after
another has been tried, and it has been found in this as in all other
appliances for treating auriferous pyrites and auriferous quartz, that
the simplest plan is invariably the most efficacious. An inclined
reverberatory furnace on the following plan is highly approved of in
Australia:—It consists of a fire-box from which the heat and products
of combustion pass over a hearth into condensing chambers. The charge
is supplied through a hopper and gradually drawn down over the hearth
by rakes until it reaches a channel near the fire-box, whence it is
drawn into a pit at the side of the furnace. Four or five small doors
are provided into which the rakes are inserted. As the charge is drawn
downwards to the bottom of the hearth it becomes gradually heated more
and more, and to such a degree as to decompose the sulphides. The roof
of the furnace over the hearth should be arched.* In a well-constructed
reverberatory (oxidating) furnace all the sulphides thrown into the bed
are completely decomposed in a short time.
A
dull red heat is maintained throughout, and until the sand is actually
raked out into the pit, there is a continuous stream of heated air
playing upon the pyritous minerals.
Metlwds of testing aurijerous quartz and auriferous pyrites.-.-Only
by a careful chemical analysis can the proportion of gold in any given
quantity of quartz, pyrites, or other mineral be ascertained; but for
all practical purposes, assays by amalgamation are sufficient. The
methods pursued in treating quartz taken from the reefs in the Wynaad
have been as follows:—
When
a reef was tested in sections, the stone from each section was taken
out in far larger quantities than were required for testing; each heap
of quartz was broken into small pieces, and the whole was well mixed,
and from the heap so mixed the portion to be treated was taken. The
stone was broken still smaller and weighed, and it was then ground very
fine on a suitable stone, another stone being used as a muller. The
finely pulverised stone was put into a clean iron pan and roasted until
fumes were no longer given off until it was certain that it was in a
lit state for amalgamation. If after roasting the pulverised material
seemed to require it, it was again ground on the stone. The heat at all
times was so regulated as to prevent the possibility of " glazing."
The roasted stone was put into an enamelled dish, and a proper proportion of quicksilver was added.
The
whole was then thoroughly rubbed by hand, at first dry; subsequently a
little cold or hot water was poured in until a paste was formed; more
water was added, the stuff being thoroughly rubbed all the time, and
finally the amalgam or (if there was no gold) the quicksilver was
washed off, the utmost care being taken to prevent the loss of any
quicksilver. The water and sludge from the enamelled dish were poured
into other vessels and these were most carefully examined subsequently,
and if there was doubt,—and sometimes when there was no doubt as to the
results, the s,ludge-sand, &c, in these were treated again,— the
amalgam or quicksilver was most often placqd under the flame of the
blowpipe, but at other times nitric acid was used and the gold (if any
and of sufficient quantity) was weighed.
The results were calculated in the ordinary way,
It
happened occasionally that the roasting was not sufficiently
protracted, or that all the material subjected to the roasting was not
ground as fine as it ought to have been, and then it became necessary
to subject it to further roasting, or to use acids to decompose the
sulphides. In all cases the utmost care was taken to preserve the
conditions necessary for effective amalgamation.
The quartz collected in various localities was treated in the same manner as that taken from sections of the reefs.
In order the better to illustrate the value of this system of testing quartz, \ took a quantity of sludge and sand from which the gold had been extracted, and which had been saved in pans, and into this I put half a grain of very
