ELECTROPLATING AND ALLIED ARTS
§ 127. Electroplating.
This is an art which is usually deemed worthy of a treatise to itself,
but for ordinary laboratory purposes it is a very simple matter--so
simple, indeed, that the multiplicity of receipts as given in
treatises are rather a source of embarrassment than otherwise.
The fundamental principles of the art are:-
(1) Dirty work cannot be electroplated.
(2) Electroplated surfaces may be rougher, but will not be smoother
than the original unplated surface.
(3) The art of electroplating being in advance of the science, it is
necessary to be careful as to carrying out instructions in detail.
This particularly applies to the conditions which determine whether a
metallic deposit shall come down in a reguline or in a crystalline
manner.
§ 128. The Dipping Bath.
An acid dipping bath is one of the most useful adjuncts to the
laboratory, not only for cleansing metals for electroplating, but for
cleaning up apparatus made out of bits of brass tube and sheet, and
particularly for quickly cleaning binding screws, etc, where it is
necessary to ensure good electrical contact.
The cheapest and most satisfactory way in the end is to make up two or
three rather large baths to begin with. The glass boxes of storage
batteries do very nicely for the purpose, and being generally ground
pretty flat at the top, they may be covered by sheets of patent plate
glass, and thus preserved from the action of the air.
First Bath. A 30 or 40 per cent solution of commercial caustic soda.
Objects may be cleansed from grease in this bath by heating them as
hot as is consistent with individual circumstances, and plunging them
into it.
It is a considerable advantage to begin by removing grease from
articles subsequently to be dipped in an acid bath, both because it
saves time and acid, and because more uniform results are obtainable
when this is done than when it is omitted. It is a great advantage to
have the caustic soda solution hot. This is always done in factories
where nickel-plating is carried on, but it is inconvenient in the
laboratory. The articles after dipping in the alkali are swilled with
water, and may even be scrubbed with a brush, so as to remove greasy
matters that have been softened but not entirely removed.
Acid Bath. A convenient bath for laboratory purposes is made by
mixing two volumes of strong commercial nitric acid with one of strong
sulphuric acid in a cell measuring, say, 12 X 10 X 15 inches.
Copper or brass articles are dipped in this bath for a few seconds,
then rinsed with water, then dipped again for a second or two, or
until they appear equally white all over, and then withdrawn as
rapidly as possible and plunged into a large quantity of clean water.
Care must be taken to transfer the articles from the bath to the water
as quickly as possible, for if time be allowed for gas to be evolved,
the surfaces become mat instead of bright.
In order to save acid it is advisable to make up a third bath, using
those odds and ends of acids which gradually accumulate in the
laboratory. Sulphuric acid from the balance cases, for instance,
mixed with its own volume of commercial nitric acid, does very well.
The objects to be dipped receive a preliminary cleansing by a dip in
this bath, the strong bath being reserved for the final dip. Sheet
brass and drawn tube, as it comes from the makers, possesses a really
fine surface, though this is generally obscured by grease and oxide.
Work executed in these materials, cleaned in alkali, and dipped in
really strong acid, will be found to present a much better appearance
than work which has been filed, unless the latter be afterwards
elaborately polished.
On no account must paraffin be allowed to get into any of the baths.
When the final bath gets weak it must be relegated to a subordinate
position and a new bath set up. A weak acid bath leaves an ugly
mottled surface on brass work.
§ 129. A metallic surface which it is intended to electroplate must,
as has been mentioned, be scrupulously clean. If the metal is not too
valuable or delicate, cleaning by dipping is easy and effectual. The
following notes will be found to apply to special cases which often
occur.
(1) Silver Surfaces intended to be gilt. These are first washed
clean with soap and hot water, and polished with whitening. They are
then dipped for a moment in a boiling solution of potassium cyanide.
A 20 per cent solution of common commercial cyanide does well, but the
exact strength is quite immaterial. The cyanide is washed away in a
large volume of soft water, and the articles are kept under water till
they are scratch-brushed.
Mat surfaces are readily produced on standard silver by dipping in hot
strong sulphuric acid. The appearance of new silver coins, which is
familiar to everybody, is obtained by this process.
(2) Finely turned and finished Brass Work. If it is intended to
nickel-plate such work, and if it is desirable to obtain brightly
polished nickel surfaces, the work must be perfectly polished to begin
with. Full details as to polishing may be found in workshop books or
treatises on watch-making. It will suffice here to say that the brass
work is first smoothed by the application of successive grades of
emery and oil, or by very fine "dead" smooth files covered with chalk.
Polishing is carried out by means of rotten stone and oil applied on
leather.
In polishing turned work care must be taken to move the file, emery,
or rotten stone to and fro over the work with great regularity, or the
surface will end by looking scratchy and irregular. The first process
of cleaning is, of course, to remove grease, and this is accomplished
best by dipping in a bath of strong hot caustic soda solution, and
less perfectly by heating the work and dipping it in the cold caustic
soda bath.
During this process a certain amount of chemical action often occurs
leading to the brass surface exhibiting some discoloration. The best
way of remedying this is to dip the brass into a hot bath of cyanide
of potassium solution. If it is inconvenient to employ hot baths or
to heat the brass work, good results may be obtained by rubbing the
articles over with a large rough cork plentifully lubricated with a
strong solution of an alkali.
If the surfaces are very soiled or dirty, a paste of alkali and fine
slaked lime may be applied on a cork rubber, and this in my experience
has always been most effective and satisfactory in every way, except
that it is difficult to get into crevices. If the alkali stains the
work, a little cyanide of potassium may be rubbed over the surface in
a similar manner.
Brass work treated by either of these methods is to be washed in clean
water till the alkali is entirely removed, and may then be
nickel-plated without any preliminary scratch-brushing. The treatment
in hot baths of alkali and cyanide is the method generally employed in
American factories as a preliminary to the nickelling of small brass
work for sewing machines, etc.
(3) Copper either for use as the kathode in electrolysis calibration
experiments or otherwise is most conveniently prepared by dipping in
the acid bath, rinsing quickly in cold water, scratch-brushing under
cold water, and transferring at once to the plating bath. In the case
where the copper plates require to be weighed they are dipped into
very hot distilled water after scratch-brushing, and then dried at
once by means of a clean glass cloth.
(4) Aluminium (which, however, does not readily lend itself to plating
operations [Footnote: This difficulty has now been overcome. See
note, section 138.] ) is best treated by alkali rubbed on with a cork,
or by a hot alkaline carbonate where rubbing is inexpedient. The
clean aluminium is scratch-brushed under water, and at once
transferred to the plating bath.
(5) Iron for Nickel-plating. According to Dr. Gore
(Electra-metallurgy, p. 319) the best bath for cleaning iron is made
as follows: "One gallon of water and one pound of sulphuric acid are
mixed with one or two ounces of zinc (which of course dissolves); to
this is added half a pound of nitric acid." The writer has been
accustomed to clean iron by mechanical means, to deprive it of grease
by caustic alkali, and to finish it off by, means of a hard scratch
brush. This process has always worked satisfactorily.
(6) Articles soldered with soft solder containing lead and tin do not
readily lend themselves to electrolytic processes, the solder
generally becoming black and refusing to be coated with the
electro-deposit. Moreover, if soldered articles are boiled for any
length of time in caustic alkali during the preliminary cleansing,
enough tin will dissolve to form a solution of stannate of potash or
soda--strong enough to deposit tin on brass or copper. A method of
coppering soldered articles will be described later on.
§ 130. Scratch-brushing.
This process is generally indispensable, and to its omission is to be
traced most laboratory failures in electroplating. Scratch-brushes
may be bought at those interesting shops where "watchmakers' supplies"
are sold. It will be well, therefore, to purchase a selection of
scratch brushes, for they are made to suit particular kinds of work.
They are all made of brass wire, and vary both in hardness and in the
fineness of the wire. The simplest kind of scratch brush consists
merely of a bundle of wires bound up tightly by another wire, and
somewhat "frizzed" out at the ends (Fig. 90). A more useful kind is
made just like a rotating brush, and has to be mounted on a lathe
(Fig. 91).
Fig. 90. Fig. 91.
The scratch brush is generally, if not always, applied wet; the
lubricant generally recommended is stale beer, but this may be
replaced by water containing a small quantity of glue, or any other
form of gelatine in solution--a mere trace (say .1 per cent) is quite
sufficient. Very fair results may be got by using either pure or
soapy water. The rotating brushes require to be mounted on a lathe,
and may be run at the same speed as would be employed for turning
wooden objects of the same dimensions.
Since the brush has to be kept wet by allowing water or its equivalent
to drip upon it, it is usual to make a tin trough over which the brush
can revolve, and to further protect this by a tin hood to keep the
liquid from being thrown all over the room. In many works the brush
is arranged to lie partly in the liquid, and this does very well if
the hood is effective.
There is a superstition that electro-deposits stick better to
scratch-brushed surfaces than to surfaces which have not been so
treated, and consequently it is usual to scratch-brush surfaces before
electro-deposit. However this may be, there is no doubt that
adherence and solidity are promoted by frequent scratch-brushing
during the process of depositing metal, especially when the latter
tends to come down in a spongy manner.
Gilt surfaces--if the gilding is at all heavy--are generally dull
yellow, or even brown, when they come from the bath, and require the
scratch brush to cause the gold to brighten, an office which it
performs in a quite striking manner. The same remark applies to
silvered surfaces, which generally leave the bath a dead white--at
all events if the deposit is thick, and if ordinary solutions are
employed. In either case the touch of the scratch brush is magical.
§ 131. Burnishing.
Burnishers of steel, agate, or bloodstone can be bought at the shops
where scratch brushes are sold, and are used to produce the same
brightening effect as can be got by scratch-brushing. The same
solutions are employed, but rather stronger, and the burnisher is
swept over the surface so as to compress the deposited metal.
Burnishing is rather an art, but when well done gives a harder and
more brilliant (because smoother) surface than the scratch brush. On
the whole, steel burnishers are the most convenient if in constant
use.
If the burnishing tools have to lie about, steel is apt to rust,
unless carefully protected by being plunged in quicklime or thickly
smeared with vaseline, and the least speck of rust is fatal to a
burnisher. In any case the steel requires to be occasionally
repolished by rouge and water on a bit of cloth or felt. The process
of burnishing is necessarily somewhat slow and tedious, and as a rule
is not worth troubling about except in cases where great permanence is
required.
The burnisher is moved over the work somewhat like a pencil with
considerable pressure, and care is taken to make the strokes as
uniform in direction as possible; otherwise the surface looks
non-uniform, and has to be further polished by tripoli, whitening,
etc, before it is presentable.
§ 132. Silver-plating.
The most convenient solution for general purposes is an 8 to 10 per
cent solution of the double cyanide of silver and potassium together
with 1 or 2 per cent of "free" potassium cyanide. Great latitude is
permissible in the strength of solution and density of current. As
commercial cyanide of potassium generally contains an unknown
percentage of other salts, which, however, do not interfere with its
value for the purpose of silver-plating, the simplest procedure is as
follows.
For every 100 c.c. of plating solution about 7 grms. of dry
crystallised silver nitrate are required. The equivalent amount of
potassium cyanide (if dry and pure) is 5.2 grms, but commercial
cyanide may contain from 50 per cent upwards to 96 per cent in the
best fused cyanide made from ferrocyanide only. An approximate idea
of the cyanide content can be obtained from the dealers when the salt
is purchased, and this is all that is required.
A quantity slightly in excess of the computed amount of cyanide is
dissolved in distilled water, and this is cautiously added to the
solution of the silver nitrate till precipitation is just complete.
The supernatant liquors are then drained away, and the precipitate
dissolved by adding a sufficiency of the remaining cyanide; this
process is assisted by warming and stirring.
An allowance of about one-tenth of the whole cyanide employed may be
added to form "free" cyanide, and the solution made up to the strength
named. It is advisable to begin with the cyanide in a moderately
strong solution, for the sake of ease in dissolving the precipitate.
This solution will deposit silver upon articles of copper or brass
immersed in it even without the battery, but the coat will be thin.
The solution is used cold, with a current density of about 10 to 20
ampères per square foot. The articles to be silvered are
scratch-brushed, washed, and electroplated, till they begin to look
undesirably rough. They are then taken out of the bath, rebrushed,
and the process continued till a sufficiency of silver is deposited.
Four grammes weight of silver (nearly) is deposited per ampère hour.
It is best to use a fine silver anode, so that the solution, does not
get contaminated by copper.
In most factories it is usual to "quicken" the objects to be silvered
before placing them in the electrolysis vats, because the deposit is
said to adhere better in consequence of this treatment. I have never
found it any improvement for laboratory purposes, but it is easy to
do. A dilute (say 2 per cent) solution of cyanide of mercury is
required containing a little free cyanide. The objects to be
"quickened" are scratch-brushed and dipped into the cyanide of mercury
solution till they are uniformly white; it is generally agreed that
the less the mercury deposited the better, so long as a perfect
coating is obtained. The objects are rinsed after quickening, and put
in the depositing bath at once.
The mat surface of silver obtained by electrolysis of the cyanide is
very beautiful--one of the most beautiful things in nature--shining
with incomparable crystalline whiteness. So delicate is it, however,
for so great is the surface it exposes, that it is generally rapidly
deteriorated by exposure to the air. It may be protected to some
extent by lacquering with pale lacquer, but it loses some of its
brilliancy and purity in the process. The deposit is generally
scratch-brushed or burnished down to a regular reflecting surface.
§ 133. Cold Silvering.
A thin but brilliant coat of silver may be readily applied to small
articles of brass or copper in the following way. A saturated
solution of sodium sulphite (neutral) is prepared, and into this a 10
per cent solution of nitrate of silver is poured so long as the
precipitate formed is redissolved. A good deal of silver may be got
into solution in this way. Articles to be silvered need only to be
cleaned, brushed, and dipped in this solution till a coat of the
required thickness is obtained.
I must admit, however, that the coating thus laid on does not appear
to be so permanent as one deposited by simple immersion from the
cyanide solution, even though it is thicker. The cyanide plating
solution will itself give a good coat of silver if it is used boiling,
and if a little potassium cyanide be added.
For purposes of instrument construction, however, a thin coat of
silver is seldom to be recommended, on account of its liability to
tarnish and its rapid destruction when any attempt is made to repolish
it. For these reasons, nickel or gold plating is much to be
preferred.
§ 134. Gilding.
This art deserves to be much more widely practised than is usual in
laboratories. Regarded as a means of preserving brass, copper, or
steel, it is not appreciably more "time robbing" than lacquering, and
gives infinitely better results. Moreover, it is not much more
expensive. Strange as it may seem, the costliness of gilding seldom
lies in the value of the gold deposited; the chief cost is in the
chemicals employed to clean the work, and in interest on the not
inconsiderable outlay on the solution and anode.
The easiest metal to gild is silver, and it is not unusual to give
base metals a thin coating of silver or copper, or both, one after the
other, before gilding, in order to secure uniformity. To illustrate
the virtue of a thin layer of gold, I will mention the following
experiment. About three years ago I learned for the first time that
to "clean" the silver used in a small household required at least an
hour's labour per diem. I further ascertained that most of this time
is spent on the polishing part of the process.
As this seemed a waste of labour, I decided to try the effect of
gilding. In order to give the proposal a fair trial I gilt the
following articles: half a dozen table spoons and forks, a dozen
dessert forks and spoons, and a dozen tea spoons. These were all
common electroplated ware. They were weighed before and after
gilding, and it was with difficulty that the increase of weight was
detected, even though a fine bullion balance was employed. On
calculating back to money, it appeared that the value of the gold
deposited was about threepence. Assuming that an equal weight of
silver had been accidentally dissolved by the free cyanide during the
plating--which is unlikely--the total amount of gold deposited would
be worth, say, sixpence.
After three years' continuous use the gilding is still perfect, except
at the points on which the spoons and forks rest, where it is
certainly rather shabby. Meanwhile the "gold" plate only requires to
be washed with hot water and soap to keep it in perfect order, a much
more cleanly and expeditious process than that of silver cleaning.
§ 135. Preparing Surfaces for Gilding.
Ordinary brass work--rough or smooth--may for purposes of
preservation be dipped, scratch-brushed, and gilt at once. Seven
years ago the writer gilt the inside of the head of a copper water
still, and simply scratch-brushed it; it is to-day in as good order as
when it was first done. If it is intended to gild work from the
first, with the view of making an exceptionally fine job of it,
"gilding metal," i.e. brass containing one to one and a quarter
ounces of zinc to the pound of copper may be specified. From its
costliness, however, this is only desirable for small work.
Iron and steel are generally given a preliminary coating of copper,
but this may be dispensed with though with no advantage--by using a
particular process of gilding.
Base metals, zinc, pewter, lead, etc, are first coppered in a cyanide
of copper solution, as will be described under the head of
Copper-plating. If it is intended to gild soldered articles,
the preliminary coating of copper is essential.
The most convenient vessel for holding a gilding solution is
undoubtedly one formed of enamelled iron. Particularly useful are the
buckets and "billies" (i.e. cylindrical cans) made of this material.
These vessels may be heated without any fear of a smash, and do not
appear to be appreciably affected by gilding solutions--at all events
during several days or weeks. The avoidance of all risk of breakage
when twenty or thirty pounds' worth of solution is in question is a
matter of importance.
Under no circumstances is it desirable to use anything but the purest
gold and best fused cyanide (called "gold" cyanide) in the preparation
of the solutions. The appearance of a pure gold deposit is far richer
than of one containing silver, and its resistance to the atmosphere is
perfect; moreover, in chemico-physical processes one has the
satisfaction of knowing what one is dealing with.
§ 136. Gilding Solutions.
The strength of solution necessary for gilding brass, copper, and
silver is not very material. About one to two pounds of "gold"
potassium cyanide (? 96 per cent KCN) per gallon does very well. The
gold is best introduced by electrolysing from a large to a small gold
electrode. One purchases a plate of pure gold either from the mint or
from reliable metallurgists (say Messrs. Johnson and Matthey of
London), and from this electrodes are cut.
The relative areas of the electrodes do not really much matter. I
have used an anode of four times the area of the cathode. The
solution is preferably heated to a temperature of about 50° C, and a
strong current is sent through it, say twenty amperes to the square
foot of anode. The electrodes must be suspended below the surface of
the solution by means of platinum wires. If the gold plates are only
partly immersed, they dissolve much more rapidly where they cut the
surface, possibly on account of the effect of convection currents,
though so far as the writer is aware no proper explanation has yet
been given.
After a time gold begins to be deposited on the cathode in a powdery
form, for which reason it is a good plan to begin by wrapping the
latter in filter paper. The process has gone on for a sufficient time
when a clean bit of platinum foil immersed in the place of the cathode
becomes properly gilt at a current density of about ten amperes per
square foot.
The powdery gold deposited on the cathode while preparing the solution
can be scraped off and melted for further use, or the whole cathode
may now be used as an anode. The platinum foil testing cathode may
also be "stripped" by making it an anode, and is for this reason
preferable to German silver or copper, which would contaminate the
solution while the "stripping" process was in progress.
For general purposes a current density of say ten to fifteen amperes
per square foot may be used, but this may be considerably varied, so
long as the upper limit is not greatly overpassed. During
gold-plating there is a considerable advantage in keeping the
electrodes moving or the solution stirred.
After immersing the cleaned and scratch-brushed articles, depositing
may go on for about three minutes, after which they are removed from
the bath and examined, in order to detect any want of uniformity in
the deposit.
The articles should be entirely immersed; if this is not done,
irregularity is apt to appear at the surface. Platinum wires employed
as suspenders, and coated along with the articles to be gilt, may also
be cleaned without loss by making them anodes. If, on examination,
all is found to be going on well, reimmerse the cathodes, and continue
plating till they appear of a dull yellowish brown (this will occur in
about four minutes), then remove them, rinse and scratch-brush them,
and replace them in the bath.
When a second coat appears to be getting rather brown than yellowish
brown, i.e. of the colour of wet wash-leather, the removal, followed
by scratch-brushing, may be repeated, and for nearly all laboratory
purposes, the articles are now fully gilt.
The coating of gold deposited from a hot cyanide solution is spongy in
the extreme, and if the maximum wear-resisting effect is to be
obtained, it is advisable to burnish the gold rather than to rely upon
the scratch brush alone.
If the area of the cathode exceeds that of the anode the solution is
said to grow weaker, and vice versa. This may be remedied in the
former case by an obvious readjustment; the latter introduces no
difficulty so far as I know except when plating iron or steel.
The student need not be troubled at the poor appearance of the deposit
before it is scratch-brushed. Heavy gold deposits are almost always
dull, not to say dirty, in appearance till the burnisher or scratch
brush is applied. On the other hand, the deposit ought not to get
anything like black in colour.
The following indications of defects may be noted--they are taken from
Gore. I have never been really troubled with them.
The deposit is blackish. This is caused by too strong a current in
too weak a bath. This may be remedied to some extent by stirring or
keeping the cathode in motion. The obvious remedy is to add a little
cyanide of gold.
The gold anode gets incrusted. This is a sign that the bath is
deficient in potassium cyanide. The gold anode gets black and gives
off gas. The solution is deficient in cyanide, and too large a
current is being passed.
If a bright surface is desired direct from the bath, some caustic
potash (say 2 per cent) may, according to Gore, be added, or the
articles may be plated only slightly by using a weak current and
taking them out directly they show signs of getting dull. By a weak
current I mean one of about five amperes per square foot.
The deposit is said to be denser if the solution be heated as
directed; but the bath will gild, though not quite so freely when
cold.
To gild iron or steel directly, dilute the bath as above recommended
some five or six times, add about 1 per cent of potassium cyanide, and
gild with a very weak current (say two or three amperes per square
foot) in the cold. Frequent scratch-brushing will be found requisite
to secure proper adherence.
It is generally recommended to gild brass or German silver in
solutions which are rather weak, but in the small practice which
occurs in the laboratory a solution prepared as suggested does
perfectly for everything except iron or steel. The scratch-brushing
should be done over a large photographic developing dish to avoid loss
of gold. It is a good plan to rinse the articles after leaving the
bath in a limited quantity of distilled water, which is afterwards
placed in a "residue" bottle, and then to scratch-brush them by hand
over the dish to catch fine gold. When any loose dust is removed the
articles may be scratched in the lathe without appreciable further
loss.
Silver-gilt articles tend to get discoloured by use, but this
discoloration can be removed by soap and water. After long use a gold
cyanide bath tends to alter greatly in composition, In general, the
bath tends to grow weaker, from the fact that there is a strong
temptation to gild as many articles at once as possible.
It is therefore a good plan to keep a rough profit and loss account of
the gold in order to find the quantity in solution. Fifty dwts. per
gallon (or 78 grms. per 4.5 litres) is recommended. A gallon of
solution of this strength is worth about eleven pounds sterling in
gold and cyanide, and a serviceable anode will be worth about 10
pounds. (Fine gold is worth nominally four pounds four shillings and
eleven pence ha'penny per oz.) Gold may be easily obtained containing
less impurity than one part in ten thousand.
§ 137. Plating with Copper.
Copper may be deposited from almost any of its salts in reguline form,
the sulphate and nitrate being most usually employed. In the
laboratory a nearly saturated solution of sulphate of copper with 1 or
2 per cent of sulphuric acid will answer most purposes. A current
density of, at most, fifteen amperes per square foot may be used,
either for obtaining solid deposits for constructional purposes or for
calibrating current measuring instruments by electrolysis. A copper
anode is of course employed.
When coppering with a view to obtaining thick deposits it is a good
plan to place the electrodes several inches apart, and, if possible,
to keep the liquid stirred, as there is a considerable tendency on the
part of copper deposits to grow out into mossy masses wherever the
current density exceeds the limit mentioned. As the masses grow
towards the anode the defect naturally tends to increase of itself,
hence the necessity for care. The phenomenon is particularly marked
at the edges and corners of the cathode.
If the deposit becomes markedly irregular, the best plan is to stop
the process and file the face of the deposit down to approximate
smoothness. In coppering it is of the utmost importance that the
cathode be clean and free from grease; it must never be touched (by
the finger, for instance) from the time it is scratch-brushed till it
is immersed in the plating bath. Any grease or oxidation tends to
prevent the copper deposit adhering properly.
A copper deposit oxidises very easily when exposed to the air.
Consequently if the surface be required free from oxide, as, for
instance, when it is to be silvered or gilt, it must be quickly washed
when withdrawn from the coppering bath, scratch-brushed, and
transferred immediately to the silvering or gilding bath.
If the surface is to be dried, as in electrolysis calibrations, it
must be rinsed quickly with boiling water and pressed between sheets
of filter paper. Another method which has been recommended is to
rinse the copper in--water slightly acidulated with sulphuric acid
(which prevents oxidation), then in distilled water, and to dry by
blotting paper and in front of a fire, taking care not to make the
plate too hot. The wash water is sufficiently acidulated by the
addition of two or three drops of acid per litre. So far as I know,
the method of washing in acidulated water was first proposed by Mr. T.
Gray.
§ 138. Coppering Aluminium.
A good adherent deposit of copper on aluminium used to be considered a
desideratum in the days when it afforded the only means of soldering
the latter. Many receipts have been published from time to time, and
I have tried, I think, most of them. On no occasion, however, till
this year (1896), have I succeeded in obtaining a deposit which would
not strip after it was tinned and soldered, though it is not difficult
to get apparently adherent deposits so long as they are not operated
upon by the soldering iron. The best of the many solutions which have
been proposed in years gone by is very dilute cupric nitrate with
about 5 per cent of free nitric acid.
The problem of electroplating aluminium which I have indicated as
awaiting a solution has at last found one. In the Archives des
Sciences physiques et naturelles de Genève for December 1895 (vol.
xxxiv. p. 563) there is a paper by M. Margot on the subject, which
discloses a perfectly successful method of plating aluminium with
copper. The paper itself deals in an interesting way with the theory
of the matter--however, the result is as follows.
(1) The aluminium articles are boiled for a few minutes in a strong
solution of ordinary washing soda. The aluminium surface is thus
corroded somewhat, and rendered favourable to the deposit of an
adherent film of copper. After removal from the soda solution the
aluminium is well washed and brushed in running water.
(2) The articles are dipped for thirty seconds or so in a hot 5 per
cent solution of pure hydrochloric acid.
(3) After dipping in the hydrochloric acid, the work is instantly
plunged into clean water for about one second, so as to remove nearly,
but not quite, all of the aluminium chloride.
(4) The work is transferred to a cold dilute (say 5 per cent) solution
of cupric sulphate slightly acidulated with sulphuric acid. The
degree of acidulation does not appear to be very important, but about
one-tenth per cent of strong acid does well.
If the preliminary processes have been properly carried out the
aluminium will become coated with copper, and the process is
accompanied by the disengagement of gas. It appears to be a rule that
if gas is not given off, the film of copper deposited is non-adherent.
The work must be left in the copper sulphate solution till it has
received a uniform coating of copper.
(5) When this is the case the work is removed--well washed so as to
get rid of the rest of the aluminium chloride, and then electroplated
by the battery in the ordinary copper sulphate bath.
If the operation (4) does not appear to give a uniform coat, or if gas
is not evolved from every part of the aluminium surface, I find that
operations (2) and (3) may be repeated without danger, provided that
the dip in the hydrochloric acid is shortened to two or three
seconds.
The copper layer obtained by Margot's method is perfectly
adherent--even when used as a base for ordinary solder--though in
this case it can be stripped if sufficient force is applied.
Since the solder recommended by M. Margot for aluminium contains zinc,
it does not run well when used to unite aluminium to copper, brass,
iron, etc. In this case, therefore, I have found the most
advantageous method of soldering to be by way of a preliminary
copper-plating.
The success of M. Margot's method depends in my experience on
obtaining just the proper amount of aluminium chloride in contact with
the aluminium when the latter is immersed in the copper sulphate
solution.
§ 139. The process of copper-plating from sulphate or nitrate may,
according to Mr. Swan (Journal of the Royal Institution, 1892, p.
630), be considerably accelerated by the addition of a trace of
gelatine to the solution. As success appears to depend upon hitting
the exact percentage amount of the gelatine, which must in any case be
but a fraction of one per cent, and as Mr. Swan refrains from stating
what the amount is, I am unable to give more precise instructions. A
few experiments made on the subject failed, doubtless through the
gelatine content not having been rightly adjusted. Mr. Swan claims to
be able to get a hard deposit of copper with a current density of 1000
amperes per square foot, but seems to recommend about one-tenth of
that amount for general use.
The solution employed is a mixture of nitrate of copper and ammonium
chloride--proportions not stated. Electrolytic copper, as generally
prepared, is very pure, but this is a mere accident depending on the
impurities which, as a rule, have to be got rid of. Electrolysis
seems to have no effect in purifying from arsenic, for instance.
Roughly speaking, about 11 grms. of copper are deposited per ampere
hour from cupric salt solutions. When the current density is too high
the anode suffers by oxidation, and this introduces a large and very
variable resistance into the circuit.
§ 140. Alkaline Coppering Solution
Coppering Base Metals. It is often desirable to coat lead, zinc,
pewter, iron, etc, with a firm and uniform layer of copper
preparatory to gilding or silvering. If copper or brass articles are
soldered with soft solder it is found that the solder does not become
silvered or gilt along with the rest of the material, but remains
uncoated and of an ugly dark colour. This defect is got over by
giving a preliminary coating of copper.
This is done in an alkaline solution, generally containing cyanogen
and ammonia. The following method has succeeded remarkably well with
me. The receipt was taken originally from Gore's Electro-metallurgy,
p. 208. A solution is made of 50 grms. of potassium cyanide
(ordinary commercial, say, 75 per cent) and 30 grms. of sodium
bisulphite in I.5 litres of water. Thirty-five grammes of cupric
acetate are dissolved in a litre of water, and 20 cubic centimetres of
the strongest liquid ammonia are added. The precipitate formed must
be more or less dissolved to a strong blue solution. The cyanide and
bisulphite solution is then added with warming till the blue colour is
destroyed. This usually requires the exact amount of cyanide and
bisulphite mentioned, but I have not found it essential to entirely
destroy the colour.
The solution contains cuprocyanide of sodium and ammonium (?), which
is not very soluble, and this salt tends to be deposited in granular
crystalline masses on standing. However, at a temperature of 50° C.
the above receipt gives an excellent coppering liquid, which will coat
zinc with a fine reguline deposit. Brass or copper partly smeared
with solder will receive a deposit of copper on the latter as well as
on the former, and, moreover, a deposit which appears to be perfectly
uniform.
In using the bath the anode tends, as a rule, to become incrusted, and
this rapidly increases the resistance of the cell, so that the current
falls off quickly. The articles should be scratch-brushed and plated
for about two minutes with a current density of about ten ampères per
square foot.
As soon as the deposit begins to look red the articles are to be
removed and rebrushed, after which the process may be continued.
About five minutes' plating will give a copper deposit quite thick
enough after scratch-brushing to allow of a very even gilding or
silvering.
Aluminium appears to be fairly coated, but, as usual, the copper
strips after soldering. Iron receives an excellent and adherent coat.
I do not think that the formation of a crust upon the anode can be
entirely prevented. According to Gore, its formation is due to the
solution being too poor in copper, but I have added a solution of the
acetate of copper and ammonium till the colour was bright blue without
in any way reducing the incrustation. If the solutions become
violently blue it is perhaps as well to add a little more cyanide and
bisulphite, but I have not found such an addition necessary. The
process is one of the easiest and most satisfactory in
electro-metallurgy.
§ 141. Nickel-plating.
An examination of several American samples of nickel-plated goods has
disclosed that the coating of nickel is, as a rule, exceedingly thin.
This is what one would expect from laboratory repetition of the
processes employed.
Commercial practice in the matter of the composition of nickelling
solutions appears to vary a good deal. Thin coatings of nickel may be
readily given in a solution of the double sulphate of nickel and
ammonia, which does rather better if slightly alkaline. Deposits from
this solution, however, become gray if of any thickness, and,
moreover, are-apt to flake off the work. The following solution has
given very good results with me. It is mentioned, together with
others, in the Electrical Review, 7th June 1895.
The ingredients are:-
Nickel sulphate 5 parts
Ammonia sufficient to neutralise the nickel salt.
Ammonium tartrate 3.75 parts
Tannin 0.025 parts
Water 100 parts
The nickel sulphate and ammonia are dissolved in half the water, the
ammonium tartrate in the other half with the tannin. The solutions
are mixed and filtered at about 40° C. This solution works well at
ordinary temperatures, or slightly warm, with a current density of ten
ampères per square foot. In an experiment made for the purpose I
found that plating may go on for an hour in this solution before the
deposit begins to show signs of flaking off. The deposit is of a fine
white colour.
The resistance of the bath is rather high and rather variable,
consequently it is as well to have a current indicator in circuit, and
it may well happen that five or six volts will be found requisite to
get the current up to the value stated. For nickelling small objects
of brass, such as binding screws, etc, it is very necessary to be
careful as to the state of polish and uniformity of their surfaces
before placing them in the plating bath. A polished surface will
appear when coated as a polished surface, and a mat surface as a mat
surface; moreover, any local irregularity, such as a speck of a
foreign metal, will give rise to an ugly spot in the nickelling bath.
For this reason it is often advisable to commence with a coat of
copper laid on in an alkaline solution and scratch-brushed to absolute
uniformity.
An examination of the work will, however, disclose whether such a
course is desirable or not; it is not done in American practice, at
all events for small brass objects. These are cleaned in alkali and
in boiling cyanide, which does not render a polished surface mat, as
weak acid is apt to do, and are then coated with a current density of
about ten ampères per square foot.
In spite of what is to be found in books as to the ease with which
nickel deposits may be polished, I find that the mat surface obtained
by plating on an imperfectly polished cathode of iron is by no means
easily polished either by fine emery, tripoli, or rouge.
Consequently, as in the case of brass, if a polished surface is
desired, it must be first prepared on the unplated cathode. In this
case, even if the deposit appears dull, but not gray, it may be easily
polished by tripoli and water, using a cork as the polisher.
Scratch-brushing with brass wire, however, though possibly not with
German silver wire, brightens the deposit, but discolours it. When
the deposit becomes gray I have not succeeded in polishing it
satisfactorily.
Soldered brass or iron may be satisfactorily coated with nickel by
giving it a preliminary coating of copper in the cyanide bath. On the
whole, I recommend in general that iron be first coated with copper in
the alkaline bath, scratch-brushed, and then nickel-plated, and this
whether the iron appears to be uniform or not. Much smoother,
thicker, and stronger coats of nickel are obtained upon the
copper-plated surface than on the iron one, and the coating does not
become discoloured (? by iron rust) in the same way that a coating on
bare iron does. The copper surface may be plated for at least an hour
at a density of ten ampères per square foot without scaling.
Scales or circles divided on brass may be greatly improved in
durability by nickel--plating. For this purpose the brass must be
highly polished and divided before it is nickelled.
The plating should be continued for a few minutes only, when a very
bright but thin coat of nickel will be deposited; it then only
remains to wash and dry the work, and this must be done at once. If
the nickel is deposited before the scale or circle is engraved, very
fine and legible divisions are obtained, but there is a risk that
flakes of nickel may become detached here and there in the process of
engraving.
142. Miscellaneous Notes on Electroplating.
Occasionally it is desirable to make a metallic mould or other object
of complex shape. The quickest way to do this is to carve the object
out of hard paraffin, and then copy it by electrotyping. Electrotype
moulds can be made in many ways. The easiest way perhaps is to take a
casting in plaster of Paris, or by means of pressure in warm
gutta-percha.
In cases where the mould will not draw, recourse must be had to the
devices of iron-founders, i.e. the plaster cast must be made in
suitable pieces, and these afterwards fitted together. This process
can occasionally be replaced by another in which the moulding material
is a mixture of treacle and glue. The glue is soaked in cold water
till it is completely soft. The superfluous water thrown away,
one-fourth part by volume of thick treacle is added, and the mixture
is melted on the water bath; during which process stirring has to be
resorted to, to produce a uniform mixture.
This liquid forms the moulding mixture, and it is allowed to flow
round the object to be copied, contained in a suitable box, whose
sides have been slightly oiled. The object to be copied should also
be oiled. After some hours, when the glue mixture has set, it will be
found to be highly elastic, so that it may be pulled away from the
mould, and afterwards resume very nearly its original form.
One drawback to the use of these moulds lies in the fact that the
gelatine will rarely stand the plating solution without undergoing
change, but this may be partially obviated by dipping it for a few
seconds in a 10 per cent solution of bichromate of potash, exposing it
to the sunlight for a few minutes, and then rinsing it.
In order to render the surface conducting, it is washed over with a
solution of a gold or silver salt, and the latter reduced in situ to
metal by a suitable reagent. A solution of phosphorus is the most
usual one (see Gore, Electro-metallurgy, p. 216). Such a mould may
be copper-plated in the sulphate bath, connection being made by wires
suitably thrust into the material.
Plaster of Paris moulds require to be dried and waxed by standing on a
hot plate in melted wax before they are immersed in the plating bath.
In this case the surface is best made conducting either by silvering
it by the silvering process used for mirrors, or by brushing it over
with good black lead rendered more conducting by moistening with an
ethereal solution of chloride of gold and then drying in the sun.
The brushing requires a stiff camel's-hair pencil of large size cut so
that the hairs project to a distance of about a quarter of an inch
from the holder. The brushing must continue till the surface is
bright, and is often a lengthy process.
The same process of blackleading may be employed to get a coat of
deposited metal which will strip easily from the cathode.
In all cases where extensive deposits of copper are required, the
growth takes place too rapidly at the corners. Consequently it is
often desirable to localise the action of the deposit. A "stopping"
of ordinary copal varnish seems to be the usual thing, but a thin coat
of wax or paraffin or photographic (black) varnish does practically as
well.
I do not propose to deal with the subject of electrotyping to any
extent, for if practised as an art, a good many little precautions are
required, as the student may read in Gore's Electro-metallurgy. The
above instructions will be found sufficient for the occasional use of
the process in the construction of apparatus, etc. There is no
advantage in attempting to hurry the process, a current density of
about ten ampères per square foot being quite suitable and
sufficiently low to ensure a solid deposit.
§ 143. Blacking Brass Surfaces.
A really uniform dead-black surface is difficult to produce on brass
by chemical means. A paste of nitrate of copper and nitrate of silver
heated on the brass is said to give a dead-black surface, but I have
not succeeded in making it act uniformly. For optical purposes the
best plan is to use a paint made up of "drop" black, ground very fine
with a little shellac varnish, and diluted for use with alcohol. No
more varnish than is necessary to cause the black to hold together
should be employed.
In general, if the paint be ground to the consistency of very thick
cream with ordinary shellac varnish it will be found to work well when
reduced by alcohol to a free painting consistency.
A very fine gray and black finish, with a rather metallic lustre, may
be easily given to brass work. For this purpose a dilute solution of
platinum tetrachloride (not stronger than 1 per cent) is prepared by
dissolving the salt in distilled water. The polished brass work is
cleaned by rubbing with a cork and strong potash till all grease has
disappeared, as shown by water standing uniformly on the metal and
draining away without gathering into drops.
After copious washing the work is wholly immersed in a considerable
volume of the platinum tetrachloride solution at the ordinary
temperature. After about a quarter of an hour the brass may be taken
out and washed. The surface will be found to be nicely and uniformly
coated if the above instructions have been carried out, but any
finger-marks or otherwise dirty places will cause irregularity of
deposit. If the process has been successful it will be found that the
deposit adheres perfectly, hardly any of it being removed by vigorous
rubbing with a cloth. If the deposit is allowed to thicken--either
by leaving the articles in the solution too long or heating the
solution, or having it too strong--it will merely rub off and leave
an irregular surface.
This process succeeds well with yellow brass and Muntz metal, either
cast or rolled, but it does not give quite such uniform (though still
good) results with gun-metal, on which, however, the deposit is darker
and deader in appearance.
A book might be written (several have been written) on the art of
metal colouring, but though doubtless a beautiful and delicate art, it
is of little service in the laboratory. For further information the
reader may consult a work by Hiorns.
§ 144. Sieves.
Properly graded sieves with meshes of a reliable size are often of
great use. They should be made out of proper "bolting" cloth, a
beautiful material made for flour-millers. Messrs. Henry Simon and
Company of Manchester have kindly furnished me with the following
table of materials used in flour-milling.
Sieves made of these materials will be found to work much more quickly
and satisfactorily than those made from ordinary muslin or wire gauze.
Relative Bolting Value of Silk, Wire, and Grit Gauze
Threads per inch Trade No. Trade No. Trade No. of
Approximate. of Silk. of Wire. Grit Gauze.
18 0000 18 16
22 000 20 20
28 00 26 26
38 0 32 34
48 1 40 44
52 2 45 50
56 3 50 54
60 4 56 58
64 5 60 60
72 6 64 66
80 7 70 70
84 8 80 80
94 9
106 10
114 11
124 12
130 13
139 14
148 15
156 16
163 17
167 18
170 19
173 20
§ 145. Pottery making in the Laboratory.
When large pieces of earthenware of any special design are required,
recourse must be had to a pottery. Small vessels, plates, parts of
machines, etc, can often be made in the laboratory in less time than
it would take to explain to the potter what is required. For this
purpose any good pipeclay may be employed. I have used a white
pipe-clay dug up in the laboratory garden with complete success.
The clay should be kneaded with water and squeezed through a cloth to
separate grit. It is then mixed with its own volume or thereabouts of
powdered porcelain evaporating basins, broken basins being kept for
this purpose. The smoothness of the resulting earthenware will depend
on the fineness to which the porcelain fragments have been reduced. I
have found that fragments passing a sieve of sixty threads to the inch
run, do very well, though the resulting earthenware is decidedly
rough.
The porcelain and clay being thoroughly incorporated by kneading, the
articles are moulded, it being borne in mind that they will contract
somewhat on firing. [Footnote: The contraction depends on the
temperature attained as well as on the time. An allowance of one part
in twelve will be suitable in the case considered.] The clay should
be as stiff as is convenient to work, and after moulding must be
allowed to get thoroughly dry by standing in an airy place; the
drying must not be forced, especially at first, or the clay will
crack.
Small articles are readily fired in a Fletcher's crucible furnace
supplied with a gas blow-pipe; the furnace is heated gradually to
begin with. When a dull red heat is attained, the full power of the
blast may be turned on, and the furnace kept at its maximum
temperature for three or four hours at least, though on an emergency
shorter periods may be made to do.
The articles are supported on a bed of white sand; after firing, the
crucible furnace must be allowed to cool slowly. It must be
remembered that the furnace walls will get hot externally after the
first few hours, consequently the furnace must be supported on bricks,
to protect the bench.
The pottery when cold may be dressed on a grindstone if necessary.
This amateur pottery will be found of service in making small fittings
for switch-boards, commutators, and in electrical work generally.
Pottery made as described is very hard and strong, the hardness and
strength depending in a great degree on the proportion of powdered
porcelain added to the clay, as well, of course, as on the quality of
both of these materials.
It is a good plan to knead a considerable quantity of the mixture,
which may then be placed in a well-covered jar, and kept damp by the
addition of a little water.
Pottery thus made does not require to be glazed, but, of course, a
glaze can be obtained by any of the methods described in works on
pottery manufacture. The following glaze has been recommended to me
by a very competent potter:-
Litharge
7 parts by weight
Ground flint
2 parts by weight
Cornish stone or felspar
1 parts by weight
These ingredients are to be ground up till they will pass the finest
sieve--say 180 threads to the inch. They are then mixed with water
till they form a paste of the consistency of cream. They must, of
course, be mixed together perfectly. The ware to be glazed is dipped
into the cream after the first firing; it is then dried as before and
refired. The glaze will melt at a bright red heat, but it will crack
if not fired harder; the harder it is fired the less likely is it to
crack.
If colouring matters are added they must be ground in a mill free from
iron till they are so fine that a thick blanket filter will not filter
them when suspended in water. This remark applies particularly to
oxide of cobalt.
APPENDIX
PLATINISING GLASS
IN the Philosophical Magazine for July 1888 (vol. xxvi. p. 1) there
is a paper by Professor Kundt translated from the Sitzungsberichte of
the Prussian Academy. This paper deals with the indices of refraction
of metals. Thin prisms were obtained by depositing metals
electrolytically on glass surfaces coated with platinum. The
preparation of these surfaces is troublesome. Kundt recounts that no
less than two thousand trials were made before success was attained.
A detailed account of the preparation of these surfaces is not given
by Kundt, but one is promised--a promise unfortunately unfulfilled so
far as I am able to discover. A hunt through the literature led to
the discovery of the following references: Central Zeitung fuer Optik
und Mechanik, p. 142 (1888); Dingler's Polytechnik Journal, Vol.
cxcv. p. 464; Comptes Rendus, vol. lxx. (1870).
The original communication is a paper by Jouglet in the Comptes
Rendus, of which the other references are abstracts. The account in
Dingier is a literal translation of the original paper, and the note
in the Central Zeitung is abbreviated sufficiently to be of no value.
The details are briefly as follows:-
One hundred grams of platinum are dissolved in aqua regia and the
solution is dried on the sand bath, without, however, producing
decomposition. Though the instructions are not definite, I presume
that the formation of PtCl4 is contemplated.
The dried salt is added little by little to rectified oil of lavender,
placed on a glass paint-grinding plate, and the salt and oil are
ground together with a muller. Care is required to prevent any
appreciable rise of temperature which would decompose the compound
aimed at, and it is for this reason that the salt is to be added
gradually. Of course the absorption of water from the air must be
prevented from taking place as far as possible. Finally, the compound
is diluted by adding oil of lavender up to a total weight of 1400
grams (of oil).
The liquid is poured into a porcelain dish and left absolutely at rest
for eight days. It is then decanted and filtered, left six days at
rest, and again decanted (if necessary). The liquid should have a
specific gravity of 5° on the acid hydrometer. (If by this the Baumé
scale is intended, the corresponding specific gravity would be 1.037.)
A second liquid is prepared by grinding up 25 grams of litharge with
25 grams of borate of lead and 8 to 10 grams of oil of lavender. The
grinding must be thoroughly carried out.
This liquid is to be added to the one first described, and the whole
well mixed. The resulting fluid constitutes the platinising liquid,
and is applied as follows:-
A sheet of clean glass is held vertically, and the liquid is painted
over it, carrying the brush from the lower to the upper edge. The
layer of oil dries slowly, and when it is dry the painting is again
proceeded with, moving the brush this time from right to left; and
similarly the process is repeated twice, the brush being carried from
top to bottom and left to right. This is with the object of securing
great uniformity in the coating. Nothing is said as to the manner in
which the glass is to be dried.
The dried glass is finally heated to a temperature of dull redness in
a muffle furnace. The resinous layer burns away without running or
bubbling, and leaves a dull metallic surface. As the temperature
rises this suddenly brightens, and we obtain the desired surface
(which probably consists of an alloy of lead and platinum). It is
bright only on the surface away from the glass.
I have not had an opportunity of trying this process since I
discovered the detailed account given by Jouglet; but many
modifications have been tried in the laboratory of the Sydney
University by Mr. Pollock, starting from the imperfect note in the
Central Zeitung, which led to no real success.
It was found that it is perfectly easy to obtain brilliant films of
platinum by the following process, provided that the presence of a few
pin-holes does not matter.
The platinum salt employed is what is bought under the name of
platinic chloride; it is, however, probably a mixture of this salt
and hydro-chloro-platinic acid, and has all the appearance of having
been obtained by evaporating a solution of platinum in aqua regia to
dryness on the water bath. A solution of this salt in distilled water
is prepared; the strength does not seem to matter very much, but
perhaps one of salt to ninety-nine water may be regarded as a standard
proportion. To this solution is added a few drops of ordinary gum
water (i.e. a solution of dextrin). The exact quantity does not
matter, but perhaps about 2 per cent may be mentioned as giving good
results.
The glass is painted over with this solution and dried slowly on the
water bath. When the glass is dry, and covered with a uniform hard
film of gum and platinum salt free from bubble holes, it is heated to
redness in a muffle furnace. The necessary and sufficient temperature
is reached as soon as the glass is just sensibly red-hot.
The mirrors obtained in this way are very brilliant on the free
platinum surface. If the gum be omitted, the platinum will have a mat
surface; and if too much gum be used, the platinum will get spotty by
bubbles bursting. There does not appear to be any advantage in using
lead.
It is quite essential that the film be dry and hard before the glass
is fired.