+------------------------------+-------+-------+-------+-------+-------+------+--------+
| | | | | | | | Fe2O3 |
| | SiO2. | K2O. | PbO. | Na2O. | CaO. | MgO. | and |
| | | | | | | | Al2O3. |
+------------------------------+-------+-------+-------+-------+-------+------+--------+
| Potash-lead (flint) glass | 53.17 | 13.88 | 32.95 | .. | .. | .. | .. |
| Soda-lime (Venetian) glass | 73.40 | .. | .. | 18.58 | 5.06 | .. | 2.48 |
| Potash-lime (Bohemian) glass | 71.70 | 12.70 | .. | 2.50 | 10.30 | .. | 0.90 |
+------------------------------+-------+-------+-------+-------+-------+------+--------+
For melting the leadless glasses, open, bowl-shaped crucibles are used,
ranging from 12 to 40 in. in diameter. Glass mixtures containing lead
are melted in covered, beehive-shaped crucibles holding from 12 to 18
cwt. of glass. They have a hooded opening on one side near the top. This
opening serves for the introduction of the glass-mixture, for the
removal of the melted glass and as a source of heat for the processes of
manipulation.
The Venetian furnaces in the island of Murano are small low structures
heated with wood. The heat passes from the melting furnace into the
annealing kiln. In Germany, Austria and the United States, gas furnaces
are generally used. In England directly-heated coal furnaces are still
in common use, which in many cases are stoked by mechanical feeders.
There are two systems of annealing. The manufactured goods are either
removed gradually from a constant source of heat by means of a train of
small iron trucks drawn along a tramway by an endless chain, or are
placed in a heated kiln in which the fire is allowed gradually to die
out. The second system is especially used for annealing large and heavy
objects. The manufacture of table-ware is carried on by small gangs of
men and boys. In England each "gang" or "chair" consists of three men
and one boy. In works, however, in which most of the goods are moulded,
and where less skilled labour is required, the proportion of boy labour
is increased. There are generally two shifts of workmen, each shift
working six hours, and the work is carried on continuously from Monday
morning until Friday morning. Directly work is suspended the glass
remaining in the crucibles is ladled into water, drained and dried. It
is then mixed with the glass mixture and broken glass ("cullet"), and
replaced in the crucibles. The furnaces are driven to a white heat in
order to fuse the mixture and expel bubbles of gas and air. Before work
begins the temperature is lowered sufficiently to render the glass
viscous. In the viscous state a mass of glass can be coiled upon the
heated end of an iron rod, and if the rod is hollow can be blown into a
hollow bulb. The tools used are extremely primitive--hollow iron
blowing-rods, solid rods for holding vessels during manipulation, spring
tools, resembling sugar-tongs in shape, with steel or wooden blades for
fashioning the viscous glass, callipers, measure-sticks, and a variety
of moulds of wood, carbon, cast iron, gun-metal and plaster of Paris
(figs. 16 and 17). The most important tool, however, is the bench or
"chair" on which the workman sits, which serves as his lathe. He sits
between two rigid parallel arms, projecting forwards and backwards and
sloping slightly from back to front. Across the arms he balances the
iron rod to which the glass bulb adheres, and rolling it backwards and
forwards with the fingers of his left hand fashions the glass between
the blades of his sugar-tongs tool, grasped in his right hand. The
hollow bulb is worked into the shape it is intended to assume, partly by
blowing, partly by gravitation, and partly by the workman's tool. If the
blowing iron is held vertically with the bulb uppermost the bulb becomes
flattened and shallow, if the bulb is allowed to hang downwards it
becomes elongated and reduced in diameter, and if the end of the bulb is
pierced and the iron is held horizontally and sharply trundled, as a mop
is trundled, the bulb opens out into a flattened disk.
[Illustration: FIG. 16.--Pontils and Blowing Iron. a, Puntee; b, spring
puntee; c, blowing iron.]
[Illustration: FIG. 17.--Shaping and Measuring Tools.
d, "Sugar-tongs" tool with wooden ends.
e,e, "Sugar-tongs" tools with cutting edges.
f, Pincers.
g, Scissors.
h, Battledore.
i, Marking compass.]
During the process of manipulation, whether on the chair or whilst the
glass is being reheated, the rod must be constantly and gently trundled
to prevent the collapse of the bulb or vessel. Every natural development
of the spherical form can be obtained by blowing and fashioning by hand.
A non-spherical form can only be produced by blowing the hollow bulb
into a mould of the required shape. Moulds are used both for giving
shape to vessels and also for impressing patterns on their surface.
Although spherical forms can be obtained without the use of moulds,
moulds are now largely used for even the simplest kinds of table-ware in
order to economize time and skilled labour. In France, Germany and the
United States it is rare to find a piece of table-ware which has not
received its shape in a mould. The old and the new systems of making a
wine-glass illustrate almost all the ordinary processes of glass
working. Sufficient glass is first "gathered" on the end of a blowing
iron to form the bowl of the wine-glass. The mere act of coiling an
exact weight of molten glass round the end of a rod 4 ft. in length
requires considerable skill. The mass of glass is rolled on a polished
slab of iron, the "marvor," to solidify it, and it is then slightly
hollowed by blowing. Under the old system the form of the bowl is
gradually developed by blowing and by shaping the bulb with the
sugar-tongs tool. The leg is either pulled out from the substance of the
base of the bowl, or from a small lump of glass added to the base. The
foot starts as a small independent bulb on a separate blowing iron. One
extremity of this bulb is made to adhere to the end of the leg, and the
other extremity is broken away from its blowing iron. The fractured end
is heated, and by the combined action of heat and centrifugal force
opens out into a flat foot. The bowl is now severed from its blowing
iron and the unfinished wine-glass is supported by its foot, which is
attached to the end of a working rod by a metal clip or by a seal of
glass. The fractured edge of the bowl is heated, trimmed with scissors
and melted so as to be perfectly smooth and even, and the bowl itself
receives its final form from the sugar-tongs tool.
Under the new system the bowl is fashioned by blowing the slightly
hollowed mass of glass into a mould. The leg is formed and a small lump
of molten glass is attached to its extremity to form the foot. The
blowing iron is constantly trundled, and the small lump of glass is
squeezed and flattened into the shape of a foot, either between two
slabs of wood hinged together, or by pressure against an upright board.
The bowl is severed from the blowing iron, and the wine-glass is sent to
the annealing oven with a bowl, longer than that of the finished glass,
and with a rough fractured edge. When the glass is cold the surplus is
removed either by grinding, or by applying heat to a line scratched with
a diamond round the bowl. The fractured edge is smoothed by the impact
of a gas flame.
In the manufacture of a wine-glass the ductility of glass is illustrated
on a small scale by the process of pulling out the leg. It is more
strikingly illustrated in the manufacture of glass cane and tube. Cane
is produced from a solid mass of molten glass, tube from a mass hollowed
by blowing. One workman holds the blowing iron with the mass of glass
attached to it, and another fixes an iron rod by means of a seal of
glass to the extremity of the mass. The two workmen face each other and
walk backwards. The diameter of the cane or tube is regulated by the
weight of glass carried, and by the distance covered by the two workmen.
It is a curious property of viscous glass that whatever form is given to
the mass of glass before it is drawn out is retained by the finished
cane or tube, however small its section may be. Owing to this property,
tubes or canes can be produced with a square, oblong, oval or triangular
section. Exceedingly fine canes of milk-white glass play an important
part in the masterpieces produced by the Venetian glass-makers of the
16th century. Vases and drinking cups were produced of extreme
lightness, in the walls of which were embedded patterns rivalling
lace-work in fineness and intricacy. The canes from which the patterns
are formed are either simple or complex. The latter are made by dipping
a small mass of molten colourless glass into an iron cup around the
inner wall of which short lengths of white cane have been arranged at
regular intervals. The canes adhere to the molten glass, and the mass
is first twisted and then drawn out into fine cane, which contains white
threads arranged in endless spirals. The process can be almost
indefinitely repeated and canes formed of extreme complexity. A vase
decorated with these simple or complex canes is produced by embedding
short lengths of the cane on the surface of a mass of molten glass and
blowing and fashioning the mass into the required shape.
Table-ware and vases may be wholly coloured or merely decorated with
colour. Touches of colour may be added to vessels in course of
manufacture by means of seals of molten glass, applied like sealing-wax;
or by causing vessels to wrap themselves round with threads or coils of
coloured glass. By the application of a pointed iron hook, while the
glass is still ductile, the parallel coils can be distorted into bends,
loops or zigzags. The surface of vessels may be spangled with gold or
platinum by rolling the hot glass on metallic leaf, or iridescent, by
the deposition of metallic tin, or by the corrosion caused by the
chemical action of acid fumes. Gilding and enamel decoration are applied
to vessels when cold, and fixed by heat.
_Cutting_ and _engraving_ are mechanical processes for producing
decorative effects by abrading the surface of the glass when cold. The
abrasion is effected by pressing the glass against the edge of wheels,
or disks, of hard material revolving on horizontal spindles. The
spindles of cutting wheels are driven by steam or electric power. The
wheels for making deep cuts are made of iron, and are fed with sand and
water. The wheels range in diameter from 18 in. to 3 in. Wheels of
carborundum are also used. Wheels of fine sandstone fed with water are
used for making slighter cuts and for smoothing the rough surface left
by the iron wheels. Polishing is effected by wooden wheels fed with wet
pumice-powder and rottenstone and by brushes fed with moistened
putty-powder. Patterns are produced by combining straight and curved
cuts. Cutting brings out the brilliancy of glass, which is one of its
intrinsic qualities. At the end of the 18th century English cut glass
was unrivalled for design and beauty. Gradually, however, the process
was applied without restraint and the products lost all artistic
quality. At the present time cut glass is steadily regaining favour.
_Engraving_ is a process of drawing on glass by means of small copper
wheels. The wheels range from 1/2 in. to 2 in. in diameter, and are fed
with a mixture of fine emery and oil. The spindles to which the wheels
are attached revolve in a lathe worked by a foot treadle. The true use
of engraving is to add interest to vessels by means of coats of arms,
crests, monograms, inscriptions and graceful outlines. The improper use
of engraving is to hide defective material. There are two other
processes of marking patterns on glass, but they possess no artistic
value. In the "sandblast" process the surface of the glass is exposed to
a stream of sharp sand driven by compressed air. The parts of the
surface which are not to be blasted are covered by adhesive paper. In
the "etching" process the surface of the glass is etched by the chemical
action of hydrofluoric acid, the parts which are not to be attacked
being covered with a resinous paint. The glass is first dipped in this
protective liquid, and when the paint has set the pattern is scratched
through it with a sharp point. The glass is then exposed to the acid.
_Glass stoppers_ are fitted to bottles by grinding. The mouth of the
bottle is ground by a revolving iron cone, or mandrel, fed with sand and
water and driven by steam. The head of the stopper is fastened in a
chuck and the peg is ground to the size of the mouth of the bottle by
means of sand and water pressed against the glass by bent strips of thin
sheet iron. The mouth of the bottle is then pressed by hand on the peg
of the stopper, and the mouth and peg are ground together with a medium
of very fine emery and water until an air-tight joint is secured.
The revival in recent years of the craft of glass-blowing in England
must be attributed to William Morris and T.G. Jackson, R.A. (Pl. II.
figs. 11 and 12). They, at any rate, seem to have been the first to
grasp the idea that a wine-glass is not merely a bowl, a stem and a
foot, but that, whilst retaining simplicity of form, it may nevertheless
possess decorative effect. They, moreover, suggested the introduction
for the manufacture of table-glass of a material similar in texture to
that used by the Venetians, both colourless and tinted.
The colours previously available for English table-glass were ruby,
canary-yellow, emerald-green, dark peacock-green, light peacock-blue,
dark purple-blue and a dark purple. About 1870 the "Jackson" table-glass
was made in a light, dull green glass. The dull green was followed
successively by amber, white opal, blue opal, straw opal, sea-green,
horn colour and various pale tints of soda-lime glass, ranging from
yellow to blue. Experiments were also tried with a violet-coloured
glass, a violet opal, a transparent black and with glasses shading from
red to blue, red to amber and blue to green.
In the Paris Exhibition of 1900 surface decoration was the prominent
feature of all the exhibits of table-glass. The carved or "cameo" glass,
introduced by Thomas Webb of Stourbridge in 1878, had been copied with
varying success by glass-makers of all nations. In many specimens there
were three or more layers of differently coloured glass, and curious
effects of blended colour were obtained by cutting through, or partly
through, the different layers. The surface of the glass had usually been
treated with hydrofluoric acid so as to have a satin-like gloss. Some
vases of this character, shown by Emile Galle and Daum Freres of Nancy,
possessed considerable beauty. The "Favrile" glass of Louis C. Tiffany
of New York (Pl. II. fig. 13) owes its effect entirely to surface colour
and lustre. The happiest specimens of this glass almost rival the wings
of butterflies in the brilliancy of their iridescent colours. The vases
of Karl Koepping of Berlin are so fantastic and so fragile that they
appear to be creations of the lamp rather than of the furnace. An
illustration is also given of some of Powell's "Whitefriars" glass,
shown at the St Louis Exhibition, 1904 (Pl. II. fig. 14). The specimens
of "pate de verre" exhibited by A. L. Dammouse, of Sevres, in the Musee
des Arts decoratifs in Paris, and at the London Franco-British
Exhibition in 1908, deserve attention. They have a semi-opaque body with
an "egg-shell" surface and are delicately tinted with colour. The shapes
are exceedingly simple, but some of the pieces possess great beauty. The
material and technique suggest a close relationship to porcelain.
(B) _Tube._--The process of making tube has already been described.
Although the bore of the thermometer-tube is exceedingly small, it is
made in the same way as ordinary tube. The white line of enamel, which
is seen in some thermometers behind the bore, is introduced before the
mass of glass is pulled out. A flattened cake of viscous glass-enamel is
welded on to one side of the mass of glass after it has been hollowed by
blowing. The mass, with the enamel attached, is dipped into the crucible
and covered with a layer of transparent glass; the whole mass is then
pulled out into tube. If the section of the finished tube is to be a
triangle, with the enamel and bore at the base, the molten mass is
pressed into a V-shaped mould before it is pulled out.
In modern thermometry instruments of extreme accuracy are required, and
researches have been made, especially in Germany and France, to
ascertain the causes of variability in mercurial thermometers, and how
such variability is to be removed or reduced. In all mercurial
thermometers there is a slight depression of the ice-point after
exposure to high temperatures; it is also not uncommon to find that the
readings of two thermometers between the ice- and boiling-points fail to
agree at any intermediate temperature, although the ice- and
boiling-points of both have been determined together with perfect
accuracy, and the intervening spaces have been equally divided. It has
been proved that these variations depend to a great extent on the
chemical nature of the glass of which the thermometer is made. Special
glasses have therefore been produced by Tonnelot in France and at the
Jena glass-works in Germany expressly for the manufacture of
thermometers for accurate physical measurements; the analyses of these
are shown in Table III.