Carbonate of lead 1A.5 "
Persulphate of iron (calcined) 1 "
Flint glass 3 "
The use of enamels--that is, substances which impart various colours
to the glass, when placed on its surface by their fusion--is not
admissible in windows which pretend to belong to any of the earlier
styles of glass painting; though enamel painting is used for the
decoration of houses, and sometimes, as I consider very improperly,
for the decoration of church windows. One sheet of glass, colourless
and transparent, or it may have its surface ground, is usually
employed. A subject is painted on it with enamel colours, much as
subjects are painted upon porcelain. When the work is completed, the
glass plate is fired, and thus the colours become semi-transparent,
and perfectly adherent to the plate; but they are not clear and
bright, and transparent, as are the colours of glass which is coloured
in the pot, and therefore have not the same brilliancy, nor do they
allow of the same bold and effective treatment.
It is much to be desired that amateurs who can draw, and who have a
feeling for this particular style of art, should devote a portion of
their time to its execution. They will find it to be extremely
agreeable and pleasant, and the few difficulties which they meet with
in their first attempts will be readily overcome by perseverance, or
by applying for assistance and advice to gentlemen engaged in the
pursuit of this interesting profession.
_Moulded and Cut Glass._--Flint glass is now very commonly blown
in moulds, and this art has been brought to such perfection that
moulded decanters and tumblers have an appearance very similar to that
of cut glass. The moulds are always made of metal, and so constructed,
that they open out into two or more pieces, which are generally hinged
to the bottom of the mould. The workman places it on the ground, and
fixes it by standing on projections from its side. He then gathers a
suitable quantity of glass on the end of his blowpipe, which he places
in the mould, and the side of the glass touching it will thus have
impressed upon it whatever form is engraved on it. After the glass has
become hard, the mould is opened, and the glass vessel is removed and
annealed.
When it is desired to cut a design on the outside of a tumbler or
wine-glass, the vessel is, in the first instance, blown of a thicker
substance than if it is to be left uncut. The necessary shapes, which
are usually in facets, are cut upon it by the action of sand and
water, a lathe of a very simple construction being used to give a
rotary motion to cutting discs, made of stone and kept continually
moist by water dripping on them, so that when the glass is pressed
against them, the required portion of its surface is worn away. The
usual diameter of these stones is about 10 inches. After the rougher
stone has been used, a finer kind of sandstone disc is employed, or a
disc of slate, upon which sand and water are allowed to drop, and the
already roughly cut surface is, by their action, partly polished.
Copper discs with flattened circumference are used for polishing the
glass, and for this purpose, emery mixed with oil, is applied to the
edges of their circumference.
_Ground Glass_ is made by rubbing the surface of glass with sand
and water, just as in the first operation of plate glass polishing.
But a very ingenious method is now generally adopted for grinding
glass, by placing it in a cradle, so that it can swing from side to
side; sand and water are placed upon the glass, and it grinds itself,
so to speak, by this operation.
_Annealing and Devitrification._--As the word "annealing" has been
often used in this article, it will be well to explain what is its
action. If a piece of molten glass be dropped into water, it will
assume an oblong shape, the lower end of which will be round, while
the other will taper off into a fine point. These drops, which have
received the name of Prince Rupert's drops, look like pieces of
ordinary glass, and if the small end of one of them be broken off, a
sort of explosion takes place, and the whole mass flies into a
thousand minute pieces, some of which will be found to have been
driven to a considerable distance. Here then it appears, that when the
skin, which is perfect and entire in the Rupert drop, is broken, the
bond which held together the constituent particles is broken also, and
so they are acted on by a repellent force, and fly away from one
another. If hot water be poured into a thick common tumbler, it very
generally cracks it: but if the tumbler be thin and of better
manufacture, it will bear almost boiling water without cracking. In
the first case it has been badly annealed; and besides this, glass
being a bad conductor of heat, from its thickness, the heat imparted
by the hot water expands the inner surface, while the outer coating,
not being warmed, does not expand, and, retaining its original form,
is burst. If, however, a tumbler be thick and properly annealed, there
is not so much danger of its breaking, when a portion of it is exposed
to a considerable rise of temperature. In the case of the Rupert
drops, they are not annealed at all, and so there is no cohesive bond
between the particles, such as there would be if they were properly
annealed, that is, if, instead of being cooled suddenly from the
molten state, they were allowed to cool in a heated chamber very
slowly. After glass has been heated, the particles of which it is
composed take a long time to rearrange themselves, so that in the
manufacture of thermometers, it is necessary, after sealing up the
bulb and tube which contain the mercury, to allow them to remain for a
long time; otherwise the pressure of the air on the outside of the
bulb, not being supported by any air on the inside, causes the
particles of glass to become more compact, and thus renders the
capacity of the thermometer bulb and tube smaller than it was, when
the thermometer was first sealed. It seems that the process of
annealing glass gives time for the particles to arrange themselves in
such a way, that when the glass is cold, it will not be so liable to
fracture from sudden changes of temperature.
Considerable curiosity has been excited of late by a new invention,
which has resulted from the investigations of a Frenchman. We have
been told that tumblers and wine-glasses, and other glass utensils,
could be so treated that they would never break; and experiments
performed upon many samples of these glasses led one to suppose, that
the object had been attained. There is no doubt whatever, that some
who have had experience of what is termed toughened glass know, that
in many cases very uncertain results are obtained in the resisting
power of the glass to the action of a violent blow. Before, however,
entering into some researches which I have made on the subject, it
will be well to state what is the nature of the change which the
toughening process produces in the glass, and this seems to be a fit
place for this consideration, as the method of making, and the
behaviour, of Prince Rupert's drops, have just been discussed.
The physical properties of these Rupert's drops have been examined
with great care by M. Victor de Luynes, and the results of his
experiments have been communicated to the _SociA(C)tA(C) de Secours des
Amis des Sciences_. For the purposes of this article, many of
his experiments have been repeated, confirming in general his
observations, and others have also been instituted. The toughness and
hardness of these drops are remarkable; the thick pear-shaped portion
will bear a sharp stroke with a hammer without breaking; nor can it be
scratched with a diamond. To break the tapering thread or tail, as it
may be conveniently called, requires considerable force. To find out
what weight was required to do this, a series of experiments was
performed, the results of which are given in the table following. The
tail of a drop was placed over a small hole bored in the top of a
table; a hook was then adjusted round a part of the tail which
measured 19 on a Birmingham wire gauge; below the table and attached
to this hook, a scale-pan was hung. This pan was then carefully
loaded, all shock being avoided, until the thread was ruptured and the
weight required to effect this was then noted:
White Glass Rupert's Drops.
Gauge. Strain.
19 16 lb. 0 oz.
19 15-1/2 " 0 "
19 16 " 0 "
19 (poor) 9-1/4 " 0 "
Green Glass.
Gauge. Strain.
19 18-3/4 " 0 "
19 (poor) 9 " 0 "
19 28 " 6 "
16 26-1/4 " 0 "
It will be observed that the drops made from green bottle glass
withstood a greater strain than those made from crown glass; the
latter, in fact, did not break throughout their mass, but left a
portion of the bulb unbroken, showing some fault in the tempering. It
was with difficulty that the workmen could be induced to make drops
out of this kind of glass, as they knew by experience that they
usually failed to break perfectly, and they stated that it was quite
impossible to make them with lead glass. To ascertain what force was
required to fracture a thread of like dimensions that had not been
tempered, one of the drops was heated to redness, and annealed by
allowing it to cool very gradually. When subjected to the same trial,
it was fractured by a weight of 12 ozs., and the drop did not break
into small fragments, but behaved exactly like ordinary glass, thus
showing that the glass had been _un_tempered by the heating process. A
piece of glass rod, drawn out into a thread in a gas flame, when
subjected to the same conditions, bore a strain of 10 oz. A
sewing-needle of the same thickness was broken by a weight of 3 lb. 14
oz., thus showing that the tail of the Rupert's drop was very much
tougher than tempered steel. By suspending a Rupert's drop in such a
manner as to allow the tail to dip into hydrofluoric acid, it is
found, that when the surface or skin is eaten away to a certain depth,
rupture takes place exactly in the same manner as when the tail is
broken. In whatever way fractured, the particles, when examined by the
microscope, show a crystalline structure, and do not at all resemble
pieces of ordinary glass; when rubbed between the palms of the hands,
they do not cut, nor scratch, nor penetrate the cuticle. If a drop be
enclosed in plaster of Paris so as to leave a portion of the tail
exposed, it may then be broken and all the particles will remain _in
situ_. On removing the plaster, it will be found that the drop has
been broken up into thousands of minute needle-shaped particles
arranged in cones, the apices being in the direction of the tail. It
would appear then from these experiments, and from observations with
polarized light, that the glass in the interior of a Rupert's drop
exists under enormous tension, and that it is only prevented from
bursting into fragments by the outer skin; on its being broken in any
part, the bond which holds together the constituent particles is
broken also, and so, being acted upon by a repellent force, they fly
away from one another. There is another kind of toy resembling in some
respects the Rupert's drop, known as the Bologna bottle or
philosopher's flask. It has the form of a soda-water bottle with the
neck cut off, the bottom being rounded off and very much thicker than
the walls. These flasks are sometimes formed accidentally in
glass-works by the workman, who, in order to examine the quality of
the glass, takes out a portion from the pot on the end of his
blowpipe, and blows a small quantity of air into the mass,
manipulating it in the usual manner. Whilst still at a very high
temperature, it is detached from the blowpipe, and is probably allowed
to fall on the ground in a place where there is a current of cold air,
the exterior thus becoming suddenly chilled. When cold, these flasks
will bear very rough handling, and will withstand the blow of a hammer
on the outside, it being almost impossible to break them by striking
the bottom; the interior will also bear the blow of a leaden bullet
falling into it from a considerable height, but if a few grains of
sand be allowed to fall into it, or if the inside skin be slightly
scratched, the mass splits into fragments in the same manner as a
Rupert's drop. The examination of these curious phenomena leads us to
the subject of "toughened glass," as it has been termed. The invention
of rendering articles of glass less fragile, which has given rise to
so much public attention during the last year, is due to M. Alfred de
la Bastie, a French engineer. His process consists in heating the
glass to be toughened to a temperature close upon its softening point,
and then plunging it into a bath of oil, or into a mixture of
oleaginous substances kept at a much lower temperature. When this
operation is successfully performed, the glass acquires properties
very similar to those of Rupert's drops; it becomes much less fragile
than ordinary glass, but when sufficient force is employed to fracture
it, the whole flies into small pieces. It cannot be cut with a
diamond, but is immediately disintegrated when the outer skin is
scratched to a certain depth.
It is to be observed, however, that in particular cases it is possible
both to saw and pierce the toughened glass. M. de Luynes reports, that
when a square of St. Gobain plate glass that had been submitted to the
process of tempering was examined by polarized light, it showed the
appearance of a black cross, the arms of which were parallel to the
sides of the square. The glass was sawed in two, along the line of the
stem of the cross, without causing fracture. On examining the divided
glass with polarized light, black bands and fringes of colour were
observed, which, by their position, proved that the molecular
condition of the glass had changed; on placing one half of the divided
glass on the other half, the fringes and black bands disappeared--on
folding one half on to the other, the black bands presented the
appearance that would have been produced by glass of double the
thickness. These facts show, that the molecular forces on the glass
were arranged symmetrically in reference to the line of parting: and
we may conclude that toughened glass being in a state of tension,
similar to that of the Rupert drop, may be divided or pierced,
provided that the molecules of the pieces produced are able to
rearrange themselves into a stable equilibrium. Polarized light shows
the directions on which the division can be made with safety.
M. de Luynes, in his communication referred to above, gives an account
of some experiments performed on plates of glass of the same quality,
tempered by this process, and untempered; one or two examples will
suffice. A tempered plate measuring about[1] 6-1/2 inches by 5 inches,
and 2/10 inch thick, was placed between two wooden frames, and a
weight of over 3-1/2 ounces (100 grammes[2]) was allowed to drop upon
it from a height of more than 13 feet (4 mA"tres[3]) without breaking
it. It only broke, when double the weight was employed from the same
height. A piece of ordinary glass under the same conditions broke,
with the weight of 3-1/2 oz. dropped upon it from a height 16 inches
(0A.40 mA"tre). Plates of toughened glass were allowed to fall on the
floor from a height, or were thrown to a distance, without breaking. A
rectangular piece of ordinary window glass, about 1/10 inch in
thickness, was bent into the form of a bridge, and then subjected to
the tempering process; placed upon the ground; it bore the weight of a
man easily without breaking. A commission, instituted by the French
naval authorities, to inquire into this process of M. de la Bastie,
has reported at some length on the subject. The following series of
experiments were tried with a view of ascertaining the comparative
power of resistance of tempered and ordinary glass. The plates
experimented upon were placed loosely in wooden frames constructed for
the purpose.
[1] These numbers are approximate translations of the numbers
given in the communication: no object could be gained in
giving complex fractions.
[2] 1 ounce avoirdupois weighs 28A.349 grammes.
[3] 1 mA"tre equals 39A.37 English inches.
_Rectangular plates about 21 inches_ (0A.525 m.) _by 10 inches_ (0A.248
m.) _and 1/6 inch_ (0A.004 m.) _thick_.
The frame with the glass inserted was laid on the ground, and in the
middle of the plate a weight of more than 10 lbs. (5 kilogrammes[4])
was placed, and upon it as a base, other weights were placed, care
being taken to avoid all shock.
[4] 1 kilogramme = 2A.2 lbs. avoirdupois.
1Aš _Ordinary glass_, broke with a weight of about 70 lb. (35 kilos.)
having resisted weights of from 30 to 50 lb.
2Aš _Toughened glass_ resisted fracture until a weight of more than
510 lb. (255 kilos.) had been added, and then was not broken. The
experiment was not carried to its limit for want of weights.
_Rectangular plates, about 13 inches_ (0A.325 m.) _by 10 inches_ (0A.248
m.) _and 1/5 inch_ (0A.005 m.) _thick_.
These plates were allowed to fall flat on to a floor of wood or thrown
to a distance and allowed to fall.
1Aš _Ordinary glass_ allowed to fall flat from a height of 1-2/10 inch
(0A.03 m.) was broken at the first trial.
2Aš _Toughened glass._ Thrown to a height 6 feet 6 inches (2 mA"tres)
and to a distance of 13 feet (4 mA"tres) was also broken at the first
trial. The piece, however, which had sustained the weight of 510 lb.
did not break till the fourth trial.
_Rectangular plates, about 10 inches_ (0A.245 m.) _by 6 inches_ (0A.157
m.) _and 1/4 inch_ (0A.007 m.) _thick_.
These plates were subjected to the same kind of tests as the
foregoing. After raising them to a given height they were allowed to
fall flat upon a wooden floor.
1Aš _Ordinary glass_ raised to a height of 20 inches (0A.50 m.) was
broken on falling.
2Aš _Toughened glass_ resisted successive falls of from 20 inches
(0A.50 m.), 32 inches (0A.80 m.), 5 feet (1A.50 m.), and 5 feet 7 inches
(1A.70 m.), but was broken when dropped from a height of 6 feet 6
inches (2A.0 m.).
_Rectangular plates about 10 inches_ (0A.245 m.) _by 6 inches_ (0A.157
m.) _and 1/5 inch_ (0A.006 m.) _thick_.
Placed in the frames, they were held in position in the rabbets by
laths nailed to the sides so as to prevent any play. The frames were
raised to different heights and allowed to fall in such a manner as to
cause as much vibration as possible.
1Aš _Ordinary glass_ was broken with a fall of about 2 feet (0A.60 m.).
2Aš _Toughened glass_ resisted falls from heights of 3 feet 3 inches (1
mA"tre), 6 feet 6 inches (2 mA"tres), 8 feet (2A.50 m.), 9 feet 9 inches
(3 mA"tres), and 14 feet 6 inches (4A.50 m.). It was only broken by a
fall of 19 feet 6 inches (6 mA"tres).
_Rectangular plates 6 inches_ (0A.158 m.) _by 4-3/4 inches_ (0A.120 m.)
_and 1/5 inch_ (0A.006 m.) _thick_.
These plates were placed in the frame on the ground, as has been
previously explained. Known weights falling from known heights were
made to strike the plates exactly in the centre. The weights consisted
of bronze spheres, one weighing 3-1/2 oz. (100 grammes) and another of
twice that weight.
1st. _Ordinary glass_ resisted the weight of 3-1/2 oz., falling from
heights of 8 inches (0A.20 m.), 12 inches (0A.30 m.), 16 inches (0A.40
m.), but was broken by a fall of 20 inches (0A.50 m.).
2nd. _Toughened glass_ resisted the blow of the 3-1/2 oz. weight
falling from heights of 20 inches (0A.50 m.), 40 inches (1 mA"tre), 60
inches (1A.50 m.), and 6 feet 6 inches (2 mA"tres). The 7 oz. weight
(200 grammes) being substituted, the plate was broken by it, falling
from a height of 60 inches (1A.50 m.).
_Rectangular plates, 6 inches_ (0A.158 m.) _by 4-3/4 inches_ (0A.120 m.)
_and 1/6 inch_ (0A.004 m.) _thick_.
The same conditions were maintained as in the previous trial.
1st. _Ordinary glass._ The 3-1/2 oz. weight was allowed to fall from
heights of 1 foot (0A.30), and 16 inches (0A.40 m). It was broken by the
second blow.
2nd. _Toughened glass._ This resisted the 7 oz. weight falling from
heights of 2 feet 4 inches (0A.70 m.), and 2 feet 8 inches (0A.80 m.),
but broke when the weight fell from 39 inches (1 mA"tre).
It appears then from these experiments, that toughened glass will
resist a blow five times as great as ordinary glass, and will bear
seven times as great a weight.
I have now detailed most of the useful experiments which have been
made by competent observers upon toughened glass, as well as some
which have been conducted in my own laboratory. The result of my own
personal investigations I will now lay before the reader. I was
consulted some time ago by a gentleman interested in the introduction
of toughened glass into this country, as to whether this kind would
become untoughened in time. I feel no hesitation in stating that when
the process has been perfectly done, the glass will remain in the same
state for any length of time, provided it be not treated in any way
which is calculated to rupture the external hard bond that holds
together the inner particles of the glass. I feel quite sure, that no
fear of this kind need interfere with the benefits, whatever they may
be, which are to be derived from submitting glass articles to the
toughening process.
A tumbler which had been toughened in Monsieur de la Bastie's works,
was, in my presence, thrown upon the ground, yet it did not break. It
was a large soda water glass. I kept it for some time, and after
considering the matter carefully, I felt, that if it were thrown down
in such a way that the whole of its side, from base to rim, came in
contact with the ground at once, and it then stood this test, it would
prove that the whole of the glass was in the condition of the Rupert's
Drops, and would therefore bear the concussion without fracture. I
held the glass and let it fall, so that it actually reached the hard
floor on its side. It immediately broke all to pieces. Now on the
first occasion when this glass was thrown down, it was tossed somewhat
upwards into the air, and the bottom being heavier reached the ground
first, and it did not break. I have also seen in glass-houses, where
the tempering process is carried on, tumblers thrown down in a similar
manner, and I noticed, that whenever they fell upon their bottoms,
they were uninjured, as also in cases where they fell upon their rims
in such a manner, that the curve of the rim acted as an arch, as in
the old trick of turning a wine-glass off the table so as not to
break; but in other cases where the tumblers fell flat upon their
sides, fracture followed. I carefully gathered together the pieces of
the large tumbler which I broke myself in this manner, and examined
them, and found that the solid bottom was broken in the same manner as
the Prince Rupert's drops break, viz., into a large number of small
pieces, having in all respects similar properties. The glass for an
inch or two above the bottom broke into small pieces, but larger than
those into which the bottom itself broke, and the upper portion of the
tumbler was fractured just as an ordinary tumbler would be. On careful
examination, microscopic and otherwise, the small pieces were found to
have the character of Prince Rupert's, whereas the larger from the
upper part of the glass had none of these characteristics in the
slightest degree.
These observations led me to perform an experiment. A toughened
tumbler was filled with plaster of Paris, which was allowed to set.
Its outside was then encased in plaster of Paris, and when the whole
was hardened, a pair of pincers were applied to a portion of the
tumbler's rim, and with a violent wrench the tumbler was broken. A
rather smart shock was communicated to the arm of the operator, very
much resembling, as he said, the shock of an electrifying machine. On
removing the plaster of Paris, it was found that the whole of the
tumbler was fractured, and, as will be seen by the accompanying
illustration, in a manner similar to that which has already been
described.
From this and other similar experiments, I was led to the conclusion
that none of the toughened articles which have cavities in them, have
thoroughly undergone the toughening process.
Having been requested to attend a series of experiments performed by a
glass manufacturer in London, which consisted in the manufacture of a
number of toughened glass tumblers, I noticed certain facts which led
me to form conclusions as to how it was that the tumblers, the
fracture of which I already explained, break in this peculiar manner.
I will first describe the way in which these tumblers were made and
toughened. By the side of the glass blower there stood a metal vessel,
about three feet six inches high, and, perhaps, from two to two feet
six inches in diameter. This was filled with melted fat or oil of some
kind at a temperature of about 80A deg. Fahr. Inside this vessel, which was
open at the top, there was a wire cage, with a trap door at the bottom
about one foot in diameter, and of about the same depth. The glass
blower, after finishing his tumbler on the pontil, held the pontil in
a horizontal position over this metal vessel, struck it a smart tap,
and the glass tumbled off into the wire cage. The glass was at a very
high temperature. In almost every instance the glass fell into the
melted fat, as a glass thrown in a similar manner will fall into
water. It sank gradually bottom downwards, and the liquid guggled into
it as it sank. Here, then, it is clear that every portion of the hot
tumbler did not come in contact with the oil at the same moment, in
fact there was an appreciable lapse of time before the tumbler
disappeared beneath the surface of the liquid. Now there must be a
limit as to the temperature of the article to be tempered and of the
liquid by which it is to be tempered, that is to say, if at a certain
temperature glass can be tempered by being plunged into the liquid of
a certain temperature, if these temperatures are varied similar
results will not follow. The upper portions of the glass coming in
contact with the tempering liquid at a lower temperature, as they must
have done, were not properly tempered, and this I have clearly proved
by the facts I have already stated. From these remarks it seems
tolerably clear that, until some method is devised of bringing all the
parts of the heated glass in contact with the cooling liquid
simultaneously, the tempering of the article cannot be perfect
throughout its whole surface. As I desire, and very sincerely, that
these processes should be brought to perfection so as to render them
useful, I willingly give this result of somewhat lengthened
investigations to those whom it may commercially concern, and I hope
that they will find, on investigating the matter, that my observations
have been tolerably correct, and that they will be able to devise a
method which will remedy in many cases manifest imperfections of their
present system. All the accidents which have happened to tempered
glass, which have been recorded in the newspapers, can be accounted
for on the principle which I have just endeavoured to explain, for
there must be instability, where the bonding material of the internal
particles of the glass is in different states of hardness; so that
there is no difficulty in conceiving how a gas globe could break
apparently spontaneously, for a portion of it which was not fairly
toughened might be exposed to a somewhat sudden rise of temperature,
produced, it may be, from a draught blowing the flame upon that
particular spot. Articles such as saucers, made of glass, which, being
flat, or nearly so, can be plunged into the tempering liquid with
great rapidity, are usually tempered all over, and these, when
toughened, can be thrown about and allowed to fall on hard floors with
impunity, thus proving the facts which I have endeavoured to
establish. I hope to be able to continue my investigations, and should
they be worth anything, will give the results of them to the public.
Before quitting this subject, I shall make a few remarks upon the
process for toughening glass, which is said to have been purchased by
the Prussian Government.
This process is described as consisting in the application of
superheated steam to the glass, brought up to a temperature near to
its melting point. Having facilities for making experiments of this
kind, I have had them tried with great care, but in no case have I met
with a satisfactory result. This probably is owing to the fact, that I
did not comply strictly with the condition of the experiments
performed by the German chemist who is said to have made the
invention, nor do I see from analogy how this process is likely to
effect a change in the glass similar to that arising from M. de la
Bastie's dipping process.
If glass, instead of being taken from the annealing kiln at the proper
time, be left exposed in the hot part of it, at a temperature just
below that at which it softens, it will be found to become gradually
opaque on its surface. Some experiments were performed many years ago
by RA(C)aumur, who exposed pieces of glass, packed in plaster of Paris,
to a red heat, which became gradually opaque, and lost altogether the
character of glass, the texture of their material becoming
crystalline, and also effected by sudden changes of temperature. Glass
treated in this way was called RA(C)aumur's porcelain. All glasses do not
undergo this change with equal rapidity, and some do not experience it
at all; but the commoner kinds, such as bottle glass, are the best to
experiment upon, for the more alumina that it contains--and it is
known that bottle glass contains a considerable quantity--the more
readily does it undergo this change, which is called _devitrification_.
In what it consists, is not at present well understood, but it offers
a field for investigation, which may produce results of very
considerable benefit to manufacturers of glass.
_Soluble Silicates._--An article on glass in a modern scientific work
like the present would not be complete without a notice of the
manufacture of soluble glass and the uses to which it has been and may
be applied. It has already been mentioned that when silica or sand is
fused with an excess of alkali, the resulting glass is soluble in
water.
Soluble glass is made on a large scale in three different ways. First
of all, if flints, that is, black flints, which are found in chalk, be
heated to a white heat, they lose their black colour and their
hardness, and are easily crushed to small pieces; and if flint in this
condition be placed in a wire cage and put into a jacketed iron
digester, that is, an iron digester which has an inner and an outer
skin, with a free space between the two, so that steam may be forced
into it from a boiler under pressure; and if the digester be screwed
down tightly with an iron cover, and steam then be allowed to pass
into the space between the two, the temperature can be raised at
pleasure, according to the pressure under which the steam is
introduced. If the valve of the boiler be loaded with a 60-lb. weight,
the temperature of the water warmed by the steam will rise
considerably higher than that of ordinary boiling water; and if this
water be saturated with caustic soda, it will dissolve the flints
slowly, forming silicate of soda, that is to say, the silicic acid of
the flint will unite directly with the soda of the solution, and
silicate of soda will thus be obtained. For certain applications, the
silicate so formed is not sufficiently pure, because the soda used
often contains a certain amount of sulphate, which will remain with it
in the solution of silicate that is drawn off from the digester. This
sulphate is very objectionable for certain applications of silicates,
because it crystallizes out, and so destroys the substance, which the
silicate is intended to preserve.
Another and a much better method is to heat together the silica in the
form of sand with alkali, either potash or soda, in a reverberatory
furnace, and as the glass becomes formed, to rake it out into water,
and then gradually to dissolve it by boiling in suitable vessels. Here
the sulphate, if it existed in the alkali, is decomposed by the
silicic acid, and the sulphuric acid passes off through the flues of
the reverberatory furnace.
There is also a very ingenious way of making silicate of soda,
discovered by Mr. Gossage, and performed as follows: common salt is
heated to a high temperature and volatilized, and in this condition is
brought into contact with steam also at a high temperature, when a
double decomposition takes place. Steam is composed of oxygen and
hydrogen; common salt, of sodium and chlorine. The chlorine of the
common salt unites with the hydrogen of the steam, and the oxygen of
the steam with the sodium, so that hydrochloric acid and oxide of
sodium are formed. Now, if these two substances at this high
temperature were allowed to cool together, the action would be
reversed, and the re-formation of steam and chloride of sodium would
be the result; but in the strong chamber lined with fire-clay, in
which these vapours are brought into contact, silica is placed in the
form of sand made up into masses, and when the oxide of sodium is
formed, it unites with the sand to make silicate of soda, and thus is
removed from the action of the hydrochloric acid, not entirely, but
sufficiently to produce a large yield of silicate of soda.
The properties of silicate of soda, as applied to the arts, are
somewhat different from those of silicate of potash, so that one
cannot always be substituted for the other. Both these substances are,
when in solution and concentrated, thick and viscid, and have the
property of causing paper, wood, &c., to adhere when applied as a gum
or glue, and hence have been called "mineral glue." In a dilute state
they can be used for coating stone, brick, or cement, and have the
power of rendering them for a time waterproof, or nearly so, and of
preventing the action of atmospheric influences, which too often
produce the decay of some of the softer stones used for building as
well as for cement. It has already been stated, that when carbonic
acid is passed through a solution of silicate of soda, silica will be
precipitated. Now, inasmuch as there is carbonic acid in atmospheric
air, when these solutions are applied to the surfaces of a building,
they will be acted upon slowly by the acid, and silica will be
precipitated in the pores of the material to which the silicates are
applied. But this operation is extremely slow, and, before it can be
thoroughly completed, the silicates, being soluble, will get in part
dissolved out by rain and moisture, and it is therefore advisable to
use with them some material which will, by a double decomposition,
form a silicate insoluble in water. The silicate, however, which is
formed, should have cohesion amongst its particles, so that it will
not only adhere to the stone itself, but its own particles will adhere
to one another when it gets dry. Various methods have been tried to
cause this insoluble substance to be formed upon the surface of
stones, so as to fill up its pores and to make a protecting cover for
it; but most of them have signally failed, because the new silicate
produced by double decomposition has not had the necessary coherence
amongst its particles. If a solution of chloride of calcium be added
to one of silicate of soda, a silicate of calcium will be
precipitated, and it was therefore thought, that by applying to a
stone successive washes of silicate of soda and chloride of calcium,
an insoluble silicate of calcium would be produced in the pores and on
its surface. It is true that such a silicate is precipitated, and
that, if the silicate employed be in excess of the chloride of
calcium, the particles will be glued together by the adhesive powers
of this silicate when it dries; but then the action of moisture upon
it is to cause it to run down the surface of the building, and set
free the particles of silicate of calcium which it held in
combination. Other processes of the same kind have been tried, and
with similar results; one great difficulty in the way of the success
of this method of applying silicates being that, from the peculiar
colloidal or gluey nature of the silicate, it does not penetrate to
any considerable depth into the stone, and, if laid on first, prevents
the penetration, as far even as it has itself gone, of the solution of
chloride of calcium. If the chloride of calcium be used before the
silicate, it will penetrate farther than the solution of silicate is
able to reach, so that it is impossible to obtain, even supposing the
substance to be used in equivalent proportions, a complete
decomposition of the one by the other.
The great object to be attained in the preservation of stone by
any silicious process, is to use _one_ solution possessing the
substances which, when the water has evaporated, will form a
perfectly coherent mass for the protection of the stone surface. The
depth of penetration, if it is sufficient to protect the outside of
the stone from the disintegrating action of the atmosphere, need not
be carried much more than one-sixteenth of an inch below the surface,
for when old stones which have long been in positions in buildings,
and which have not decayed at all, are examined, it will be found
that they are covered with an extremely thin film of a hard
substance, not thicker than a sheet of writing paper, which has for
ages protected and preserved them from decay. This film is produced
by a determination from the inside to the outside of the stone of a
silicious water, which existed in it in the quarry, and which, when
the stone was placed in the building, gradually came to the surface,
the water evaporating and leaving behind it a thin film of silica, or
of a nitrate--most likely the latter.
If alumina be fused with potash, aluminate of potash, soluble in
water, is made; if, however the solution is too concentrated, a
certain quantity of the alumina will be precipitated; but if it be
dilute, the whole of the alumina will remain in solution. When
aluminate of potash of specific gravity 1A.12 is mixed with a solution
of silicate of potash of specific gravity 1A.2, no precipitate or
gelatinization will take place for some hours; the more dilute the
solution, the longer will it remain without gelatinization, and of
course the thinner it will be, and the greater power of penetration it
will have when applied to a porous surface. When solutions of
aluminate of potash and of silicate of potash of greater density are
mixed together, a jelly-like substance is almost immediately formed,
and sometimes even the whole mass gelatinizes. If this jelly be
allowed to dry slowly, it will contract, and at last a substance will
be left behind sufficiently hard to mark glass, though the time for
this hardening may be from one to two years; and on examination it is
found that this substance has very nearly the same chemical
composition as felspar, and is perfectly insoluble in ordinary mineral
acids. Now, suppose a dilute solution of this mixture to be applied to
the surface of stone, the silicate and aluminate of potash will
gradually harden and fill up the interstices of the stone; and as both
the substances entering into combination are contained in the same
solution, they will both penetrate to the same depth. Inasmuch as the
artificial felspar is not acted upon by destructive agents which would
disintegrate the stone, it becomes a bonding material for its loosened
particles, and at the same time gives a case-hardening to the stone,
which no doubt will as effectually protect it against atmospheric
influences as in the case of the hardening of the natural one. We have
a tolerable guarantee that this will be so, if we consider the number
of enduring minerals into the composition of which silica, alumina,
and potash enter, and also of the almost imperishable character of
granite, which is so largely composed of felspar. Many experiments
have been performed on an exhaustive scale with these materials, and
in every case it has been found that they have answered the
expectation of those who have thus tested them. It is, however,
necessary to state, that in making these experiments, great care must
be used to employ the mixed substance in solution before
gelatinization has set in, for if this has occurred, even to the
slightest extent, a surface coating is formed on the stone, which, not
having formed a bond with it, easily rubs off.
Another application of soluble silicates in this or other forms is to
render walls of buildings which are porous, waterproof. A colourless,
transparent material which can effect this object is doubtless
desirable, as anything like an opaque wash, if applied to brick-work,
would destroy the colour of the bricks, and therefore the character of
the building constructed with them. The silico-aluminate of potash may
be used for this purpose, as above directed; and even silicate of
potash alone, provided it be in sufficient quantities, will answer
well, if from year to year, for two or three years, the application be
renewed, so as to fill in spaces, wherever the silicate may have been
in part dissolved out. When the silicate of potash alone is used, the
action of the carbonic acid of the air in precipitating the silica is
depended on, and while this action is going on, portions of the
silicate not acted on will be dissolved out.
Many years ago, an effort was made in Germany to revive the ancient
art of fresco painting, and with very considerable success. It was
found, however, that our climate is not suited to the permanence of
this method of decoration, nor indeed is any climate absolutely
suitable, because in fresco painting, the surface only of the lime is
coloured with pigments laid on, so that any influence which would
destroy the lime surface would cause the removal of the pigments; and
from the porous nature of the surface of the work after it is
completed, absorption of moisture will from time to time take place,
causing the adhesion of dirt and other foreign substances which may
fall upon it, and which it is almost impossible to remove without
detriment to the picture. Dr. Fuchs, of Munich, discovered a method of
painting with soluble silicates, which has been tried with
considerable success in Berlin by the late Professor Kaulbach. On a
properly prepared ground, the painting was executed in colours mixed
with water, which, when dry and the painting finished, were fixed to
the wall by the application of soluble silicates. For the preservation
of the work, Dr. Fuchs mainly relied upon the action of atmospheric
carbonic acid. Now, when carbonic acid acts upon silicate of soda or
silicate of potash, we have already seen that the silicic acid is
precipitated in the hydrated form, and that the carbonic acid has
united with the soda or potash to form carbonate of soda or carbonate
of potash. These substances being left in the painting and penetrating
to a certain depth beneath its surface, must find their way out, and
in almost every instance have done so in the form of an efflorescent
substance, which has caused the picture to have the appearance of
being mildewed over its surface. Sometimes, however, sulphates occur
in the ground, and then sulphates of soda and of potash have been
formed, injurious to the permanence of the surface of the picture,
because they crystallize and force off portions of the lime and sand
of which the surface is composed. The effect of the efflorescence of
the carbonates on the surface of a silicious painting may be seen in
the famous picture of the meeting of Wellington and Blucher, in the
House of Lords, painted by the late Mr. Maclise, R.A. When, however,
the solution of aluminate and silicate of potash is used with the
pigments on a properly prepared ground, there is no fear of this
efflorescence taking place, and paintings executed with it have stood
for many years, without giving any signs whatever of decay.
To those interested in this subject, it is desirable that they should
perform a series of experiments themselves, and ascertain the best
methods of practically applying this vehicle in the execution of large
mural paintings. They will find that, although at first they may meet
with some difficulties, yet after a while these difficulties will
vanish, and they will have a material to work with, which will meet
all their requirements.
In an article so brief as the present, it is impossible to enter fully
into all the details of the manipulation of this particular process of
painting; it is, however, most desirable to give a short account of
the method of preparing the ground and of applying the colours,
leaving the rest to be learned from practical experience.
Angular fresh-water river sand, well washed, should be mixed with
sufficient lime to cause it to adhere to the wall on which it is
placed, and this in all cases should be freshly plastered in the
ordinary way. No plaster of Paris (which is sulphate of lime) should
be used in the preparation of the groundwork. The coating of fine sand
and lime is laid on to a depth of about an eighth of an inch, and when
dry, an application of dilute silicate of potash should be made, in
order to bond together the particles of sand which, owing to the
employment of so small a quantity of lime, can be readily brushed off.
As soon as these particles are well fixed together and do not come off
when the hand is passed over the surface of the wall, the ground is in
a fit state for the commencement of the painting. The colour should be
used with zinc white, and not with lead white, and, of course, they
must be in the state of fine powder, and not ground up with oil or any
such material. The artist can use his mixture of silicate of alumina
and aluminate of potash of the strength already described; he may,
when desirable, dilute it to a certain extent with water, but he
should not do so too much. He can then paint with it just as he would
with water in water-colour painting; and if he finds that any portion
of his colours, after they are dry, are not sufficiently fixed upon
the wall, he can then with a brush pass over them a coating of the
clear liquid, used a little stronger. When the whole work is finished,
it will perhaps be desirable to give it one or two coats of a very
dilute solution of silicate of alumina and aluminate of potash. After
a time, owing to the contraction in drying of this material, it would
be advisable--say, after the lapse of two or three months--to again
apply a coat of it somewhat stronger; and again, if after a year, or
more than a year, it should appear that any portions of the surface
were becoming loose, another application of the mixed silicate of
alumina and aluminate of potash to these loosened parts alone will be
desirable. This repetition may appear to some to be an objection to
the process, but it is not so, however; for in the formation of those
natural substances, such as flints, which we find so hard, no doubt a
very great lapse of time occurred in the induration of the gelatinous
silica which formed them. Neither do we object from time to time, at
intervals of years to renew the coats of varnish on oil paintings, in
order to preserve them or to bring out afresh the brilliancy of their
colours.
The soluble silicates are frequently used as bonding materials in the
manufacture of artificial stone and cement, very good results having
been attained. The objection, however, to their employment for these
purposes is the expense of the material of which they form a
constituent part, and it seems almost impossible ever to bring it into
competition with dressed natural stone. But for ornamental purposes,
from the plastic nature of the substance when in the wet state, it can
be pressed into moulds, and wherever plaster mouldings are admissible,
no doubt this material would be useful for certain kinds of
ornamentation. Some years ago, Mr. Ransome, of Ipswich, after having
made his artificial stone with sand and silicate of soda, heated it in
ovens, so as to produce a hard and semi-vitrified mass. A church, the
mouldings of which are made of this stone, may be seen at the bottom
of Pentonville Hill, London; and certainly as to durability, there is
no doubt that the substance has answered very well. But from
difficulties in manipulation and other reasons, that gentleman gave up
this method of making artificial stone, and is now working another
process which yields far better results. Silicate of soda is mixed
with sand (generally Aylesford sand), and after the mixture is moulded
and dried, it is exposed to the action _in vacuo_ of chloride of
calcium in solution. Whether the whole mass is placed in a vacuum
chamber and then charged with chloride of calcium; or whether a vacuum
is formed on the under side of the substance, and the chloride of
calcium solution caused by suction to filter through it, is uncertain.
However, whatever be the manipulative processes, the result is the
same, and appears to be extremely satisfactory.
Soluble silicates produce very remarkable results when mixed with
certain substances. If silicate of soda or potash be mixed with white
lead, in a very short time it sets into a hard substance, just as does
plaster of Paris when mixed with water. If powdered pumice-stone or
sand, in the proportion of eight parts to one of carbonate of lead, be
mixed together with soluble silicate, a very hard and coherent mass is
obtained, and there seems no reason why a mixture of this kind, in
which pumice-stone is used, should not be employed for the purpose to
which pumice-stone is usually applied. It would have the advantage of
being easily moulded into forms, so as to suit mouldings, which might
by it be much more accurately and expeditiously smoothed down (as in
the case especially of picture-frame mouldings), than they can be by
the ordinary pumice-stone.
Another very important application of soluble silicates is the
rendering of wood incombustible. Many experiments have been performed
which show that when wood is thoroughly impregnated to a depth of a
quarter of an inch or more with silicate of soda, it will not flame,
but will only char. Now, supposing that the constructive timbers of a
house were worked, and then placed in suitable vessels and saturated
with silicate of soda, they would then be rendered practically
fireproof, or at least it would take a very prolonged exposure to heat
to cause them to smoulder away, while at no period of this time would
they burst into flame. From the peculiarly gluey nature of these
soluble silicates, they do not penetrate readily into porous
substances; it has therefore been suggested that the impregnation of
the wood should take place in vacuum chambers, just in the manner that
the creosoting process for preserving railway sleepers is at present
performed. It is most certainly advisable that the wood should be
worked before being exposed to the silicating process, for that would
render it so hard, that it would considerably increase the cost of
labour in cutting and planing it.
At the commencement of this article, it was stated that silicic acid,
or silica, could be made soluble in water. Some very interesting
experiments were performed by the late Dr. Graham, Master of the Mint,
which gave rise to the discovery of the process of dialysis. If some
silicate of soda be mixed with water, so that not more than 5 per
cent. of silica be in the solution (rather less is better), and if
some hydrochloric acid be then added in sufficient quantity to make
the liquid distinctly acid, and the mixture be placed in a dialyzing
apparatus, the chloride of sodium formed by the union of the chlorine
of the hydrochloric acid with the sodium of the silicate of soda will
pass out through this dialyzing membrane, leaving hydrated silica
behind, which will remain in solution in the water with which the
silicate was mixed. The dialyzing apparatus is constructed in the
following manner; a sort of tambourine ring is made with gutta percha,
in place of wood, from 8 to 10 inches or even more in diameter, the
depth, being about 2 inches. Another ring of gutta percha, of about an
inch deep or even less, is made so as to fit tightly outside the
tambourine; a piece of vegetable parchment is then moistened and
placed over the tambourine, and the thinner ring is pressed over it,
so as to secure it tightly. This is the dialyzing vessel, and it is
into this that the mixture of silicate and hydrochloric acid must be
put. The solution should not be more than an inch deep in the
dialyzing vessel, which is then made to float upon distilled water in
a larger vessel of suitable size. The distilled water should be
changed every day, until no precipitate can be obtained in it with
nitrate of silver, and when this point is arrived at, all the chloride
of sodium will have passed through the vegetable parchment into the
larger vessel of water, and nothing but silicic hydrate will remain
behind in solution. If this liquid be allowed to stand for some time,
it will gelatinize, and later on the jelly will contract, becoming
extremely hard, so that lumps of it, when broken, will in their
fracture resemble that of flint. No doubt, at some future period, some
one will discover a method of rendering this condition of silica
useful in the arts.
Soluble silicates are very useful as detergents. A small quantity of
silicate of soda mixed with hard water renders it valuable for washing
purposes. Silicate of soda is also used in the manufacture of the
cheaper kinds of soap. We can hardly speak of it as an adulteration,
because it renders the soap with which it is combined much more
powerful in its cleansing action. I suggest to those interested in the
application of science to the arts, that this subject will no doubt
well repay experimental investigations.
It is much to be wished that those engaged in this branch of art and
manufacture, and who have some knowledge of chemistry, would turn
their attention to getting a better and more perfect method of making
coloured pot-metal glass. I have been engaged for some time, and still
am engaged, in experiments to effect this object. But inasmuch as my
engagements are very numerous, and I cannot give the proper time to it
I desire, I therefore take the liberty of suggesting to others the
ways in which I am working, that they may be able to arrive at good
results more speedily probably than I shall be able to do. If sulphate
of copper be mixed with silicate of potash, silicate of copper will be
precipitated. Now, if this be carefully washed and dried, it will be a
silicate of a definite composition, and I propose to use such
silicates as these with ordinary glass mixtures, in order to impart
the particular colour which the oxide employed has been already
described as giving to the glass. Silicate of manganese is prepared in
a similar way to the silicate of copper; silicate of cobalt, and other
silicates, can be used as staining materials for colouring glass.
These mixed in due proportion would give tints, and would, I do not
feel the slightest doubt, produce colours with much greater certainty
than they are now produced, and tints hitherto unknown could be made
to the great benefit of the glass-painter.