The Cupellation of Silver Ores.
This interesting process is performed in a reverberatory furnace of
a very peculiar construction, the cupel employed on the large scale
differing somewhat from the ordinary one, being considerably larger and
varying also in form. It consists of a strong oval wrought-iron ring,
with a part of the full shape omitted, as shown in accompanying sketch,
in order to allow of the overflow of lead during the process, in the
form of litharge. This iron ring, known as the _test ring_, contains
the cupel, and in order to prepare the latter, the frame, which
measures about 60 ins. in its longest diameter, 40 ins. in breadth, and
6 ins. in depth, is strengthened by having a number of broad strips of
iron seamed across the bottom by riveting to the sides of it. The cupel
itself is prepared for use by taking finely ground bone-ash, together
with a little carbonate of potash, and working them up with just
sufficient water to make the mass cohere properly; the carbonate of
potash may be advantageously dissolved in the water; the latter is then
applied in small quantities at a time to the bone-ash until the proper
coherency has been obtained; of the total quantity of bone-ash employed
in the operation, 2 per cent. of potash will be _quantum sufficit_ to
mix with it. The iron frame, or test, is then filled with the mixture,
and it is pressed down into a solid compact mass, the centre part
being hollowed out with a small trowel, the sides sloping towards the
concavity in the middle; the hollow should not however be extended more
than within 1 to 1-1/2 in. of the bottom of the frame, and above the
iron bars. The cupel forms the hearth of the furnace we have spoken
of, and of which Fig. 13 is a sectional view; it is removable, and not
a fixture in the furnace. It must be left for several days to dry,
after having been constructed as described, when it is ready for use,
and only requires firmly wedging in its place beneath the arch of the
furnace.
[Illustration: Fig. 13. Cupels, section and perspective views.]
The fire should be only very moderate at the commencement of the
operation, and the furnace slowly raised in temperature, lest the cupel
should crack by being too quickly heated. As the temperature increases,
if without any apparent defects in the bone-ash cupel, or hearth, which
it may now be termed, the wind or blast, generally driven by a fan, is
thrown in through a nozzle, or an aperture in the furnace, which, for
facilitating the immediate removal of the bone-ash hearth, is placed
upon an iron car, and runs beneath the vault of the furnace on rails,
so that it may thus be very readily withdrawn when found necessary. The
admission of a current of air into the furnace oxidizes the excess of
lead, in combination with the silver, producing litharge on the surface
of the molten mass; the formation of the litharge takes place rapidly,
and it is continually blown forward by the strength of the blast as
fast as it is produced, running through a gap or channel specially made
for the purpose in the mouth of the cupel into a movable iron pot which
is placed for its reception. The continual oxidization and flow-off of
the lead alters the respective proportions of the metals in the cupel.
For this reason it is always kept full of lead ore, which is effected
by taking it in its fused state from a kettle in which it is ready
melted by means of a long-handled ladle; and thus about 500 or 600 lbs.
of metal are constantly kept in this bone-ash cupel or hearth.
As the silver necessarily increases in the hearth, it will require to
be occasionally withdrawn, in order to make room for a further supply
of lead ore. This process is adopted when it reaches about from 8 to 10
per cent. of silver to the ton (between 2,000 and 3,000 ozs.), and may
be effectually performed by drilling a hole underneath the cupel, and
letting the silver flow through it into a receptacle placed to receive
it. Of course the operations of the furnace are arrested while these
manipulations are being carried on. After the withdrawal of the silver,
the hole is closed up again with a plug of moistened bone-ash prepared
as before; when the process may be continued a second time by giving
500 or 600 lbs. of fresh lead ore to the cupel. Thus a single cupel
will often last 48 hours, and 6 or 7 tons of lead may be oxidized upon
it.
We have already observed that the prolongation of the cupelling
process increases the richness of the remaining alloy, and this very
rich silver-lead alloy is again subjected to a second operation in
cupelling. This process of assaying or refining is similar in every
respect to the former, and is often performed in the same furnace, the
cupel being first of all brought to almost a bright red heat, when
about 600 lbs. of the silver-lead alloy are added, and a strong current
of air given in order to oxidize the remaining lead in combination with
the silver. In this operation the material under treatment, previous
to its introduction to the cupel, should be melted in a kettle easy of
access, and added in its fused state. The current of air in connection
with the heat of the furnace immediately begins to purify the silver by
oxidizing the lead, and forms litharge, which passes off through the
channel provided in the mouth of the cupel; as this proceeds, fresh
silver-lead alloy is added, to keep the level of the metal always at
the same height. This is continued until some three tons of the alloy
from the first cupellation have been put in, and when about 600 or 700
lbs. of silver are collected in the cupel.
When the cupel has received the above proportion of metal, the addition
of the alloy ceases, and the silver is allowed to purify. The litharge
which passes off towards the close of this process will be richer
in silver than in the former one; consequently it is found best in
practical metallurgical operations to treat in a special manner the
last part of silver cupelling on the large scale, for it needs very
careful management indeed to secure all the silver, especially to do so
in a fine state. Towards the completion of the process the fire should
be increased considerably, in order to keep the silver thoroughly
melted, and also to oxidize and completely remove every trace of lead
that is possible. As it begins to purify itself from the remaining lead
a characteristic brightness will be perceived. When this takes place
the fire must be lowered, the wind or blast stopped, and the metal left
to cool gradually. This latter proceeding is of some importance, as a
too sudden cooling of the surface causes the interior of the metal to
expand and shoot, by which means little globules of silver may be lost;
therefore it should be allowed to cool very slowly.
The iron ring encircling the cupel with its contents may now be drawn
from beneath the arch of the furnace, and the cake of silver taken from
its bed in the bone-ash which formed the vessel, and cleaned of any
impurity; when it may be re-melted in a plumbago crucible, and cast
into ingot moulds. These moulds should be made of iron, and should
always, when used for this purpose, be warmed and greased a little,
previous to the introduction of the melted material, to prevent the
metal from spitting and adhering to it. If skilfully treated during
the process of cupellation, the desilvered lead seldom contains more
than ·002 to ·003 per cent. of silver to the test assay of 200 grs., or
between six and ten pennyweights to the ton, beyond which point it is
unprofitable to carry on the operation.
The litharge which is formed and passes off during the process
gradually grows richer in silver towards the end of the cupellation. It
probably contains after concentration about thirty to forty ounces of
silver to the ton of litharge. This is again subjected to the several
operations of the same kind for the recovery of the silver.
It is somewhat remarkable that the present method of recovering and
purifying this metal bears a strong resemblance to that employed in
ancient times, and which is spoken of in the Holy Scriptures by the
prophet Ezekiel (xxii. 18 and 20): "Son of man, the house of Israel is
to me become dross: all they are brass, and tin, and iron, and lead,
in the midst of the furnace; they are even the dross of silver." And
also, "As they gather silver, and brass, and iron, and lead, and tin,
into the midst of the furnace, to blow the fire upon it, to melt it; so
will I gather you in mine anger and in my fury, and I will leave you
there, and melt you." The celebrated metallurgist Dr. Lamborn says,
"Only those who have seen, beneath the glowing arch at the smelting
works, flames surging wave after wave across the surface of the liquid
metal, carrying all the substances, here called dross, from the pure
silver; and only those who have heard the roar of the fiery blast,
that ceases neither day nor night, until its task of purification is
accomplished,--can appreciate the terrible force of the figure made use
of by the prophet." According to the above scriptural passage it is
evident that the ancients were in possession of the first rudiments of
assaying, and understood to some extent the purification of metals; but
scriptural testimony does not point out with what amount of skill and
success these operations were performed. Judging from the appliances
which have been handed down from generation to generation, we are
inclined to think they must have been practised somewhat rudely; for
it has been left to the present school of scientific and practical
metallurgists to found and develop the art in the direction of that
commercial success to which it has at the present day attained.
This plan of cupellation which we have just described is still adopted
in many continental works in the assaying of silver-lead ores. In
England the system has been almost entirely superseded by one invented
by the late Mr. Pattinson of Newcastle, and which is confidently stated
to be far more convenient in practice.