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October 2007
By
Michael
H. Piatt
After
miners broke mineral-bearing ore from the surrounding rock and transported
it from a mine, specialized machinery pulverized and ground the ore, then
separated gold and silver from waste. Buildings
that contained this machinery were known as “stamp mills” or “quartz mills.”
Processing quartz ore required heavy piston-like vertical rods called
“stamps” that dropped repeatedly on the rock, crushing it to powder.
The number of stamps served as the measure of a mill’s capacity.
Nine quartz mills operated at Bodie during its boom: the Syndicate mill, 16 stamps, built in 1865,
increased to 20 stamps in 1878; Standard mill, 20 stamps, built in 1877; Miners
mill, 4 stamps, 1878; Bodie mill, 10 stamps, 1878; Noonday mill, 30 stamps,
1879, increased to 40 stamps in 1881; Bulwer-Standard mill, 30 stamps, 1880;
Spaulding mill, 10 stamps, 1880; Silver Hill mill, 10 stamps, 1881; and the
Bodie Tunnel mill, 15 stamps, 1881.
The challenges facing Bodie’s millmen centered on recovering an acceptable amount of gold (alloyed with silver) and silver (combined in sulphurets) within time constraints at minimum cost from tons of ore. Gold and silver sulphurets share high specific gravities, a property that allowed milling machinery to separate them from waste rock by gravity. When washed in water, gold, silver sulphurets, and rock particles that contain them, tend to settle more quickly than barren rock particles, allowing the precious metals to be trapped.
Another useful characteristic of gold, and to a lesser
degree silver sulphurets, is an affinity for
mercury. Gold particles readily adhere
to mercury, forming an “amalgam.”
Mercury, known among western miners as “quicksilver,” is a metal that is
liquid at room temperature, extremely dense, and sinks in water. Mercury also repels sand, assuring that it
will attach to gold and silver-bearing compounds then separate from barren rock
particles.
Sulphurets,
while enjoying high specific gravities, tend to resist amalgamating with
mercury. Their aversion
to mercury complicated removing them from crushed ore, and millmen
went to considerable lengths to coax sulphurets to
amalgamate.
Employing 1870s technology, all nine quartz mills (and
two tailings mills) at Bodie used a form of “pan amalgamation” known as the
“Washoe Process,” developed at Washoe two decades earlier to handle the
Comstock’s obstinate silver ores. Pan
amalgamation borrowed methods from
Other large-scale, industrialized ore-milling methods had
been implemented in silver-mining districts beyond the Comstock, but the Washoe
Process consumed the least amount of energy—an attribute essential at Bodie,
where the ore contained silver as well as gold, and which was isolated from a
fuel source. Other available processes,
such as the Reese River Process, lixiviation, and leaching would have attained
higher yields at Bodie, but they required roasting furnaces that were
prohibitively expensive to fuel. Also on
the list of rejected processes were those requiring large volumes of costly
chemicals, such as chlorination.(1) Smelting would have attained the best
results, but was not used at Bodie because it required a fuel supply vast
enough to sustain temperatures that melted the target metals. Transportation costs, whether importing fuel
or chemicals, or exporting rocks, proved to be a decisive factor for the remote
district.
Bodie’s millmen thoroughly
weighed the benefits of better results against higher costs,
given that the district’s highest-grade ore contained only minute quantities of
precious metals. This was demonstrated
in 1881 during the Bodie Mine’s bonanza, when a ton of rock bore only about 2
ounces of gold and 30 ounces of silver.
The expense of transporting cordwood
and a desire to capture silver as well as gold, led Bodie’s
millmen to select the Washoe Process, which employed
basic crushing, grinding, and amalgamation.
Only enough heat and chemicals were introduced to enhance amalgamation
and costly furnaces were omitted. Ores,
such as Bodie’s, that responded to treatment without
roasting were considered “free-milling.”
Other districts were less fortunate.
The complexity of their ores required more expensive milling methods,
and the mines had to deliver ore high enough in grade to cover the added cost.
Although
the Washoe Process minimized fuel consumption, driving the machinery inside a
Bodie quartz mill still required a tremendous amount of energy. A stationary steam engine furnished motive
power, delivered throughout the mill by belts, gears, and shafts. Steam also supplied heat to hasten
amalgamation. Voracious wood-burning
boilers produced the steam, and their fuel requirements were considerable. At the Bulwer-Standard mill, four boilers
burned 11-1/2 cords every 24 hours to power 30 stamps, 24 pans, 8 settlers, and
2 agitators. Two boilers in the Standard
mill consumed 7 cords every day running 20 stamps, 16 pans, 8 settlers, and 2
agitators.
Washoe Process. Equipment
for the Washoe Process was readily available from manufacturers in
Mining companies built quartz mills on hillsides,
preferably below the mines, so that ore wagons traveled downhill to dump their
heavy cargo. Bins above the mill held
enough rock to sustain milling during lapses in shipments.
Rock Breaker. The first step sorted the ore by size. Fist-size rocks (2-1/2 inches in diameter)
and smaller fell between parallel iron bars that formed a “grizzly.” Rocks too large to pass through the grizzly
were broken, either with sledge hammers by hand or by a mechanical rock breaker. Most rock breakers at Bodie were “jaw
crushers” designed by Eli Whitney Blake (nephew of inventor Eli Whitney), whose
products had been in
Stamps. Correctly sized ore fragments dropped into
another bin inside the mill, after which a mechanical ore-feeder fed them into
a cast-iron “mortar” in which huge stamps rose and fell. The largest stamps at Bodie, installed at the
Noonday mill in 1881, weighed 950 pounds each and dropped almost 100 times per
minute. Depending on rock hardness,
stamp weight, and drop frequency, one stamp crushed about 3 tons of ore in 24
hours.
A “battery” referred to a group of
stamps contained within one mortar, plus all their mechanical components and
supporting timbers. Most batteries
comprised five stamps side-by-side, though some batteries contained two, four,
or even seven stamps. Batteries were
placed end-to-end in a row. The
Standard’s 20-stamp mill contained four end-to-end batteries, each with five
stamps. The 30-stamp Bulwer-Standard
mill ran two parallel rows of three, five-stamp batteries
end-to-end—essentially two 15-stamp mills back-to-back.
Water
flooded the mortar as pounding stamps pulverized the ore. Because the stamps’ reciprocating action
pushed and sloshed the ore and water mixture, mill operators strove to find a
drop sequence that would not starve one stamp and choke another. Opinions flourished about the best sequence,
a persistent subject of debate among millmen.
Wire cloth or a perforated iron sheet called a “screen”
covered the mortar’s discharge side, allowing only correctly sized particles to
escape the pounding stamps. Particles
passing through the tiny apertures varied from about poppy-seed size (0.030
inch) to very fine dust. Because one
side of each mortar was essentially porous, quartz mills required an abundant
and reliable water source, the primary concern in choosing a millsite. The
Standard mill used 1,300 gallons of water for every ton of ore crushed, and
supplying enough water became problematic.
Completed in July 1877, the Standard mill drew water from a nearby
spring, which quickly proved inadequate.
In October 1878 a 4,500-foot pipe tapped a second spring to the
north. Supplementing these sources in
July 1879, the company constructed a reservoir atop the hill to receive water
pumped from the Mono Mine. After the
Mono closed in July 1880, the Standard pumped water from its own main shaft,
beginning in November, and sent it through the Bulwer Tunnel to the Standard
and the Bulwer-Standard mills. After
industrial-scale mining ceased in 1913, a well dug in the millyard
provided water during the mill’s final years.
Settling Tanks. Sluices
or “launders” directed the watery, sandy mixture, known as “pulp” (as in
“beaten to a pulp”), from the batteries to rectangular wooden tanks, where the
heaviest particles settled to the bottom, forcing excess water over the
rim. This overflow carried away very
small dust-like particulates, such as clay, silt, and fine sand that were slow
to settle. Called “slimes,” the murky
water also contained gold and silver sulphurets that
remained in suspension. Allowing
sufficient time for the miniscule particles to settle delayed milling and
conflicted with the overriding urge to move as much ore through the machinery
as quickly as possible. Believing that
clay and silt also inhibited amalgamation, mill operators willingly discarded
slimes despite assays showing that values could be substantial.
After slimes and excess water had been removed from the
crushed ore, pulp remaining in the settling tanks was now the consistency of
mortar. Laborers then hand shoveled the
“thickened” pulp from the settling tanks into amalgamating pans.
Amalgamating Pans. Central to the milling process were the
pans. Developed in 1858 to amalgamate
gold-bearing ores from
Because heat accelerated amalgamation, steam injected
into the pans elevated the pulp’s temperature almost to the boiling point. After the desired temperature had been
reached and fine grinding completed, a handwheel
raised the muller, which continued swirling as
mercury was added to the mixture.
Chemicals to facilitate amalgamation were also introduced, including
hydrochloric acid, potash, nitric acid, sulfuric acid, soda (from marshes on
Mono Basin), and no doubt an occasional cigar butt or a spattering of tobacco
juice. Most effective, however, were
salt and sulfate of copper (bluestone).
Fine grinding, heating, and prolonged exposure to mercury required 5 to
8 hours for each charge. During this
time, most of the ore’s precious metals infiltrated the mercury, to be
recovered later.
Settlers. High-pressure water flushed the
mercury-saturated pulp from the pan through a pipe into a large vat called a
“settler.” Also known as a “separator,”
the device was aptly named because it separated the gold- and silver-rich
amalgam from water and ground waste rock by allowing the heavier particles to
settle. Settlers were cylindrical tanks
7 to 10 feet in diameter, 3 to 6 feet deep, often made of wood with cast iron
bottoms. Revolving mullers
gently stirred the pulp about 12 to 15 revolutions per minute.
Slow stirring facilitated separation, assuring that the
dense amalgam (quicksilver, gold, and silver sulphurets)
worked its way downward while lighter waste material (very fine rocky particles
and water) rose. Amalgam collecting on
the settler’s bottom drained through an inverted siphon into a receiving tank
that was safeguarded by lock and key.
Operators carefully controlled the amount of water and the stirring
speed. If the pulp were too thick, or
agitation too violent, the valuable metals remained suspended. Too much water or
excessively slow stirring allowed ground rock to settle with the amalgam. Separation in settlers required 2 to 4 hours,
after which holes in the vat’s side drained off depleted pulp. Because settlers were larger than pans and
required less time, one settler usually handled charges from several pans.
Agitators. After settlers had removed a large percentage
of the gold- and silver-laden amalgam, many mills subjected the pulp to another
settling machine called an “agitator” to extract even more amalgam. Agitators were similar to settlers, though larger, usually 8 to 12 feet in diameter, 3 to 6 feet deep,
and the pulp flowed through them continuously instead of in charges. A muller rotating
about 12 revolutions per minute gently stirred the pulp. Quartz mills usually contained fewer
agitators than settlers, because agitators required less time and handled more
pulp. After agitators had removed the
last bit of amalgam, the outflow known as “tailings” discharged into “tailings
ponds.” Meanwhile, assayed samples from
points along the way monitored the mill’s effectiveness. Given this information, operators carefully
employed experience, intuition, and ritual to adjust the mill’s machinery and
optimize yield.
Cleanup Pan. About
once a month, or after completing a shipment of ore, the mill’s machinery was
stopped, disassembled, and carefully cleaned.
Workers scraped and washed pulp and amalgam from surfaces and gathered
gold nuggets from the mortars. Recovered
pulp and amalgam were carefully treated in a separate “cleanup pan.” A mill’s “cleanup” could substantially
increase the value of an ore shipment, and anxious mine managers and investors
eagerly awaited results.
Retort. After the
amalgam had been strained through chamois or flannel filters to remove excess
mercury, a special furnace or “retort” reached 675 degrees Fahrenheit to boil
away remaining quicksilver. Sealed to
prevent vaporized mercury from escaping, the retort condensed highly toxic
mercury gases, permitting liquid quicksilver to be reused by the mill. Mercury was expensive, and millmen taxed their abilities to improve its
retention. Nonetheless, some quicksilver
always wound up in the tailings. Mercury
loss was a constant concern, and replacing it cut significantly into profits.
Melting Furnace. “Sponge,” the product of retorting, consisted
of mercury-free gold and silver. A
coke-fired “melting furnace” exceeding 2,000 degrees Fahrenheit melted the
sponge in crucibles, from which a gold and silver alloy was poured into molds
to form bars of “bullion.” After
cooling, the solid bars were shipped to the
After the ore had been crushed, ground, heated,
amalgamated, settled, agitated, retorted, and melted into bullion, one might
assume that everything of value had been removed. But despite the best intentions of
manufacturers and operators, mills missed a large percentage of the precious
metals. Tailings and slimes contained a
considerable amount of gold, silver sulphurets, and
quicksilver. Because silver was more
difficult to recover than gold, mills treating Comstock silver ore routinely
missed between 25% and 35% of the ore’s assay value. Bodie’s mills,
handling ore higher in gold content, missed only about 10% to 20% of the
values.
Mill owners, according to tradition, owned tailings and
slimes: “. . . that which was caught
inside the [mill] building belonged to the mine, whatever was caught outside
belonged to the mill.” (Engineering and Mining Journal 14
February 1891, 206) Assays revealed that
tailings ponds often contained substantial values, inspiring millmen to discover ways of profiting from wasted
material. Periodically tailings and
slimes were excavated, then run through the mill
again. Second parties also worked
tailings, either purchasing the material or agreeing to pay a percentage of
their yield to the mill owners. These
operators dug and hauled tailings to “tailings mills” that employed machinery
similar to an ordinary quartz mill.
Stamps, however, were unnecessary because tailings had already been
finely ground. Bodie’s
two tailings mills were rudimentary affairs, employing only amalgamating pans,
which doubled as settlers. Boilers
produced steam to power mullers and heat the
amalgam. Thereafter, blanket sluices
took advantage of high specific gravities to catch gold, silver sulphurets, and mercury that had eluded amalgamation.
Boss Continuous Process. The Washoe Process remained Bodie’s primary ore treatment method through 1890. One exception was the Noonday mill, which
adopted an experimental variation. Just
five months after the 30-stamp mill was completed in 1880, its machinery was
rearranged into the “Boss Continuous Process.”
Patented the following year, the process Marvin P. Boss tested at Bodie
promised to automate milling and improve returns, especially from poor
ore. Boss reduced the amount of water in
the stamp batteries, permitting him to do away with the slow, labor-intensive
settling tanks that lost a high percentage of the ore’s value as slimes. Instead, crushed ore flowed from the stamps
into the first pan of a series.
Subsequent pans, settlers, and agitators, connected in sequence, allowed
the pulp to pass from one machine to the next, flowing continuously through
each. The first pans ground the crushed
ore progressively finer while incrementally increasing its temperature. Quicksilver and chemicals were added about
midway, after which more pans effected amalgamation. Separation occurred as the finely ground ore,
water, and mercury mixture passed through settlers then agitators, each drawing
off amalgam through siphons. Yields
improved largely because very fine gold and silver particulates, normally
expelled as slimes, were retained.
Mortars equipped with larger than usual screens permitted the stamps to
crush more ore in less time, increasing the mill’s capacity. The process also lessened manual labor and
reduced fuel costs.
The continuous process’s major benefits turned out to be
drawbacks. Because of its inherent
inflexibility, the system was only practical in mills that ran steadily on ore
with uniform properties. As a matter of
survival, most mills at Bodie crushed ore from different mines, and their millmen preferred the Washoe Process’s versatility. No mill at Bodie, other than the Noonday,
adopted the continuous system until the Standard mill’s renovation a decade
later.
Combination Process. When the Standard Company renovated its
20-stamp mill in 1890, it adopted Boss’s continuous process to augment a new
ore treatment method introduced by Superintendent Arthur Macy, a recently hired
college-educated mining engineer. Based
on scientific theory and meticulous calculations, Macy selected a combination
of machines that increased the mill’s volume, improved yield, and reduced fuel
costs. Macy’s informed modifications
began a new chapter in Bodie’s history by
demonstrating that profits could be made from ore so poor that it had been left
underground. Mills, such as the
renovated Standard, were called “combination mills,” because they integrated
components with specific capabilities.
Macy outfitted the mill with amalgamating plates and mechanical
concentrators, a distinctive combination that was becoming so pervasive in
Amalgamating Plates. Crushed ore splashing around inside the
mortars washed through the screens then flowed over long inclined copper sheets
that had been plated with silver then coated with quicksilver. Flowing pulp spread over the entire plate,
streaming downward in waves that rolled the sandy particles over the
surface. Particles of metallic gold
adhered to the mercury coating, forming an amalgam. Captured gold was often
referred to as “coarse gold,” a term somewhat misleading because everything
leaving the battery was smaller than a grain of salt. Workers periodically stopped the batteries,
called “hanging up the stamps,” and shut off the water. They scraped the putty-like amalgam from the
9-1/2-foot-long copper plates and retorted it, recovering about 80% of the
ore’s gold. They re-coated the plates
with mercury and restarted the stamps.
Pulp,
after flowing over the plate, still contained sulphurets
that had not been captured. The sandy
substance traveled through pipes to machines that would recover its silver-rich
compounds.
Concentrators. During western mining’s early years,
“concentration” meant isolating specific particles of crushed ore. Initially, blanket sluices or riffles caught
gold-laden sand missed by a mill’s machinery.
Later, mechanical concentrating devices captured particles dense with
valuable compounds, mainly silver. But
these particles required additional treatment to recover their precious
metals. Concentration proved especially
valuable in districts where unusually complex ore called for costly treatment
or transportation to a distant smelter.
By discarding worthless rock at the mill, only the richest particles, or
“concentrates,” received the expensive treatment.
In 1864 patented concentrating machines began offering
decided improvements over riffles and blanket sluices. “Concentrators” combined the forces of
gravity, washing, shaking, and surface tension in ways that persuaded light
waste particles to migrate in one direction while heavy particles containing
the desired metals moved in another. At
least one early model subjected a 5-foot diameter dish of water and crushed ore
to simultaneous rotating and oscillating motions that were so similar to a
prospector panning gold that the resemblance became its chief selling point.
By 1882 concentrators were manufactured in a bewildering
number of designs, each guaranteed by its manufacturer to be superior.
There
are end-shake and side-shake machines and those which oscillate. . . . Some have slow motions, others run
rapidly--in fact, almost every conceivable device to effect a separation of
heavy minerals from the light can be found already in existence, and others
continue to arrive with astonishing frequency.
(Mining and Scientific Press 4
November 1905, 305)
One popular concentrating machine, patented in 1867, consisted
of a slightly inclined shaking table over which a broad rubber belt moved. Also known as a vanner,
from the Cornish word “van” that describes testing crushed tin ore by washing a
sample in a shallow bowl-shaped shovel, the machine applied appropriate forces
to crushed ore travelling on the belt.
The
distributor spreads the pulp evenly over the surface of . . . the belt, which
is moving continuously up hill or toward the head of the machine. The side shake or lateral motion given the
bed causes the sulphurets and valuable metallic
portions of the ore to settle and lie upon the surface of the belt, and, as
they pass up hill . . . they come under the water box delivering clear water in
fine streams upon the belt; and, as the pulp passes through these streams, the
worthless or lighter portion of the ore is . . . washed from the valuable
portions, and passes down the belt into the tailings sluice. The valuable portions of the ore, which still
adhere to the belt, pass on over the head . . . and deposit themselves in [a]
small box. . . . When the small box is
filled with sulphurets it is removed and another
substituted, thus making the process of concentration continuous and
automatic. (Engineering and Mining Journal 11 July 1896, 29)
Concentrators worked exceptionally well, and having
proved themselves elsewhere were first tried at Bodie
in 1881. Manufactured according to
William B. Frue’s design, the two vanners
installed in the Silver Hill mill to concentrate sulphuret
ore from the Oro Mine were technically a success, but the closest smelter was
the Selby works in
Macy avoided paying transportation and smelting costs by
treating concentrates locally at the Standard mill, where concentrates yielded
tolerable profits when subjected to lengthy and extraordinarily harsh chemical
treatment in an amalgamating pan and settler that were “specially adapted [for]
slow treatment.” (Mining and Scientific Press 7 May 1892, 358)
After
amalgamating plates had recovered the coarse gold and vanners
collected the sulphurets, the old Washoe pans and
settlers, rearranged into the Boss Continuous Process, ground, heated,
amalgamated, settled, and agitated the pulp as it flowed from one machine to
the next. The outdated machines
recovered the last bit of value before discharging the waste into the tailings
ponds.
Four years later Macy’s successor at the Standard, Thomas
H. Leggett, brought in the cyanide process and removed the mill’s pans and
settlers. Thereafter, cyaniding became a
critical step in the ore treating process, a huge improvement over pan
amalgamation. The Washoe and Boss
processes were gone, but one pan and a settler remained to handle
concentrates. The combination of plate
amalgamation and mechanical concentration, followed by cyaniding, worked so
well that it was reproduced in 1899, when the Standard mill that survives today
at Bodie State Historic Park replaced the original mill destroyed by fire.
Amalgamating Plates Alone. After Bodie banker J. S. Cain took over the
Standard Company’s property in 1915, the milling process changed again. Cain decided not to oversee mining ventures
himself. Instead, he threw open the
mines to leasing. During the ensuing two
decades, most of the ore crushed in the Standard mill had been mined by
small-scale lessees, locally known as “leasers.” These two- or three-man operations rarely
produced enough rock to sustain the mill more than four or five days at a time,
even though only 10 of the mill’s 20 stamps were operational—kept working with parts
scavenged from the northern two batteries.(3)
The mill also stood idle for long periods between ore shipments. The mechanical concentrators and cyanide
plant were abandoned, reflecting the expense of running a high-volume process
and the financial constraints on Bodie’s residents of
that era. Avoiding what they called
“sulfide rock,” leasers selectively mined ore that they thought was rich in
gold and would pay the highest return considering their limitations. They also felt that silver was of little
value because it diluted their gold bullion.
Plate amalgamation alone recovered about 80% of the ore’s assay
value. Because sulphuret
ore was avoided, a shift occurred in the ratio of gold to silver in the
bullion. During this period, which
lasted until the mill ran for the last time in 1935, Bodie’s
bullion contained about 60% gold by weight, 40% silver.
NOTES
1.
One exception was an experimental chlorine gas
and lime solution leaching plant built on Booker Flat in 1884 to treat tailings
from the defunct Noonday mill.
Preliminary tests led to construction of a 40-ton per day plant that ran
without roasting furnaces. Sketchy
documentation suggests the operation lasted until 1889. The degree of success is unknown, but the
process was not adopted by any other Bodie mill. (Mining
and Scientific Press 22 November 1884, 328; Engineering and Mining Journal 22 November 1884, 351; California
State Mining Bureau 1890, 337-338)
2.
Copper plates covered with mercury had been used
on the Mother Lode for years to capture free gold from quartz ore, but the
plates tended to be only about 20 inches wide and were known as amalgamating
“sluices.” The Standard mill’s new
design employed plates about 4 feet wide, or the full width of the battery. The Standard also departed from the Mother
Lode’s practice of “battery
amalgamation” by not introducing mercury into the mortar during stamping.
3.
Between 1929 and 1931, the highly financed mining
conglomerate Treadwell-Yukon reopened the Red Cloud Mine and repaired the
Standard mill. The company added Union
concentrators to handle the south end’s sulphuret rock and built an ore bin on the hill behind the
mill to receive rock delivered by truck.
A new wooden trestle conveyed ore in mine cars to the mill, replacing
the inclined trestle that once delivered ore from the Bulwer Tunnel.
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