SPECIFIC GRAVITY.
The relation of the weight of a substance to its volume should be kept
in mind in all cases where both weight and volume are dealt with.
Students are apt to imagine that on mixing equal volumes of, say,
sulphuric acid and water, an acid of half the strength must be obtained.
If the statement of strength is in parts by weight this will lead to
considerable error. For example, 100 c.c. of sulphuric acid containing
98 per cent. by weight of real acid, will, if diluted with 100 c.c. of
water, yield a solution containing not 49 per cent. by weight, but about
63.5 per cent. of the acid. The reason is this: the 100 c.c. of
sulphuric acid weighs 184 grams, and contains 180.32 grams of real acid,
while the 100 c.c. of water weighs only 100 grams; the mixed water and
acid weighs 284 grams, and contains 180.32 of real acid, which is
equivalent to nearly 63.5 per cent. by weight. If, however, the method
of statement be volumetric, it would be correct to say that doubling the
volume halves the strength: if 100 c.c. of brine contains 10 grams of
salt, and is diluted with water to 200 c.c., it would be of one-half the
former strength, that is, 100 c.c. of the solution would contain 5 grams
of salt.
This confusion is avoided by always stating the strengths as so many
grams or "c.c." in 100 c.c. of the liquid. But obviously it would be
advantageous to be able to determine quickly the weight of any
particular substance corresponding to 1 c.c. or some other given volume.
Moreover, in descriptions of processes the strengths of acids and
solutions are frequently defined neither by their gravimetric nor
volumetric composition, but by a statement either of specific gravity or
of the degrees registered by Twaddell's or Beaumé's hydrometer. Thus, in
the description of the process of gold parting, one writer gives: "The
acid should be of 1.2 specific gravity"; and another says: "The acid
must not be stronger than 32° Beaumé."
These considerations justify an account of the subject in such a work as
this. And on other grounds the determination of a specific gravity is
one of the operations with which an assayer should be familiar.
The meaning of "specific gravity" is present in the mind of every one
who uses the sentence "lead is heavier than water." This is meaningless
except some such phrase as "bulk for bulk" be added. Make the sentence
quantitative by saying: "bulk for bulk lead is 11.36 times heavier than
water," and one has the exact meaning of: "the specific gravity of lead
is 11.36." A table of the specific gravities of liquids and solids shows
how many times heavier the substances are than water.
It is better, however, to look upon the specific gravity (written
shortly, sp. g.) as the weight of a substance divided by its volume. In
the metric system, 1 c.c. of water at 4° C. weighs with sufficient
exactness 1 gram; consequently, the sp. g., which states how many times
heavier than water the substance is, also expresses the weight in grams
of one c.c. of it. So that if a 100 c.c. flask of nitric acid weighs,
after the weight of the flask has been deducted, 120 grams, 1 c.c. of
the acid weighs 1.2 gram, and the sp. g. is 1.2. The specific gravity,
then, may be determined by dividing the weight of a substance in grams
by its volume in c.c.; but it is more convenient in practice to
determine it by dividing _the weight of the substance by the weight of
an equal volume of water_. And since the volumes of all substances,
water included, vary with the temperature, the temperature at which the
sp. g. is determined should be recorded. Even then there is room for
ambiguity to the extent that such a statement as the following, "the
specific gravity of the substance at 50° C. is 0.9010," may mean when
compared with water at 50° C. or 4° C., or even 15.5° C. For practical
purposes it should mean the first of these, for in the actual
experiments the water and the substance are compared at the same
temperature, and it is well to give the statement of results without any
superfluous calculation. In the metric system the standard temperature
is 4° C., for it is at this point that 1 c.c. of water weighs exactly 1
gram. In England, the standard temperature is 60° F. (15.5° C.), which
is supposed to be an average temperature of the balance-room. The
convenience of the English standard, however, is merely apparent; it
demands warming sometimes and sometimes cooling. For most purposes it is
more convenient to select a temperature sufficiently high to avoid the
necessity of cooling at any time. Warming to the required temperature
gives very little trouble.
~Determination of Specific Gravity.~--There is a quick and easy method
of determining the density or sp. g. of a liquid, based upon the fact
that a floating body is buoyed up more by a heavy liquid than by a light
one. The method is more remarkable for speed than accuracy, but still
is sufficiently exact. The piece of apparatus used for the purpose is
endowed with a variety of names--sp. g. spindle, hydrometer, areometer,
salimeter, alcoholimeter, lactometer, and so on, according to the
special liquid upon which it is intended to be used. It consists of a
float with a sinker at one end and a graduated tube or rod at the other.
It is made of metal or glass. Generally two are required, one for
liquids ranging in sp. g. from 1.000 to 2.000, and another, which will
indicate a sp. g. between 0.700 and 1.000. The range depends on the size
of the instrument. For special work, in which variations within narrow
limits are to be determined, more delicate instruments with a narrower
range are made.
[Illustration: FIG. 34.]
In using a hydrometer, the liquid to be tested is placed in a cylinder
(fig. 34) tall enough to allow the instrument to float, and not too
narrow. The temperature is taken, and the hydrometer is immersed in the
fluid. The mark on the hydrometer stem, level with the surface of the
liquid, is read off. With transparent liquids it is best to read the
mark under and over the water surface and take the mean.
The graduation of hydrometers is not made to any uniform system. Those
marked in degrees Baumé or Twaddell, or according to specific gravity,
are most commonly used. The degrees on Baumé's hydrometer agree among
themselves in being at equal distances along the stem; but they are
proportional neither to the specific gravity, nor to the percentage of
salt in the solution. They may be converted into an ordinary statement
of specific gravity by the following formulæ:--
Sp. g. = 144.3/(144.3-degrees Baumé.)
or putting the rule in words, subtract the degrees Baumé from 144.3, and
divide 144.3 with the number thus obtained. For example: 32° Baumé
equals a sp. g. of 1.285.
144.3/(144.3-32) = 144.3/(112.3) = 1.285
This rule is for liquids heavier than water; for the lighter liquids the
rule is as follows:--
Sp. g. = 146/(136 + degrees Baumé.)
or in words divide 146 by the number of degrees Baumé added to 136. For
example: ammonia of 30° Beaumé has a sp. g. of 0.880 (nearly).
146/(136+30) = 146/166 = 0.8795
A simple series of calculations enables one to convert a Beaumé
hydrometer into one showing the actual sp. g. Graduation, according to
sp. g. is the most convenient for general purposes. In these instruments
the distances between the divisions become less as the densities
increase.
Twaddell's hydrometer is graduated in this way: Each degree Twaddell is
0.005 in excess of unity. To convert into sp. g. multiply the degrees
Twaddell by 0.005, and add 1. For example: 25° Twaddell equals a sp. g.
of 1.125.
25×.005 = 0.125; + 1.000 = 1.125.
There is a practice which ignores the decimal point and speaks of a sp.
g. of 1125 instead of 1.125. In some cases it is convenient, and
inasmuch as no substance has a real sp. g. of much over 20, it can lead
to no confusion. The figures expressed in this way represent the weight
of a litre in grams.
Some hydrometers are graduated so as to show at a glance the percentage
composition of the liquid they are intended to be used with. Gay-Lussac
designed one to show the alcoholic strength of mixtures of alcohol and
water; the construction of others upon the same principle is easy and
perhaps useful. But when the principle is applied to complex liquids and
mixed solutions, it is misleading.
The various methods of graduation ought all to give place to one showing
a simple statement of the sp. g.
The method of determining sp. g. with the hydrometer is obviously
inapplicable to the case of solids, and in the case of liquids it should
not be used where exact figures are required. There are several other
methods which may be used, but on the whole those with the specific
gravity bottle are most convenient.
[Illustration: FIG. 35.]
~The specific gravity bottle~ (fig. 35) is a light flask of about 25
c.c. capacity, provided with a well-fitting perforated stopper. It is
essentially a graduated flask, which measures a constant volume, but it
does not much matter what the volume is.
_In taking the sp. g. of a liquid_ (_or, what is the same thing, a fused
solid_) there is wanted the weights (1) of the flaskful of water and (2)
of the flaskful of the liquid. Dividing the second by the first gives
the required sp. g. The actual weighings required are--
(1) of the dry and empty flask,
(2) of the flask filled with water, and
(3) of the flask filled with the liquid.
The weighing of the flask once made need not be often repeated. It is
well to do so now and then for safety's sake; but one weighing will
serve for a large number of determinations. The same remarks apply to
the weighing of the bottle filled with water. The bottle is dried by
rinsing out first with alcohol and afterwards with ether; ether is very
volatile, and a short exposure in a warm place will soon drive off the
little remaining about the sides. The ether vapour should be sucked out
through a glass tube. See that the bore of the stopper is dry as well as
the bottle. Let the dry bottle stand in the box of the balance for a
minute or two before weighing. The weight is, strictly speaking, not
that of the empty bottle, but of the bottle filled with air. The empty
bottle would weigh from 20 to 30 milligrams less. Correcting for this
would, in most cases, only make a difference in the fourth place of
decimals,[8] so that it is better to ignore the error.
The weight of the flask filled with water is got by filling it with
distilled water, and inserting the stopper. The excess of water will
overflow at the margin and through the bore. The bottle is wiped with a
soft, dry cloth, taking care not to squeeze or warm the bottle. The
bottle will remain filled to the top of the stopper. It is allowed to
stand in the balance box for a minute or two, and then weighed.
Distilled water, as stated, should be used; the use of ordinary water
may increase the weight by 5 or 6 milligrams. Many waters, if they have
not previously been boiled, give off bubbles of air which render the
weighing worthless.
The temperature of the water is of greater importance; lowering the
temperature 2° will increase the weight by 10 or 12 milligrams. A beaker
of water may be warmed or cooled to the required temperature; then the
bottle is filled from it, and quickly weighed. If the balance-room is
cooler than the water, the latter will draw back into the bottle, and a
few small bubbles of air will enter; but even in extreme cases this will
only increase the weight by a very small fraction of a milligram. There
is more trouble caused when the room is warmer, for the liquid then
expands and protrudes as a drop resting on the top of the stopper.
There will in this case be loss by evaporation, which in the case of the
more volatile liquids, such as alcohol, is serious. To prevent this
loss, as well as any that may arise by overflow, the stopper should be
dilated above into a small cup, A (fig. 36), which may itself be
stoppered. In a bottle of this kind the neck of the stopper is
graduated, and the bottle is considered full when the liquid stands at
the level of the mark in the neck. On inserting the stopper, the liquid
rises into the cup, and is reduced to the level of the mark by
absorption with pieces of filter-paper.
[Illustration: FIG. 36.]
For most purposes, however, there is no need for cooling and allowing
room for subsequent expansion. The assayer, as a rule, can select his
own standard temperature, and may choose one which will always
necessitate warming. It will be handier in this case to have a bottle
with a thermometer stopper. Of the two types shown in fig. 37, that with
the external thermometer tube (A) is more generally useful.
[Illustration: FIG. 37.]
The bottle is filled at a lower temperature, and is then gently warmed
so as to slowly raise the temperature to the required degree. The
superfluous liquid is then at once wiped off, and the bottle cooled and
weighed.
The weight of the flask filled with the liquid whose sp. g. has to be
determined is ascertained in a similar way. Of course the temperature
must be the same. If the liquid does not mix with water, the bottle
should be dried before filling, but otherwise the flask need only be
rinsed out two or three times with the liquid.
Having obtained the three weighings, deduct the weight of the bottle
from each of the others to get the weights of the water and liquid
respectively. Divide the latter by the former, the result shows the sp.
g. As an example, take the following, in which a rather large sp. g.
bottle was used:--
1. Weight of bottle 39.299 gram
2. Weight of bottle and water 81.884 "
3. Weight of bottle and paraffin 73.146 "
By subtracting 1 from 2 and 3 the result is as follows:--
81.884 grams 73.146 grams
39.299 " 39.299 "
------ ------
42.585 of water. 33.847 of paraffin.
Divide the weight of the paraffin by that of the water--
42.585)33.8470(0.7948
29.8095
-------
.......
The sp. g. of the paraffin is 0.7948.
_The sp. g. of a fusible solid_ may be obtained in the same way at a
temperature some degrees above its fusing point.
_The sp. g. of a solid in powder or gravel sufficiently fine to pass
through the neck of the bottle_ is easily determined. If the bottle
filled with water weighs 50 grams, and there is placed on the pan
alongside of it 20 grams of a sand, the weight of the two together will
of course be 70 grams. But if the sand is put in the bottle, it
evidently displaces its own bulk of water; and if, on again weighing,
the weight is found to be 62 instead of 70 grams, it is because the 20
grams of sand has displaced 8 grams of water. Bulk for bulk, the sand is
2-1/2 times as heavy.
In practice, the weight of the bottle filled with water will probably be
already known; if not, it must be determined. A certain quantity, say 20
grams, of the powdered substance is then transferred carefully to the
bottle. The bottle need not be dry inside, but its neck and outside must
be. In making this transference a careful worker will make no loss, and
the mode of working saves a little time. But it is better to weigh the
dry flask; put into it 10 to 20 grams of the powder, and weigh again.
The increase in weight gives accurately the weight of powder in the
bottle. About two-thirds fill the bottle with distilled water, and mix
with the powder by gentle shaking. Air bubbles will disentangle
themselves, and rise to the surface of the water. Wash back anything
adhering to the stopper with a jet of water, and fill the bottle almost
to overflowing. Allow it to stand for a minute or so; replace the
stopper; warm to the required temperature; take off the superfluous
moisture; wipe and weigh. As an example, take the following:--
1. Weight of bottle 12.681 grams
2. " " bottle filled with water 37.708 "
3. " " bottle with wolfram 40.821 "
4. " " bottle with wolfram and water 61.199 "
Subtract (1) from (3) to get the weight of wolfram taken:
40.821 grams
12.681 "
------
28.140 "
add the weight of the wolfram to the weight of the bottle filled with
water:
28.140 grams
37.708 "
------
65.848 "
subtract (4) from this to get the weight of water displaced:
65.848 grams
61.199 "
------
4.649 "
Divide the weight of the wolfram by the weight of the water displaced to
get sp. g.:
4.649)28.140(6.053
27.894
------
......
_If the solid is soluble in water, or has a tendency to float_, some
liquid other than water is used. Paraffin oil or oil of turpentine will
do. The process is as follows:--The weight of the dry and empty bottle
having been determined, add a sufficiency of the substance and weigh
again to find how much has been added. Fill up with paraffin oil and
weigh again. Clean out the substance by rinsing with paraffin; fill up
and weigh. Calculate the sp. g. as if water had been used, and multiply
by the sp. g. of the paraffin.
For example:
1. Weight of bottle 39.299 grams
2. " " bottle and nitre 57.830 "
3. " " bottle and paraffin 73.146 "
4. " " bottle and paraffin and nitre 84.665 "
5. " " bottle and water 81.884 "
First from (1),(3), and (5), calculate the sp. g. of the paraffin as
already shown. It will be 0.7948. Deduct (1) from (2) to get the weight
of the nitre:
57.830 grams
39.299 "
------
18.531 "
add this to (3):
18.531 grams
73.146 "
------
91.677 "
and deduct (4) to find the weight of the equal bulk of paraffin.
91.677 grams
84.665 "
------
7.012 "
divide the weight of the nitre by the weight of the paraffin:
7.012)18.531(2.6427
------
......
The sp. g., taking paraffin as the standard instead of water, is 2.6427.
Multiply this by the sp. g. of paraffin, 0.7948, and the result is
2.1004 as the sp. g. of nitre compared with water.
Similarly, a sp. g. compared with water at say 50° C. can be converted
into one compared with water at standard temperature, by multiplying by
the sp. g. of water at 50° C. The following table gives the sp. g. of
water at various temperatures:--
-----------+------++-----------+------++-----------+-------
Degrees | || Degrees | || Degrees |
Centigrade.|Sp. G.||Centigrade.|Sp. G.||Centigrade.|Sp. G.
-----------+------++-----------+------++-----------+-------
4° |1.0000|| 20° |0.9982|| 40° |0.9923
10° |0.9997|| 25° |0.9971|| 50° |0.9881
15° |0.9991|| 30° |0.9957|| 100° |0.9586
-----------+------++-----------+------++-----------+-------
If, for example, a substance at 50° C. has a sp. g. of 0.9010 as
compared with water at 50° C., it will have (compared with water at 4°
C.) a sp. g. of 0.9010×0.9881; or 0.8903. The figures 0.8903 represent
the sp. g. of the substance at 50° C. compared with water at 4° C.
Except in comparing the sp. gravities of the same substance at different
temperatures, a calculation of this kind serves no useful purpose.
_In taking the specific gravity of a solid not in powder_, a lump of it
is freed from loose particles and its exact weight determined. By means
of a horse hair with a slip knot it is suspended to the balance, and
beneath it is placed, out of contact with the balance pan, a beaker of
distilled water. The horse hair must be long enough to keep the mineral
well beneath the surface of the water so as to allow the balance to
vibrate. Air bubbles are removed by touching with a camel-hair pencil.
Whilst the mineral is suspended in water the weight is again taken. It
will weigh less than before, and the difference between the two
weighings gives the weight of water (and consequently the volume)
displaced by the mineral. The weight in air divided by the difference is
the specific gravity. Thus
Weight in air 3.2170 grams
Weight in water 2.7050 "
------
Difference 0.5120 gram
3.2170/0.5120 equals 6.28, the sp. g.
The sp. g. of a substance depends mainly on its composition, but is
affected by certain conditions. The effect of temperature has been
already considered. Air holes and empty spaces lessen the specific
gravity of otherwise solid bodies; and metals, which after fusion become
imperfect solids, have their density increased by hammering or rolling.
But metals when free from pores have their density diminished when
rolled, without annealing. The effects of these conditions are slight
when compared with those due to the presence of impurities.
For simple substances, or mixtures of only two substances, a
determination of sp. g. is a sufficient check on the composition for
many practical purposes; and with more complex mixtures, such as slags
and some of the products of dressing operations in which the material
does not differ much in its nature from time to time, such a
determination will yield information of considerable value, and afford a
check upon the proper working of a process.
When the mixing of two substances is accompanied by a change in volume,
the sp. g. of the mixture can only be learnt by experiment. But when the
substances have no such action on each other the resulting sp. g. can be
calculated. Some of these calculations have a practical interest as well
as an educational value. Students should practise them so as to become
familiar with the relations between weight and volume.
_When substances are mixed by volume_, the sp. g. of the mixture is the
mean of those of its constituents, and may be calculated in the usual
way for obtaining averages. 1 c.c. of a substance having a sp. g. of 1.4
mixed with 1 c.c. of another having a sp. g. of 1.0 will yield 2 c.c. of
a substance having a sp. g. of 1.2. If, however, we write gram instead
of c.c. in the above statement, the resulting sp. g. will be 1.16. The
simplest plan is to remember that the sp. g. is the weight divided by
the volume (sp. g. = w/v) and the sp. g. of a mixture is the sum of the
weights divided by the sum of the volumes (sp. g. = (w + w' + w",
&c.)/(v + v' + v", &c.)). In the above example the sum of the volumes is
2 c.c.; the weights (got by multiplying each volume by its
corresponding sp. g.) are 1.4 gram and 1 gram. The sum of the weights
divided by the sum of the volumes is 2.4/2 or 1.2.
The sp. g. of a mixture of 10 c.c. of a substance having a sp. g. of
1.2, with 15 c.c. of another having a sp. g. of 1.5 may be thus found:--
sp. g. = (12+22.5)/(10+15) = 1.38
multiply each volume by its sp. g. to get its weight:
10×1.2 = 12 15×1.5 = 22.5
add these together (12+22.5 = 34.5) and divide by the sum of the volumes
(10+15 = 25):
25)34.5(1.38
25
--
95, &c.
The sp. g. will be 1.38, provided the mixture is not accompanied by any
change of volume.
The same formula will serve when the proportion of the ingredients is
given by weight. A mixture of 4 parts by weight of galena (sp. g. 7.5)
with 5 parts of blende (sp. g. 4) will have a sp. g. of 5.06:
sp. g. = (4+5)/(0.53+1.25) = 9/1.78 = 5.06
It is necessary in this case to calculate the volumes of the galena and
of the blende, which is done by dividing the weights by the sp.
gravities: thus, 4 divided by 7.5 gives 0.53 and 5 divided by 4 gives
1.25.
The converse problem is a little more difficult. Given the sp. g. of a
mixture and of each of the two ingredients, the percentage by weight of
the heavier ingredient may be ascertained by the following rule, which
is best expressed as a formula. There are three sp. gravities given; if
the highest be written H, the lowest L and that of the mixture M, then:
Percentage of heavier mineral = (100×H×(M-L))/(M×(H-L))
Suppose a sample of tailings has a sp. g. of 3.0, and is made up of
quartz (sp. g. 2.6) and pyrites (sp. g. 5.1): then the percentage of
pyrites is 27:
(100×5.1×(3-2.6))/(3×(5.1-2.6)) = (510×0.4)/(3×2.5) = 204/7.5 = 27.2
The same problem could be solved with the help of a little algebra by
the rule already given, as thus: the sp. g. of a mixture equals the sum
of the _weights_ of the constituents divided by the sum of the
_volumes_. Then 100 grams of the tailings with _x_ per cent. of pyrites
contain 100-_x_ per cent. of quartz. The sum of the weights is 100. The
volume of the pyrites is _x_/5.1 and of the quartz (100-_x_)/2.6.
Then we have by the rule
3 = 100/((_x_/5.1)+(100-_x_)/2.6)
3 = 1326/(510-2.5_x_)
204 = 7.5_x_
and _x_ = 27.2
If the percentage (P) and sp. g. (H) of one constituent and the sp. g.
(M) of the mixture are known, the sp. g. of the other constituent may be
calculated by the following formula, in which _x_ is the required sp.
g.:
_x_ = ((100-P)×M×H)/((100×H)-(P×M))
For example, "tailings" (sp. g. 3.0) containing 27.2 per cent. of
pyrites (sp. g. 5.1) will contain (100-27.2), 72.8 per cent. of earthy
matter having a mean sp. g. of _x_:
_x_ = ((100-27.2)×3×5.1)/((100×5.1)-(27.2×3))
= 1113.84/428.4 = 2.6
The differences in sp. g. corresponding to differences in strength have
been carefully determined and tabulated in the case of the stronger
acids and of many other liquids. Such tables are given at the end of
this book.
_To Calculate the Weight of a Measured Volume of Mineral or
Rock._--Multiply the cubic feet by 62.4 and then multiply by the sp. g.
of the stuff, the answer gives the weight in pounds. For example, 100
cubic feet of quartz weighs 100×62.4×2.6 = 16,224 lbs. The weight of any
mass of mineral of known extent and sp. g. is ascertained in this way.
The following table gives the specific gravities of some of the commoner
minerals.
Barytes 4.5
Blende 4.0
Calcite 2.6
Cassiterite 6.9
Chalybite 3.8
Copper pyrites 4.2
Fluor 3.1
Galena 7.5
Hæmatite 5.0
Mispickel 6.2
Pyrites 5.0
Quartz 2.6
FOOTNOTES:
[8] The difference of 20 or 30 milligrams is disregarded here because it
detracts equally from the actual weight of the water and liquid to be
determined. If the liquid is a heavy one the difference shows itself in
the third or second place of decimals. The correction may be made by
deducting from the weight of the flask 0.0012 grams for each gram of
water it holds.
PART II.--THE METALS.
CHAPTER IX.
SILVER, GOLD, CYANIDES, PLATINUM, MERCURY.
SILVER.
Silver is widely diffused, and has been found in most mining districts.
It occurs native in sufficient quantity to constitute one of the chief
ores of the metal. It also occurs combined with sulphur (as in
argentite), with sulphur and antimony (as in stephanite or brittle
silver ore, and in pyrargyrite or ruby silver), and with copper,
sulphur, antimony, and arsenic, as in polybasite. Chloride of silver
occurs native as horn silver or kerargyrite. Silver is found in the ores
of other metals, such as fahlerz, which sometimes contains from two to
ten per cent. of the metal, and galena, which is an important source of
it; in fact, galena is never found entirely free from silver. It is
present also in greater or less quantity in the ores of copper and zinc.
Silver dissolves readily in nitric acid, forming silver nitrate. It only
forms one family of salts, and of these the chloride and nitrate are of
chief importance to the assayer. The formation of the chloride of silver
on the addition of hydrochloric acid or a soluble chloride to the nitric
acid solution, serves for the recognition and separation of silver. The
precipitated chloride is white (becoming violet on exposure to light),
insoluble in nitric acid, soluble in ammonia, hyposulphite of soda, or
concentrated solutions of chlorides. The best confirmatory test is made
by wrapping the precipitate in a little sheet lead, and cupelling, when
the silver will be left in the metallic state, and is easily recognized.
~Dry Assay.~--This assay is made up of two parts: (1) the concentration
of the silver in a button of lead; and (2) the cupellation of the
resulting alloy. The concentration of the button of lead may be
effected either by scorification or by fusion in a crucible.
The scorification assay is performed in a scorifier, which is a shallow
open-mouthed dish about 2-1/2 inches across, with a very thick bottom to
enable it to withstand the corrosive action of the slag. A charge of
more than 3 or 5 grams of the ore cannot be worked in one, and with such
small charges the unavoidable variations have a serious effect on the
figures reported. A difference of one milligram on the weight of the
button of silver got represents a difference of 6 or 10 ounces per ton.
With rich ores such variation is unavoidable under any conditions, and
the only safe plan is to take the mean of several assays. But with
poorer ores the accuracy of the assay, as well as convenience in
working, is much increased by working in a crucible with larger charges.
In scorification the proportion of lead required for scorifying 1 gram
of ore is in average cases from 10 to 15 grams, sinking in the case of
galena to 2 grams, and rising with earthy and refractory substances to
from 30 to 40 grams. But by fusing in a crucible with well-selected
fluxes, a proportion of 4 of flux to 1 of ore is generally sufficient;
and not only is the proportion of added matter less, but it is also
easier to manipulate large quantities in crucibles, so that, although in
some cases the crucible assay is more troublesome and less satisfactory,
yet with poor and earthy ores it is the best method of dealing with
them; while when properly worked it yields results as accurate as
scorification does. As a general rule, if more than 5 grams of ore must
be taken, the crucible assay should be adopted.
[Illustration: FIG. 38.]
~Scorification Assay.~--The charge of ore is usually 3 grams, sometimes
5; the lead varies from 30 to 70 grams, and the quantity of soda, borax,
or powdered glass added varies from 0.3 to 3 or 4 grams. It is generally
recommended to have the lead granulated,[9] and to mix the ore with
about half of it in the scorifier; then to put on the rest of the lead;
and finally to sprinkle the borax or glass on the top. It answers just
as well, however, to use the lead in the shape of foil, and wrap the ore
up in it; and if the ore contains much sulphur, the borax may with
advantage be added (wrapped in a little tissue paper) some five or ten
minutes after the operation has started.
[Illustration: FIG. 39.]
The process of scorification is as follows:--A scorifier (fig. 38) of
convenient size having been selected (one 2-1/2 inches across is most
generally useful), it is dried at a gentle heat for about ten minutes.
The charge is then put into it, and it is introduced, with the help of a
scorifier tongs (fig. 39), into a muffle heated considerably above
redness. The muffle is then closed, and when the metal has melted down,
it is opened, but the temperature is kept up. A ring of slag will, after
a time, form around the metal, and when this appearance (known as the
eye) presents itself, the temperature may be lowered. When the eye has
disappeared--that is, when the layer of slag has quite closed in--a
pinch of powdered culm wrapped in tissue paper is added. As soon as the
slag has again become tranquil, the scorifier is taken out, and its
contents are poured into a mould (fig. 40), the slag is detached, and
saved. If the button of metal weighs more than 30 grams, its size is
reduced by another scorification in the same scorifier, which should
have been replaced in the muffle immediately after the contents had been
poured out. If the ore is not a very rich one, the button of lead will
carry practically all the silver; but with rich ores it is more
satisfactory to save the slag, and subsequently to melt it down with the
cupel on which the lead has been treated, so as to recover the silver
lost in the slag, together with that absorbed in the cupel, at one
operation. Or, if the cupellation loss is neglected or calculated in
some other manner, the slag or slags from the scorifier may be powdered
and mixed with 20 grams of oxide of lead, 5 grams of borax, and 1 gram
of charcoal. This should be melted down in a small crucible, and the
resulting button of lead cupelled.
[Illustration: FIG. 40.]
If the scorification has been unsatisfactory, the quantity of silver
obtained from the slag will be by no means inconsiderable. The usual
explanation is that with sulphury ores compounds of metallic oxides and
sulphides (oxysulphides) are formed, which remain in the slag, retaining
considerable quantities of the precious metal. It is said that under
certain conditions such a slag may contain as much as 10 per cent. of
silver. An excess of lead and a high temperature prevents the formation
of these oxysulphides. But if much silver is present in the ore, the
slag cannot be safely thrown away, even if sulphur is absent, and the
process has been satisfactorily performed.
If the crust which appears on the surface of the lead does not clear,
add a small lump of borax and 20 grams more lead; then close the muffle,
and keep the temperature as high as possible. If the slag forms
properly, but shows unfused or only half-fused lumps, even when the
scorification has proceeded for some time, add more borax, and stir with
an iron rod. The slag adhering to the rod must be detached by hammering,
and replaced in the scorifier.
If the ore consists largely of quartz, soda should be added instead of
borax; or, if it contains much copper, powdered quartz may be used. If
the scorifier at the end of an operation is more than usually corroded,
the borax should be replaced in subsequent assays on similar ores by
powdered glass or quartz.
If a fairly fluid slag is formed which does not clear from the metal and
show the eye, more lead and a higher temperature is wanted.
As a general rule, it may be stated that when a scorification is
unsatisfactory, what is wanted is more heat, more lead, or more borax.
It is a safe plan when work has to be done on a strange ore, to make
three or four assays with varying quantities of lead. The proportion of
lead is right when a further addition does not yield a higher result.
The proper proportion having been found, a note of it should be made for
future use.
POT ASSAYS.
The object of the fusion in a crucible, like that of scorification, is
to concentrate the silver in a button of lead which is to be
subsequently cupelled; and to retain the earthy and waste matters in the
slag. It is necessary to consider the quality of the slag and the weight
and quality of the lead. The slag when fused should be liquid and
homogeneous, and not too corrosive on the crucible. The button of lead
should be soft, malleable, and free from a coating of regulus.[10] In
weight it should not differ much from the ore taken. With 20 grams of
ore, for example, a button of lead weighing from 18 to 25 grams will be
satisfactory: less than this would leave an undue proportion of silver
in the slag; and more would be unnecessarily large for cupelling, and
would increase the loss in that operation.
With average ores, take 20 grams of the powdered ore and mix with 30
grams of "soda," 40 grams of red-lead or litharge, 5 grams of borax, and
from 2 to 2.5 grams of flour, and place in an E crucible (Battersea
round). Put these in the furnace at a red heat, cover the crucible, and
gradually raise the temperature until the whole charge has melted down
and is in a state of tranquil fusion. Pour into a mould, and replace the
crucible in the furnace. As soon as the lead is solid, detach the slag
and put it back into the crucible; and when it is again fluid, charge on
to it with a copper scoop a mixture of 20 grams of oxide of lead, and 1
gram of charcoal: when fusion has again become tranquil, pour and detach
the button of lead. The lead buttons should be hammered into discs with
rounded edges, and be freed from slag; if too big for a cupel they may
be scorified together in a small scorifier, but it is better to cupel
them separately.
~Ores containing Metallic Oxides.~--Peroxides of iron, manganese, and
copper interfere by counteracting the effect of the charcoal or flour,
and thus reducing the size of the lead button. Peroxide of iron will
reduce the weight of lead by a little more than its own weight; and
peroxide of manganese has about twice this effect. When these oxides are
present an additional quantity of flour must be used, and precautions
must be taken to prevent reoxidation of the slag by the furnace gases.
This may best be prevented by using a layer of common salt as a cover to
the charge. When the ores contain a good deal of quartz or stony matter,
the fluxes just given (for average ores) will do; but the proportion of
soda should be diminished, and that of the borax, oxide of lead, and
flour increased as the quantity of metallic oxides become greater. If
the ore contains practically no quartz, the soda may be altogether
omitted, and some glass or powdered quartz added. The following charge
may be taken as an example: weigh up 20 grams of the powdered ore, 15
grams each of "soda" and borax, 60 grams of oxide of lead, and 5 grams
of flour. Mix and place them in an E crucible, and cover with a layer of
from a quarter to half an inch of common salt. Place in the furnace as
before. The salt will give off a considerable amount of fume, which
will, to a certain extent, conceal the state of the charge: when the
crucible has been in the furnace for about 25 minutes remove it and pour
out the contents immediately. With ores that produce a thick slag the
addition of 5 grams of fluor spar will be an advantage. It may happen
that with an unknown ore the first assay will be more or less
unsatisfactory: but from it the necessity for adding more or less flour
will be learnt, and a second assay, with the necessary modification of
the charge, should give a good result.
~Ores containing much Sulphides.~--Ores of this class may be easily
recognized, either by the appearance of the minerals they contain or by
the odour of sulphurous oxide (SO_{2}) which they evolve when roasted on
a spatula. The sulphides most commonly present, in addition to the
sulphurized minerals of silver, are pyrites, galena, blende, and
mispickel. When they are present in only a moderate amount, their effect
is simply to increase the weight of the button of lead; and this is
easily counteracted by reducing the amount of flour, or by omitting it.
When in larger amounts, they not only yield large buttons, but also
render the metal sulphury, sometimes even giving a button of regulus
instead of lead. This last evil may be remedied (1) by putting in a rod
of iron as soon as the charge has fused, or (2) it may be counteracted
by a proper addition of nitre, or (3) when the sulphides present are
only those of iron or copper the sulphur may be removed by calcining,
and the ore converted into one of the class containing metallic oxides.
The calcination is effected as follows:--Weigh up 20 grams of the
powdered ore and place it in a wide-mouthed crucible sufficiently large
to perform the subsequent melting down in. The roasting must be done at
a gentle heat at first, so as to avoid clotting: the mouth of the
crucible should project considerably above the coke, and should slope
forward towards the worker. The charge must be occasionally stirred with
the stirrer (fig. 10) so as to expose fresh surfaces to the action of
the air, and to prevent adhesion to the sides of the crucible. The
stirrer should not be removed till the calcination is finished. The
temperature should be raised at the end to a good red heat; and (to
ensure the decomposition of any sulphate that may be formed) the roasted
ore should be rubbed up in a mortar with a pinch of anthracite, and
again calcined. It is then mixed with fluxes as described, and fused in
the same crucible.
The calcination of an ore is a work occupying a good deal of time, and,
in most cases, it is better to take advantage of the desulphurizing
power of red lead or nitre. Red lead by itself will do, but a large
quantity of it will be required; 1 part of a metallic sulphide needs
from 20 to 50 parts of red lead to yield a button free from sulphur;
whereas at most from 2 to 2-1/2 parts of nitre are sufficient. There is
sometimes an advantage in having a considerable excess of oxide of lead
in the slag, but where there is no such reason, 2 parts of red lead to 1
of ore is enough. A charge which will do for most sulphides is the
following: 20 grams of ore, 40 to 100 grams of red lead, 20 grams of
"soda," 5 of borax, and sufficient nitre (or perhaps flour) to give a
button of about 25 grams of lead. How much this must be (if not already
known) may be approximately determined by fusing 3 grams of the ore and
3 grams of "soda" in a small crucible (C) with 50 grams of litharge (not
red lead) under a cover of salt, and weighing the resulting button of
lead. Subtract 3 from the weight of lead obtained, and the difference
multiplied by 1.3 will give the quantity in grams of nitre required. If
the button of lead weighs less than 3 grams flour must be added. If this
is not satisfactory repeat the assay, adding an extra gram of nitre for
each 4 grams of lead in excess of that required, or 1 gram of flour for
a 12-gram deficiency.
In the method in which iron is used as a de-sulphurising agent, only as
much oxide of lead should be added as will give a button of lead of the
required size. Rather a large button of lead should be got, and the slag
should be strongly alkaline; if the ore does not already carry a large
amount of sulphur some should be added. The fusion should be performed
at a low temperature (similar to that for a galena assay), and should be
continued for some time after it has become tranquil. Take 20 grams of
the ore, 40 grams of "soda," 40 grams of oxide of lead, and 5 or 10
grams of borax; place this mixture in a crucible (with a rod of iron, as
in the galena assay), cover, and fuse for about half an hour. Take out
the rod, washing it in the slag, and, in a minute or two, pour. Clean
and cupel the button of lead.
~General Remarks on the Fusion.~--Other things being equal, the smaller
the quantity of the slag the better, provided there is sufficient to
cover the metal. The presence of peroxides of the heavy metals is
prejudicial, since they tend to increase the quantity of silver retained
in the slag. It may be given as a general rule that when iron, copper,
manganese, &c., are present, there is a more than ordinary need for
cleaning the slags, and care must be taken to keep these metals in the
state of lower oxide.
In selecting the fluxes, it should be remembered that soda is the best
for quartz, and borax for lime and metallic oxides. And that with ores
almost free from gangue some quartz or glass should be added to protect
the crucible. Two parts of soda are enough to flux 1 part of quartz;
whilst of borax, or oxide of lead, 4 parts are barely sufficient. Oxide
of lead has the advantage of being heavy and so does not occupy much
space in the crucible; on the other hand, if the melting down be
performed too quickly, or if oxide of lead only is used, this high
specific gravity is a disadvantage, for the lighter earthy matter floats
as a pasty mass on the more fluid oxide of lead, and thus escapes its
action.
When metallic sulphides are present in the ore, an excess of oxide of
lead helps to keep the sulphur out of the button of metal. In addition
to the oxide of lead required as a flux, some will be required to
provide the lead in which the silver is to be collected. Oxide of lead,
mixed with charcoal or flour, yields, when heated, a multitude of minute
buttons of metal uniformly distributed through the mass of the charge;
as the charge melts down these run together and fall to the bottom; this
shower of lead collects the silver more easily than a single button at
the bottom of the crucible could do. Only that portion of the oxide of
lead which remains in the slag can be considered as a flux; very often
the first indication of an excessive reduction of lead is the pastiness
of the slag rendered thick by the withdrawal of the oxide of lead which
would have kept it fluid. If, in an assay, it is found that 5 parts of
flux are not sufficient for 1 part of ore, the remedy lies in using a
different flux rather than in taking a larger quantity.
_On the Reducing Effect of Charcoal, Flour, and Tartar._--The weight to
be got from a given charge will depend (provided sufficient oxide of
lead is present) upon the proportion of the reducing agents in it. We
have thought it well to illustrate this part of the subject by a series
of experiments which the learner will do well to practise for himself
before proceeding to the assay of actual ores. Take 80 grams of litharge
and 20 grams of a mixture of borax and soda. Fuse three lots (1) with
1.5 gram of charcoal, (2) with 3 grams of flour, and (3) with 7.5 grams
of tartar. Weigh the buttons of lead obtained, and divide each by the
weight of reducing agent used. The results will differ somewhat with the
dryness and quality of the flour, etc., used; in one series of
experiments they were as follows:--
Gram. Grams. Gram. Grams.
1.5 charcoal gave 34.0 lead .'. 1 charcoal = 22.6 lead.
3.0 flour " 33.5 " .'. 1 flour = 11.2 "
7.5 tartar " 38.0 " .'. 1 tartar = 5.0 "
The use of flour as a reducing agent has many advantages, and it is well
to remember that _1 gram of flour reduces about 11 grams of lead_; and
that charcoal has twice, and tartar one-half, this reducing effect.
_On the Reducing Effect of Charcoal, &c., on Red Lead._--It is often
easier to obtain red lead of good quality than it is litharge, and by a
large number of assayers red lead is the form of oxide of lead always
used. Red lead, however, contains an excess of oxygen which will use up
some of the reducing agent before lead separates out. On making a series
of experiments (similar to the last, but using 80 grams of red lead
instead of the litharge) the results were, with the same quantities of
the reducing agents:--
With charcoal, 18 grams of lead.
" flour, 18 " "
" tartar, 22 " "
Comparing these with the results with litharge, in the previous table it
will be seen that the same quantity of reducing agent has in each case
brought down 16 grams less of lead, so that a larger amount of the
reducing agent must be added to get a button of the same weight as that
obtained with litharge. To get a button of a desired weight, say 22
grams, we must add reducing agent sufficient to throw down 22 + 16 or 38
grams of lead, which would require 3.4 grams of flour. If this amount of
flour is fused with 80 grams of red lead, a button of lead weighing 22
grams will be formed, the other 16 grams being kept up by the oxygen of
the red lead.
If the quantity of red lead differs from 80 grams, this rule must be
modified. With 40 grams of red lead, for example, we should add an
excess of reducing agent sufficient to throw down 8 grams of lead
instead of 16. Similarly, with 160 grams of red lead, we should add
enough to throw down 32 grams.
The following rule will enable one to calculate the weight of flour
required to produce a button of lead of any desired weight from any
given quantity of red lead. Each 5 grams of red lead present diminishes
the weight of the lead by 1 gram. If then we _divide the weight of red
lead in a charge by 5, and add this to the weight of lead required, the
sum divided by 11 will give the weight of flour which must be added_.
Using 80 grams of red lead and wanting a button of 20 grams, we should
add 3.3 grams of flour.
80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly.
The following are some results obtained which will illustrate the
rule:--
Red Lead used. Flour used. Lead got.
40 grams 3 grams 25.0 grams
100 " 3 " 13.5 "
80 " 4 " 30.0 "
80 " 5 " 40.0 "
_On the Reducing Effect of Metallic Sulphides, and the Counteracting
Effect of Nitre._--The sulphides found in ores will reduce a button of
lead from oxide of lead just as flour does; and, as charcoal, flour and
tartar differ in their reducing power, so equal weights of the different
mineral sulphides throw down different weights of lead.
One gram of iron pyrites yields about 11 grams of lead. One gram of
copper pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of
lead, whilst 1 gram of antimonite will give 6, and 1 gram of galena only
a little over 3 grams. It is evident that if an ore carries much of
these sulphides, the quantity of lead reduced will be very much larger
than that required for an assay. To counteract this effect nitre is
added; _1 gram is added for each 4 grams of lead in excess of that
required_. For example: with a 20-gram charge of an ore containing 50
per cent. of pyrites, if no nitre were added, 110 grams of lead would be
got; or, if there was not sufficient oxide of lead to yield this
quantity of metal, the button would be sulphury. To reduce the weight of
the button by 80 grammes, we should add 20 grams of nitre, if litharge
were used; or if red lead were used, we should add 16 grams of nitre,
_since the oxidizing effect of 20 grams of red lead is equivalent to
that of 1 of nitre_, and since 80 grams of red lead are generally used
in a charge. Two assays of an ore of this kind with these quantities of
nitre gave 26.0 grams of lead with litharge, and 22.5 grams with red
lead.
It is best to use in these assays 80 grams of red lead, 20 of soda, and
5 of borax, with 20 grams of the ore. If the lead got by the preliminary
fusion in a small crucible with litharge (described under "_ores
containing much sulphides_") is known, the following table will indicate
the quantity of nitre, or flour, to be added with this charge:--
-------------------------------+-------------------+------------------
Lead got in Preliminary Fusion | Flour to be added | Nitre to be added
with 3 grams of Ore. | to the Assay. | to the Assay.
-------------------------------+-------------------+------------------
0.0 gram | 3.3 grams | none
3.0 grams | 1.3 gram | --
6.0 " | none | 4.0 grams
9.0 " | -- | 9.0 "
12.0 " | -- | 14.0 "
15.0 " | -- | 19.0 "
18.0 " | -- | 24.0 "
21.0 " | -- | 29.0 "
-------------------------------+-------------------+------------------
If litharge is used in the assay instead of red lead 4 grams more nitre,
or 1.5 gram less flour must be used. When more than a few grams of nitre
are added to a charge the proportion of "soda" and borax should be
increased, because one of the products of the reaction of nitre upon
sulphides in the presence of soda is sulphate of soda, and because the
"soda" thus used up no longer serves as a flux; more borax should be
added, as it is the best flux for the metallic oxides which are formed
in the process. If in an assay too large a button of lead is got, even
after this calculation has been made, and the assay is repeated, add 1
gram more nitre for each 4 grams of lead in excess. Sometimes the assay
appears tranquil before the nitre has produced its full effect; in such
cases it is well to seize the crucible with the tongs and mix its fused
contents by rotating them; if this causes an effervescence, the crucible
should be replaced in the fire and the fusion continued. The following
experiments will illustrate the extent to which the above rules may be
relied on. In all of them the standard flux was used, viz.:--80 grams of
red lead, 20 of soda, and 5 of borax.
_Pyrites_ 5 5 5 5 2.5 5 10 15 20
Quartz -- 20 -- 20 17.5 15 10 5
Nitre -- -- 5 5 -- 4 16 28.5 41
Lead got 42.5 36.0 16.0 19.0 11.5 22.5 22.5 26.5 27.5
_Copper Pyrites_ 8 8 8 8
Quartz -- 12 -- 12
Nitre -- -- 4 4
Lead got 47.5 34.0 33.0 26.0
_Antimonite_ 8 8 8 8
Quartz -- 12 -- 12
Nitre -- -- 4 4
Lead got 29.0 26.0 13.0 13.0
_Galena_ 10 10 10 10 15 20
Quartz. -- 15 -- 15 5 --
Nitre -- -- 3 3 3.5 7
Lead got 17.0 19.0 8.0 8.0 18.5 18.5
A similar set of experiments, with 80 grams of litharge instead of 80
grams of red lead, gave:--
_Pyrites_ 4 4 4 4 7 10
Quartz -- 15 -- 15 13 10
Nitre -- -- 5 5 12.5 20
Lead got 46.5 40.5 25.5 24.5 27.0 26.5
_Copper Pyrites_ 5 5 5 5
Quartz -- 15 -- 15
Nitre -- -- 5 5
Lead got 44.5 32.5 23.0 25.0
_Blende_ 5 5 5 5 10
Quartz -- 15 -- 15 10
Nitre -- -- 5 5 15
Lead got 41.5 38.5 21.5 22.5 21.6
_Antimonite_ 5 5 5 5 10
Quartz -- 15 -- 15 10
Nitre -- -- 5 5 10
Lead got 31.0 32.5 11.5 12.5 18.7
_Galena_ 10 10 10 10 15 20
Quartz -- 15 -- 15 5 --
Nitre -- -- 5 5 7.5 11
Lead got 33.5 33.5 13.0 14.0 19.5 22.7
The variation in some of these experiments, in which we might have
expected similar results, is due to the fact that the sulphur, and in
some cases the metals, are capable of two degrees of oxidation. For
example: theoretically 1 gram of iron pyrites (FeS_{2}) would yield 8.6
grams of lead if the sulphur were oxidised to sulphurous oxide (SO_{2}),
and the iron to ferrous oxide (FeO); whilst if the sulphur were oxidised
to sulphate (SO_{3}), and the iron to ferric oxide, 12.9 grams of lead
will be thrown down. Similarly the yield with copper pyrites would be
7.5 or 11.6; with blende, 6.4 or 8.5; with antimonite, 5.5 or 8; and
with galena, 2.6 or 3.4. As regards the metals, the lower oxide will
always be formed if the assay is carried out properly (fused under a
cover, and with a sufficiency of reducing agent). But the proportion of
sulphur oxidised completely will vary with the conditions of the assay.
With a slag containing much soda the tendency will be to form sulphate,
and, in consequence, a big reduction of lead; whilst with an acid slag
containing much quartz the tendency will be for the sulphur to go off as
sulphurous oxide (SO_{2}). In a fusion with litharge alone all the
sulphur will be liberated as the lower oxide, whilst with much soda it
will be wholly converted into sulphate. For example: 3 grams of an ore
containing a good deal of pyrites and a little galena, gave, when fused
with litharge, 16.5 grams of lead. A similar charge, containing in
addition 20.0 grams of soda, gave 22.5 grams of lead.
It will be noted from the experiments that 1 gram of nitre kept up on
the average 4 grams of lead; the range being from 3.2 with acid slags to
5.3 with very basic ones. These facts serve to explain some apparently
irregular results got in practice.
CUPELLATION.
The process is as follows:--The cupels, which should have been made some
time before and stored in a dry place, are first cleaned by gentle
rubbing with the finger and blowing off the loose dust; and then placed
in a hot muffle and heated to redness for from 5 to 10 minutes before
the alloy to be cupelled is placed on them. The reasons for this are
sufficiently obvious: the sudden evolution of much steam will blow a
cupel to pieces; and, if the whole of the water has not been removed
before the cupel is filled with molten lead, the escaping steam will
bubble through, and scatter about particles of the metal. If some
particles of unburnt carbon remain in the bone ash, a similar result
will be produced by the escape of bubbles of carbonic acid as soon as
the fused litharge comes in contact with them. The cupels having been
prepared are arranged in a definite order in the muffle, and the assay
buttons are arranged in a corresponding order on some suitable tray
(cupel tray, fig. 41); the heat of the muffle being at bright redness.
Then with the help of the tongs (fig. 42) the assay buttons should be
placed each in its proper cupel; a note having been previously made of
the position it is to occupy, and the door of the muffle closed.
[Illustration: FIG. 41.]
This part of the work should be done promptly, so as not to unduly cool
the muffle: the start requires a fairly high temperature, and is a
critical part of the process. A black crust forms at once on the surface
of the lead; but this ought soon to fuse and flow in greasy drops from
off the face of the metal, so as to leave the latter fluid with a
well-defined outline, and much brighter than the cupel. If this clearing
does not take place, the buttons are said to be frozen; in which case
the temperature must be raised, some pieces of charcoal put in the
muffle, and the door closed. If they still do not clear, the heat must
have been much too low, and it is best to reject them and repeat the
assays.
[Illustration: FIG. 42.]
When the buttons have cleared it is well to check the draught of the
furnace, and to partly open the door of the muffle, so as to work at as
low a temperature as is compatible with the continuation of the
process.[11] Too low a temperature is indicated by the freezing of the
buttons and the consequent spoiling of the assays. Experience soon
enables one to judge when the heat is getting too low. A commoner error
is to have the heat too high: it should be remembered that that which
was high enough to clear the buttons at starting is more than sufficient
to keep the process going. At the finish a higher temperature is again
required: therefore the door of the muffle should be closed and the
furnace urged. The finish is easily recognised. The drops of litharge
which in the earlier stages flow steadily from the surface of the alloy,
thin off later to a luminous film. At the end this film appears in
commotion, then presents a brilliant play of colours, and, with a sudden
extinction, the operation is finished. The metal again glows for an
instant whilst becoming solid.
If the button is a small one the cupel is withdrawn at once and placed
on that square of the cupel tray which corresponds to the position it
occupied in the muffle. If, however, it is fairly large precautions must
be taken to prevent spirting.
Molten silver dissolves oxygen from the air and gives it off on
solidifying; the escape of the gas on sudden cooling is violent and, by
throwing off particles of the metal, may cause loss. This is called
"vegetation" or "spirting." The silver is apparently solid when spirting
takes place; the crust breaks suddenly and some of the metal is forced
out. The evil is best guarded against by slow cooling and avoiding
draughts. With large buttons of silver precautions should never be
omitted. One plan is to allow the cupels to cool in the muffle itself,
the mouth being closed with hot charcoal. Another is to cover the cupel
with another cupel previously heated to redness; in this case the silver
cools between two hot cupels, and, of course, cools slowly. A third plan
is to withdraw the cupel to the door of the muffle, holding it until it
begins to get solid and then immediately to put it back into the hotter
part of the muffle.
Silver remains after cupellation in flattened elliptical buttons,
adhering but only slightly to the cupel. Its upper surface should show
faint markings as if it were crystalline. The presence of platinum
renders it still more crystalline, but removes the characteristic lustre
and renders the metal dull and grey. Copper, if not completely removed,
has a very marked effect on the appearance of the button: the metal is
spread out, damping, as it were, and firmly adhering to the cupel, which
latter in the neighbourhood of the metal is almost black with oxide of
copper. Sometimes the silver button is globular, or even more sharply
rounded on its under than on its upper surface; it is said that this is
due to the presence of lead. Gold may be present even to the extent of
50 per cent. without showing any yellow colour.
The appearance of the cupel affords some useful information. The
presence of cracks evidently due to shrinkage indicates a badly made
cupel. If, however, they are accompanied by a peculiar unfolding of the
cupel, the margin losing its distinctness, it is because of the presence
of antimony. When lead is the only easily oxidisable metal present, the
stained portion of cupel is yellow when cold. A greenish tint may be due
to small quantities of copper or, perhaps, nickel, cobalt, or platinum.
Larger quantities of copper give a greenish grey or almost black colour.
A dark green and corroded cupel may be due to iron. Rings of
pale-coloured scoria may be due to tin, zinc, antimony, or arsenic. When
the cupel shows signs of the presence of these metals in objectionable
quantity, it is well to repeat the assay and scorify so as to remove
them before cupellation.
The button should be detached from the cold cupel by seizing with a
pair of pliers: the under surface should be distorted by squeezing or
hammering the button so as to loosen the adhering bone ash. The cleaning
is easily completed by rubbing with a clean hard brush. After cleaning
the buttons are best put on a tray of marked watch-glasses, and then
taken to the balance and weighed. The weight of silver got needs a small
correction; (1) by deducting for the amount of silver introduced by the
lead or oxide of lead used in the assay;[12] and (2) by adding for the
cupellation loss.
~Loss in Cupellation.~--During the whole process of cupelling a silver
lead alloy a more or less abundant fume may be observed rising from the
cupel. This furnishes an evident loss of lead and a possible loss of
silver; for although silver at the temperature of cupellation gives off
no appreciable vapour, it is known that such fume formed on a large
scale contains silver. It is, however, difficult to believe that the
small amount of lead vapourised carries with it a weighable amount of
silver. That it does not do so in the ordinary way of working is shown
by the fact that a button of silver equal in weight to the silver lost
in cupelling may be got by smelting the cupel and cupelling the
resulting button of lead. The loss of silver by volatilisation is
altogether inconsiderable, unless the temperature at which the operation
is performed is much too high.
Another possible source of loss is the infiltration of small particles
of alloy into the cupel. The cupel is necessarily porous, and particles
of metal may perhaps drain into it, more especially if the bone ash is
not in fine powder; but if this is the main source of loss it is hard to
see why, in cupelling equal weights of silver and gold, the loss is not
equal in each case. It is not easy to believe that the mere filtration
of the fused alloy will effect such a change in the proportion of the
metals as that which actually occurs. For example: a cupel on which an
alloy consisting of 0.80 gram of silver, 0.47 gram of gold, and 25 grams
of lead had been cupelled, was found to contain 7-1/2 milligrams of
silver, and rather less than half a milligram of gold. Assuming, for the
sake of argument, that the gold present had filtered into the cupel in
the form of small drops of alloy, it would have been accompanied by less
than a milligram of silver, and the presence of the extra 6 or 7
milligrams of silver must have been due to a different cause. There can,
thus, be little doubt that the cause of the greater part of the
"cupellation loss" is a chemical one and cannot be counteracted by a
mechanical contrivance.[13] In cupellation, then, there is a loss,
apart from imperfect working, inherent in the process itself; and as the
amount of this loss varies under different conditions, it is necessary
to study it somewhat in detail.
The following experiments are taken without selection from the work of
one student. Three experiments were made for each determination, and the
mean result is given. By "range" is meant the difference between the
highest and lowest result and the percentage loss is calculated on the
silver present. The silver added in the lead used has been deducted.
~Effect of Varying Lead.~--In each experiment 0.4 gram of silver was
taken and cupelled with the lead. The silver loss and "range" are
expressed in milligrams.
------------+--------------+--------+------------------
Lead Used. | Silver Lost. | Range. | Percentage Loss.
------------+--------------+--------+------------------
Grams. | | |
10 | 6.5 | 1.0 | 1.62
20 | 7.0 | 1.0 | 1.75
40 | 12.0 | 1.5 | 3.00
60 | 12.7 | 0.5 | 3.17
------------+--------------+--------+------------------
The loss increases with the lead used.
~Effect of Varying Temperature.~--0.4 gram of silver was cupelled with
20 grams of lead.
Temperature. Silver Lost. Range. Percentage Loss.
Bright red 7.0 1.0 1.75
Clear yellow 17.3 1.7 4.32
The difference in temperature in these experiments was much greater than
would occur even with careless work.
~Effect of Varying Silver.~--20 grams of lead were used in each
cupellation.
---------------+--------------+--------+------------------
Silver Taken. | Silver Lost. | Range. | Percentage Loss.
----------------------------------------------------------
Milligrams. | | |
12.5 | 0.7 | 0.2 | 5.6
25.0 | 1.4 | 0.1 | 5.6
50.0 | 1.6 | 0.4 | 3.2
100.0 | 2.9 | 0.3 | 2.9
200.0 | 5.6 | 0.5 | 2.8
400.0 | 7.0 | 1.0 | 1.7
800.0 | 9.7 | 1.0 | 1.2
---------------+--------------+--------+------------------
It will be seen that, although the quantity of silver lost increases
with the silver present, the percentage loss is greater on the smaller
buttons.
The following results are often quoted:--Cupelling 1 grain of silver
with 10 grains of lead, the loss was 1.22 per cent.; 10 grains of silver
with 100 grains of lead, loss 1.13 per cent.; 25 grains of silver
cupelled with 250 grains of lead, lost 1.07 per cent. The proportion of
silver to lead was the same in the three experiments, and the largest
button gave the best result. Evidently, if the quantities of lead had
been the same in the three experiments (say, 250 grains in each case),
the loss on the smaller quantities of silver would appear worse in the
comparison.
In judging these results, it must be borne in mind that it is difficult
to regulate the temperature, &c., in consecutive experiments so as to
get exactly similar results, so that the range in consecutive
cupellations is greater than that in a batch cupelled side by side.
~Effect of Copper and Antimony.~--0.1 gram of silver was cupelled with
20 grams of lead, and to one batch 0.5 gram of antimony, and to another
0.5 gram of copper was added.
Loss in
Silver Lost. Range. Percentage.
Without addition 2.9 0.3 2.9
With antimony 3.2 0.2 3.2
With copper 4.9 1.7 4.9
Perhaps the antimony has so small an effect because it is eliminated in
the earlier part of the process, while the silver is still alloyed with,
and protected by, a large proportion of lead; whilst the copper on the
other hand makes its fiercest attack towards the close, when the silver
is least capable of resisting it. The ill effects of copper are most
strongly felt when the quantity of lead present is not sufficient to
remove it: the coppery button of silver got under these conditions is
very considerably less than the weight of silver originally taken.
Although the above is a fair statement of the loss attending average
work, it will not do in very important and exact work to place too much
reliance on the figures given, or, indeed, on any other set of figures,
with the object of correcting the result of an assay. Each man must rely
on his own work.
It is easy to determine what allowance must be made for the loss in
cupellation by cupelling side by side with the assay piece an alloy of
similar and _known_ composition. For, if the two pieces are very nearly
alike, we may justly conclude that the loss on each will be the same;
and if, further, we take the average of three or four such
determinations we shall get results accurate within 0.1 per cent. The
method of getting such results may be best explained by one or two
illustrations. This method of working is termed "assaying by checks."
Suppose we have an alloy of silver and lead in unknown proportions and
that by cupelling two lots of 10 grams each there is got from I. 0.1226
gram of silver, and from II. 0.1229 gram. We should know from general
experience that the actual quantity of silver present was from 2 to 4
milligrams more than this. To determine more exactly what the loss is,
the following plan is recommended:--The two silver buttons are wrapped
up each in 10 grams of lead, and cupelled side by side with two other
lots of 10 grams of the original alloy. If now the two buttons I. and
II. weigh 0.1202 and 0.1203, they will have suffered in this second
cupellation an average loss of 2.5 milligrams. Suppose the two fresh
lots of alloy gave 0.1233 and 0.1235 of silver, the average loss on
these would also be 2.5 milligrams. Add this loss to each result, and
take the mean; which is in this case 0.1259.
If copper is present in the alloy as well as silver, it is necessary to
add about the same quantity of copper to the checks as is supposed, or
known, to be present in the assays. If the substance to be assayed is an
alloy of silver and copper, first cupel 0.5 gram of it, with, say, 10
grams of lead, and weigh the resulting button of silver, in order to get
an approximate knowledge of its composition. Suppose the button weighs
0.3935 gram. We know that this is below the truth: for the sake of round
numbers take it as 0.4, and assume that the rest of the alloy (0.1 gram)
was copper. Two check pieces are then weighed out, each containing 0.4
gram silver and 0.1 gram of copper wrapped in 5 grams of lead. Of course
the silver must be pure. And there is also weighed out two (or better,
four) assay pieces each containing half a gram of the alloy wrapped in 5
grams of lead. The whole lot are then cupelled as nearly as possible
under the same conditions. With four assay pieces, the cupels should be
placed close together in two rows of three across the muffle; the two
check pieces are put in the middle cupels. Suppose the buttons of silver
got weighed as follows:--
Check pieces I. 0.3940 II. 0.3945
Assay pieces I. 0.3905 II. 0.3912
III. 0.3910 IV. 0.3909
The average loss on the two check pieces is 5.7 milligrams, and the
average result of the four assay pieces is 0.3909. Add the average loss
to the average result, and there is got the corrected result, 0.3966.
And if 0.5 gram of alloy contain 0.3966 of silver, 1000 will contain
793.2 of silver, and this is the degree of fineness.
A correction for the loss in cupellation is always made in this way
when rich alloys are being assayed; and in the case of rich ores it may
be done after the manner of the first of the above illustrations. There
is another method of working which relies more on experiment. This is to
smelt the cupel as described further on (p. 114), and to again cupel the
resulting button of lead. The button of silver got in this second
cupellation is added to that first obtained. It will sometimes, but not
often, happen that the two buttons together will slightly exceed in
weight the silver which was actually present. This is because of the
retention in the buttons of a small quantity of lead. It has been stated
that the proportion of lead thus retained may be as much as 1% of the
silver present; this, however, can only be under exceptional conditions.
A determination of the actual silver in the buttons got in the series of
cupellations quoted on pages 102, 103, gave an average percentage of
99.85, so that even with the larger buttons the effect of the retained
lead would be only to increase the weight by about 1 milligram. In the
method of working with checks, the retained lead has no disturbing
influence.
~The proportion of lead required~ for the cupellation of any particular
alloy requires consideration. With too much lead the time occupied in
the process is increased, and so is the loss of silver; on the other
hand, too little lead is of greater disadvantage than too much. From 8
to 16 parts of lead are required for each part of silver alloy, or, if
gold is present, about twice as much as this must be used. For the
cupellation of 1 gram of a silver copper alloy containing different
percentages of copper, the following quantities of lead should be
used:--
Percentage of Copper
in Alloy. Lead Required.
5 6 grams
10 8 "
20 10 "
30 12 "
40 14 "
50-100 16-18 "
The alloy, in not too large pieces, is wrapped in the required weight of
lead foil and charged into the cupel at once; or the lead may be put in
first, and, when the cupellation has fairly started, the alloy may be
added wrapped in tissue paper; or a portion of the lead may be first
started and the alloy wrapped in the remaining lead and subsequently
added. The cupellation of large quantities of alloy or of alloys which
contain tin, antimony, iron, or any substance which produces a scoria,
or corrodes the cupel, must be preceded by a scorification. The
advantages of this are that the slag is poorer in precious metal than
that found on a cupel and is more easily collected and cleaned; that
larger quantities of metal can be treated, and that, even if the
substance is in part infusible, or produces at the start a clinkery mass
or scoria, the oxide of lead gradually accumulates, fluxes the solid
matters, and produces a good final result; but if the oxide of lead by
itself is not sufficient for the purpose, borax or some other flux can
be easily added.
If the button of silver got is very small its weight may be estimated
from its size; but it must be remembered that the weight varies as the
cube of the diameter. If one button has twice the diameter of another it
is eight times as heavy and so on. Scales specially constructed for
measuring silver and gold buttons may be purchased; but it is much
better to make the measurement with the help of a microscope provided
with an eyepiece micrometer.
If the length of the long diameter of a silver button be taken the
following table will give the corresponding weight in milligrams:--
------------+-----------++------------+---------
Diameter. | Weight. || Diameter. | Weight.
------------------------------------------------
0.04 inch | 3.6 || 0.015 inch | 0.19
0.035 " | 2.4 || 0.014 " | 0.15
0.03 " | 1.5 || 0.013 " | 0.12
0.025 " | 0.9 || 0.012 " | 0.097
0.02 " | 0.45 || 0.011 " | 0.075
0.019 " | 0.4 || 0.010 " | 0.056
0.018 " | 0.33 || 0.008 " | 0.028
0.017 " | 0.27 || 0.006 " | 0.012
0.016 " | 0.23 || 0.004 " | 0.004
------------+-----------++------------+---------
The weight of a corresponding button of gold is got by multiplying by
2.25. These figures are based on those given by Plattner, and apply only
to buttons of such shape as those left after cupellation. A sphere of
silver 0.01 inch in diameter would weigh 0.09 milligram, and a similar
sphere of gold weighs 0.167 milligram.
It is safer, however, to compare with a micrometer the diameter of the
button whose weight has to be determined with that of a standard button
of nearly equal size whose weight is known. The weights of the two
buttons are proportional to the cubes of their diameters. This plan of
working is described more fully in Appendix B., page 440.
~Calculation of the Results.~--After deducting for the silver added, and
correcting for the cupellation loss, the calculation is made in the
usual way; reporting as so many parts per thousand in the case of rich
alloys and as so many ounces and pennyweights, or better as ounces and
decimals of an ounce, in the case of poor alloys and ores.
In this last case, however, it is less fatiguing to refer to a set of
tables which give, either directly or by means of simple addition, the
produce corresponding to any weight obtained from certain given weights
of the substance. The following table gives the produce in ounces and
decimals of an ounce per ton of 2240 pounds:--
------------+----------------------------------------------------------
| Weight of Ore taken.
Weight of |----------+----------+-----------+-----------+------------
Metal got. | 3 grams. | 5 grams. | 20 grams. | 50 grams. | 100 grams.
------------+----------+----------+-----------+-----------+------------
0.0001 | 1.09 | 0.65 | 0.16 | 0.06 | 0.03
0.0002 | 2.18 | 1.31 | 0.33 | 0.13 | 0.06
0.0003 | 3.27 | 1.96 | 0.49 | 0.20 | 0.10
0.0004 | 4.36 | 2.61 | 0.65 | 0.26 | 0.13
0.0005 | 5.44 | 3.27 | 0.82 | 0.33 | 0.16
0.0006 | 6.53 | 3.92 | 0.98 | 0.39 | 0.19
0.0007 | 7.62 | 4.57 | 1.14 | 0.46 | 0.23
0.0008 | 8.71 | 5.23 | 1.31 | 0.52 | 0.26
0.0009 | 9.80 | 5.88 | 1.47 | 0.59 | 0.29
0.001 | 10.89 | 6.53 | 1.63 | 0.65 | 0.33
0.002 | 21.78 | 13.07 | 3.27 | 1.31 | 0.65
0.003 | 32.67 | 19.60 | 4.90 | 1.96 | 0.98
0.004 | 43.56 | 26.13 | 6.53 | 2.61 | 1.31
0.005 | 54.44 | 32.67 | 8.17 | 3.27 | 1.63
0.006 | 65.33 | 39.20 | 9.80 | 3.92 | 1.96
0.007 | 76.22 | 45.73 | 11.43 | 4.57 | 2.29
0.008 | 87.11 | 52.27 | 13.07 | 5.23 | 2.61
0.009 | 98.00 | 58.80 | 14.70 | 5.88 | 2.94
0.01 | 108.89 | 65.33 | 16.33 | 6.53 | 3.27
0.02 | 217.78 | 130.67 | 32.67 | 13.07 | 6.53
0.03 | 326.67 | 196.00 | 49.00 | 19.60 | 9.80
0.04 | 435.56 | 261.33 | 65.33 | 26.13 | 13.07
0.05 | 544.44 | 326.67 | 81.67 | 32.67 | 16.33
0.06 | 653.33 | 392.00 | 98.00 | 39.20 | 19.60
0.07 | 762.22 | 457.33 | 114.33 | 45.73 | 22.87
0.08 | 871.11 | 522.67 | 130.67 | 52.27 | 26.13
0.09 | 980.00 | 588.00 | 147.00 | 58.80 | 29.40
0.1 | 1088.89 | 653.33 | 163.33 | 65.33 | 32.67
0.2 | 2177.78 | 1306.67 | 326.67 | 130.67 | 65.33
0.3 | 3266.67 | 1960.00 | 490.00 | 196.00 | 98.00
0.4 | 4355.56 | 2613.33 | 653.33 | 261.33 | 130.67
0.5 | 5444.44 | 3266.67 | 816.67 | 326.67 | 163.33
0.6 | 6533.33 | 3920.00 | 980.00 | 392.00 | 196.00
0.7 | 7622.22 | 4573.33 | 1143.33 | 457.33 | 228.67
0.8 | 8711.11 | 5226.67 | 1306.67 | 522.67 | 261.33
0.9 | 9800.00 | 5880.00 | 1470.00 | 588.00 | 294.00
1.0 | 10888.89 | 6533.33 | 1633.33 | 653.33 | 326.67
------------+----------+----------+-----------+-----------+------------
When, as in this table, the fraction of an ounce is expressed by two
places of decimals, it may be reduced to pennyweights (dwts.) by
dividing by 5. For example, 0.40 of an ounce is 8 dwts. The fraction of
a dwt. similarly expressed may be converted into grains with sufficient
exactness by dividing by 4. Thus, 1.63 ozs. equal 1 oz. 12.60 dwts., or
1 oz. 12 dwts. 15 grains. In England it is usual to report in ounces and
decimals of an ounce.
The way to use the table is best shown by an example. Suppose a button
of silver weighing 0.0435 gram was obtained from 20 grams of ore. Look
down the 20-gram column of the table, and select the values
corresponding to each figure of the weight, thus:--
0.04 = 65.33 ozs. to the ton
0.003 = 4.90 "
0.0005 = 0.82 "
------ -----
0.0435 = 71.05 "
Add these together. The produce is 71.05 ozs., or 71 ozs. 1 dwt. to the
ton.
Or, suppose an ore is known to contain 1.24 per cent. of silver. Look
down the 100-gram column, select the values, and add them together as
before.
1.0 = 326.67 ozs. per ton
0.2 = 65.33 "
0.04 = 13.07 "
---- ------
1.24 = 405.07 "
This gives 405 ozs. 1 dwt. 10 grains to the ton.
The calculation becomes more complicated when, as is frequently the
case, the ore contains metallic particles. These show themselves by
refusing to pass through the sieve when the ore is powdered. When they
are present, a large portion, or if feasible the whole, of the sample is
powdered and sifted. The weights of the sifted portion and of the
"metallics," or prills, are taken; the sum of these weights gives that
of the whole of the sample taken. It is very important that nothing be
lost during the operation of powdering.
Each portion has to be assayed separately. It is usual to assay a
portion of the sifted sample, say, 20 or 50 grams, and to add to the
produce of this its share of the "metallics." This way of calculating,
which is more convenient than correct, is illustrated by the following
example:--
Weight of whole sample 400 grams
Made up of sifted portions 399 "
" "Metallics" 1 "
---
400 "
Twenty grams of the sifted portion, when assayed, gave 0.1050 gram of
silver. The whole of the "metallics" scorified and cupelled gave 0.842
gram of silver. Since the 20 grams assayed was 1-20th of the whole,
1-20th part of the 0.842 gram of silver (from the metallics) must be
added to its produce. We thus get 0.1471 gram (0.1050 + 0.0421).
Referring to the 20 gram column, we get--
0.1 = 163.33
0.04 = 65.33
0.007 = 11.43
0.0001 = 0.16
------ ------
0.1471 = 240.25 ounces per ton.
A more legitimate method of calculation is as follows:--Calculate
separately the produce of each fraction as if they were from different
ores. Multiply each produce (best stated in per cents.) by the weight of
the corresponding fraction. Add together the products, and divide by the
weight of the whole sample. Taking the same example for illustration, we
have:--
_Metallics._--Weight 1 gram.
1 gram of it yielded 0.842 grams of silver.
.'. Produce = 84.2 per cent.
Produce multiplied by the weight is still ~84.2~.
_Sifted Portion._--Weight 399 grams.
20 grams of it yielded 0.105 gram of silver.
.'. Produce = 0.525 per cent.
Produce multiplied by weight (0.525 × 399) is ~209.475~.
Add together; and divide by 400, the weight of the whole sample--
84.2
209.475
-------
400) 293.675 (0.7342
0.7342 is the total produce of the ore in per cents.
Referring to the 100-gram column in the table we find 239.84 ounces to
the ton as the produce.
0.7 = 228.67
0.03 = 9.80
0.004 = 1.31
0.0002 = 0.06
------
239.84
Comparing this with the result calculated by the first method--viz.,
240.26, we see that that was 0.38 oz., or between 7 and 8 dwts. too
high.
With ores containing "metallics" it is of great importance to powder the
whole of the selected sample without loss during the process; and of
even greater importance to well mix the sifted portion, of which the
last portions to come through the sieve are apt to be more than
ordinarily rich through the grinding down of some portions of the
metallic prills.
~Remarks on Cupellation.~--Cupellation is at once the neatest and the
most important of the dry methods of assaying. Its purpose is to remove
easily oxidisable metals, such as lead and copper, from silver and gold,
which are oxidisable with difficulty. Metals of the first class are
often spoken of as _base_, and gold and silver as _noble_ metals.
When lead is exposed to the action of air at a temperature a little
above redness, it combines with the oxygen of the air to form litharge,
an oxide of lead, which at the temperature of its formation is a
_liquid_. Consequently, if the lead rests on a porous support, which
allows the fused litharge to drain away as fast as it is formed, a fresh
surface of the lead will be continually exposed to the action of the
air, and the operation goes on until the whole of the lead has been
removed. Silver or gold exposed to similar treatment does not oxidise,
but retains its metallic condition; so that an alloy of lead and silver
similarly treated would yield its lead as oxide, which would sink into
the support, while the silver would remain as a button of metal.
The porous support, which is called _a cupel_(fig. 5), should absorb the
slag (oxide of lead, etc.) just as a sponge absorbs water, but must be
sufficiently fine-grained to be impervious to the molten metal. At first
sight it appears difficult to filter, as it were, a fluid slag from a
fluid metal; but an ordinary filter-paper damped with oil will allow
oils to run through and yet retain the water; but damped with water it
will allow water to run through and retain oils. Similarly, fused slags
damp and filter through a cupel, but the molten metal not damping it
withdraws itself into a button, which is retained. Although, of course,
if the cupel is very coarse-grained the metal may sink into the hollows.
Copper, antimony, tin, and most other metals, form powdery oxides, which
are not of themselves easily fusible, and it is necessary when these are
present to add some solvent or flux to render the oxide sufficiently
fluid. Fortunately, oxide of lead is sufficient for the purpose; hence,
mixed oxides of copper and lead, provided the lead is present in proper
proportion, form a fluid slag. In separating copper from silver or gold,
advantage is taken of this fact; for, although we cannot cupel an alloy
of copper and silver, it is easy to cupel an alloy of copper, silver and
lead. If, however, the lead is not present in sufficient quantity, the
whole of the copper will not be removed, and the button of silver, still
retaining copper, will be found embedded in a coating of black oxide of
copper. Copper oxidises less easily than lead does; and, consequently,
the alloy which is being cupelled becomes relatively richer in copper
as the operation proceeds. It is on this account that the ill-effects of
the copper make themselves felt at the close of the operation, and that
the oxide of copper is found accumulated around the button of silver.
Tin and antimony, on the other hand, are more easily oxidised; and the
tendency of their oxides to thicken the slag makes itself felt at the
commencement: if the button of alloy once frees itself from the ring or
crust of unfused oxide first formed, the cupellation proceeds quietly,
and leaves a clean button of silver in the centre. But in either case
the cupellation is imperfect, and should be repeated with a larger
proportion of lead. An unfused and, consequently, unabsorbed slag tends
to retain small buttons of alloy or metal, and thus cause serious loss.
There is a principle underlying many of the phenomena of dry silver
assaying which the student should endeavour to understand; and which
serves to emphasise and explain some facts which without an explanation
may present difficulties. If a button of melted lead be covered with a
layer of slag rich in oxide of lead, and a second metal be added, this
other metal distributes itself between the metal and slag in proportions
which depend mainly upon the ease with which it is oxidised, and to a
large extent upon the relative quantities of material present. Easily
oxidisable metals such as zinc, iron, antimony and tin, will go mainly
into the slag, and, if the proportion of the slag is large, very little
will go into the metal. On the other hand, with metals oxidisable with
difficulty, such as silver, gold, and platinum, the reverse holds true;
nearly the whole of the metals will go into the lead, and very little
into the slag. If, however, the slag be very rich, say in antimony, the
lead will contain antimony; and, on the other hand, if the lead be very
rich in silver, the slag will contain silver in appreciable quantity.
Copper, which is near lead in the facility with which it is oxidised,
will serve for the purpose of a detailed example. The results of actual
analyses of metal and slag formed in contact with each other are shown
in the following table:--
---------------------------+--------------------------
Percentage Composition | Percentage Composition
of the Metal. | of the Slag.
----------+----------------+-----------+--------------
Lead. | Copper. | Lead. | Copper.
6.8 | 93.2 | 71.4 | 21.4
20.0 | 80.0 | 78.0 | 17.0
28.0 | 72.0 | 80.0 | 12.5
32.0 | 68.0 | 86.0 | 6.7
85.0 | 15.0 | 90.0 | 3.6
----------+----------------+-----------+--------------
It will be seen from this table that the slag is always much richer in
lead and poorer in copper than the metal with which it is in contact.
The ratio of lead to copper in these five samples is:--
In the Metal. In the Slag.
1 : 14 1 : 0.3
1 : 4 1 : 0.2
1 : 2.5 1 : 0.16
1 : 2 1 : 0.08
1 : 0.16 1 : 0.04
Assuming these figures to be correct, the following statement is
approximately true. On oxidising an alloy of 10 grams of copper and 10
grams of lead, and pouring off the slag when 3 grams of lead have gone
into it, there will be a loss of (owing to the slag carrying it off)
about 0.2 gram of copper. On repeating the operation, the next 3 grams
of lead will carry with them about 0.5 gram of copper; and on again
repeating, 3 grams of lead will remove 0.8 gram of copper. Finally, the
last gram of lead will carry with it 0.3 gram of copper, and there will
be left a button of copper weighing 8.3 grams. The slag will have
carried off altogether 1.7 gram of copper, which is 17 per cent. of the
metal originally present.
With the more perfect exposure to the air, and quicker removal of the
slag, which results from heating on a cupel, the loss would be heavier.
Karsten got by actual experiment on cupelling copper and lead in equal
proportions, a loss of 21.25 per cent.
Going back to the example: if the slag were collected and fused with a
suitable reducing agent so as to convert, say, half of it into metal,
that half would contain nearly the whole of the copper (such a reduction
is called "cleaning the slag"). On reoxidising this metal, another
button of copper is formed which, added to the first, would reduce the
loss from 17 per cent. to, say, 7 or 8 per cent. And it is conceivable
that by a series of similar operations, almost the whole of the 10 grams
of copper originally taken might be recovered. In practice the problem
is (as far as the copper is concerned) not how to save, but how most
easily to remove it; and since the removal of this metal is quicker from
an alloy containing not too much lead, it is evident that two or three
operations with small quantities of lead will be more effectual than a
single treatment with a larger quantity. With those metals (tin,
antimony, &c.) which pass quickly into the slag, the contrary is true;
hence with these it is necessary to have enough lead present, so that
the slag formed at the outset shall contain enough oxide of lead to make
it fluid. As silver is so much less easily oxidised than copper, we
should reasonably expect that the proportion of silver carried off in
the oxide of lead would be considerably less than that of the copper
indicated in the above example. Indeed, there are one or two facts
which tend to encourage the hope that the operation may be conducted
without any loss. If a piece of pure silver foil is exposed on a cupel
to air at the usual temperature of cupellation, it undergoes very little
change; it does not even fuse; it loses nothing in weight, and does not
oxidise. In fact, even if oxide of silver were formed under these
conditions, it could not continue to exist, for it is decomposed into
silver and oxygen at a temperature considerably below redness. On the
other hand, oxide of silver is not reduced to metal by heat alone, when
mixed with an excess of oxide of lead; while metallic silver is
converted into oxide when heated with the higher oxides of lead, copper,
and some other metals. That silver, and even gold (which is more
difficult to oxidise than silver), may be carried off in the slag in
this way, is in agreement with general experience. If 10 grams of silver
are cupelled with 10 grams of lead, there will be a loss of about 50
milligrams of silver, which is in round numbers 1-30th of the
corresponding copper loss; with 10 grams of gold and 10 grams of lead,
the loss will be 4 or 5 milligrams, which is about 1-12th of the
corresponding silver loss.
~Determination of Silver in Assay Lead.~--Scorify 50 grams of the lead
with 0.5 gram of powdered quartz or glass at not too high a temperature.
When the eye has "closed in," pour; reject the slag, and cupel the
button of lead. Remove the cupel from the muffle immediately the
operation is finished. Weigh, and make a prominent note of the result in
the assay book, as so many milligrams of silver contained in 100 grams
of lead.
~Determination of Silver in Red Lead or Litharge.~--Fuse 100 grams of
the oxide with from 10 to 20 grams of borax; and in the case of litharge
with 2 grams or with red lead 4 grams of flour. Cupel the lead, and
weigh the button of silver. Note the result as in the last case.
~Determination of Silver in Argentiferous Lead.~--Be careful in taking
the sample, since with rich silver lead alloys the error from bad
sampling may amount to several parts per cent. Cupel two lots of 20
grams each, and weigh the buttons of silver. Add to these the estimated
cupel loss, and calculate the result. Or wrap each button of silver in
20 grams of assay lead, and re-cupel side by side with two fresh lots of
20 grams each of the alloy. Calculate the loss incurred, and add on to
the weight of the two fresh buttons got.
~Determination of Silver in Bullion.~--The remarks made under the last
heading as to the importance of correct sampling apply with equal force
here. Make a preliminary assay by cupelling 0.1 gram of the alloy with 1
gram of assay lead; calculate the percentage composition. Refer to the
table on page 105 to find what weight of lead is required for cupelling
1 gram of alloy.
Weigh out four lots of 1 gram each, and wrap them in the required
quantity of lead. Make two check pieces by weighing up two lots of fine
silver equal to that which you believe to be present in the assay
pieces; add copper to make up the weight to 1 gram, and wrap in the same
quantity of lead as was used for the assays.
[Illustration: FIG. 43.]
Prepare six cupels and charge them in the annexed order (fig. 43), and
cupel. Guard against spirting. Clean and weigh the buttons of silver.
Add the mean loss on the two check pieces to the mean weight of the four
assay pieces; this multiplied by 1000 will give the degree of fineness.
~Determination of Silver in Copper.~--The silver is best separated in
the wet way before cupelling, but if the proportion is not too small, it
can be found by cupellation. Weigh up 3 grams of the metal, wrap in 30
grams of sheet lead, and cupel; when the cupellation has proceeded for
fifteen minutes, add 20 grams more lead, and continue till finished.
Weigh the button of silver.
The cupellation loss will be five or six per cent. of the silver
present. Determine it by powdering the saturated portion of the cupel
and fusing in a large Cornish crucible with 30 grams each of soda and
borax, 10 grams of fluor spar, and 1-1/2 gram of charcoal. Cupel the
resulting button of lead, and add 10 grams more of lead towards the
close of the operation. Deduct the weight of silver contained in the
lead used from the weight of the two buttons, and calculate to ounces to
the ton.
In an experiment in which 0.1975 gram of silver was present, the weight
of the button from the first cupellation was 0.1867, and that of the
button from the second, after correcting for the lead added, was 0.0110
gram.
~Determination of Silver in Galena.~ _By Pot Assay._--Mix 20 grams of
the powdered ore with 30 grams of red lead, 20 grams of soda, and 5
grams of borax, as also with from 7 to 10 grams of nitre. Fuse and pour.
Clean the slag if the ore is rich. Cupel the buttons of lead. Make the
usual corrections and calculate in ounces to the ton.
_By Scorification._--Take 10 grams of the ore, 30 grams of lead, and
0.5 gram of borax. Scorify, clean the slag by adding anthracite after
the "eye" has closed in: cupel the button of lead. Weigh the button of
silver, make the necessary corrections, and calculate to ounces to the
ton.
The determination may also be made by cupelling the button of lead got
in the dry lead assay.
A sample of galena determined by the three methods gave the following
results:--
By pot assay 7.18 ozs. per ton.
" scorification 7.02 "
" lead assay 6.72 "
~Determination of Silver in an Ore.~ _By Pot Assay._--Take 20 grams of
the powdered ore and mix with 30 grams of soda, 40 grams of red lead,
and 5 grams of borax, as also with from 2 to 3 grams of flour. Fuse:
pour. Clean the slag by fusing with 20 grams of red lead and two grams
of flour. Cupel the buttons of lead; weigh; make the necessary
corrections, and calculate to ounces to the ton.
_By Scorification._--Take 5 grams of the powdered ore, 50 grams of lead,
and 0.5 gram of "soda" or borax. Scorify. Clean the slag by fusing in a
crucible as in the pot assay. Cupel, &c.
_Examples._--_By Pot Assay._--Ore taken 20 grams.
Silver got 0.2893 gram
Silver from slag 0.0060 "
Silver lost in cupellation 0.0100 "
------
0.3053 "
Deduct silver in red lead 0.0017 "
------
Silver in ore 0.3036 " = 495.9 ozs. per ton.
_By Scorification._--Ore taken, 3 grams.
Silver got. 0.0425 gram
Silver from slag 0.0022 "
Silver lost in cupellation 0.0020 "
------
0.0467 "
Deduct silver in lead 0.0015 "
------
Silver in ore 0.0452 " = 492.2 ozs. per ton.
~Determination of Silver in Silver Precipitate.~--This substance
contains, in addition to metallic silver and gold, sulphates of lead and
lime; oxides of zinc, copper, and iron; and more or less organic matter.
The sample as received is generally free from "water at 100° C."; and,
since it rapidly absorbs water, care should be taken in weighing it.
Since it contains combined water it is not suited for scorifying;
therefore the determination of silver and gold (fine metal) is made by
pot assay. Weigh up 5 grams of the precipitate, mix with 100 grams of
litharge and 1 gram of charcoal. Melt in a crucible at a moderate heat
and pour. Detach the slag, replace in the crucible, and, when fused, add
a mixture of 20 grams of litharge and 1 gram of charcoal. When the
fusion is again tranquil, pour; and cupel the two buttons of lead.
In a sample worked in this manner the mean of four determinations gave
0.6819 gram of "fine metal"; deducting 1 milligram for the silver
contained in the oxide of lead, and adding 8 milligrams for the
cupellation loss, there is got 0.6889 gram or 13.778 per cent. of silver
(and gold) in the sample.
~Determination of Silver in Burnt Ores.~ _By Pot Assay._--Roasted
cupriferous pyrites containing small quantities of gold and silver comes
under this heading. The following mixture will give a fluid slag which
is heavy and tough when cold:--
Ore. Borax. Sand. Litharge. Charcoal.
100 50 50 100 7
Mix; place in a large crucible; cover with salt; and melt down under
cover. When fused drop in an iron rod for a few minutes, and about a
couple of minutes after its withdrawal, pour the charge quickly into a
large conical mould. The button of lead should weigh about 50 grams.
Cupel and weigh the silver. The litharge may be replaced by red lead, in
which case another gram of charcoal powder must be added.
In our experience the results obtained by this method are about 20 per
cent. less than the actual content of the ore. The results of two
assays, after deducting for the silver in the litharge used, were 3.9
and 4.1 milligrams; and a third assay, in which 5.4 milligrams of silver
had been added, gave 9.2, which, after deducting the added silver,
leaves 3.8 milligrams. The average of the three results is 3.9
milligrams from the 100 grams of ore.
Two lots of 100 grams of the same ore treated in the wet way gave 5.2
and 5.0 milligrams of silver. Burnt ores from Spanish pyrites carry
about 0.005 per cent. of silver.
WET METHODS.
Silver is got into solution from its ores by attacking with nitric acid,
but it is best, after dissolving, to cautiously add dilute hydrochloric
acid, and to carefully avoid excess. If the quantity of silver is very
small the solution is allowed to stand twenty-four hours, but,
otherwise, it is warmed and filtered as soon as it clears. Dry the
residue and concentrate the silver in a button of lead by pot method or
scorification, according to the amount of stony matter present. Cupel
the lead, and the resulting button will be free from all metals, except
perhaps gold. It may be weighed; or dissolved in nitric acid, and the
silver determined gravimetrically in the diluted and filtered solution.
It is better to weigh the metal and afterwards to determine the gold in
it, estimating the silver by difference. Silver alloys are dissolved in
dilute nitric acid (free from chlorides), diluted, and filtered. The
solution is then ready for gravimetric determination.
Sulphuretted hydrogen precipitates silver (like copper), completely,
even from fairly acid solutions.
GRAVIMETRIC DETERMINATION.
Add dilute hydrochloric acid in small excess to the hot dilute solution,
which must contain free nitric acid. Heat and stir until the solution
clears. Decant through a small filter, and wash with hot water,
acidulated at first with a little nitric acid if bismuth is suspected to
be present. Dry quickly, transfer as much as possible of the precipitate
to a watch-glass; burn and ignite the filter paper, treating the ash
first with two drops of nitric acid and then with one of hydrochloric,
and again dry. Add the rest of the silver chloride and heat slowly over
a Bunsen burner until it begins to fuse. Cool and weigh.
The precipitate is silver chloride (AgCl) and contains 75.27 per cent.
of silver. The moist precipitate is heavy and curdy; it is decomposed by
direct sunlight, becoming violet under its influence. When heated it is
yellowish; and, since it is volatile at a high temperature, it must not,
in drying, be heated above its fusing point. The fused chloride can be
removed from the crucible (to which it adheres strongly) by digesting
with dilute acid and zinc.
For the determination of silver in nearly pure bullion the following
process is used:--Weigh up 1.5054 gram of the alloy. With this amount of
alloy each 2 milligrams of silver chloride formed is equivalent to 1
degree of fineness, so that the weight of the silver chloride obtained
(stated in milligrams and divided by 2) will give the degree of
fineness. Transfer to a bottle (known as "bottles for the Indian mint
assay") and dissolve in 10 c.c. of dilute nitric acid, then make up with
water to 200 c.c. and add 3 c.c. of dilute hydrochloric acid. Allow to
stand a few minutes and then shake. Fill the bottle completely with
water, allow to settle, and syphon off the clear liquid; pour on more
water, shake gently to break up the lumps, and again fill the bottle
with water. Invert over the mouth of the bottle a porous Wedgwood
crucible, somewhat similar to those used in gold parting. Take firm hold
of the crucible and bottle, and invert promptly so that the silver
chloride may be collected in the crucible. Allow to stand a little while
for the precipitate to settle, and then carefully remove the crucible
under water.[14] Drain off most of the water and break up the silver
chloride with the help of a well-rounded glass rod. This greatly
facilitates the subsequent drying. Dry first on the water bath and then
on the iron plate. Remove the dried silver chloride, by inverting the
crucible, and weigh it.
As an example, 3 determinations of silver in a coin carried out in this
way gave:--
(1) 1.8500 gram AgCl = 925.0 fineness.
(2) 1.8498 " = 924.9 "
(3) 1.8502 " = 925.1 "
~Determination of Silver in Burnt Ores.~--Take 100 grams of the ore and
place in a large beaker of 2-1/2 litres capacity, and cover with 375
c.c. of hydrochloric acid. Boil for half an hour until the oxides are
dissolved and the residue looks like sand and pyrites; then add 20 c.c.
of nitric acid, and boil till free from nitrous fumes. Dilute to 2
litres with water, and pass a current of sulphuretted hydrogen till the
iron is reduced, the copper and silver precipitated, and the liquor
smells of the gas. This takes about one hour and a half.
Filter off the precipitate (rejecting the solution) and wash with warm
water. Dry and transfer to an evaporating dish, adding the ashes of the
filter paper. Heat gently with a Bunsen burner until the sulphur burns,
and then calcine until no more sulphurous oxide comes off. When cold add
30 c.c. of nitric acid, boil and dilute to 100 c.c. Add 1 c.c. of very
dilute hydrochloric acid (1 to 100),[15] stir well, and allow to stand
overnight. Decant on to a Swedish filter paper, dry and calcine.
Mix the ashes with 100 grams of litharge and 1 gram of charcoal, and
fuse in a small crucible. Detach the button of lead and cupel. Weigh and
make the usual corrections. As an example, 100 grams of ore treated in
this way gave 5.8 milligrams of silver; deducting 0.8 for the silver
added in the oxide of lead leaves 5 milligrams obtained from the ore.
Another experiment on 100 grams of the same ore to which 5 milligrams of
silver had been added gave 11.0 milligrams. Deduct 5.8 for the silver
added; this leaves 5.2 milligrams as the silver obtained from the ore.
These give, as a mean result, 0.0051 per cent., or 1.66 ounce per ton.
~Determination of Silver in Commercial Copper.~--For the method of doing
this, with an example and experiment, see under the heading of
_Examination of Commercial Copper_.
VOLUMETRIC METHODS.
There are two of these, one adapted for the determination of silver in
alloys of approximately known composition, and the other of more general
application. The first of these, generally known as "Gay-Lussac's"
method is, as regards its working, perfect in principle; but it requires
a practically constant quantity of silver, that is, one which varies by
a few milligrams only in each determination. It is a confirmatory method
rather than a determinative one. The other is known as "Volhard's," and
resembles in principle and method an ordinary volumetric process.
~Gay-Lussac's method~ is based on the precipitation of silver from a
nitric acid solution by a solution of sodium chloride. The point at
which the whole of the silver is precipitated being recognised by the
standard solution ceasing to give a precipitate. The process depends for
its success upon, (1) the ease which silver chloride separates out from
the solution leaving it clear after shaking, and, (2), the cloudiness
produced by the reaction of very small quantities of silver nitrate and
sodium chloride. In working, a quantity of the sodium chloride solution
equal to 1 gram of silver is added at once to the assay; and, when the
solution has been rendered clear by shaking, the residual silver (which
should not exceed a few milligrams) is estimated with the help of a
weaker solution of sodium chloride. The success in working evidently
depends upon the accuracy with which the first addition of the salt
solution is made. On this account the standard solution is run in from a
special pipette capable of delivering a practically invariable volume of
solution. It is not so important that this shall deliver exactly 100
c.c. as that in two consecutive deliveries the volume shall not differ
by more than 0.05 c.c. The dilute salt solution is one-tenth of the
strength of that first run in, and 1 c.c. of it is equivalent to 1
milligram of silver. Ordinarily it is run in 1 c.c. at a time (and an
ordinary burette may be used for this purpose), shaking between each
addition until it ceases to give a precipitate. If many such additions
have to be made the operation not only becomes tedious, but the
solution also ceases to clear after shaking, so that it becomes
impossible to determine the finishing point.
If the assay contains less than one gram of silver the first addition of
the dilute salt solution of course produces no precipitate. Five
milligrams of silver in solution (5 c.c.) is then added, and the assay
proceeded with in the usual way; 5 milligrams of silver being deducted
from the amount found.
There is required for the assay a _standard solution of sodium
chloride_, which is prepared by dissolving 5.4162 grams of the salt
(made by neutralizing carbonate of soda with hydrochloric acid) in water
and diluting to one litre. 100 c.c. of this is equivalent to 1 gram of
silver.
The weaker solution of salt is made by diluting 100 c.c. of the stronger
one to one litre. One c.c. of this will equal 1 milligram of silver, or
0.1 c.c. of the stronger solution.
A _standard solution of silver_ equivalent to the dilute salt solution
is made by dissolving 1 gram of fine silver in 10 c.c. of dilute nitric
acid, and diluting with water to one litre.
[Illustration: FIG. 44.]
The solution of salt is standardised as follows:--Weigh up 1.003 gram of
fine silver and dissolve in 25 c.c. of dilute nitric acid in a bottle
provided with a well-fitting flat-headed stopper. Heat on the water bath
to assist solution, resting the bottle in an inclined position. When
dissolved blow out the nitrous fumes with the help of a glass tube bent
at right angles. Run in from a stoppered pipette (as shown in fig. 44)
100 c.c. of the standard salt solution, and shake vigorously until the
solution clears. Fill an ordinary burette with the weaker standard salt
solution, and run 1 c.c. into the assay bottle, letting it run down the
side so that it forms a layer resting on the assay solution. If any
silver remains in solution a cloudy layer will be formed at the junction
where the two liquids meet. This is best observed against a black
background If a cloudiness is seen, shake, to clear the liquid, and run
in another c.c. of salt, and continue this until a cloudiness is no
longer visible. Deduct 1.5 c.c. from the amount of the weaker sodium
chloride solution run in. Divide the corrected reading by 10, and add to
the 100 c.c. This will give the volume of strong salt solution
equivalent to the silver taken.
If the first addition of the weaker salt solution causes no cloudiness
add 5 c.c. of the silver solution from an ordinary pipette, shake, and
then run in the weaker salt solution, working as before. These 5
milligrams of silver added must be allowed for before calculating. As
an example:--1.0100 gram of fine silver was taken for standardising a
solution and 4 c.c. of the weaker salt solution were run in. Deducting
1.5 and dividing by 10 gives 0.25 c.c. to be added to the 100 c.c.
100.25 : 1.0100 :: 100 : _x_
_x_ = 1.0075
which is the standard of the salt solution.
The method of working an assay may be gathered from the following
example:--In the determination of silver in some buttons left after
cupellation, it was assumed that these would contain 99.5 per cent. of
silver. For the assay it was necessary to take a quantity that should
contain a little more than 1.0075 grams of silver; then
99.5 : 100 :: 1.0075 : _x_
_x_ = 1.0125
To ensure a slight excess, there was taken 1.0150 gram of the buttons,
which was treated in exactly the same way as for the standardising. The
quantity of the weaker salt solution required was 7 c.c.; deducting 1.5
c.c., and dividing by 10, gives 100.55 c.c. of strong salt solution,
which is equivalent to 1.0130 gram of silver. This being obtained from
1.015 gram of alloy, is equal to 99.8 per cent., or 998.0 fine.
~The Effect of Temperature.~--The standardising and the assay must be
done at the same time, since a difference of 5° C. makes a difference of
0.1 c.c. in measuring the 100 c.c. of strong solution of salt. It is
always best to prepare a standard with each batch of assays.
~SULPHOCYANATE METHOD.~--Volhard's process is based upon the
precipitation of silver in nitric acid solutions with potassium
sulphocyanate, the finishing point being the development of a
reddish-brown colour, produced by the action of the excess of
sulphocyanate upon ferric sulphate. The white sulphocyanate settles
readily, leaving the liquor clear; and a persistent brown coloration in
the liquid indicates the finish. The assay must be carried out in the
cold; and water free from chlorides[16] must be used.
_The standard sulphocyanate of potassium_ solution is made by dissolving
4-1/2 or 5 grams of the salt (KCyS) in water, and diluting to 1 litre.
100 c.c. are about equivalent to 0.5 gram of silver.
_The standard silver nitrate solution_ is made by dissolving 5 grams of
fine silver in 50 c.c. of dilute nitric acid, boiling off nitrous fumes,
and diluting to 1 litre.
The _indicator_ is a saturated solution of iron alum, or a solution of
ferric sulphate of equivalent strength made by titrating acid ferrous
sulphate with potassium permanganate. Use 2 c.c. for each assay.
The sulphocyanate solution is standardised by placing 50 c.c. of the
silver nitrate solution in a flask with 20 c.c. of dilute nitric acid,
diluting to 100 c.c. with water, and running in the sulphocyanate until
the greater part of the silver is precipitated; then adding 2 c.c. of
the ferric indicator, and continuing the titration until a reddish-brown
colour is developed, and remains permanent after shaking continuously.
The assay is similarly performed, the silver being used in the state of
a nitric acid solution.
The effect of variations in the conditions of the assay may be seen from
the following experiments, in which 20 c.c. of standard silver nitrate
were used:--
~Effect of Varying Temperature~:--
Temperature 10° C. 30° C. 70° C. 100° C.
Sulphocyanate reqd. 19.6 c.c. 19.3 c.c. 19.0 c.c. 18.6 c.c.
~Effect of Varying Nitric Acid~:--Varying nitric acid has no effect,
except that with a fairly acid solution the finishing point is somewhat
sharper.
Nitric acid added 5 c.c. 10 c.c. 20 c.c. 50 c.c.
Sulphocyanate reqd. 19.6 c.c. 19.5 c.c. 19.6 c.c. 19.6 c.c.
~Effect of Varying Bulk~:--
Bulk 50 c.c. 100 c.c. 200 c.c. 300 c.c.
Sulphocyanate reqd. 19.5 c.c. 19.6 c.c. 19.6 c.c. 19.7 c.c.
~Effect of Varying Ammonic Nitrate~:--
Ammonic nitrate 0 gram 1 gram 5 grams 10 grams
Sulphocyanate reqd. 19.6 c.c. 19.6 c.c. 19.7 c.c. 19.9 c.c.
~Effect of Varying Silver~:--
Silver added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.
Sulphocyanate reqd. 1.0 c.c. 9.70 c.c. 19.6 c.c. 49.4 c.c. 99.0 c.c.
This method is valuable for determining silver in salts, alloys, and
solutions, where no more than an ordinary degree of accuracy is
demanded. It is easy, and applicable under most of the usual conditions.
Its greatest disadvantage is the brown coloration produced by the
sulphocyanate when the assay is nearly, but not quite, finished; and the
slowness with which this is removed on shaking up with the precipitate.
This is worse with large quantities of precipitate, and if about 1 gram
of silver is present, it gives an indefiniteness to the finish which
lowers the precision of the process to about 1 in 500; this is useless
for the assays of bullion. One writer states that this inconvenience is
due to portions of liquid being entangled in the precipitate, but it
appears much more likely to be due to the action of the precipitate
itself. In attempting to apply the process to the assay of bullion by
working it on the principle of a Gay-Lussac assay, it was found that a
very considerable excess of silver was required to complete the
reaction. In these experiments 100 c.c. of "sulphocyanate" (very
accurately measured) was run into the solution containing the weighed
portion of bullion (fine silver) and, after shaking the solution, was
filtered. In the filtrate the remaining silver, if there should be any,
was determined by the ordinary titration, but with "sulphocyanate" of
one-tenth the strength. This final titration was quite satisfactory. The
amount of silver precipitated by the first 100 c.c., however, varied
with the quantity of silver present as in the following series.[17]
Silver present. Silver precipitated.
1.1342 gram. 1.1322 gram.
1.1375 " 1.1335 "
1.1405 " 1.1351 "
1.1484 " 1.1379 "
These, of course, preclude a method of the kind aimed at, and at the
same time emphasise the importance of uniformity of work in the ordinary
process. In the determination of chlorides in sea-water, Dittmar used a
combined method: precipitating the bulk of the silver as chloride, and
after filtering, determining the small excess of silver by
sulphocyanate. This modification answers admirably when applied to the
assay of bullion. In the ordinary Gay-Lussac method, the precipitation
of the bulk of the silver by the 100 c.c. of salt solution leaves
nothing to be desired, either as to ease in working or accuracy of
result; the silver precipitate settles quickly, and leaves a clear
liquor admirably fitted for the determination of the few milligrams of
silver remaining in solution. But the method of determining this
residual silver by adding successive small quantities of salt so long as
they continue to give a precipitate is unsatisfactory, and, judged on
its own merits apart from the rest of the process, could hardly escape
condemnation. It is clumsy in practice, for the continued adding of
small portions of salt solution is laborious and becomes impossible with
more than a few milligrams of silver in solution. The proposed
modification is simple; having precipitated the silver with the 100 c.c.
of salt solution, as described under Gay-Lussac's method (page 120),
shake till the liquor clears, and filter into a flask, washing with a
little distilled water. Add 2 c.c. of "ferric indicator" to the filtrate
and titrate with a standard "sulphocyanate solution" made by diluting
the ordinary standard solution to such an extent that 100 c.c. after
diluting shall be equivalent to 0.1 gram of silver.[18] Calculate the
weight of silver found by "sulphocyanate" and add it to the weight which
100 c.c. of the salt solution will precipitate.
An advantage of this modification is that an excess of 15 milligrams may
be determined as easily and exactly as 5. In standardising the salt
solution, then, weigh up, say 1.0150 gram of pure silver, dissolve and
titrate. Suppose 13.5 c.c. of "sulphocyanate" required; then these are
equivalent to .0135 gram of silver, (100 c.c. = .1); the silver
precipitated by the salt is 1.0150-.0135--_i.e._, 1.0015 gram, which is
the standard.
~Application of the Method to Assays for Arsenic.~--If silver nitrate be
added to a neutral solution of an arsenate of one of the alkali metals,
silver arsenate (Ag_{3}AsO_{4}), is thrown down as a dark-red
precipitate. If, after adding excess of silver nitrate to insure a
complete precipitation, the arsenate of silver be filtered off, the
weight of the arsenic could be estimated from the weight of silver
arsenate formed. But this may be done much more conveniently by
dissolving the precipitate in nitric acid, and titrating with
sulphocyanate; the silver found will be to the arsenic present as 324
(108×3) is to 75.
The mineral is best treated by the method given in the third paragraph
on page 382; but the solution, after being acidified with nitric acid,
should be made exactly neutral with ammonia. A small excess of silver
nitrate should then be added, and since acid is liberated in the
reaction, the liquor must again be neutralised.[19] The precipitate must
then be filtered off, and washed with distilled water. Then dissolve it
in the paper by slowly running over it 20 c.c. of dilute nitric acid.
Wash the filter with distilled water, collecting with the filtrate in a
small flask. Add 2 c.c. of "ferric indicator" and titrate.
If the sulphocyanate solution be made up with 11 or 12 grams of the
potassium salt to the litre, and be then standardised and diluted, so
that for 100 c.c. it shall equal 1.08 gram of silver, (see p. 38), then
it will also equal .25 gram of arsenic (As). Except for ores rich in
arsenic, it will be better to work with a solution one half this
strength. The standard as calculated from an experiment with pure silver
should be checked by another using pure resublimed white arsenic,
As_{2}O_{3}, which contains 75.75 % of the metal. The quantity of white
arsenic taken, .1 or .2 gram, should contain about as much arsenic as
will be present in the assays. It is converted into sodium arsenate by
evaporating to a small bulk with nitric acid and neutralising with soda.
The precipitation and titration of the silver arsenate should be exactly
as in the assays.
The difficulty of the method is in the neutralising; which has to be
very carefully done since silver arsenate is soluble in even faintly
acid solutions; one drop of nitric acid in 100 c.c. of water is enough
to produce an absolutely worthless result; and an excess of acid much
less than this is still very prejudicial. The addition of a little
sodium acetate to the solution after the final neutralising has a good
effect.
~Arsenic in Mispickel.~--Weigh up .250 gram of the finely-powdered ore,
and place in a Berlin crucible about 1-1/4 or 1-1/2 inch in diameter.
Treat with 10 or 12 drops, one drop at a time, of strong nitric acid,
warm very gently, but avoid much heating. Put on a thin layer of nitre,
and rather more than half fill the crucible with a mixture of equal
parts of soda and nitre. Heat quickly in the blow-pipe flame, and when
the mass is fused and effervescing, withdraw and allow to cool. Boil out
with water, filter and wash. Insert a piece of litmus paper and
cautiously neutralise with nitric acid, using ammonia to neutralise any
accidental excess of the acid. Add a gram or so of ammonium nitrate and
silver nitrate in excess, neutralise again with ammonia and add two or
three grams of sodium acetate. Filter off the precipitate, wash and
titrate. In the fusion care should be taken to avoid much effervescence
(an excess of the soda mitigates this) and the operation should be
stopped as soon as the whole has entered into fusion.
COLORIMETRIC DETERMINATION.
There is, properly speaking, no colorimetric method, but the following,
which is sometimes used, is based on similar principles. It is useful
for the determination of small quantities of silver in substances which
yield clear solutions with nitric acid.
Dissolve a weighed quantity of the substance in nitric acid, and dilute
to a definite bulk. Divide into two equal parts. To one, add a drop or
two of dilute hydrochloric acid, stir and filter. To the other, add a
similar amount of dilute acid, and then to the filtered portion run in
from a burette standard silver nitrate (1 c.c. = 0.5 milligram silver)
until the solutions are equally turbid. Calculate in the usual way.
GOLD.
Gold occurs in nature chiefly as metal. It always contains more or less
silver, and, in alluvial sands, &c., may be associated with platinum and
iridium.
Gold is insoluble in hydrochloric or nitric acid, but is dissolved by
aqua regia or by solutions of iodine, bromine, or chlorine. It is taken
up by mercury, forming an amalgam, from which the mercury may be driven
off by heat.
When gold occurs in particles of any size, it is readily detected by its
appearance, but when finely disseminated through a large quantity of
rock, it is separated and detected by the amalgamation assay--described
below--or by a process of washing somewhat similar to vanning, or by the
following test:--Powder and, if necessary, roast 50 to 100 grams of the
ore, put on it three or four crystals of iodine and enough alcohol to
cover it; allow to stand for half an hour; a piece of filter paper
moistened with the liquid and burnt leaves an ash with a distinctly
purple tint if any gold is present. It is better, however, to filter off
the solution, evaporate, and ignite. Then, either take up with mercury,
and ignite the amalgam so as to get a speck of the metallic gold; or
treat with a few drops of aqua regia, and test the solution with
stannous chloride: a purple coloration indicates gold.
~AMALGAMATION ASSAY.~--This does not attempt to give the total produce
of gold, but rather the quantity which can be extracted on a large
scale; therefore it should imitate as closely as possible the process
adopted in the mine or district for extracting the metal.
Take 2 lbs of the ore in powder and roast; make into a stiff paste with
hot water and rub up for an hour or so with a little mercury. Wash off
the sand carefully, and collect the amalgam. Drive off the mercury by
heat, and weigh the residual gold. It is best to cupel it with lead
before weighing.
In an experiment on a lot of ore which contained 0.189 gram of gold,
0.179 gram was obtained by the above process, equal to about 94-1/2 per
cent. recovered. With ores generally, the yield may be from 80 to 90 per
cent. of the actual gold present.
DRY ASSAY.
The dry assay of gold ores resembles in its main particulars the dry
assay for silver by the crucible method; and for much that is of
importance in its discussion the student is referred to what is written
under Silver on pp. 90-113.
~Size of Assay Charges.~--Gold ores rarely contain more than a few
ounces, often only a few pennyweights of gold to the ton; consequently,
the button of gold obtainable from such quantities of ore as may be
conveniently worked by assaying methods is often so small as to require
more than ordinary care in its manipulation. One milligram of gold forms
a button of about the size of one of the full-stops on this page, and
compared with a million similar particles of quartz (about four ounces),
represents a produce of a quarter of an ounce to the ton: a proportion
such as the assayer is frequently called on to determine. It is evident,
therefore, that a charge of half an ounce or less of the ore, such as is
usual with silver ores, would demand of the worker both skill and care
in the handling of the minute quantity of gold to be obtained from it.
Fortunately the work is simple and precise, so that in practised hands
and with only a 5-gram charge the assay of a 5-dwt. ore is practicable;
with so small a charge, however, the result is barely perceptible on a
sensitive balance: the button of gold should be measured under a
microscope. It follows, therefore, that larger charges of say 50, 100,
or even 200 grams, have an advantage in that they lessen the strain on
the worker's attention, and, except in the case of the poorest mineral,
bring the button of gold within the scope of the balance. On the other
hand, the inconvenience of the larger charges lies in the amount of
fluxes and consequent size of the crucibles required to flux them.