Sand 100 parts.
Dry sulphate of soda 50 "
Quicklime 20 "
Carbon, as charcoal 4 "
Different makers have different mixtures. This by M. Gehlen is given
as _about_ the proportions of the several constituents employed.
The charging of the pots is conducted in this manner: they are filled
with lumps of fritt, and the heat of the furnace is raised as rapidly
as possible, until, in about eight or nine hours the fritt has run
down or melted into glass. More fritt is then added, which also melts,
and from time to time this is repeated, till the pot contains a
sufficient quantity. After about sixteen hours the whole has become
converted into glass, and the surface of the molten mass is covered
with liquid salt and sulphate of soda. This scum is called glass-gall
or sandiver, and is carefully removed with iron ladles. Some broken
glass, or cullet, is now thrown into the glass-pot, a little at a
time, the object being to cause any salt which may remain in the pot
to rise to the surface, which is then removed, and so the glass is in
this manner purified, after it has been further heated for some hours,
to expel gases.
When the glass is made, and its temperature so reduced that it is in a
doughy or pasty state, it is then worked off by the blowers into
either sheets or tables, as is desired. The blowing of sheet and crown
glass is a work of considerable difficulty and labour, and one which
cannot be successfully performed, except by a workman who has been
brought up from boyhood in a glass-house. A quantity of the soft glass
is collected or gathered on the end of a blowpipe, and the workman
then blows into it, and distends it into a globular form. Now it is
necessary, in making sheet glass, that that globular form should be
elongated; the workman therefore holds his blowpipe, which is about
five feet long, in a vertical direction, and the softened globe
becomes pear-shaped. By dexterously swinging the blowpipe from side to
side, which he does while standing on a plank placed over a sort of
pit, and by causing it to rise on either side, he converts the
pear-shape into a true cylinder, having rounded ends. When the
cylinder has assumed the exact shape desired, he places his thumb on
the end of the blowpipe, and holds the opposite end of the cylinder in
the mouth of the furnace. The glass softens at the heated end, and the
expanding air causes it to burst the opening. It is then shaped with a
suitable tool, so that it is of the diameter of the cylinder. When the
latter is cooled, a piece of hot glass is applied to its shoulder with
a pontee, and is drawn out into a thread around it. This makes the
glass hot. The thread of glass is removed, a cold instrument is
applied rapidly, and the shoulder of the blowing is cut off. The glass
is next detached from the blowpipe, and its ends removed, and it is
then annealed for a short time, and cut down lengthways internally by
a diamond. It is afterwards placed, with the long cut uppermost, in
what is called a flattening kiln, that is, in a sort of oven or
furnace heated to a high temperature and having a perfectly smooth
stone floor; after a short exposure the glass softens, and a workman,
with suitable wooden tools, opens it out where it was cut by the
diamond, and causes it to lie flat upon the stone. It is then rubbed
by a wooden tool, and in this way is flattened, removed from the
flattening stone kiln, and placed in a hot chamber, in which it is
allowed to cool slowly, for the purpose of "annealing."
Sheet glass, formerly called broad glass, was originally made on the
Continent; but its manufacture, first established in this country by
the introduction of foreign workmen, has extended to very large
dimensions, and the quality of English sheet is now quite equal, if
not superior, to anything that is produced abroad. The advantage which
it possesses over crown glass is, that much larger sheets can be made,
and this is very easily noticed if we examine the larger dimensions of
common window panes compared with those which were formerly made. Even
now the workmen employed in this class of manufacture are generally
Belgians. A sheet glass blower must be very strong, and have great
skill in handling his blowpipe, for the cylinders which he blows are
frequently sixty inches long, and their weight is very considerable.
Glass shades are blown by sheet blowers. These sometimes are very
large, and require great skill. When their shape is to be that of a
cylinder with a dome top, they are made as in the ordinary course of
blowing a cylinder of sheet glass, but instead of one end being burst
as described, they are simply detached from the blowpipe. When they
have to be oval or square at their bases, they are blown into wooden
moulds of the required form, which have their insides charred. The
gathered mass of glass is placed inside such a mould, and is then
blown into until it touches the sides. This is an operation requiring
great strength and delicacy; strength to blow with sufficient force to
bring the softened glass to touch the mould in all its parts, and
delicacy to prevent the pressure from being so great as to cause the
outside of the glass shade to receive marks on its surface from the
mould.
The shaping of the molten glass into tables of _crown_ is different in
detail. The globular mass formed by the first blowings is held by a
workman vertically over his head. An assistant gathers a small
quantity of soft glass from the furnace on the end of a pointed iron
rod, and causes it to adhere to the flattened surface, at a point
opposite to that to which the blowpipe is attached. The glass near the
blowpipe, while hot, is touched with a cold instrument, and
immediately cracks around its neck, detaching the blowpipe from the
mass. The pointel is taken by the blower, and the opening formed by
the removal of the blowpipe is placed opposite to what is called a
"flashing" furnace, that is, a furnace with a large circular opening
in its front, and which is heated to such an intense degree, that it
is impossible for a person unaccustomed to it to approach within
several feet of the furnace-mouth. The workman generally wears a
shield or screen to protect the upper part of his body and face. The
glass becomes softened by the heat, and the workman gives his pointel
a rotary motion, somewhat similar to that which a housemaid gives to a
mop when she trundles it; and as the glass softens, the opening gets
larger and larger, until at last the softened mass instantaneously
flashes out into a circular sheet, an operation which produces a very
startling effect upon the eyes of anyone beholding it for the first
time. The circular crown table thus made is detached from the pointel,
and the mass of glass which caused it to adhere forms what is known by
the name of the bull's eye. The table thus made is, like the sheet,
placed in an annealing furnace, and there left for a proper length of
time.
The manufacture of _plate_ glass is altogether different from that of
crown and sheet. First of all, much greater care is taken in the
selection of the materials, the sand used being of a purer kind than
that employed in the manufacture of common window glass; the alkali is
of a better quality; and more caution is taken in all the manipulative
processes prior to the melting of the mixture. Arsenious acid is more
frequently used than manganese for the correction of the iron
impurity. It has been noticed that in the plate glass-pots, there are
grooves placed around their sides, and these are intended to receive
metal claspers, by means of which the pot can be removed bodily from
the furnace. In former times the glass was made in large pots, and
then ladled out into smaller ones, of a square form called _cuvettes_,
and in these it was left exposed to the heat of the furnace for a
length of time, in order that it might be refined, by the rising of
impurities to the surface and by the escape of air bubbles. The use of
these cuvettes is now discontinued, and the pot in which the glass is
founded is removed from the furnace and its contents poured upon the
tables on which the plate is formed, by the action of rollers. A plate
glass table is made of iron; its surface is smooth and of the size
required to make a large plate, and it is placed upon wheels and run
upon a tramway from one part of the glass-house to another, so as to
be opposite to the mouth of the furnace from which the glass-pot has
to be removed. Along the sides of this table, taken lengthways,
moveable strips of iron are placed, rising above it to a sufficient
height to secure the desired thickness for the glass plate, and on
these strips runs a roller, so adapted that it can be made to pass
pretty readily from one end of the table to the other. The contents of
the glass-pot, when placed over the table by means of a crane and
tilted up, fall out somewhat as a lump of dough would fall from a
kneading trough if it were inverted, for it must be borne in mind that
the glass in this process is not in a very fluid state. The roller is
made to pass rapidly over the softened glass, and in this way spreads
it over the table, until it comes in contact with the strips placed
along the edge, which serve as gauges for determining the thickness of
the plate. After the plate is formed, it immediately sets, and is
removed while hot into an annealing furnace, which is always so placed
that the glass can be transferred to it from the table with the least
possible delay. In this furnace several plates of fresh-made glass are
deposited, and are allowed to cool extremely slowly, in order that the
glass may be properly annealed. When this process is completed, the
plates are removed, the edges are trimmed off with a diamond, and one
plate, bedded in plaster of Paris, is placed upon a flat stone
receptacle; another plate, also coated on one of its sides with
plaster of Paris, is made to adhere to a piece of machinery placed
directly above the other plate, and is so situated, with respect to
this latter, that the two surfaces are perfectly parallel one to the
other.
It should be here mentioned, that the side of the plate which touches
the table is always rough, and has no polish, while that over which
the roller is passed is slightly undulating, and has a bright polish
similar to that of a sheet of blown glass, and which is technically
known as "fire" polish. The machine to which the upper plate is
attached is so arranged that, when set in motion, it causes it to move
in just the same direction that a plate would do if moved by the human
arm; this is therefore called an elbow motion. Boys stand by the sides
of the two plates, and throw fine sand and water on the lower one, so
that the opposed surfaces mutually grind one another, and when this
process is completed on one side, they are reversed, and the same
operation is performed on the other side. The plates have now the
appearance of ground glass, and the surfaces are further ground by
fine emery powder, which causes them to be much smoother and more
ready for the final polishing. Formerly this was entirely done by
hand, women generally being the operators, and oxide of iron, called
crocus, mixed with water, the material employed for polishing. Now,
however, a more rapid and perfect method is adopted by the use of
machinery. A table is prepared which moves from side to side, giving
to the plate a lateral motion; and above is a beam, in which holes are
drilled at intervals, through which short iron rods, nearly an inch in
diameter, pass. On these are padded iron buffers, covered on their
under surface with leather; while, pressing down these rods, and
therefore the buffers, are springs, which act with considerable force,
but which are able to yield to pressure caused by any inequality over
which the buffers may pass. The glass plate is fixed upon this table,
and its upper surface is exposed to the action of the buffers, while
oxide of iron, in a very fine state of division and mixed with water,
is allowed to come upon its surface. The glass travelling from side to
side is rubbed by the buffers in a lateral direction, and has also a
longitudinal motion, so that every portion of it is rubbed equally. If
any inequalities occur on the glass, the springs which press down the
buffers give way and allow them to rise over it, and this process is
continued for some time, until at last the plate receives the polish
so characteristic of plate glass. It is then removed from the table
and examined by skilled persons, and whatever defects can be removed
by hand, are remedied.
Another kind of plate glass, called "patent rolled plate," is made by
ladling out from a pot molten glass in the proper state of
consistence. The ladle is brought over a small glass table, and a
similar operation is performed to that already described. This patent
rolled plate is sometimes made with grooves on one of its surfaces, or
with patterns in imitation of diamond quarry glazing, and, in fact,
with any designs, according to the taste of the manufacturer. These
designs are all engraved upon the table, and communicate their
patterns to the soft glass; but the smooth surface of such glass which
comes in contact with the roller is slightly undulating, though
polished. This method of glass making was invented and patented by Mr.
Hartley, the noted manufacturer, of Sunderland.
A lighter kind of plate glass, which is principally used for glazing
the better class of pictures and engravings, and called "patent"
plate, is simply sheet glass polished after the manner of plate glass.
Crown glass, which only admits of being cut into small squares, is
also used for picture glazing, but is more carefully prepared, and is
called by the name of "flatted crown."
_Looking Glasses._--Plate glass is employed for making looking
glasses, and two processes are now in use for silvering them, the
first of which consists in applying a sheet of tinfoil saturated with
quicksilver to one side of the glass. The operation is conducted as
follows: on a perfectly smooth table a sheet of stout tinfoil is laid,
and on it is poured quicksilver, which is distributed evenly over the
surface with a hare's foot. When the whole sheet is amalgamated with
the quicksilver, more of that substance is poured over it, until it
flows quite freely. The glass plate to be silvered, having been made
perfectly clean, is floated upon the surface of the quicksilver, an
operation requiring care, and is then covered all over with weights,
by which means the excess of quicksilver is pressed out, and the glass
comes in contact with the amalgamated sheet of tinfoil, to which it
adheres entirely. This ancient method of silvering glass has some
advantages over the one next to be described. The colour of the plate
is, according to artistic taste, better, and with care the plate will
not lose its brilliancy for years. I have in my possession some old
glasses, the silvering of which is very beautiful, except where it has
suffered from mechanical injuries. Silver can be precipitated from a
solution of nitrate of silver in several ways, and in some of these
specimens was like a bright film. If a crystal of nitrate of silver be
put into a test-tube with some bitartrate of lime, and the mixture be
rendered ammoniacal and gently warmed (it being kept in motion during
the experiment), its sides will be covered with a very brilliant
deposit of metallic silver. Oil of cloves and grape sugar have also
the power of reducing metallic silver from ammoniacal solutions of the
nitrate, when gently warmed; but the mixtures must not be made too
hot. In silvering plates of glass, they are first well cleaned, then
placed in a perfectly level position, and the silvering liquid is
poured over the surface, the room in which the operation is performed
being kept sufficiently warm to assist the deposition. When enough
silver has been deposited on the glass, the liquid is poured off and
the plate dried, while the silver film is protected by being coated
with a suitable hard varnish. The composition of the mixtures used by
different persons is generally kept secret, though the chemical
principle of the reduction of the silver salt is the same. Glasses
silvered by this process sometimes lose their brilliancy, by becoming
covered on their silvered side with small spots. It is however stated
that this results either from a bad system of deposition, or from the
film of silver not being sufficiently thick and solid.
_Flint Glass_, although called by this name, is not made from flint,
but from the best sand, of pure and dazzling whiteness, obtained from
Alum Bay, in the Isle of Wight, and from Fontainebleau, in France. The
cost per ton is from 1_l._ to 1_l._ 15_s._, whereas the price of
the sand used for making plate glass is about one-eighth of that
amount. The alkali employed is generally extremely good carbonate of
potash, whereas soda is used in the manufacture of the other kinds of
glass which have been described. The addition of a small quantity of
black oxide of manganese is sometimes necessary to correct the slight
tint imparted by iron, which seems to be always present in minute
quantities, even in the purest samples of sand. Oxide of lead in the
form of red lead, in this sort of glass, takes the place of lime. The
advantages derived from using the oxide are, that it makes the mixture
more fusible, and also imparts that particular brilliancy and lustre
so peculiarly characteristic of well-made flint glass. In different
works, various mixtures are made for the composition of the glass; but
to give an idea of the proportions in which the materials are mixed,
it will be well to quote the statement of M. Payen, who says that of
the finest crystal flint glass, the following is the composition:
sand, 3; red lead 2 to 2-1/4; carbonate of potash, 1-1/2 to 1-2/3. A
little nitre or saltpetre is used as an oxidizing agent. The
glass-pots employed in this branch of the manufacture are covered, so
that the flames of the furnace do not come in contact with the
materials, the object in thus isolating them from direct contact with
the flame being to prevent the entrance of impurities, by which the
colour might be injured. On account of the pots being covered, the
materials take a much longer time to get hot, and require quite double
the time in founding that sheet or plate glass does; the presence of
oxide of lead materially assisting the rapidity of the fusion. When
flint glass is ready for working, the time required to work off a pot
of it is much longer than that which is required for a pot of crown or
sheet; and it is a matter of considerable importance, that the
furnace-man should so manage his fires as to keep the glass in a
proper working condition, that is, he should not let it get too cold
(therefore too solid) nor too fluid. Flint glass is worked off by the
blower into wine-glasses, tumblers, decanters, and other suitable
vessels. Let us take a wine-glass as an illustration of the method of
working. A small quantity of glass is gathered on the blowpipe, which
is much smaller than that used in making sheet, and is blown into a
bulb, which may be slightly elongated or globular, the forms being
given to it by the motion which the workman imparts to his blowpipe
while he is blowing, or after he has blown, into the mass. In the case
of a wine-glass, an assistant boy gathers a small quantity of glass on
the end of a small pointel, or solid iron rod. This is placed on the
side of the globe opposite that which is in connection with the
blowpipe, which is then detached by touching the glass nearest it with
a piece of iron, wetted with cold water: this causes a crack, and a
gentle tap causes separation. The workman then moulds the opening made
by detaching the blowpipe, in order to do which, he has to apply the
glass often to the mouth of the furnace, to soften it. He then opens
out the globe into the shape of a cup with a pair of small iron tongs,
with legs uniform in shape, slightly tapering and smooth, and he uses
a peculiar kind of scissors for trimming the edges. The other parts of
the glass are moulded with the tongs, accuracy of size being obtained
by means of measuring compasses and a scale. The workman sits during
this operation in a seat with arms, laying the pontee on them, and
turning it, so as to make it move backwards and forwards with his left
hand, while with the tongs in his right he gives the glass the desired
form.
Before passing on to a description of the manufacture and composition
of coloured glasses, it is necessary that I should make a few remarks
on the difficulties under which our English glass makers labour, owing
to not paying sufficient attention to the scientific treatment of
their mixtures. It has already been stated that glass is composed of a
mixture of silicates, which are definite chemical compounds. Some are
much more dense than others, and are therefore liable to sink, so that
the glass taken from one part of the pot will be very different in
composition from that taken from another part; besides this, it is
found on examination, that other portions of the materials employed
are present in such proportions, that they cannot possibly exist in
the form of true silicates. M. Dumas, the distinguished French
chemist, asserts, and with truth, that glass ought to be a true
chemical compound. This, however, does not seem to be the opinion
here; and sufficient attention is not paid by English manufacturers to
mixing their materials, so as to form definite silicates, the result
being that glass is produced with a striated effect. This is easy to
be seen in the common kinds, as in bottle glass; but owing to the more
careful and prolonged fusion of the finer varieties, such as plate
glass, this defect is to a considerable extent remedied, though not
altogether overcome. In the French manufacture of plate glass, more
attention has been paid to the chemical composition of the various
silicates which enter into it. At St. Gobain, a plate glass, is
produced which, on analysis, is found to contain definite silicates,
and without any excess of material which does not enter into chemical
combination; and the consequence is, that this glass is more perfect
and homogeneous than that made in this country. No doubt this superior
quality is owing to the fact, that the famous chemist, Gay-Lussac,
devoted much of his time to assisting in the manufacture carried on at
these works. We cannot over-estimate the importance of a scientific
superintendence, not only of glass-works, but of all other
manufactures in which chemical reactions take place; for although
experience may lead a cautious observer to produce substances of
nearly correct composition, yet the assistance of a scientific
observer is of the greatest importance, because, what under other
circumstances must be simply empirical, is under his guidance carried
on according to definite and fixed laws.
Mention has already been made of how, in the case of mixing carbonates
of soda and potash, the one assists the fusibility of the other, and
this is more particularly true in the mixture of silicates in the
composition of the ordinary glass. Silicates of soda and potash are
separately much more infusible than a mixture of the two, and the
addition of other silicates to them renders them more fusible still;
silicate of lead, as has already been mentioned, causing the glass
into whose composition it enters to fuse at a much lower temperature
than it would do if that silicate were absent. Again, if the silicate
of lead be present in too large proportions, and if great care be not
taken in the manufacture of lead glass, the silicate of lead, from its
greater density, will sink lower among the molten silicates, and will
therefore cause a larger proportion of lead to be in the glass at the
bottom of the pot than there is at the top. We often notice in
tumblers and decanters of the cheaper kind, that there are very
distinct striA| running through the whole substance in some particular
portion of the glass. Now this is owing to the greater density of the
lead silicate, which sinks lower down in the collected mass of glass,
and therefore imparts to it this peculiar effect. When a pot of flint
glass is worked off, that which remains at the bottom usually contains
more lead than that which is worked off in the earlier part of the
day.
_Coloured Glasses._--It has been before shown that silica unites with
metallic oxides; in fact, glass is nothing but a compound brought
about by the union. With certain metallic oxides, silica forms
coloured silicates or glasses; and these, when fused with colourless
glasses, impart to them the colour of the silicate. Oxide of iron
colours glass either green or yellow, according to the nature of the
oxide; the silicate of the protoxide of iron being green, and that of
the peroxide, yellow of a slightly brownish tint. Copper forms two
oxides, the suboxide and the protoxide; the suboxide colours glass
red, while the protoxide renders it green. Black oxide of manganese
colours glass purple; but if large quantities be used, it makes it
perfectly black. Sesquioxide of chromium imparts a beautiful green
colour to glass, while oxide of uranium produces an opalescent effect
of yellow with a tinge of green. This latter, by the way, has the
power of reducing the ultra-violet rays of the spectrum to luminous
rays, and, when held in the rays of a spectrum obtained by the
electric light, produces an extremely beautiful effect, which is
called fluorescence. A small quantity of the oxide of gold tints glass
pink, but the colour becomes extremely rich and ruby-like, when a
larger quantity of the oxide is employed. Oxide of cobalt in very
small quantities yields, with silicic acid, an intensely blue
silicate. This substance, carefully prepared in a special manner and
ground to a fine powder, forms the well known water-colour pigment
called smalt. Oxide of silver stains glass from a delicate lemon tint
to a deep orange, in proportion to the quantity of the oxide employed.
With the exception of the last-named colouring material, the above
mentioned are mixed together with the substances which form the glass,
and are melted in the usual way in glass-pots, except that they are
treated with considerably more care, in order that their tints may be
true. Oxide of silver, however, is never mixed with the materials of
which the glass is made, but is applied to the surface in the
following manner: a solution of nitrate of silver mixed with some
substance, such, for instance, as chalk, may be painted upon the parts
of the glass which it is desired to stain, and these are heated to a
dull red heat, in what is called a "muffle." Wherever the oxide of
silver, which is reduced from the nitrate by heat, comes in contact
with the glass, the latter is stained more or less intensely,
according to the quantity of silver present. Pure metallic silver may
be melted with metallic antimony, and the mass ground to a fine powder
in water. This powder, after being mixed with some Venetian red and
gum water, is applied to the surface of the glass, which is, when dry,
heated to a dull red heat in a muffle, producing the yellow stain,
which can be seen after the Venetian red and the excess of silver have
been scraped off. The reason why silver, or oxide of silver, is not
mixed with the glass materials and fused with them, is because it does
not readily unite with oxygen, and, when it has done so, it loses its
oxygen again at a high temperature, and becomes reduced to the
metallic state; and inasmuch as metals have no effect whatever in
staining silicates, glass made in this way would not have the yellow
colour which it has, when the silver is heated upon its surface to a
much lower temperature in a muffle; for the temperature to which the
constituents of the glass must be heated, so as to cause them to burn
it in, would be so high, that the oxide of silver first formed at a
lower temperature would be reduced to the reguline or metallic state.
Gold also, like silver, does not unite with oxygen readily, or remain
in union with it at high temperature; therefore great care is required
in the preparation of glass to be coloured by oxide of gold; the form
in which it is used being generally that of the purple of Cassius,
made by precipitating a salt of tin with a salt of gold. This
substance is mixed with the glass to be coloured, and heated in a
suitable glass-pot. Portions of it are gathered and allowed to cool,
these being generally of a yellowish, brownish, and sometimes reddish
tint, though they have not in any case the same beautiful red colour
which they produce when applied, as will be immediately described, to
the surface of white glass. A certain quantity of white glass is
gathered from the glass-pot in the soft state with one of these pieces
of gold glass; the whole mass is heated until both become soft, and is
then blown and formed into sheet, which, on examination, will be found
to consist mainly of white glass, with its surface thinly covered with
the glass stained with oxide of gold, while the beautiful ruby colour,
which the gold imparts to the glass, appears pure and distinct. If
such glass as this be heated to too high a temperature, as when it is
used in the manufacture of stained glass windows, the ruby colour is
in part, and sometimes altogether, destroyed, for the oxide of gold
loses its oxygen, and metallic gold is left behind, which does not
yield a colour to the silicate. I have in my possession a piece of
French glass of a pale sapphire tint, which, when heated in the
oxidizing flame of the blowpipe, assumes a brilliant and intense ruby
colour, showing that in the first condition, the gold is not in a
state of oxidation sufficient to impart colour to the glass.
When the suboxide of copper is mixed and fused with the glass which it
is intended to colour, the result is an opaque substance, almost like
red bottle-sealing-wax, which is treated in a manner exactly similar
to the gold glass; viz. it is coated with white glass, and blown and
shaped into sheets, which owe their intense ruby colour to a thin film
of the coloured glass closely adhering to the mass of the white upon
which it is placed. Glass made in this way is called "coated," and
sometimes "flashed" glass, and is extremely useful for ornamental
purposes, for by the action upon the coloured surface of hydrofluoric
acid, the ruby coating can be eaten away, and the white glass beneath
left entire. If the backgrounds of the patterns be painted upon the
ruby side with a material like Brunswick black, which is able to
resist the action of hydrofluoric acid, and if the plate of glass, on
its ruby side, be exposed to the action of the vapour of this acid, or
to the action of the acid in solution in water, in a short space of
time the pattern will be eaten away; and if the Brunswick black
coating be removed with turpentine, a sheet of ruby glass will be
obtained with a white pattern etched upon it.
Owing to the powerful colouring properties which oxide of cobalt
exerts, a very deep-coloured blue glass can be made, which can be
treated like the red copper glass, and may be made to coat and cover
in the same way the surface of plates of white glass. Purple glass,
coloured with oxide of manganese, and green glass are also sometimes
used as coating materials for white glass, but other colours are never
employed in this way.
It is manifest that if different metallic oxides be used with the same
glass, mixed tints will be produced, so that by mingling small
quantities of oxide of cobalt and protoxide of copper, a blue glass
having a greenish hue may be obtained. The revival of glass painting
has caused manufacturers to turn their attention to these mixtures, in
order to produce tints resembling those of ancient stained glass.
Messrs. Powell and Son, of Whitefriars, were the first to perform
experiments on these mixtures, and after much laborious attention and
patience their efforts have been crowned with great success, for they
have been enabled to produce glass as beautiful in tint and in texture
as the best specimens of ancient manufacture. Their example has been
followed by others, such as Messrs. Hartley of Sunderland, and Messrs.
Chance and Co. of Birmingham.
While treating of the effect produced by different metallic oxides
upon colour, it may be well to mention that the opaque glasses used
for such purposes, as the enamelling of watch-faces, are made by
mixing with the materials a certain quantity of arsenious acid (or
white arsenic), in much larger quantities than when it is employed
simply to correct the tint imparted to glass by the iron impurities in
the sand. Oxide of tin also renders glass white and opaque, and a
certain quantity of bone ash will produce a similar effect, though not
in so satisfactory a manner.
_Glass painting_ first became general in this country at the time
when the Early English style of architecture prevailed, and some of
the best specimens were executed during that period. By the best
specimens is not meant, that the figures painted upon those windows
were artistically as correct as similar works of a later date, but
that they were designed and executed in accordance with those
principles, which should always govern the adaptation of a substance
like glass to ornamental purposes. The earlier mediA|val artists
depended for effect more upon the boldness of their outline, than upon
the intensity of their shading or the delicacy of their manipulation.
The form of a thirteenth-century figure is merely indicated by a few
bold and well drawn outlines, the features being formed by lines, the
pupils of the eyes by simple well-shaped masses of opaque pigment; and
such a treatment as this was quite sufficient to convey what was, to
the observer, more or less a symbolical, than a truthful
representation of the Scripture history which they were intended to
illustrate. These artists remembered that windows are openings in a
building, through which light has to pass, and they did not,
therefore, like many of the later imitators, render them opaque by
masses of intense shadow, which perfectly obscure the colour of the
glass upon which the picture is painted, and render the passage of
light through it simply impossible. The thirteenth-century glass
painters, too, in the treatment of their shadows, bore this great
principle in mind, and instead of daubing and stippling them on,
usually indicated them with a thin wash of enamel colour, intensified
in parts by lines crossing one another, and therefore called
cross-hatching, through the interstices of which the light, although
subdued, was able, in a measure, to pass.
But as the object of this article is not to discuss the merits of the
various styles of glass painting, however much I might desire to
enlarge upon it, I pass on to a description of the methods employed in
the manufacture of stained glass windows. In the first place, after a
design has been drawn, in which the effect of the window as a whole
can be carefully considered, cartoons of the figures and ornament are
made of the exact size of the intended painting. And here it should be
noted, that all the lines should be extremely clear, precise, and well
drawn, because it is from these that the workman, who is not usually
himself an artist, has to convey on the glass the feeling of the
artist. The cartoon, when completed, is laid down in pieces for
convenience-sake on a table, and fastened with small nails. The
glass-cutter then selects the various coloured glasses which are
required to be inserted in their proper places, so as to carry out the
design of the artist. For instance, a piece of white or yellow-tinted
glass is cut to the shape of the face. If the figure be a small one,
the hair also is included in this; and probably in the figure of a
saint, the nimbus which surrounds the head may be included; while in
larger figures, particularly in the earliest styles, the face was of
glass of one tint, the hair of another, and the nimbus of one or more
tints, different from either of these. Sometimes, in the later styles,
the hair, after the face was painted and burnt in, was stained with
the silver stain already described, so that when the glass was
cleaned, it was of a yellow colour. However, not to enlarge more upon
these points, which really belong more to the artistic than to the
industrial part of window painting, let us proceed to the
consideration of manipulative details. The outlines of the figures and
ornament are painted with a substance called "tracing brown," made by
mixing with a flux some oxide of iron, heating them together in a
crucible and grinding the product to a fine powder, which is mixed
with certain vehicles adapted to the particular use to which it is to
be applied. Different fluxes are employed by different glass painters;
some contain borax, because such fluxes fuse more easily, and
therefore cause the glass which is painted to be exposed for a less
time, and to a lower temperature, than when less fusible fluxes are
used.
It is always satisfactory to an author, to feel that his articles have
been of some use to those whom he hoped to benefit. Since this article
was written a letter appeared in one of the architectural journals,
complaining that the glass furnished by manufacturers to glass
painters was of inferior composition to that which was used by the
manufacturers of ancient stained glass windows. In fact, it was
asserted that modern glass was not made with due care, and that to
this was owing the unfortunate disappearance of some of the painting
and tracing of modern stained glass windows; but that this is not the
case, is manifest to all who understand the manufacture of glass. The
real reason why the colouring matter with which glass painters outline
and shade their designs, has in many instances gradually come off from
the surface of the glass, is, because the fluxes used for making it
adhere to the glass are of such a composition, that they themselves
have by the action of time become disintegrated.
Some time ago, a person engaged in the manufacture of the enamel
plates used for railway lamps, on which are written the names of the
stations, called upon me, and told me, that the enamel which he
employed had become dark, spotty, and in many cases had peeled off
from the glass. The reason of this is identical with that which occurs
in stained glass windows, viz. that the fluxes that he used were not
suitable for the purpose, considering that they had to withstand the
action of the weather. From an analysis made of these fluxes (not of
those last alluded to, but of those which have been employed in
stained glass windows), it appears that large quantities of borax have
been introduced; and, wherever this is the case, no reliance whatever
can be placed on the permanency of pictures painted with such fluxes.
I have appended a few receipts for fluxes, which can be used with
safety by any glass painter who will take the trouble to try them. But
I must strongly advise that all those who are connected with the
making of fluxes in any glass painting establishment, should master
sufficient chemical knowledge to enable them to ascertain the
behaviour of the materials, with respect to one another, as well as of
the nature of the glass upon which they are employed; for very much
indeed depends upon a correct knowledge of the character of the glass
as to whether it be hard or soft, what it contains, and of the
temperature at which the glass becomes sufficiently soft to form a
firm and enduring union with the colours fluxed upon it.