GLASS. 
851 
Plate Nat. Hist. fig. Cl 4. The seticgallensis 
is found near the Senegal, and in several 
parts of Siberia: and the hscvia is to be met 
with only in Germany'. 
GLASS, a transparent, brittle, factitious 
Lodv, produced bv the action of fire upon a 
fixed salt and sand, or stone, that readily 
melts. 
The fixed alkalies have a strong affinity 
for several of the earths, particularly for silica 
and alumina, which they dissolve in consi- 
derable quantity, when assisted by iieat. 
When a strong heat is applied to a mixture 
of fixed alkali and silica, if melts, and forms 
a transparent mass well known by the name 
of glass. 
The method of making this useful com- 
pound was known at an early period. Accord- 
ing to Pliny, the discovery was owing to an 
accident. Some merchants, with a ship-load 
of soda from Egypt, had cast anchor at the 
mouth of the river Belus in Phoenicia, and 
were dressing their dinner on the sand. 
They made use of large lumps of soda to 
Support their kettle*, and lighted fires under 
them. The heat melted the soda and the 
siliceous sand together, and the result was 
glass. 
For some time after this accidental disco- 
very, the manufacture of glass was confined 
to the river Belus. This manufacture seems 
to have been carried to a considerable de- 
gree of perfection among the antients. They 
mention drinking-glasses, glass prisms, and 
coloured glasses of various kinds. But per- 
fectly transparent glass was considered as 
very valuable; for Nero gave 50,000/. for 
two glass cups with handles; a proof that 
their processes must have been far less per- 
fect than ours. It was usual for them to melt 
the materials of their glass into a black mass 
called ammonitrum, of which statues were 
sometimes made. This ammonitrum was 
again melted and purified by refiners. Glass 
panes seem to have been first used in win- 
dows in the third century ; but they did not 
come into common use till long after. 
The materials employed in the manufac- 
tory of glass may be reduced under three 
classes, namely, alkalies, earths, and metallic 
oxides. 
The fixed alkalies may be employed in- 
differently; but soda is preferred in this 
country. The soda of commerce is usually 
mixed with common salt, and combined w ith 
carbonic acid. It is proper to purify it from 
both of these foreign bodies before using it. 
This, however, is seldom done. 
Tiie earths are silica, lime, and sometimes 
a little alumina. Silica constitutes the basis 
of glass. It is employed in the state of line 
sand or Hints; and sometimes for making 
very fine glass, rock crystal is employed. 
When sand is used, it ought if possible to be 
perfectly white; for when it is coloured with 
metallic oxides, the transparency of the glass 
is injured. Such sand can only be employed 
for very coarse glasses. It is necessary to 
free the sand from all the loose earthy par- 
ticles with which it may be mixed, which is 
done by washing it well with water. 
Lime renders glass less brittle, and en- 
ables it to withstand better the action of the 
atmosphere. It ought in no case to exceed 
the twentieth part of the silica employed, 
etlierwise it corrodes the glass pots. This 
indeed may be prevented hv throwing a little 
clay into the melted glass; but in that case 
a green glass only is obtained. 
The metallic oxides employed are the red 
oxide of lead or litharge, and the white. oxide 
of arsenic. The red oxide of lead, when 
it; hardness is very considerable ; its specific 
gravity varies from 2.3 to 4, according to 
the proportion of metallic oxide which it 
contains. When cold it is brittle; but at a 
red heat it is one of the most ductile bodies 
mown, and may be drawn out into threads 
added in sufficient quantity, enters into fusion | su fine as to be scarcely visible to the naked 
with silica, and forms a glass without the ad- ; eve. (See Ductility.) It is almost perfect - 
dition of any other ingredient. Five parts of b elastic, and of Course is one of the mod 
minium and two of silica form a glass of an ~~ ' 
orange-colour and full of stria:. Its specific 
gravity is 5. The red oxide of lead renders- 
glass less brittle and more fusible; but when 
added beyond a certain proportion, it injures 
the transparency and the whiteness of glass. 
'Fhe white oxide of arsenic answers the 
same purposes with that of lead; but on ac- 
count of its poisonous qualities it is seldom 
used. U is customary to add a little nitre to 
the white oxide of arsenic, to prevent the 
heat from reviving it, and rendering it vola- 
tile., When added beyond a certain propor- 
tion, it renders glass opaque and milky like 
the dial-plate of a watch. When any com- 
bustible body is present, it is usual in some 
manufactures to add a little white oxide of 
arsenic. This supplying oxygen, the com- 
bustible is burnt, and flies off; while the re- 
revived arsenic is at the same time volati- 
lized. 
After mixing the alkali and sand together, 
it is usual to expose them for some time to a 
moderate heat. This serves several pur- 
poses. It drives off all combustible bodies 
which may happen to be mixed with the 
sand ; it produces a commencement of com- 
bination which makes the glass afterwards 
less liable to corrode the clay pots in which 
it is melted; and the alkali, by this incipient 
combination, is not so apt to be volatilized, 
which might be the case if the materials were 
exposed at once to a violent heat. The mix- 
ture, after being thus heated, is called the 
frit. Through the domes in which the frit is 
heated, it is usual to see very thin bubbles 
of glass passing; a proof that some of the 
materials are volatilized during this first part 
of the process. 
The hit, while still hot, is introduced into 
large pots made of a mixture of pure clay 
and baked clay, and exposed to a heat suf- 
ficient to melt it completely. The fusion 
must he continued till the effervescence oc- 
casioned by (he separation of the carbonic 
acid from the soda lias subsided, and the 
opaque scum, known by the name of glass- 
sonorous of bodies. There are but few che- 
mical agents which base any action on it. 
Fluoric acid dissolves it with' great rapidity, 
and so do the fixed alkalies when assisted 
by heat. Dr. Priestley has shown also, that 
the long-continued action of hot water is ca- 
pable of decomposing it; a discovery which 
explains sufficiently the siliceous earth ob- 
tained by Boyle and Margralf, when they 
subjected water to tedious distillations in 
glass vessels. 
There are different kinds of glass manu- 
factured for different purposes : the principal 
of these are Hint glass, crown glass, and bot- 
tle glass. 
Flint glass is formed of soda", pounded flints) 
and oxide of lead. If is the densest, most 
transparent, and' most beautiful glass, and is 
often called crystal. Crown glass contains 
no lead. It is composed of soda and line 
sand. This kind is used for the panes of 
windows. Bottle glass is the coarsest of alh 
It is composed of kelp, or the refuse of soap 1 
boilers, and common sand. Its green colour 
is owing to the presence of iron. Of these 
species the most fusible is Hint glass, and the 
least fusible bottle glass. According to the 
experiments of Saussure, flint glass melts at 
the temperature of 19° Wedgewood, crown 
glass at 30°, and bottle glass at 47°. 
Glass is often tinged of various colours bv 
mixing with it while in fusion some one or 
other of the metallic oxides ; and on this pro- 
cess, "well conducted, depends the formation 
of pastes or factitious gems. 
Blue glass is formed by means of oxide of 
cobalt. 
Green, by the oxide of iron or of copper. 
Violet, by oxide of manganesq. 
Red, by a mixture of the oxides of copper 
and iron. 
Purple, by the purple oxide of gold. 
White, by t lie oxide of arsenic and of zinc. 
Yellow’, by the oxide of silver and By com- 
bustible bodies. 
Opticians, who employ glass for optical 
instruments, often Complain ot the many de- 
fects under which it labours. Tie chief of 
;all, which collects on the surface of the j these are the following : 
glass, must be removed. '1 Ins scum is oc- 
casioned by the common salt and other fo- 
reign bodies which are always mixed with 
the soda of commerce. When the fusion has 
been continued the proper lime, the furnace 
is allowed to cool a little, in that state the 
glass is exceedingly ductile, and readily as- 
sumes any shape that the workman pleases. 
It the glass vessels, after being formed, 
were cooled rapidly, they would contract 
unequally, and become in consequence so 
brittle as to fall to pieces whenever they were 
handled. To prevent this inconvenience, 
they are put into a large red-hot furnace, 
which is allowed to cool very slowly to the 
temperature of the air. This process is call- 
ed annealing. 
'1 lie properties that distinguish good glass 
are well known. It is perfectly transparent; 
5 Vs 
Streaks. These are waved lines, often vi- 
sible in glass, which interrupt distinct vision. 
They are probably owing sometimes to want 
of complete fusion, which prevents the differ- 
ent materials from combining sufficiently; 
but in some cases also they may be produced 
by the workmen lifting up, at two different 
times, the glass which is to go to the forma- 
tion, of one vessel or instrument. 
Tears. These are white specks or knots, 
occasioned by the vitrified clay of the fur- 
naces, or by the presence of some fo.eign 
salt. 
Bubbles. These are air-bubbles which have 
not been allowed to escape, 'i bev indicate 
w ant of complete fusion, either from too little 
alkali, or the application of too little heat. 
Cords. These are asperities on the sur'ace 
of the g’ass, ia consequence of too little heal, 
