entire mixture will usually suffice.
In this case as-
This invention can be made applicable also in calcining a double precipitate of
sulphate of barium, strontium, or calcium, and sulphide of zinc.
with sulphide of zinc an intimate mixture of the material with the ammonium salt
is desired. To effect this more perfectly, I sometimes precipitate the zinc from its
sulphate with a mixture of sulphide of ammonium, and sulphide of barium. This
last named mixture can be formed by adding the desired quantity of dissolved
sulphate of ammonium to an excess of dissolved sulphide of barium, and filtering
out the sulphate of barium which falls. The mixture of dissolved sulphides which
results is added to the solution of zinc, both solutions being preferably saturated,
and the entire mixture both liquid and precipitate evaporated to dryness. The
resulting solid mass is then calcined as much as possible out of contact of air. The
sulphate of ammonia distils off and can be condensed in any ordinary manner and
used for a fresh operation.
The ammonia liquor of gas works, bone works, coke ovens, and the ammoniacal
liquors resulting from any other manufacturing process may be made available for
this purpose by first purifying them and then treating them with the sulphide of
an alkaline earth to remove the carbonic acid. The mode I prefer for purifying
from tarry and other matters is as follows: I agitate it with china clay, fuller's
earth, saw dust, or other absorbent powder not having a chemical action on the gas
liquor and then filter. The solution of sulphide of ammonium and other salts thus
formed can be used to precipitate the zinc from its solution.
Other rather volatile salts not liable to decomposition or to act chemically on
the sulphide of zinc would answer the purpose, for it is obvious that where the sole
action of the ammoniacal salt in calcining is to form an artificial atmosphere to
prevent the sulphide colorizing at a high temperature, any neutral salt that will
volatilize at about the same temperature might be used, but I have found that
the sulphate or chloride of ammonium is on the whole the best and cheapest for
this purpose.
Having now described my invention and the best mode known to me of carrying
it into effect, I claim under the Letters Patent :-
1st. The process of manufacturing pigment containing calcined sulphide of zinc,
which consists in calcining the said sulphide with or without other substances
mixed with sulphate of ammonium or other salt having no chemical or coloring

page_text: action on the said sulphide or mixture, and that volatilizes at a little below
the degree of heat at which the sulphide becomes colorized in air, or steam, or
both, and thus forming a protecting atmosphere round the sulphide or mixture,
substantially as described.
d. The manufacture of a white pigment by precipitating zinc as a sulphide
from its solutions by means of purified ammonia liquor, substantially as
described.
In witness whereof I the said Thomas Griffiths have hereunto set my hand
d seal this ninth day of August in the year of our Lord one thousand eight
ndred and eighty three