A VALUABLE PAPER ON THE MANUFACTURE OF GLASS. 
345 
ON THE MANUFACTURE AND USE 
I OF SOLUBLE GLASS. 
(Teaiislntcd from “ Traiie de Chimie nppliqtiee 
|.| aiix Arts, par Mr. Dumas,” by James Renwick, 
I I LL. D., Professor of Nat- Exp. Philosophy 
I aixi Chermistry ill Columbia College, New Yolk. ) 
Soluble glass is a simple silicate of potassa 
or soda, which unites perfect solubility in boil- 
ing water to some of the general properties of 
! common glass: besides, although the uses 
to which soluble glass is applied are very 
different from those of common glass, the 
study of it will furnish such exact and close 
analogies to other descriptions of glass, that we 
: are compelled to include it in the groupofche- 
j mical compounds which they form. 
The discovery of soluble glass and of its 
j uses, is due to a distinguished Germanche- 
misf, from whom we derive all we have to say 
in relation to it. This glass, when dissolved 
in water, forms a liquid which may be applied 
to cloth or wood, for the purpose of render- 
ing them incombustible. In fact, by the 
1 evaporation of the water in which it is dis- 
solved, a layer of asubstance capable effusing 
when heated, is deposited on these bodies, 
which is capable of protecting them from the 
contact of air necessary for their combustion. 
Preparation,— Soluble glass may be obtain- 
ed by dissolving pure silea, obtained by pre- 
cipitation, in a boiling solution of caustic po- 
tassa ; but, this process, being both inconve- 
nient and costly, cannot be practised upon a 
large scale. 
When s^ind and carbonate of potassa are 
heated together, the carbonic acid is never 
wholly driven off, except when the sand is in 
excess, but the whole of the carbonic acid 
may be expelled by adding powdered char- 
coal to the mixture, in such proportion that 
the carbonic acid of that part of the carbo- 
nate which is not decomposed may meet with 
a sufficient quantity of carbon to convert it 
into carbonic oxide. In this way the silica 
first forms a silicate in the proportions con- 
tained in common glass, and drives off the 
appropriate equivalent of carbonic acid ; then, 
ata high heat, the rest of the carbonate of potas- 
sa is decomposed by the carbon, the carbonic 
oxideescapes, and the potassa, thus freed, either 
sublimes, or combines with the glass already 
formed. 
In order to obtain soluble glass of good and 
uniform quality, certain precautions are 
necessary. The caibonate of potassa em- 
ployed, must be purified.^ If itcontainrauch 
chloride of potassium, the product will 
not be entirely soluble in water, and a glu- 
tinous residuum will be left. In addition, the 
glass will be liable to effloresce. Sulphate of 
potassa does not produce any bad effect, be- 
cause it is decomposed by the carbon, when 
the matter continues sufficiently long in 
fusion ; but without this precaution, the glass 
will contain sulphuret of potassium, which 
also has a tendency to efflorescence. 
• Pearlash being the purer form, we shall use 
its name in the practical part. 
The sand must be pure, or at any rate must 
not contain any notable proportion of lime 
or alumina, for these earths render a part of 
the glass insoluble. A small portion of oxide 
of iron has no influence on the qualities of 
the glass. 
The sand and carbonate of potassa (pearl- 
ash) are taken in the proportion of two of the 
latter to three of the former, and to 10 parts 
of pearlash and 15 of sand, 4 parts of charcoal 
are added. A less portion of charcoal must 
not be taken ; on the contrary, if the form of 
potash employed be not sufficiently pure, a 
larger proportion of charcoal may be advan- 
tageously employed. This substance accele- 
rates the fusion of the glass, and separates 
from it all the carbonic acid, of which there 
would otherwise remain a small quantity, 
which would have an injurious effect. 
In other respects the same precautions that 
are employed in the manufacture of common 
glass, are to be observed. The materials must 
be first well mixed, then fritted, and finally 
melted in a glass pot, until the mass be- 
comes liquid and homogeneous. The melted 
matter is taken out of the pot with an iron 
ladle, and the pot is then filled with fresh /nf. 
Thirty pounds of pearlash, 45 of sand, and 
l21bs. of powdered charcoal may be taken for 
a charge ; with this quantity the heat must 
be continued for 5 or 6 hours. 
The crude glass thus obtained is usually full 
of air bubbles ; it is as hard as common glass, 
ofabiackish gray colour, andtransparentat the 
edges ; sometimes it has a colour approaching 
to whiteness, and at others is yellowish or red- 
ish ; these are indications that the quantity 
of charcoal has not been sufficient. 
If it be exposed for some weeks to the air, it 
undergoes slight changes, which rather tend 
to improve, than injure its qualities. It at- 
tracts a little moisture from the air which 
slowly penetrates its mass, without changing 
its aggregation or its appearance, it merely 
cracks, and a slight efflorescence appears at 
its surface. If it be exposed to heat, after it 
has undergone this change, it swells up, 
owing to the escape of the aqueous matter it 
has absorbed. 
In order to prepare it for solution in boil- 
ing water, it must be reduced to powder by 
stampers ; if this were not done, it would 
dissolve too slowly. One part of glass requires 
from 4 to 5 of water for its solution. 
The water is first heated to ebullition in an 
open boiler, the powdered glass is then added 
by degrees, and must be continually stirred, 
to prevent it from adhering to the bottom. 
The ebullition must be continued three or four 
hours, until no more glass is dissolved : tha 
liquor will then have acquired the proper 
degree of concentration, li the ebullition be 
checked before this state is attained, carbonic 
acid will be absorbed by the potassa from the 
air, which will produce an injurious effect ; 
for the same reason, too great a quantity of 
water must not be employed, for, during the 
long evaporation which will then become 
necessary, the carbonic acid of the water will 
readily combine with the potassa, and cause a 
precipitation of the silica. 
