carries
away the bulk of the lighter material, leaving a black residue
consisting of magnetic iron ore and other heavy minerals, together with
any gold which may originally have been present in the mass. The
washing is repeated until enough of the enriched sand is collected,
when the gold is finally recovered by carefuj washing or "panning out"
in a smaller pan. In Mexico and South America, instead of the pan, a
wooden dish or trough, variously shaped in different districts, and
known as " batea," is used.
The
" cradle," a simple appliance for treating somewhat larger quantities,
varies in length from 3 feet 6 inches to 7 feet, but the shorter length
is that usually adopted. Its nature will be evident from fig. I, in
which a is a movable hopper with a perforated bottom of sheet iron in
which the " pay dirt" is placed. Water is poured on the dirt, and the
rocking motion imparted to the cradle causes finer particles, to pass
through the holes in the hopper on to the screen 6, which is of
canvas, and thence to the base of the cradle, where to the auriferous
particles accumulate on the transverse bars of wood f, called "
riffles." Washing by the cradle, which is now but little used except in
preliminary workings, is' tedious and expensive.
The
" torn" is a sort of cradle with an extended sluice placed on an
incline of about I foot in 12. The upper end contains a perforated
riddle plate which is placed directly over the riffle box, and under
certain circumstances mercury may be placed behind the riffles Copper
plates amalgamated with mercury are also used when the gold is very
fine, and even in some instances amalgamated silver coins have been
used for the same purpose. Sometimes the stuff is disintegrated with
water in a " puddling machine," which is used, especially in Australia,
when the earthy matters are tenacious and water scarce. The machine
frequently resembles a brickmaker's washmill, and is worked by horse or
steam power.
In
workings on a larger scale, where the supply of water is abundant, as
in California, sluices are generally employed. They are shallow troughs
about 12 feet long, about 16 to 20 inches wide, and I foot in depth.
The troughs taper slightly, sa that they can be joined in series, the
total length often reaching several hundred feet. The incline of the
sluice varies with the conformation of the ground and the tenacity of
the stuff to be washed, from I in 16 to 1 in 8.
Fig
2 represents one of the simplest forms of sluice as used in river
diggings in the north-west of America. A rectangular trough of boards,
whose dimensions depend chiefly on the size of the planks available, is
set up on the higher part of the ground at one side of the claim to be
worked, upon trestles or piers of rough stone-work, at such an
inclination that the stream may carry of all but the largest stones,
which are kept back by a grating of boards about 2 inches apart at a. The
gravel, which in this particular instance is from 12 to :6 feet thick,
and with an average breadth to the river of 25 to 30 feet, is dug by
hand and thrown in at the upper end, the stones kept back being removed
at intervals by two men with four-pronged steel forks. The floor of
-the sluice is laid with riffles made of strips of wood 2 inches square
laid parallel to the direction of the current (as at b, and in cross section at c), and at other points d with
boards having transverse notches filled with mercury. These were known
originally as Hungarian riffles The bottom of the working, which is
below the drainage level of the valley, is kept dry by a Chinese bucket
pir.r.p e, attached to a rough undershot wheel driven by the
current in the sluice. The sluice boxes are made in lengths, and united
together spigot and faucet fashion, so that they may easily be removed
and re-erected as the different parts of the claim are progressively
exhausted.
In
the larger and more permanent erections used in hydraulic mining, the
upper ends of the sluices are often cut in rock or lined with stone
btocks, the grating stopping the larger stones being known as a"
grizzly."
