Economical Processes.
In all silversmiths' establishments, the economical or waste-saving
processes, as they are termed, require special and careful attention,
so that the actual working loss, or that portion of it which is
entirely irrecoverable by the manufacturer, may be reduced to the
lowest possible degree. It may not be known to the general reader, or
to the beginner in the precious metal trades, that there always takes
place in the working up of the metal a loss of material, a portion of
which the manufacturer is unable to recover, however cautious may be
the means employed for that purpose. In the best regulated workshops,
this loss will amount at the lowest estimate to about 2-1/2 per cent.
of the whole quantity worked up in the establishment. If the actual
loss can be reduced to within the above limit it is considered very
low, and highly satisfactory. Taking into consideration the loss
that is occasioned in precious metal working, and from calculations
that we have made from experience, we have long since arrived at the
conclusion, that it cannot possibly be estimated under 10 per cent.
of the total work daily performed; and this opinion is based upon
experiments, the _raw_ material being weighed before the process of
melting and after the articles were completed, a fair calculation
of course being made for unfinished work. This was including every
description of manufacture; in some branches of the trade the working
loss is not quite so great, but then there are others in which it is
exceedingly heavy, so that the estimated loss in the jewellery trades
cannot be safely put at a lower percentage than we have quoted.
It will thus be seen that the _real_ loss, such as manufacturers are
unable to recover by the means already known to them, amounts to
one-fourth part of the total working loss of the establishment. This
is easily accounted for: in the first place, a little takes place in
the melting of the various alloys, the re-melting of scrap metal,
the reduction of lemel, &c.; then there are the sundry manipulations
of working; the passage of the metal through various acids, and the
processes of finishing, each of which detaches small particles of
metal, too small to be visible to the naked eye, but all of which go
to form a portion of the loss which the manufacturer never recovers.
The unrecovered metal may be judiciously proportioned as follows:--A
portion of it works itself into the wood-work of the flooring of the
shops, lathes, boards, and other parts of workshop appliances; then
there is the refiner's profit--as purchases of the sweep, polishings,
and other refuse of precious metal workers. Instances can be recorded
in which shrewd business-men have actually taken up the floors of their
workshops and recovered a vast quantity of metal which was supposed
to be lost for ever; and instances are well remembered in which
two jewellers, upon removing into more extensive premises, availed
themselves of the opportunity, not only of removing the boards which
formed the flooring of the premises they were about to leave, but also
those of the tenants they were about to succeed. In one case, metal of
the value of £80 was recovered, and in the other it reached the large
amount of £150. The two jewellers referred to, of course, were too
un-English to refund the proceeds to the late tenants, who, when they
became aware of it, if ever they did, would be, no doubt, wiser if
sadder men.
To prevent the precious metal from finding its way into such places
as these, it is advisable to have the floors well protected with sheet
zinc or iron, in which case not the least particle could be lost in
this manner. The extra cost of laying the floors would soon be amply
repaid, by an extra quantity of the working loss being recovered; and
if other equally effective precautions were adopted in the waste-saving
processes by precious metal workers, the _real_ loss, which they cannot
avoid suffering, might even yet be reduced to the lowest possible
point. Iron or zinc covered floors may be protected from wear, by
laying over the surface small square grates of perforated iron, and
these, being removable, may be readily taken up at stated periods, for
sweeping the refuse from the floors; once a month will be found often
enough to do this. The gratings should, however, be swept over lightly
every day in order to remove the dust and particles of metal that may
accumulate upon the surface into the perforations, and also for the
removal of waste paper and other rubbish, continually accumulating in
workshops.
Floors containing no such waste-saving precautions, are commonly swept
over once, and sometimes twice each day, the refuse arising therefrom
being carefully passed through a very fine sieve, all extraneous matter
removed, and the residue remaining in the sieve being well sorted for
the detection of all the precious metal visible to the naked eye. The
whole refuse matter is then thoroughly burned in a muffle provided
specially for the purpose, and finally reduced to a fine powder in a
cast-iron mortar. When it has reached this stage of the process, it is
quite ready for the particular kind of treatment it next receives at
the hands of the refiner. Grinding by large stone rollers is now fast
superseding this mode of pulverising jewellers' waste and refuse. When
the latter plan is adopted, the refuse should be swept from the floors
every morning, carefully looked through, and then transferred to a
barrel (having the top removed, which may be used as a lid), where it
can be well kept together, and hidden from view until the time arrives
for its further treatment.
The waste which accumulates in the processes of polishing, lapping,
&c., is greater than that already referred to, consequently, it
cannot be too carefully looked after, in every stage, where a large
manufacturing trade is being carried on in various branches. It is
advisable in the practice of true economy, for the polishing, lapping,
and scratching boxes to be repeatedly cleaned out, and the contents
removed out of the temptation of every one, by being placed in a box,
well lined with either sheet lead or zinc, which ensures the perfect
safety of the material placed therein from all irregularities in the
workshop. This kind of waste on being prepared for sale is again placed
in a very strong wrought-iron box, made of a suitable size to fit the
muffle, and having a thick close lid to it. After the work of the day
has been completed, the fire in the furnace or muffle is made up,
the dampers are closed, and then the iron box containing the refuse
is at once passed in and allowed to remain there till morning, when
every particle of matter will have become thoroughly burned; a slight
pulverization after this process readily reduces it to a fine powder;
further operations then cease, and the product is in all probability in
a fine state of division, and fit for the subsequent operations of the
refiner and assayer, whose special business it is to attend to these
arrangements of precious metal workers.
The next process we have to consider is one which includes the
whole of the liquid substances variously employed in silver-working
establishments, such as the pickling solutions, washing-out waters,
whitening mixtures, and waste or spent solutions of every kind. The
whitening solutions or mixtures, when in use, should be kept apart
from the ordinary cleansing liquids, as after they have been in use for
a time, they become saturated with copper taken off the work during the
whitening processes; if the solution is then set aside for some time
the copper eventually crystallizes out from the liquor, which may be
poured into the waste-water tub, and the remaining crystals of sulphate
of copper, for such it then is, may be removed and preserved.
In small establishments one large tub, to form the receptacle for
all spent or used-up liquids, will be found sufficient; but in large
places several will be required. In the former case the water is only
drawn off at the beginning of every fresh week, which allows plenty
of time for the precipitation of the silver without any disturbance
taking place in the mixture between the close of one week and the
commencement of another; whereas in large concerns it requires to
be drawn off continually unless other vessels are provided for its
reception during a long period. Attempts have been made to recover the
silver from these solutions by simply filtering the liquid through a
coarse piece of felt or flannel; or by providing a false bottom in
the tub or other vessel containing the waste waters, arranged in the
following manner:--A tolerably large tub would be employed, being about
one-fourth filled with coarse deal sawdust, next would be placed the
false bottom perforated with numerous small holes, and upon this would
be firmly secured a piece of felt, so as to exactly fill up the space
in that part of the tub, which then serves to act as the filterer of
all solutions poured in above. The liquid after passing through the
piece of felt proceeds through the perforations in the false bottom
into the sawdust beneath, where it is allowed to run away by means
of a small hole or tap at the bottom. But the use of either of these
processes, if adopted on a large scale, where the waste products
amounted to some hundreds per annum, would be wretchedly bad economy
and tend to a serious loss of valuable metal; the boiling sulphuric
acid, used in cleansing the work and for other purposes, has the power
of dissolving minute particles of silver as well as those of the baser
metal which always enters into the composition used in the production
of the work of the silversmith; therefore, that portion of the metal
which has become dissolved and entered into the chemical state,
requires to be brought back to its original form before it can be saved
by such means as those just described. To illustrate this more clearly,
we will take the process of gold-colouring. If workmen were to notice
the rinsing waters employed in this process, subsequently allowing the
vessels containing the rinsing to stand for a very short time, upon
pouring away the surplus water, a white curdy precipitate will at once
be observed at the bottom. This is the silver removed from the surface
of the gold alloy, which has been precipitated by the muriatic acid
and the common salt employed in the colouring mixture, into the form
of _chloride of silver_. Now in this proceeding there is no gold to be
seen in any of the vessels, but it is a well-known fact that a portion
has been removed during the process from the surface of the gold
articles. Where is it? Why, it has become dissolved, and is therefore
held invisible in the solution, in consequence of the colouring mixture
forming the well-known solvent for gold, _aqua-regia_. This is exactly
the case with a portion of silver in the silversmith's solutions; small
particles are continually being dissolved by the mixtures employed,
and are thus held in solution past the power of filtering, unless
some chemical ingredient be added to it, which acts as a re-agent
upon the metal sought to be recovered. From what we have seen in the
colour water, which always contains a little silver, it is evident
that both muriatic acid and common salt will do this work for us. We
prefer common salt, on account of its cheapness, besides being easily
procurable.
The best mode of treatment for the silversmith's waste waters, after
being collected together by pouring into the receptacle specially
provided for that purpose, is to prepare a saline solution for the
precipitation of the silver. This may be made by mixing together common
salt and tepid water, in the following proportions:--
Common salt 3 oz.
Tepid water 1 pt.
The water need only be sufficiently warm to dissolve the salt, and
the proportions given do not require to be strictly adhered to; in
fact any quantity, if properly mixed, will do to effect the purpose
required, and we merely give these as a guide for the process. In small
establishments where only one tub is employed, the above proportion of
saline solution may be added (every Saturday after the completion of
the day's work) to the waste-water; the whole should then be stirred
slowly in a circular direction, and allowed to settle until Monday
morning, when all the surplus water may be drawn off and poured away.
In larger establishments the accumulation of waste-water is greater,
therefore several collecting vessels should be employed, and the
mixture for precipitation may be added to them at other times than
those stated, if required, and in accordance with workshop regulations.
The sediment produced in the collecting vessels after the supernatant
water has all been drawn off, may be removed, dried by heat in a strong
iron pan, and subsequently sold to the refiner.