é GLASS. 3 
m fora spindle to turn the glass by. The glass being to which was added pear! ashes, or an alkali of some Flint Glas. 
* now thin, soon becomes so cool as to require ing, kind, to serve as a flux, and a small quantity of arsenic, “Vv” 
" which is done by presenting the open end to the flame, The materials now used for the finest flint glass, are, Materials. 
ing the workman resting ‘the rod in the hook, and turn- 
* ing it slowly round, that the glass may become equal- 
; iy heated. When the open end is sufficiently sofas 
ed, the workman, supporting the rod over a rest or 
hook, turns the glass steadily round. The motion opens 
_ and enlarges the aperture to the form of Fig: 5; and 
» they take great care, by turning. with a regular mo- 
tion, to e the circular figure, such as is represent- 
ed in Fig. 6. In this state, being whirled rapidly 
round, the centrifugal force throws it out into a flat 
circular plate a6, of from 3 feet 6 inches to 4 feet dia- 
meter. ‘ sila leans : 
When the Sve is sufficiently cooled, the workman 
applies a cold iron to crack the neck of the central pro- 
jection which connects the plate with the rod, and he 
‘then lays the plate flat down upon a bed of ashes pre- 
-viously prepared, The rod or spindle is now separated 
by a gentle stroke, and the plate is taken up upon an 
iron tork, and conveyed to the annealing oven, where 
- a great number are set-up edgewise, and supported by 
iron frames to keep them flat. The fire of the oven is 
suffered to burn out, and the heat to diminish as slowly 
‘as possible, until the glasses become quite cold: They 
are then withdrawn from the furnace, by taking down 
a slight front wall built in the door or arch. The 
‘heat of the annealing oven, which is not sufficient to 
make the glass so sott as to bend by its weight, is al- 
lowed to die away. \ 
Window plate, also called German plate, or table 
lass, is made by the same means of blowing and rolling; 
t these are managed to produce a cylinder, which is 
cut open and spread flat upon a table. 
In order to form glass plates by the extension of a 
cylinder, the workman blows the glass into the shape of 
a pear, as at Fig. 7.. The length of this pear must be 
nearly equal to the length of the plate, and its diameter 
of such a size, that the circumference, when unfolded, 
-will be equal to the breadth of the plate desired. He 
now supports the blowing iron over a stool or iron bar, 
whilst an assistant, with a pointed iron, pierces a hole 
into the extreme end of the pear, opposite the end of 
the blowing iron: This opening is enlarged by intro- 
ducing the blade of a pair of spring tongs, as in Fig. 8. 
whilst the glass is turned round; and, by a peculiar 
ent of these, the end of the pear is at last. 
opened out to a cylinder, as in Fig. 9. The workman 
now mounts a stool, and holds the blowing-iron per- 
pendicularly, whilst his assistant cuts open one side of 
the cylinder with a pair of cutting shears, as in Fig. 9. 
eearowing iron is next broken off, and arod b, Fig. 10. 
ied to the end of the cylinder, to form a spindle to 
work the other end by. This rod has a flat circular 
fe upon the end of it, or three prongs c, c, c, which 
eing dipped in the melted glass, are applied to the 
first, a fine white transparent sand, which is termed the 
body of glass ; this is found often mixed with clay, &c. 
from which it is freed by washing, until the water 
comes quite clear off; it is then calcined, and after- 
wards sifted through a very fine sieve of from 40 to 50 
asses to the inch. The second article is red lead, or 
itharge, which serves as a powerful flux, gives greater 
density to glass, renders it more ductile, less apt to 
crack from sudden changes of temperature, and imparts 
to it a high refractive and reflective power. Litharge 
is preferable to red lead, as it gives a purer glass, is less 
liable to be adulterated. and acts more powerfully as a 
flux. The third ingredient is an alkali, either soda or 
potash ; the finest, pearl ashes dissolved, and the solu~ 
tion’ evaporated to drynesss, is considered the best, as 
it imparts no particular colour to the glass, whereas soda 
gives it a greenish-blue tinge. 
Nitre, the fourth ingredient, is also used as.a flux, and 
serves to correct the imperfections that arise from the 
lead being insufficiently calcined. A fifth article, viz. 
arsenic, is sometimes used to aid the fluxing ingredients; 
but the.quantity must be very small, lest the transpa-~ 
rency of the glass be hurt by the opake white colour. 
which it imparts when used largely. A sixth, and very 
important ingredient, is the black oxide of manganese, 
which is used to destroy any accidental foulness of colour 
that may arise in the glass, and particularly any tinge 
given by particles of iron amongst the sand; but while 
it destroys the green-yellow, or olive. colours in glass, 
by imparting to them a purple tinge, the mixture of 
colours produces a blacker glass, and of course greatly 
injures its transparency. 
When too much manganese is added, the purple co- 
lour may be destroyed by charcoal; but this can only 
be done partially, as the purple tinge is not visible un- 
til a perfect glass is formed, after which the charcoal 
cannot be mixed with the glass. 
When a batch of flint glass is prepared, it. is taken pugon, 
from the mixing-house to the glass-house, and then put 
into the pots in small quantities of ten or a dozen sho- 
velfuls at a time. When this is melted, which it 
does in two or three hours, more is added till the 
pot is full. The mouth of the pot is then completely 
closed, by putting soft clay round the stopper, except a 
small aperture, which is left to allow the sandover or 
glass-gall to escape. This substance consists of those 
salts that are contained in alkalies purified in the 
common mode, and which have no affinity for silex, 
and are thrown up to the top. From the glass, or metal 
as it is termed, being hotter at the’back of the pot than 
it is at the mouth or front, a small declivity takes 
place on the surface of the glass, and if the pot is full 
to the brim or mouth, it will be a little higher at the 
back part, consequently the liquid sandover runs off, 
end of the cylinder. By this rod the glass is carried to Flint glass requires about 48 hours to its complete fu- 
the fire, and the end from which the tube was broken sion, although the furnace (Plate CCLXXY. Fig. 3. a 
off is heated. It is then opened by the spring tongs, See Description of Plates) is carried to as intense a beat So 
and reduced to a cylinder of the same size as that at as possible. After_it is fixe, that is melted into li- aT ae 
the other end. The shears are next used to cut the cy- quid glass, and freed from all air-bubbles, prepara- 
linder open from this end, and leave it in the state of a tion is made for the working of the glass. For this 
sheet of paper rolled up. The cylinder is now laid upon 
smooth copper. table, where the spindle is detached, 
and the glass spread into a flat sheet, and annealed. 
Secr. V. On Flint Glass. 
ty v, 
” | Blint glass was formerly manufactured from flints cal- 
cined, and afterwards ground, well washed, and sifted, 
purpose the blower, in: order to make a common 
wine glass, takes a hollow tube of iron about four 
feet and a half long, which he heats red hot at the 
one end. He then dips it into the liquid metal con- 
tained in_ the , and takes up a quantity of glass, 
and forms a hollow ball, as described in Sect. II. He 
then sits down on a chair with two long arms, to one 
