+-------------+-------+-------+------+------+-------+------+------+-----+------------+
| | | | | | | | | | Depression |
| | SiO2. | Na2O. | K2O. | CaO. | Al2O3.| MgO. | B2O3.| ZnO.| of |
| | | | | | | | | | Ice-point. |
+-------------+-------+-------+------+------+-------+------+------+-----+------------+
| Tonnelot's | | | | | | | | | |
| "Verre dur"| 70.96 | 12.02 | 0.56 |14.40 | 1.44 | 0.40 | .. | .. | 0.07 |
| Jena glass--| | | | | | | | | |
| XVI.-111 | 67.5 | 14.0 | .. | 7.0 | 2.5 | .. | 2.0 | 7.0 | 0.05 |
| 59-111 | 72.0 | 11.0 | .. | 5.0 | 5.0 | .. | 12.0 | .. | 0.02 |
+-------------+-------+-------+------+------+-------+------+------+-----+------------+
Since the discovery of the Rontgen rays, experiments have been made to
ascertain the effects of the different constituents of glass on the
transparency of glass to X-rays. The oxides of lead, barium, zinc and
antimony are found perceptibly to retard the rays. The glass tubes,
therefore, from which the X-ray bulbs are to be fashioned, must not
contain any of these oxides, whereas the glass used for making the
funnel-shaped shields, which direct the rays upon the patient and at the
same time protect the hands of the operator from the action of the rays,
must contain a large proportion of lead.
Among the many developments of the Jena Works, not the least important
are the glasses made in the form of a tube, from which gas-chimneys,
gauge-glasses and chemical apparatus are fashioned, specially adapted to
resist sudden changes of temperature. One method is to form the tube of
two layers of glass, one being considerably more expansible than the
other.
(C) _Sheet and Crown-glass._--Sheet-glass is almost wholly a
soda-lime-silicate glass, containing only small quantities of iron,
alumina and other impurities. The raw materials used in this manufacture
are chosen with considerable care, since the requirements as to the
colour of the product are somewhat stringent. The materials ordinarily
employed are the following: sand, of good quality, uniform in grain and
free from any notable quantity of iron oxide; carbonate of lime,
generally in the form of a pure variety of powdered limestone; and
sulphate of soda. A certain proportion of soda ash (carbonate of soda)
is also used in some works in sheet-glass mixtures, while "decolorizers"
(substances intended to remove or reduce the colour of the glass) are
also sometimes added, those most generally used being manganese dioxide
and arsenic. Another essential ingredient of all glass mixtures
containing sulphate of soda is some form of carbon, which is added
either as coke, charcoal or anthracite coal; the carbon so introduced
aids the reducing substances contained in the atmosphere of the furnace
in bringing about the reduction of the sulphate of soda to a condition
in which it combines more readily with the silicic acid of the sand. The
proportions in which these ingredients are mixed vary according to the
exact quality of glass required and with the form and temperature of the
melting furnace employed. A good quality of sheet-glass should show, on
analysis, a composition approximating to the following: silica (SiO2),
72%; lime (CaO), 13%; soda (Na2O), 14%; and iron and alumina (Fe2O3,
Al2O3), 1%. The actual composition, however, of a mixture that will give
a glass of this composition cannot be directly calculated from these
figures and the known composition of the raw materials, owing to the
fact that considerable losses, particularly of alkali, occur during
melting.
The fusion of sheet-glass is now generally carried out in gas-fired
regenerative tank furnaces. The glass in process of fusion is contained
in a basin or tank built up of large blocks of fire-clay and is heated
by one or more powerful gas flames which enter the upper part of the
furnace chamber through suitable apertures or "ports." In Europe the gas
burnt in these furnaces is derived from special gas-producers, while in
some parts of America natural gas is utilized. With producer gas it is
necessary to pre-heat both the gas and the air which is supplied for its
combustion by passing both through heated regenerators (for an account
of the principles of the regenerative furnace see article FURNACE). In
many respects the glass-melting tank resembles the open-hearth steel
furnace, but there are certain interesting differences. Thus the
dimensions of the largest glass tanks greatly exceed those of the
largest steel furnaces; glass furnaces containing up to 250 tons of
molten glass have been successfully operated, and owing to the
relatively low density of glass this involves very large dimensions. The
temperature required in the fusion of sheet-glass and of other glasses
produced in tank furnaces is much lower than that attained in steel
furnaces, and it is consequently possible to work glass-tanks
continuously for many months together; on the other hand, glass is not
readily freed from foreign bodies that may become admixed with it, so
that the absence of detachable particles is much more essential in glass
than in steel melting. Finally, fluid steel can be run or poured off,
since it is perfectly fluid, while glass cannot be thus treated, but is
withdrawn from the furnace by means of either a ladle or a gatherer's
pipe, and the temperature required for this purpose is much lower than
that at which the glass is melted. In a sheet-glass tank there is
therefore a gradient of temperature and a continuous passage of material
from the hotter end of the furnace where the raw materials are
introduced to the cooler end where the glass, free from bubbles and raw
material, is withdrawn by the gatherers. For the purpose of the removal
of the glass, the cooler end of the furnace is provided with a number of
suitable openings, provided with movable covers or shades. The
"gatherer" approaches one of these openings, removes the shade and
introduces his previously heated "pipe." This instrument is an iron
tube, some 5 ft. long, provided at one end with an enlarged butt and at
the other with a wooden covering acting as handle and mouthpiece. The
gatherer dips the butt of the pipe into the molten "metal" and withdraws
upon it a small ball of viscous glass, which he allows to cool in the
air while constantly rotating it so as to keep the mass as nearly
spherical in shape as he can. When the first ball or "gathering" has
cooled sufficiently, the whole is again dipped into the molten glass and
a further layer adheres to the pipe-end, thus forming a larger ball.
This process is repeated, with slight modifications, until the gathering
is of the proper size and weight to yield the sheet which is to be
blown. When this is the case the gathering is carried to a block or
half-open mould in which it is rolled and blown until it acquires,
roughly, the shape of a hemisphere, the flat side being towards the pipe
and the convexity away from it; the diameter of this hemisphere is so
regulated as to be approximately that of the cylinder which is next to
be formed of the viscous mass. From the hemispherical shape the mass of
glass is now gradually blown into the form of a short cylinder, and then
the pipe with the adherent mass of glass is handed over to the blower
proper. This workman stands upon a platform in front of special furnaces
which, from their shape and purpose, are called "blowing holes." The
blower repeatedly heats the lower part of the mass of glass and keeps it
distended by blowing while he swings it over a deep trench which is
provided next to his working platform. In this way the glass is extended
into the form of a long cylinder closed at the lower end. The size of
cylinder which can be produced in this way depends chiefly upon the
dimensions of the working platform and the weight which a man is able to
handle freely. The lower end of the cylinder is opened, in the case of
small and thin cylinders, by the blower holding his thumb over the
mouthpiece of the pipe and simultaneously warming the end of the
cylinder in the furnace, the expansion of the imprisoned air and the
softening of the glass causing the end of the cylinder to burst open.
The blower then heats the end of the cylinder again and rapidly spins
the pipe about its axis; the centrifugal effect is sufficient to spread
the soft glass at the end to a radius equal to that of the rest of the
cylinder. In the case of large and thick cylinders, however, another
process of opening the ends is generally employed: an assistant attaches
a small lump of hot glass to the domed end, and the heat of this added
glass softens the cylinder sufficiently to enable the assistant to cut
the end open with a pair of shears; subsequently the open end is spun
out to the diameter of the whole as described above. The finished
cylinder is next carried to a rack and the pipe detached from it by
applying a cold iron to the neck of thick hot glass which connects
pipe-butt and cylinder, the neck cracking at the touch. Next, the rest
of the connecting neck is detached from the cylinder by the application
of a heated iron to the chilled glass. This leaves a cylinder with
roughly parallel ends; these ends are cut by the use of a diamond
applied internally and then the cylinder is split longitudinally by the
same means. The split cylinder is passed to the flattening furnace,
where it is exposed to a red heat, sufficient to soften the glass; when
soft the cylinder is laid upon a smooth flat slab and flattened down
upon it by the careful application of pressure with some form of rubbing
implement, which frequently takes the form of a block of charred wood.
When flattened, the sheet is moved away from the working opening of the
furnace, and pushed to a system of movable grids, by means of which it
is slowly moved along a tunnel, away from a source of heat nearly equal
in temperature to that of the flattening chamber. The glass thus cools
gradually as it passes down the tunnel and is thereby adequately
annealed.
The process of sheet-glass manufacture described above is typical of
that in use in a large number of works, but many modifications are to be
found, particularly in the furnaces in which the glass is melted. In
some works, the older method of melting the glass in large pots or
crucibles is still adhered to, although the old-fashioned coal-fired
furnaces have nearly everywhere given place to the use of producer gas
and regenerators. For the production of coloured sheet-glass, however,
the employment of pot furnaces is still almost universal, probably
because the quantities of glass required of any one tint are
insufficient to employ even a small tank furnace continuously; the exact
control of the colour is also more readily attained with the smaller
bulk of glass which has to be dealt with in pots. The general nature of
the colouring ingredients employed, and the colour effects produced by
them, have already been mentioned. In coloured sheet-glass, two distinct
kinds are to be recognized; in one kind the colouring matter is
contained in the body of the glass itself, while in the other the
coloured sheet consists of ordinary white glass covered upon one side
with a thin coating of intensely coloured glass. The latter kind is
known as "flashed," and is universally employed in the case of colouring
matters whose effect is so intense that in any usual thickness of glass
they would cause almost entire opacity. Flashed glass is produced by
taking either the first or the last gathering in the production of a
cylinder out of a crucible containing the coloured "metal," the other
gatherings being taken out of ordinary white sheet-glass. It is
important that the thermal expansion of the two materials which are thus
incorporated should be nearly alike, as otherwise warping of the
finished sheet is liable to result.
_Mechanical Processes for the Production of Sheet-glass._--The
complicated and indirect process of sheet-glass manufacture has led to
numerous inventions aiming at a direct method of production by more or
less mechanical means. All the earlier attempts in this direction failed
on account of the difficulty of bringing the glass to the machines
without introducing air-bells, which are always formed in molten glass
when it is ladled or poured from one vessel into another. More modern
inventors have therefore adopted the plan of drawing the glass direct
from the furnace. In an American process the glass is drawn direct from
the molten mass in the tank in a cylindrical form by means of an iron
ring previously immersed in the glass, and is kept in shape by means of
special devices for cooling it rapidly as it leaves the molten bath. In
this process, however, the entire operations of splitting and flattening
are retained, and although the mechanical process is said to be in
successful commercial operation, it has not as yet made itself felt as a
formidable rival to hand-made sheet-glass. An effort at a more direct
mechanical process is embodied in the inventions of Foucault which are
at present being developed in Germany and Belgium; in this process the
glass is drawn from the molten bath in the shape of flat sheets, by the
aid of a bar of iron, previously immersed in the glass, the glass
receiving its form by being drawn through slots in large fire-bricks,
and being kept in shape by rapid chilling produced by the action of
air-blasts. The mechanical operation is quite successful for thick
sheets, but it is not as yet available for the thinner sheets required
for the ordinary purposes of sheet-glass, since with these excessive
breakage occurs, while the sheets generally show grooves or lines
derived from small irregularities of the drawing orifice. For the
production of thick sheets which are subsequently to be polished the
process may thus claim considerable success, but it is not as yet
possible to produce satisfactory sheet-glass by such means.
_Crown-glass_ has at the present day almost disappeared from the market,
and it has been superseded by sheet-glass, the more modern processes
described above being capable of producing much larger sheets of glass,
free from the knob or "bullion" which may still be seen in old
crown-glass windows. For a few isolated purposes, however, it is
desirable to use a glass which has not been touched upon either surface
and thus preserves the lustre of its "fire polish" undiminished; this
can be attained in crown-glass but not in sheet, since one side of the
latter is always more or less marked by the rubber used in the process
of flattening. One of the few uses of crown-glass of this kind is the
glass slides upon which microscopic specimens are mounted, as well as
the thin glass slips with which such preparations are covered. A full
account of the process of blowing crown-glass will be found in all older
books and articles on the subject, so that it need only be mentioned
here that the glass, instead of being blown into a cylinder, is blown
into a flattened sphere, which is caused to burst at the point opposite
the pipe and is then, by the rapid spinning of the glass in front of a
very hot furnace-opening, caused to expand into a flat disk of large
diameter. This only requires to be annealed and is then ready for
cutting up, but the lump of glass by which the original globe was
attached to the pipe remains as the bullion in the centre of the disk of
glass.
_Coloured Glass for Mosaic Windows._--The production of coloured glass
for "mosaic" windows has become a separate branch of glass-making.
Charles Winston, after prolonged study of the coloured windows of the
13th, 14th and 15th centuries, convinced himself that no approach to the
colour effect of these windows could be made with glass which is thin
and even in section, homogeneous in texture, and made and coloured with
highly refined materials. To obtain the effect it was necessary to
reproduce as far as possible the conditions under which the early
craftsmen worked, and to create scientifically glass which is impure in
colour, irregular in section, and non-homogeneous in texture. The glass
is made in cylinders and in "crowns" or circles. The cylinders measure
about 14 in. in length by 8 in. in diameter, and vary in thickness from
1/8 to 3/8 in. The crowns are about 15 in. in diameter, and vary in
thickness from 1/8 to 1/2 in., the centre being the thickest. These
cylinders and crowns may be either solid colour or flashed. Great
variety of colour may be obtained by flashing one colour upon another,
such as blue on green, and ruby on blue, green or yellow.
E. J. Prior has introduced an ingenious method of making small oblong
and square sheets of coloured glass, which are thick in the centre and
taper towards the edges, and which have one surface slightly roughened
and one brilliantly polished. Glass is blown into an oblong box-shaped
iron mould, about 12 in. in depth and 6 in. across. A hollow rectangular
bottle is formed, the base and sides of which are converted into sheets.
The outer surface of these sheets is slightly roughened by contact with
the iron mould.
(D) _Bottles and mechanically blown Glass._--The manufacture of bottles
has become an industry of vast proportions. The demand constantly
increases, and, owing to constant improvements in material in the moulds
and in the methods of working, the supply fully keeps pace with the
demand. Except for making bottles of special colours, gas-heated tank
furnaces are in general use. Melting and working are carried on
continuously. The essential qualities of a bottle are strength and power
to resist chemical corrosion. The materials are selected with a view to
secure these qualities. For the highest quality of bottles, which are
practically colourless, sand, limestone and sulphate and carbonate of
soda are used. The following is a typical analysis of high quality
bottle-glass: SiO2, 69.15%; Na2O, 13.00%; CaO, 15.00%; Al2O3, 2.20%; and
Fe2O3, 0.65%. For the commoner grades of dark-coloured bottles the glass
mixture is cheapened by substituting common salt for part of the
sulphate of soda, and by the addition of felspar, granite, granulite,
furnace slag and other substances fusible at a high temperature. Bottle
moulds are made of cast iron, either in two pieces, hinged together at
the base or at one side, or in three pieces, one forming the body and
two pieces forming the neck.
[Illustration: FIG. 18.--Tool for moulding the inside and outside of the
neck of a bottle.
C, Bottle.
A, Conical piece of iron to form the inside of the neck.
B, B, Shaped pieces of iron, which can be pressed upon the outside of
the neck by the spring-handle H.]
A bottle gang or "shop" consists of five persons. The "gatherer" gathers
the glass from the tank furnace on the end of the blowing-iron, rolls it
on a slab of iron or stone, slightly expands the glass by blowing, and
hands the blowing iron and glass to the "blower." The blower places the
glass in the mould, closes the mould by pressing a lever with his foot,
and either blows down the blowing iron or attaches it to a tube
connected with a supply of compressed air. When the air has forced the
glass to take the form of the mould, the mould is opened and the blower
gives the blowing iron with the bottle attached to it to the "wetter
off." The wetter off touches the top of the neck of the bottle with a
moistened piece of iron and by tapping the blowing iron detaches the
bottle and drops it into a wooden trough. He then grips the body of the
bottle with a four-pronged clip, attached to an iron rod, and passes it
to the "bottle maker." The bottle maker heats the fractured neck of the
bottle, binds a band of molten glass round the end of it and
simultaneously shapes the inside and the outside of the neck by using
the tool shown in fig. 18. The finished bottle is taken by the "taker
in" to the annealing furnace. The bottles are stacked in iron trucks,
which, when full, are moved slowly away from a constant source of heat.
The processes of manipulation which have been described, although in
practice they are very rapidly performed, are destined to be replaced by
the automatic working of a machine. Bottle-making machines, based on
Ashley's original patent, are already being largely used. They ensure
absolute regularity in form and save both time and labour. A
bottle-making machine combines the process of pressing with a plunger
with that of blowing by compressed air. The neck of the bottle is first
formed by the plunger, and the body is subsequently blown by compressed
air admitted through the plunger. A sufficient weight of molten glass to
form a bottle is gathered and placed in a funnel-shaped vessel which
serves as a measure, and gives access to the mould which shapes the
outside of the neck. A plunger is forced upwards into the glass in the
neck-mould and forms the neck. The funnel is removed, and the plunger,
neck-mould and the mass of molten glass attached to the neck are
inverted. A bottle mould rises and envelops the mass of molten glass.
Compressed air admitted through the plunger forces the molten glass to
take the form of the bottle mould and completes the bottle.
In the case of the machine patented by Michael Owens of Toledo, U.S.A.,
for making tumblers, lamp-chimneys, and other goods of similar
character, the manual operations required are (1) gathering the molten
glass at the end of a blowing iron; (2) placing the blowing iron with
the glass attached to it in the machine; (3) removing the blowing iron
with the blown vessel attached. Each machine (fig. 19) consists of a
revolving table carrying five or six moulds. The moulds are opened and
closed by cams actuated by compressed air. As soon as a blowing iron is
in connexion with an air jet, the sections of the mould close upon the
molten glass, and the compressed air forces the glass to take the form
of the mould. After removal from the machine, the tumbler is severed
from the blowing iron, and its fractured edge is trimmed.
[Illustration: FIG. 19.--Owens's Glass-blowing Machine. g,g,g,
Blowing-irons.]
Compressed air or steam is also used for fashioning very large vessels,
baths, dishes and reservoirs by the "Sievert" process. Molten glass is
spread upon a large iron plate of the required shape and dimensions. The
flattened mass of glass is held by a rim, connected to the edge of the
plate. The plate with the glass attached to it is inverted, and
compressed air or steam is introduced through openings in the plate. The
mass of glass, yielding to its own weight and the pressure of air or
steam, sinks downwards and adapts itself to any mould or receptacle
beneath it.
The processes employed in the manufacture of the glass bulbs for
incandescent electric lamps, are similar to the old-fashioned processes
of bottle making. The mould is in two pieces hinged together; it is
heated and the inner surface is rubbed over with finely powdered
plumbago. When the glass is being blown in the mould the blowing iron is
twisted round and round so that the finished bulb may not be marked by
the joint of the mould.
III. MECHANICALLY PRESSED GLASS. (A) _Plate-glass._--The glass popularly
known as "plate-glass" is made by casting and rolling. The following are
typical analyses:
+---------+-------+-------+-------+--------+--------+
| | SiO2. | CaO. | Na2O. | Al2O3. | Fe2O3. |
+---------+-------+-------+-------+--------+--------+
| French. | 71.80 | 15.70 | 11.10 | 1.26 | 0.14% |
| English.| 70.64 | 16.27 | 11.47 | 0.70 | 0.49% |
+---------+-------+-------+-------+--------+--------+
The raw materials for the production of plate-glass are chosen with
great care so as to secure a product as free from colour as possible,
since the relatively great thickness of the sheets would render even a
faint tint conspicuous. The substances employed are the same as those
used for the manufacture of sheet-glass, viz. pure sand, a pure form of
carbonate of lime, and sulphate of soda, with the addition of a suitable
proportion of carbon in the form of coke, charcoal or anthracite coal.
The glass to be used for the production of plate is universally melted
in pots or crucibles and not in open tank furnaces. When the glass is
completely melted and "fine," i.e. free from bubbles, it is allowed to
cool down to a certain extent so as to become viscous or pasty. The
whole pot, with its contents of viscous glass, is then removed bodily
from the furnace by means of huge tongs and is transported to a crane,
which grips the pot, raises it, and ultimately tips it over so as to
pour the glass upon the slab of the rolling-table. In most modern works
the greater part of these operations, as well as the actual rolling of
the glass, is carried out by mechanical means, steam power and
subsequently electrical power having been successfully applied to this
purpose; the handling of the great weights of glass required for the
largest sheets of plate-glass which are produced at the present time
would, indeed, be impossible without the aid of machinery. The
casting-table usually consists of a perfectly smooth cast-iron slab,
frequently built up of a number of pieces carefully fitted together,
mounted upon a low, massive truck running upon rails, so that it can be
readily moved to any desired position in the casting-room. The viscous
mass having been thrown on the casting-table, a large and heavy roller
passes over it and spreads it out into a sheet. Rollers up to 5 tons in
weight are employed and are now generally driven by power. The width of
the sheet or plate is regulated by moving guides which are placed in
front of the roller and are pushed along by it, while its thickness is
regulated by raising or lowering the roller relatively to the surface of
the table. Since the surfaces produced by rolling have subsequently to
be ground and polished, it is essential that the glass should leave the
rolling-table with as smooth a surface as possible, so that great care
is required in this part of the process. It is, however, equally
important that the glass as a whole should be flat and remains flat
during the process of gradual cooling (annealing), otherwise great
thicknesses of glass would have to be ground away at the projecting
parts of the sheet. The annealing process is therefore carried out in a
manner differing essentially from that in use for any other variety of
flat glass and nearly resembling that used for optical glass. The rolled
sheet is left on the casting-table until it has set sufficiently to be
pushed over a flat iron plate without risk of distortion; meanwhile the
table has been placed in front of the opening of one of the large
annealing kilns and the slab of glass is carefully pushed into the kiln.
The annealing kilns are large fire-brick chambers of small height but
with sufficient floor area to accommodate four or six large slabs, and
the slabs are placed directly upon the floor of the kiln, which is built
up of carefully dressed blocks of burnt fireclay resting upon a bed of
sand; in order to avoid any risk of working or buckling in this floor
these blocks are set slightly apart and thus have room to expand freely
when heated. Before the glass is introduced, the annealing kiln is
heated to dull red by means of coal fires in grates which are provided
at the ends or sides of the kiln for that purpose. When the floor of the
kiln has been covered with slabs of glass the opening is carefully built
up and luted with fire-bricks and fire-clay, and the whole is then
allowed to cool. In the walls and floor of the kiln special cooling
channels or air passages are provided and by gradually opening these to
atmospheric circulation the cooling is considerably accelerated while a
very even distribution of temperature is obtained; by these means even
the largest slabs can now be cooled in three or four days and are
nevertheless sufficiently well annealed to be free from any serious
internal stress. From the annealing kiln the slabs of glass are
transported to the cutting room, where they are cut square, defective
slabs being rejected or cut down to smaller sizes. The glass at this
stage has a comparatively dull surface and this must now be replaced by
that brilliant and perfectly polished surface which is the chief beauty
of this variety of glass. The first step in this process is that of
grinding the surface down until all projections are removed and a close
approximation to a perfect plane is obtained. This operation, like all
the subsequent steps in the polishing of the glass, is carried out by
powerful machinery. By means of a rotating table either two surfaces of
glass, or one surface of glass and one of cast iron, are rubbed together
with the interposition of a powerful abrasive such as sand, emery or
carborundum. The machinery by which this is done has undergone numerous
modifications and improvements, all tending to produce more perfectly
plane glass, to reduce the risk of breakage, and to lessen the
expenditure of time and power required per sq. yd. of glass to be
worked. It is impossible to describe this machinery within the limits of
this article, but it is notable that the principal difficulties to be
overcome arise from the necessity of providing the glass with a
perfectly continuous and unyielding support to which it can be firmly
attached but from which it can be detached without undue difficulty.
When the surface of the glass has been ground down to a plane, the
surface itself is still "grey," i.e. deeply pitted with the marks of the
abrasive used in grinding it down; these marks are removed by the
process of smoothing, in which the surface is successively ground with
abrasives of gradually increasing fineness, leaving ultimately a very
smooth and very minutely pitted "grey" surface. This smooth surface is
then brilliantly polished by the aid of friction with a rubbing tool
covered with a soft substance like leather or felt and fed with a
polishing material, such as rouge. A few strokes of such a rubber are
sufficient to produce a decidedly "polished" appearance, but prolonged
rubbing under considerable pressure and the use of a polishing paste of
a proper consistency are required in order to remove the last trace of
pitting from the surface. This entire process must, obviously, be
applied in turn to each of the two surfaces of the slab of glass.
Plate-glass is manufactured in this manner in thicknesses varying from
3/16 in. to 1 in. or even more, while single sheets are produced
measuring more than 27 ft. by 13 ft.
_"Rolled Plate" and figured "Rolled Plate."_--Glass for this purpose,
with perhaps the exception of the best white and tinted varieties, is
now universally produced in tank-furnaces, similar in a general way to
those used for sheet-glass, except that the furnaces used for "rolled
plate" glass of the roughest kinds do not need such minutely careful
attention and do not work at so high a temperature. The composition of
these glasses is very similar to that of sheet-glass, but for the
ordinary kinds of rolled plate much less scrupulous selection need be
made in the choice of raw materials, especially of the sand.
The glass is taken from the furnace in large iron ladles, which are
carried upon slings running on overhead rails; from the ladle the glass
is thrown upon the cast-iron bed of a rolling-table, and is rolled into
sheet by an iron roller, the process being similar to that employed in
making plate-glass, but on a smaller scale. The sheet thus rolled is
roughly trimmed while hot and soft, so as to remove those portions of
glass which have been spoilt by immediate contact with the ladle, and
the sheet, still soft, is pushed into the open mouth of an annealing
tunnel or "lear," down which it is carried by a system of moving grids.
The surface of the glass produced in this way may be modified by
altering the surface of the rolling-table; if the table has a smooth
surface, the glass will also be more or less smooth, but much dented and
buckled on the surface and far from having the smooth face of blown
sheet. If the table has a pattern engraved upon it the glass will show
the same pattern in relief, the most frequent pattern of the kind being
either small parallel ridges or larger ribs crossing to form a lozenge
pattern.
The more elaborate patterns found on what is known as "figure rolled
plate" are produced in a somewhat different manner; the glass used for
this purpose is considerably whiter in colour and much softer than
ordinary rolled plate, and instead of being rolled out on a table it is
produced by rolling between two moving rollers from which the sheet
issues. The pattern is impressed upon the soft sheet by a printing
roller which is brought down upon the glass as it leaves the main rolls.
This glass shows a pattern in high relief and gives a very brilliant
effect.
The various varieties of rolled plate-glass are now produced for some
purposes with a reinforcement of wire netting which is embedded in the
mass of the glass. The wire gives the glass great advantages in the
event of fracture from a blow or from fire, but owing to the difference
in thermal expansion between wire and glass, there is a strong tendency
for such "wired glass" to crack spontaneously.
_Patent Plate-glass._--This term is applied to blown sheet-glass, whose
surface has been rendered plane and brilliant by a process of grinding
and polishing. The name "patent plate" arose from the fact that certain
patented devices originated by James Chance of Birmingham first made it
possible to polish comparatively thin glass in this way.
[Illustration: FIG. 20.--Modern American Glass-Press.]
(B) _Pressed Glass._--The technical difference between pressed and
moulded glass is that moulded glass-ware has taken its form from a mould
under the pressure of a workman's breath, or of compressed air, whereas
pressed glass-ware has taken its form from a mould under the pressure of
a plunger. Moulded glass receives the form of the mould on its interior
as well as on its exterior surface. In pressed glass the exterior
surface is modelled by the mould, whilst the interior surface is
modelled by the plunger (fig. 20).
The process of pressing glass was introduced to meet the demand for
cheap table-ware. Pressed glass, which is necessarily thick and
serviceable, has well met this legitimate demand, but it also caters for
the less legitimate taste for cheap imitations of hand-cut glass. An
American writer has expressed his satisfaction that the day-labourer can
now have on his table at a nominal price glass dishes of elaborate
design, which only an expert can distinguish from hand-cut crystal. The
deceptive effect is in some cases heightened by cutting over and
polishing by hand the pressed surface.
The glass for pressed ware must be colourless, and, when molten, must be
sufficiently fluid to adapt itself readily to the intricacies of the
moulds, which are often exceedingly complex. The materials employed are
sand, sulphate of soda, nitrate of soda, calcspar and in some works
carbonate of barium. The following is an analysis of a specimen of
English pressed glass; SiO2, 70.68%; Na2O, 18.38%; CaO, 5.45%; BaO,
4.17%; Al2O3, 0.33%; and Fe2O3, 0.20%. Tanks and pots are both used for
melting the glass. The moulds are made of cast iron. They are usually in
two main pieces, a base and an upper part or collar of hinged sections.
The plunger is generally worked by a hand lever. The operator knows by
touch when the plunger has pressed the glass far enough to exactly fill
the mould. Although the moulds are heated, the surface of the glass is
always slightly ruffled by contact with the mould. For this reason every
piece of pressed glass-ware, as soon as it is liberated from the mould,
is exposed to a sharp heat in a small subsidiary furnace in order that
the ruffled surface may be removed by melting. These small furnaces are
usually heated by an oil spray under the pressure of steam or compressed
air.
See Antonio Neri, _Ars vitraria, cum Merritti observationibus_
(Amsterdam, 1668) (Neri's work was translated into English by C.
Merritt in 1662, and the translation, _The Art of making Glass_, was
privately reprinted by Sir T. Phillipps, Bart., in 1826); Johann
Kunkel, _Vollstandige Glasmacher-Kunst_ (Nuremberg, 1785); Apsley
Pellatt, _Curiosities of Glass-making_ (London, 1849); A. Sauzay,
_Marvels of Glass-making_ (from the French) (London, 1869); G.
Bontemps, _Guide du verrier_ (Paris, 1868); E. Peligot, _Le Verre, son
histoire, sa fabrication_ (Paris, 1878); W. Stein, "Die
Glasfabrikation," in Bolley's _Technologie_, vol. iii. (Brunswick,
1862); H. E. Benrath, _Die Glasfabrikation_ (Brunswick, 1875); J.
Falck and L. Lobmeyr, _Die Glasindustrie_ (Vienna, 1875); D. H.
Hovestadt, _Jenaer Glas_ (Jena, 1900; Eng. trans. by J. D. and A.
Everett, Macmillan, 1907); J. Henrivaux, _Le Verre et le cristal_
(Paris, 1887), and _La Verrerie au XX^e siecle_ (1903); Chance, Harris
and Powell, _Principles of Glass-making_ (London, 1883); Moritz V.
Rohr, _Theorie und Geschichte der photographischen Objektive_ (Berlin,
1899); C. E. Guillaume, _Traite pratique de la thermometrie de
precision_ (Paris, 1889); Louis Coffignal, _Verres et emaux_ (Paris,
1900); R. Gerner, _Die Glasfabrikation_ (Vienna, 1897); C. Wetzel,
_Herstellung grosser Glaskorper_ (Vienna, 1900); C. Wetzel,
_Bearbeitung von Glaskorpern_ (Vienna, 1901); E. Tscheuschner,
_Handbuch der Glasfabrikation_ (Weimar, 1885); R. Dralle, _Anlage und
Betrieb der Glasfabriken_ (Leipzig, 1886); G. Tammann,
_Kristallisieren und Schmelzen_ (Leipzig, 1903); W. Rosenhain, "Some
Properties of Glass," _Trans. Optical Society_ (London, 1903),
"Possible Directions of Progress in Optical Glass," _Proc. Optical
Convention_ (London, 1905) and _Glass Manufacture_ (London, 1908);
Introduction to section 1, _Catalogue of the Optical Convention_
(London, 1905). (H. J. P.; W. Rn.)
_History of Glass Manufacture._
The great similarity in form, technique and decoration of the earliest
known specimens of glass-ware suggests that the craft of glass-making
originated from a single centre. It has been generally assumed that
Egypt was the birthplace of the glass industry. It is true that many
conditions existed in Egypt favourable to the development of the craft.
The Nile supplied a waterway for the conveyance of fuel and for the
distribution of the finished wares. Materials were available providing
the essential ingredients of glass. The Egyptian potteries afforded
experience in dealing with vitreous glazes and vitreous colours, and
from Egyptian alabaster-quarries veined vessels were wrought, which may
well have suggested the decorative arrangement of zigzag lines (see
Plate I. figs, 1, 2, 4 d) so frequently found on early specimens of
glass-ware. In Egypt, however, no traces have at present been found of
the industry in a rudimentary condition, and the vases which have been
classified as "primitive" bear witness to an elaboration of technique
far in advance of the experimental period. The earliest specimens of
glass-ware which can be definitely claimed as Egyptian productions, and
the glass manufactory discovered by Dr Flinders Petrie at Tell el
Amarna, belong to the period of the XVIIIth dynasty. The comparative
lateness of this period makes it difficult to account for the wall
painting at Beni Hasan, which accurately represents the process of
glass-blowing, and which is attributed to the period of the XIth
dynasty. Dr Petrie surmounts the difficulty by saying that the process
depicted is not glass-blowing, but some metallurgical process in which
reeds were used tipped with lumps of clay. It is possible that the
picture does not represent Egyptian glass-blowers, but is a traveller's
record of the process of glass-blowing seen in some foreign or subject
country. The scarcity of specimens of early glass-ware actually found in
Egypt, and the advanced technique of those which have been found, lead
to the supposition that glass-making was exotic and not a native
industry. The tradition, recorded by Pliny (_Nat. Hist._ xxxvi. 65),
assigns the discovery of glass to Syria, and the geographical position
of that country, its forests as a source of fuel, and its deposits of
sand add probability to the tradition. The story that Phoenician
merchants found a glass-like substance under their cooking pots, which
had been supported on blocks of natron, need not be discarded as pure
fiction. The fire may well have caused the natron, an impure form of
carbonate of soda, to combine with the surrounding sand to form silicate
of soda, which, although not a permanent glass, is sufficiently
glass-like to suggest the possibility of creating a permanent
transparent material. Moreover, Pliny (xxxvi. 66) actually records the
discovery which effected the conversion of deliquescent silicate of soda
into permanent glass. The words are "Coeptus addi magnes lapis." There
have been many conjectures as to the meaning of the words "magnes
lapis." The material has been considered by some to be magnetic iron ore
and by others oxide of manganese. Oxides of iron and manganese can only
be used in glass manufacture in comparatively small quantities for the
purpose of colouring or neutralizing colour in glass, and their
introduction would not be a matter of sufficient importance to be
specially recorded. In chapter 25 of the same book Pliny describes five
varieties of "magnes lapis." One of these he says is found in magnesia,
is white in colour, does not attract iron and is like pumice stone. This
variety must certainly be magnesian limestone. Magnesian limestone mixed
and fused with sand and an alkaline carbonate produces a permanent
glass. The scene of the discovery of glass is placed by Pliny on the
banks of the little river Belus, under the heights of Mount Carmel,
where sand suitable for glass-making exists and wood for fuel is
abundant. In this neighbourhood fragments and lumps of glass are still
constantly being dug up, and analysis proves that the glass contains a
considerable proportion of magnesia. The district was a glass-making
centre in Roman times, and it is probable that the Romans inherited and
perfected an indigenous industry of remote antiquity. Pliny has so
accurately recorded the stages by which a permanent glass was developed
that it may be assumed that he had good reason for claiming for Syria
the discovery of glass. Between Egypt and Syria there was frequent
intercourse both of conquest and commerce. It was customary for the
victor after a successful raid to carry off skilled artisans as
captives. It is recorded that Tahutmes III. sent Syrian artisans to
Egypt. Glass-blowers may have been amongst their captive craftsmen, and
may have started the industry in Egypt. The claims of Syria and Egypt
are at the present time so equally balanced that it is advisable to
regard the question of the birthplace of the glass industry as one that
has still to be settled.
The "primitive" vessels which have been found in Egypt are small in size
and consist of columnar stibium jars, flattened bottles and amphorae,
all decorated with zigzag lines, tiny wide-mouthed vases on feet and
minute jugs. The vessels of later date which have been found in
considerable quantities, principally in the coast towns and islands of
the Mediterranean, are amphorae and alabastra, also decorated with
zigzag lines. The amphorae (Plate I. figs. 1 and 2) terminate with a
point, or with an unfinished extension from the terminal point, or with
a knob. The alabastra have short necks, are slightly wider at the base
than at the shoulder and have rounded bases. Dr Petrie has called
attention to two technical peculiarities to be found in almost every
specimen of early glass-ware. The inner surface is roughened (Plate I.
fig. 4 c), and has particles of sand adhering to it, as if the vessel
had been filled with sand and subjected to heat, and the inside of the
neck has the impression of a metal rod (Plate I. fig. 4 a), which
appears to have been extracted from the neck with difficulty. From this
evidence Dr Petrie has assumed that the vessels were not blown, but
formed upon a core of sandy paste, modelled upon a copper rod, the rod
being the core of the neck (see EGYPT: _Art and Archaeology_). The
evidence, however, hardly warrants the abandonment of the simple process
of blowing in favour of a process which is so difficult that it may
almost be said to be impossible, and of which there is no record or
tradition except in connexion with the manufacture of small beads. The
technical difficulties to which Dr Petrie has called attention seem to
admit of a somewhat less heroic explanation. A modern glass-blower, when
making an amphora-shaped vase, finishes the base first, fixes an iron
rod to the finished base with a seal of glass, severs the vase from the
blowing iron, and finishes the mouth, whilst he holds the vase by the
iron attached to its base. The "primitive" glass-worker reversed this
process. Having blown the body of the vase, he finished the mouth and
neck part, and fixed a small, probably hollow, copper rod inside the
finished neck by pressing the neck upon the rod (Plate I. fig. 4 b).
Having severed the body of the vase from the blowing iron, he heated and
closed the fractured base, whilst holding the vase by means of the rod
fixed in the neck. Nearly every specimen shows traces of the pressure of
a tool on the outside of the neck, as well as signs of the base having
been closed by melting. Occasionally a knob or excrescence, formed by
the residue of the glass beyond the point at which the base has been
pinched together, remains as a silent witness of the process.
If glass-blowing had been a perfectly new invention of Graeco-Egyptian
or Roman times, some specimens illustrating the transition from
core-moulding to blowing must have been discovered. The absence of
traces of the transition strengthens the supposition that the revolution
in technique merely consisted in the discovery that it was more
convenient to finish the base of a vessel before its mouth, and such a
revolution would leave no trace behind. The roughened inner surface and
the adhering particles of sand may also be accounted for. The vessels,
especially those in which many differently coloured glasses were
incorporated, required prolonged annealing. It is probable that when the
metal rod was withdrawn the vessel was filled with sand, to prevent
collapse, and buried in heated ashes to anneal. The greater the heat of
the ashes the more would the sand adhere to and impress the inner
surface of the vessels. The decoration of zigzag lines was probably
applied directly after the body of the vase had been blown. Threads of
coloured molten glass were spirally coiled round the body, and, whilst
still viscid, were dragged into zigzags with a metal hook.
_Egypt_.--The glass industry flourished in Egypt in Graeco-Egyptian and
Roman times. All kinds of vessels were blown, both with and without
moulds, and both moulding and cutting were used as methods of
decoration. The great variety of these vessels is well shown in the
illustrated catalogue of Graeco-Egyptian glass in the Cairo museum,
edited by C. C. Edgar.
Another species of glass manufacture in which the Egyptians would appear
to have been peculiarly skilled is the so-called mosaic glass, formed by
the union of rods of various colours in such a manner as to form a
pattern; the rod so formed was then reheated and drawn out until reduced
to a very small size, 1 sq. in. or less, and divided into tablets by
being cut transversely, each of these tablets presenting the pattern
traversing its substance and visible on each face. This process was no
doubt first practised in Egypt, and is never seen in such perfection as
in objects of a decidedly Egyptian character. Very beautiful pieces of
ornament of an architectural character are met with, which probably once
served as decorations of caskets or other small pieces of furniture or
of trinkets; also tragic masks, human faces and birds. Some of the
last-named are represented with such truth of colouring and delicacy of
detail that even the separate feathers of the wings and tail are well
distinguished, although, as in an example in the British Museum, a
human-headed hawk, the piece which contains the figure may not exceed
3/4 in. in its largest dimension. Works of this description probably
belong to the period when Egypt passed under Roman domination, as
similar objects, though of inferior delicacy, appear to have been made
in Rome.
_Assyria_.--Early Assyrian glass is represented in the British Museum by
a vase of transparent greenish glass found in the north-west palace of
Nineveh. On one side of this a lion is engraved, and also a line of
cuneiform characters, in which is the name of Sargon, king of Assyria,
722 B.C. Fragments of coloured glasses were also found there, but our
materials are too scanty to enable us to form any decided opinion as to
the degree of perfection to which the art was carried in Assyria. Many
of the specimens discovered by Layard at Nineveh have all the appearance
of being Roman, and were no doubt derived from the Roman colony, Niniva
Claudiopolis, which occupied the same site.
[Illustration: PLATE I.
FIG. 1.
FIG. 2.
FIG. 3.
FIG. 4.
FIG. 5.
FIG. 6.
FIG. 7.
FIG. 8.
FIG. 9.
FIG. 10.]
[Illustration: PLATE II.
FIG. 11.
FIG. 12.
FIG. 13.
FIG. 14.]
_Roman Glass_.--In the first centuries of our era the art of
glass-making was developed at Rome and other cities under Roman rule in
a most remarkable manner, and it reached a point of excellence which in
some respects has never been excelled or even perhaps equalled. It may
appear a somewhat exaggerated assertion that glass was used for more
purposes, and in one sense more extensively, by the Romans of the
imperial period than by ourselves in the present day; but it is one
which can be borne out by evidence. It is true that the use of glass for
windows was only gradually extending itself at the time when Roman
civilization sank under the torrent of German and Hunnish barbarism, and
that its employment for optical instruments was only known in a
rudimentary stage; but for domestic purposes, for architectural
decoration and for personal ornaments glass was unquestionably much more
used than at the present day. It must be remembered that the Romans
possessed no fine porcelain decorated with lively colours and a
beautiful glaze; Samian ware was the most decorative kind of pottery
which was then made. Coloured and ornamental glass held among them much
the same place for table services, vessels for toilet use and the like,
as that held among us by porcelain. Pliny (_Nat. Hist._ xxxvi. 26, 67)
tells us that for drinking vessels it was even preferred to gold and
silver.
Glass was largely used in pavements, and in thin plates as a coating for
walls. It was used in windows, though by no means exclusively, mica,
alabaster and shells having been also employed. Glass, in flat pieces,
such as might be employed for windows, has been found in the ruins of
Roman houses, both in England and in Italy, and in the house of the faun
at Pompeii a small pane in a bronze frame remains. Most of the pieces
have evidently been made by casting, but the discovery of fragments of
sheet-glass at Silchester proves that the process of making sheet-glass
was known to the Romans. When the window openings were large, as was the
case in basilicas and other public buildings, and even in houses, the
pieces of glass were, doubtless, fixed in pierced slabs of marble or in
frames of wood or bronze. The Roman glass-blowers were masters of all
the ordinary methods of manipulation and decoration. Their craftsmanship
is proved by the large cinerary urns, by the jugs with wide, deeply
ribbed, scientifically fixed handles, and by vessels and vases as
elegant in form and light in weight as any that have been since produced
at Murano. Their moulds, both for blowing hollow vessels and for
pressing ornaments, were as perfect for the purposes for which they were
intended as those of the present time. Their decorative cutting (Plate
I. figs. 5 and 6), which took the form of simple, incised lines, or
bands of shallow oval or hexagonal hollows, was more suited to the
material than the deep prismatic cutting of comparatively recent times.
The Romans had at their command, of transparent colours, blue, green,
purple or amethystine, amber, brown and rose; of opaque colours, white,
black, red, blue, yellow, green and orange. There are many shades of
transparent blue and of opaque blue, yellow and green. In any large
collection of fragments it would be easy to find eight or ten varieties
of opaque blue, ranging from lapis lazuli to turquoise or to lavender
and six or seven of opaque green. Of red the varieties are fewer; the
finest is a crimson red of very beautiful tint, and there are various
gradations from this to a dull brick red. One variety forms the ground
of a very good imitation of porphyry; and there is a dull
semi-transparent red which, when light is passed through it, appears to
be of a dull green hue. With these colours the Roman _vitrarius_ worked,
either using them singly or blending them in almost every conceivable
combination, sometimes, it must be owned, with a rather gaudy and
inharmonious effect.
The glasses to which the Venetians gave the name "mille fiori" were
formed by arranging side by side sections of glass cane, the canes
themselves being built up of differently coloured rods of glass, and
binding them together by heat. A vast quantity of small cups and paterae
were made by this means in patterns which bear considerable resemblance
to the surfaces of madrepores. In these every colour and every shade of
colour seem to have been tried in great variety of combination with
effects more or less pleasing, but transparent violet or purple appears
to have been the most common ground colour. Although most of the vessels
of this mille fiori glass were small, some were made as large as 20 in.
in diameter. Imitations of natural stones were made by stirring together
in a crucible glasses of different colours, or by incorporating
fragments of differently coloured glasses into a mass of molten glass by
rolling. One variety is that in which transparent brown glass is so
mixed with opaque white and blue as to resemble onyx. This was sometimes
done with great success, and very perfect imitations of the natural
stone were produced. Sometimes purple glass is used in place of brown,
probably with the design of imitating the precious murrhine. Imitations
of porphyry, of serpentine, and of granite are also met with, but these
were used chiefly in pavements, and for the decoration of walls, for
which purposes the onyx-glass was likewise employed.
The famous cameo glass was formed by covering a mass of molten glass
with one or more coatings of a differently coloured glass. The usual
process was to gather, first, a small quantity of opaque white glass; to
coat this with a thick layer of translucent blue glass; and, finally, to
cover the blue glass with a coating of the white glass. The outer coat
was then removed from that portion which was to constitute the ground,
leaving the white for the figures, foliage or other ornamentation; these
were then sculptured by means of the gem-engraver's tools. Pliny no
doubt means to refer to this when he says (_Nat. Hist._ xxxvi. 26. 66),
"aliud argenti modo caelatur," contrasting it with the process of
cutting glass by the help of a wheel, to which he refers in the words
immediately preceding, "aliud torno teritur."
The Portland or Barberini vase in the British Museum is the finest
example of this kind of work which has come down to us, and was entire
until it was broken into some hundred pieces by a madman. The pieces,
however, were joined together by Mr Doubleday with extraordinary skill,
and the beauty of design and execution may still be appreciated. The two
other most remarkable examples of this cameo glass are an amphora at
Naples and the Auldjo vase. The amphora measures 1 ft. 5/8 in. in
height, 1 ft. 7-1/2 in. in circumference; it is shaped like the earthern
amphoras with a foot far too small to support it, and must no doubt have
had a stand, probably of gold; the greater part is covered with a most
exquisite design of garlands and vines, and two groups of boys gathering
and treading grapes and playing on various instruments of music; below
these is a line of sheep and goats in varied attitudes. The ground is
blue and the figures white. It was found in a house in the Street of
Tombs at Pompeii in the year 1839, and is now in the Royal Museum at
Naples. It is well engraved in Richardson's _Studies of Ornamental
Design_. The Auldjo vase, in the British Museum, is an oenochoe about 9
in. high; the ornament consists mainly of a most beautiful band of
foliage, chiefly of the vine, with bunches of grapes; the ground is blue
and the ornaments white; it was found at Pompeii in the house of the
faun. It also has been engraved by Richardson. The same process was used
in producing large tablets, employed, no doubt, for various decorative
purposes. In the South Kensington Museum is a fragment of such a tablet
or slab; the figure, a portion of which remains, could not have been
less than about 14 in. high. The ground of these cameo glasses is most
commonly transparent blue, but sometimes opaque blue, purple or dark
brown. The superimposed layer, which is sculptured, is generally opaque
white. A very few specimens have been met with in which several colours
are employed.
At a long interval after these beautiful objects come those vessels
which were ornamented either by means of coarse threads trailed over
their surfaces and forming rude patterns, or by coloured enamels merely
placed on them in lumps; and these, doubtless, were cheap and common
wares. But a modification of the first-named process was in use in the
4th and succeeding centuries, showing great ingenuity and manual
dexterity,--that, namely, in which the added portions of glass are
united to the body of the cup, not throughout, but only at points, and
then shaped either by the wheel or by the hand (Plate I. fig. 3). The
attached portions form in some instances inscriptions, as on a cup
found at Strassburg, which bears the name of the emperor Maximian (A.D.
286-310), on another in the Vereinigte Sammlungen at Munich, and on a
third in the Trivulzi collection at Milan, where the cup is white, the
inscription green and the network blue. Probably, however, the finest
example is a situla, 10-1/2 in. high by 8 in. wide at the top and 4 in.
at the bottom, preserved in the treasury of St Mark at Venice. This is
of glass of a greenish hue; on the upper part is represented, in relief,
the chase of a lion by two men on horseback accompanied by dogs; the
costume appears to be Byzantine rather than Roman, and the style is very
bad. The figures are very much undercut. The lower part has four rows of
circles united to the vessel at those points alone where the circles
touch each other. All the other examples have the lower portion covered
in like manner by a network of circles standing nearly a quarter of an
inch from the body of the cup. An example connected with the specimens
just described is the cup belonging to Baron Lionel de Rothschild;
though externally of an opaque greenish colour, it is by transmitted
light of a deep red. On the outside, in very high relief, are figures of
Bacchus with vines and panthers, some portions being hollow from within,
others fixed on the exterior. The changeability of colour may remind us
of the "calices versicolores" which Hadrian sent to Servianus.
So few examples of glass vessels of this period which have been painted
in enamel have come down to us that it has been questioned whether that
art was then practised; but several specimens have been described which
can leave no doubt on the point; decisive examples are afforded by two
cups found at Vaspelev, in Denmark, engravings of which are published in
the _Annaler for Nordisk Oldkyndeghed_ for 1861, p. 305. These are small
cups, 3 in. and 2-1/2 in. high, 3-3/4 in. and 3 in. wide, with feet and
straight sides; on the larger are a lion and a bull, on the smaller two
birds with grapes, and on each some smaller ornaments. On the latter are
the letters DVB. R. The colours are vitrified and slightly in relief;
green, blue and brown may be distinguished. They are found with Roman
bronze vessels and other articles.
The art of glass-making no doubt, like all other art, deteriorated
during the decline of the Roman empire, but it is probable that it
continued to be practised, though with constantly decreasing skill, not
only in Rome but in the provinces. Roman technique was to be found in
Byzantium and Alexandria, in Syria, in Spain, in Germany, France and
Britain.
_Early Christian and Byzantine Glass_.--The process of embedding gold
and silver leaf between two layers of glass originated as early as the
1st century, probably in Alexandria. The process consisted in spreading
the leaf on a thin film of blown glass and pressing molten glass on to
the leaf so that the molten glass cohered with the film of glass through
the pores of the metallic leaf. If before this application of the molten
glass the metallic leaf, whilst resting on the thin film of blown glass,
was etched with a sharp point, patterns, emblems, inscriptions and
pictures could be embedded and rendered permanent by the double coating
of glass. The plaques thus formed could be reheated and fashioned into
the bases of bowls and drinking vessels. In this way the so-called
"fondi d'oro" of the catacombs in Rome were made. They are the broken
bases of drinking vessels containing inscriptions, emblems, domestic
scenes and portraits etched in gold leaf. Very few have any reference to
Christianity, but they served as indestructible marks for indicating the
position of interments in the catacombs. The fondi d'oro suggested the
manufacture of plaques of gold which could be broken up into tesserae
for use in mosaics.
Some of the Roman artificers in glass no doubt migrated to
Constantinople, and it is certain that the art was practised there to a
very great extent during the middle ages. One of the gates near the port
took its name from the adjacent glass houses. St Sofia when erected by
Justinian had vaults covered with mosaics and immense windows filled
with plates of glass fitted into pierced marble frames; some of the
plates, 7 to 8 in. wide and 9 to 10 in. high, not blown but cast, which
are in the windows may possibly date from the building of the church.
It is also recorded that pierced silver disks were suspended by chains
and supported glass lamps "wrought by fire." Glass for mosaics was also
largely made and exported. In the 8th century, when peace was made
between the caliph Walid and the emperor Justinian II., the former
stipulated for a quantity of mosaic for the decoration of the new mosque
at Damascus, and in the 10th century the materials for the decoration of
the niche of the kibla at Cordova were furnished by Romanus II. In the
11th century Desiderius, abbot of Monte Casino, sent to Constantinople
for workers in mosaic.
We have in the work of the monk Theophilus, _Diversarum artium
schedula_, and in the probably earlier work of Eraclius, about the 11th
century, instructions as to the art of glass-making in general, and also
as to the production of coloured and enamelled vessels, which these
writers speak of as being practised by the Greeks. The only entire
enamelled vessel which we can confidently attribute to Byzantine art is
a small vase preserved in the treasury of St Mark's at Venice. This is
decorated with circles of rosettes of blue, green and red enamel, each
surrounded by lines of gold; within the circles are little figures
evidently suggested by antique originals, and precisely like similar
figures found on carved ivory boxes of Byzantine origin dating from the
11th or 12th century. Two inscriptions in Cufic characters surround the
vase, but they, it would seem, are merely ornamental and destitute of
meaning. The presence of these inscriptions may perhaps lead to the
inference that the vase was made in Sicily, but by Byzantine workmen.
The double-handled blue-glass vase in the British Museum, dating from
the 5th century, is probably a chalice, as it closely resembles the
chalices represented on early Christian monuments.
Of uncoloured glass brought from Constantinople several examples exist
in the treasury of St Mark's at Venice, part of the plunder of the
imperial city when taken by the crusaders in 1204. The glass in all is
greenish, very thick, with many bubbles, and has been cut with the
wheel; in some instances circles and cones, and in one the outlines of
the figure of a leopard, have been left standing up, the rest of the
surface having been laboriously cut away. The intention would seem to
have been to imitate vessels of rock crystal. The so-called "Hedwig"
glasses may also have originated in Constantinople. These are small cups
deeply and rudely cut with conventional representations of eagles, lions
and griffins. Only nine specimens are known. The specimen in the Rijks
Museum at Amsterdam has an eagle and two lions. The specimen in the
Germanic Museum at Nuremberg has two lions and a griffin.
_Saracenic Glass._--The Saracenic invasion of Syria and Egypt did not
destroy the industry of glass-making. The craft survived and flourished
under the Saracenic regime in Alexandria, Cairo, Tripoli, Tyre, Aleppo
and Damascus. In inventories of the 14th century both in England and in
France mention may frequently be found of glass vessels of the
manufacture of Damascus. A writer in the early part of the 15th century
states that "glass-making is an important industry at Haleb (Aleppo)."
Edward Dillon (_Glass_, 1902) has very properly laid stress on the
importance of the enamelled Saracenic glass of the 13th, 14th and 15th
centuries, pointing out that, whereas the Romans and Byzantine Greeks
made some crude and ineffectual experiments in enamelling, it was under
Saracenic influence that the processes of enamelling and gilding on
glass vessels were perfected. An analysis of the glass of a Cairene
mosque lamp shows that it is a soda-lime glass and contains as much as
4% of magnesia. This large proportion of magnesia undoubtedly supplied
the stability required to withstand the process of enamelling. The
enamelled Saracenic glasses take the form of flasks, vases, goblets,
beakers and mosque lamps. The enamelled decoration on the lamps is
restricted to lettering, scrolls and conventional foliage; on other
objects figure-subjects of all descriptions are freely used. C. H. Read
has pointed out a curious feature in the construction of the enamelled
beakers. The base is double but the inner lining has an opening in the
centre. Dillon has suggested that this central recess may have served to
support a wick. It is possible however, that it served no useful
purpose, but that the construction is a survival from the manufacture of
vessels with fondi d'oro. The bases containing the embedded gold leaf
must have been welded to the vessels to which they belonged, in the same
way as the bases are welded to the Saracenic beakers. The enamelling
process was probably introduced in the early part of the 13th century;
most of the enamelled mosque lamps belong to the 14th century.