Blowing: 
or flashing 
of crown 
glass. 
310 
square, having an arch thrown over it of about two 
feet in height. On the one side is a grating to con- 
tain the fire, with an ash-pit beneath. The bottom 
of this furnace is raised about 34 feet high, so as to 
be more convenient for the workmen to turn the ma- 
terials. As the neutral salt contained in the kelp 
when heated is extremely penetrating, and readily goes 
through common or even fire bricks, carrying along with 
it a considerable quantity of alkali, and thereby very 
much injuring the quality of the glass, various plans 
have been tried to prevent it, some by making large 
bricks of fire clay, and others by placing a plate of iron 
so far under the floor of the furnace, as not to be much 
affected by the heat, from an idea that when the iron 
tub or pan is filled with the neutral salt, no more will 
be lost. Another plan is to have flues for admitting a 
stream of cold air below the floor of the furnace, which 
cools it, and keeps the salt from running off. But it 
would undoubtedly be a much better plan to separate 
the alkali from all heterogeneous matter previous to 
mixing it with the sand. 
The operation of calcining, or burning frit, requires 
mauch care. After the materials are put into the fur- 
nace, they are stirred frequently, until all the earthy 
matter in the kelp is burnt away, which generally re- 
quires about three hours ; the heat is then raised to such 
a degree, as to bring the materials almost into a state of 
fusion, which must be prevented by constant stirring, 
otherwise it would get into what is termed calches, or 
small knots that contain more sand than the rest of the 
batch, occasioned by the alkali being dissipated, from 
an excess of heat. If it is continued at that high 
température for about two hours, it will be sufficiently 
calemed. It is then taken out of the furnace, and 
spread, while warm, upon a plate, and divided into 
large cakes ; this must be done before it cools, other- 
wise it would be one hard lump. It is the opinion of 
many, that frit cannot be too old. All the opulent 
manufacturers, therefore, lay up great quantities, and 
seldom use any till it is six months old. 
Crown glass is made by filling the pots on the melt- 
ing furnace with frit, adding about one-eighth part of 
broken glass. The furnace is then raised to as high 
a degree of heat as possible, and in about ten or twelve 
hours the frit is melted. The pots are again filled u 
in the same-manner. The heat is then continued, and 
if possible increased, till the metal, as it is now called, 
is completely fine, that is pure liquid glass fit to be 
made into window glass, altogether requiring from 30 
to’36 hours of intense heat. : 
After the metal is. completely fine, the founder, that 
is the workman who manages the metal making, allows 
the fire to slacken for about two hours, until it arrives at 
what is termed a working heat ; this is called settling 
the furnace, and on the proper.settling of the furnace 
the working of the metal in a great measure depends. 
For blowing or. flashing crown glass few tools are re- 
quired. The principal instrument is an iron tube, the end 
of which being heated, is dipped into the pot of melted 
gis and turned round to collect some glass upon it. 
his portion -of glass is distributed equally upon all 
sides.of the end of the tube, by rolling it upon a flat 
table of iron. It.is then heated, and again dipped into 
the melted glass, to take up an additional quantity. In 
this state, by blowing slightly through the tube, the. 
glass is made hollow within ; and the workman judges, 
from the manner in which it enlarges when, he blows, 
if the glass is so arranged round the end of the tube, 
that it may be afterwards blown out and extend itself 
GLASS. 
equally on all sides, to form a large globe, or hollow 
vessel of any other circular figure, of a _thick- 
ness, If he perceives any side to be too thin, he cor- ,7 
rects this, by rolling it on the marble. This ope-.or fas 
raration is repeated three or four times. He then of cro 
heats it well in the fire, and rolls the glass with great glass. 
care, to form’ it circularly upon the end of the tube, 
and ng: bor: it out in the manner represented b 
Fig. 1. Plate CCLXXVI. Then, by ‘blowing through Prat 
the tube; he distends the Rg into the form of a long ccLx 
hollow pear, resting it all the while upon the iron table, Fig: | 
and rolling it round; that it may preserve a correct cir- 
cular figure, and distend itself equally. To | n out, 
the neck of the pear, he rolls it, as it may require,overa = __ 
smooth iron rod, fixed up horizontally, as in Fig. 2, and Fig: 
the pressure thus caused will lengthen the neck. He then, 
blows again through the tube, supporting the glass, by. 
resting the extreme end of the pear upon the ir A 
and rolling it round at the same time: The pressure. 
upon the iron rod raises a small paint or eminence upon, 
the globe, opposite the end of the blowing iron. The 
blowing being dexterously managed, and assisted by the. 
pressure of the rolling, the glass is enlarged to the. ’ 
form of a sphere, Fig. 3, which remains attached to the Fig. 5. 
tube by a neck. ‘ 
By the time the workman has proceeded so far, and. 
produced a globe, Fig. 3. of a very regular thickness, 
the glass is generally so far cooled as to grow er a 
though it will readily yield) to a pressure, it will not, 
alter its figure by its own weight. ‘To proceed in the 
operation, it must be heated again. For this se, 
it is carried to a particular furnace, called the bottoming. 
hole, to be exposed to the heat of the flames. A slight 
wall iserected before one-half of this mouth, to screen 
the workman from the heat ; and the screen is at a suffi. 
cient distance from the wall of the furnace, to leave. 
room for the globe to pass between them. A hook is, 
fixed in this wall for the support of the blowing iron, 
which is rested in the hook, at a few inches from the. 
neck of the globe... This method admits of turning the. 
tube rapidly round as an axis, to which the PIA is. 
affixed, and therefore revolves with it. The side of the. 
a8 oe 
globe opposite to the end of the blowing iron being 
Sane fe the heat of the fire, is gradually softened ; 
but the neck of the globe, which joins the rod, and. 
all that part of the. glass which is more remote from. 
the fire, is not softened im am equa} degree. In this 
situation,. the. centrifugal force produced by the whir- 
ling, causes, the equatorial parts of the globe to fly out. 
from the centre, by stretching or enlarging itself ; but 
as this enlargement will be most powerful upon the, 
softest part of the glass, which is the end opposite to. 
the fire, the centrifugal force will cause that end to be- 
come flattened almost toa flat plate, or at least to a 
slightly convex plate, as shewn in Fig. 4. act 
In this state the glass is removed from the fire, and. 
the neck is cracked off, by resting the tube across the 
rod and turning it round, whilst the neck is touched 
with a piece of Iron wetted. in cold water. This pro- 
duces a circular crack round the neck, which is separa, 
ted from the,tube, by laying the glass. upon a tab 
shewn in Fig. 4, which is composed. of two planes d, 
inclining towards each other, so as to receive the 
glass without danger of its rolling off. In this situ- 
ation, a. slight stroke upon the red breaks off the 
neck, and leaves the glass the table. The work- 
man now dips the end of an iron rod o into the 
melted glass, and takes up as much as will make 
it adhere firmly to the prominence ¢,, so.as to serve, 
