The Liverpool Standard and General Commercial Advertiser.
Liverpool, Merseyside, England, 1838-10-12 00:00:00

American forests we should be unable to procure our potash
from vegetable sources, we might be able by the decomposition
of felspar to procure a supply of potash to last us for a long
time. Felspar consists of silica united to alumina and pot-
ash. In fact we may divide the silica into two proportions,
and then in the first place we shall have one portion united
with the potash, and in the second place another portion
united to the alumina. Now the combination of silica with
potash is a salt, and the combination of silica with alumina is
likewise a salt, therefore this felspar is a double salt of which
there exist vast numbers in chemistry, seine of which are of
great importance. Alum is an artificial product which can
scarcely be said to exist naturally, but which is of great impor-
tance in the at ts. It is a combination of sulphuric acid with alu-
mina, sodium, and water. The sulphuric acid is thus divided
into two portions, now leaving out the water, which by simply
drying the alum we-may get rid of, we shall see that alum
very much resembles the felspar which we.can scarcely form
artificially, but which abounds iu nature. Now all glass and
porcelain are double salts resembling felspar in a very great
degree. Our object in the manufacture of porcelain is to
bring silica and potash together so that they may resemble
the porcelain we wish to obtain. It is our object to get glass
of a certaip degree of fusibility, and we therefor?, unite them
in certain proportions so that the resulting salt may be manu-
factured of whatever purity we may desire. Porcelain, on
the contrary, it is our object to form in such a way that it
shall be very difficult to melt, and the easy fusibility which
is so much to be desired in glass is a great objection in
porcelain. For these processes there are different proportions
employed. The silicates of the alkalies are far more
easily melted than the silicates of the earths. Silicate of
potash makes a pure glass of an intermediate degree of fusi-
bility, but in practice it is necessary to have glass of different
degrees of fusibility, and to effect this different suostances
are used. lead is added to render it more fusible, and lime
to decrease its fusibility. We have, therefore, three varieties
of glass : flint glass, a double silicate of potash or soda and
lead, and the (Efferent varieties of crown glass, which have
introduced into them a greater or less degree of fusibility.
In the manufacture of table glass there are three ingredients:
the fine sand sometimes procured from Lyme Regis, some-
times from South America and other distant places, as it
must be of great purity; then we have the alkali, chiefly
soda, on account of its cheapness and greater powers of sa-
turation; and, in addition to these, a great quantity of red
lead. In fact, in making use of red lead, there are two im-
portant objects gained: the first is, the manufacturer makes
his glass more fusible, and saves his coals; in the next place,
the lead being a 'heavy ingredient, he increases the value of
his glass by being enabled to charge more for it. An article
made of glass containing lead weighs inure than if no lead
had been employed; and as it is paid for by weight, the pro-
fit of the manufacturer is greater. But in addition to this,
the red lead gives the glass a degree of lustre which enhances
its appearance; therefore, as an article of luxury, it is in-
creased in value. There are limits to this addition of lead,
as it is liable to be acted upon by acids, although combined
with the silicate of soda: indeed, drinking-glasses are some-
times acted upon by the liquids they may contain, and it is
well known that the cylinders of some electrical machines
can scarcely be kept dry, owing to the presence of too great
a quantity of lead. The manufacture of plate and crown
glass differs from that of flint glass, because it is necessary
to make the former much harder. In consequence of this,
cheaper materials are used: lead is left out, and the soda or
potash is replaced to a great extent by lime. The presence
of lead in glass may be readily ascertained by pounding the
glass, mixing it with charcoal, and putting it into a crucible;
when exposed to heat the lead will melt, and collect at the
bottom of the crucible. There is made at Prague, in Bo-
hemia, an article which is beautiful as an article of luxury,
and is very useful in a scientific point of view. It is a pure
silicate of soda, and requires an intense heat to melt it. With
it we can operate with ease in chemistry, as it is neither
liable to melt nor to crack with sudden variations of heat and
cold as an inferior metal. It is distinguished from the Eng-
lish glass as containing no lead, and but little alkali. In the
manufacture of different species of porcelain the chemical
union is not so decided as in glass. It is nut perfect. The
imrortant constituent of the different kinds of earthenware
is clay; but this is mixed with a certain quantity of ground
flint, the object of which is to prevent the clay from shrink-
ing and cracking on exposure to heat. If we take a piece of
fine Cornish clay, we shall find that it is a compound of silica
and alumina, that it is the clay which in felspar is united
with the silicate of potash. If we take a piece of granite we
shall find that it consists of an aggregate of three minerals,
which are quartz, (pure silica) mica, and felspar. Some of
these granites resist the action of the atmosphere for an inde-
finite length of time, but others decompose very rapidly. Of
the three constituents of the granite, it is only the felspar
which is decomposed, and from the action of the air and
water it is broken up. The silicate of potash is washed away,
and the silicate of alumina remains behind in the form of
clay. We can easily ascertain this by seeing the rock in
the process of decay. This pure clay is characterised by its
uniting with water and constituting a remarkably palpable
.substance. If we take a portion of clay, and wash it with
water, it forms a sort of substance which we can form into
any shape we wish. But this pure clay it is impossible to
make use of at once in the manufacture of porcelain, be-
cause pure clay contracts very much in drying. The amount
of contraction is so considerable, that the celebrated Wedg-
wood, to whom the porcelain manufacture is much indebted,
made use of it in the construction of the pyrometer, an in-
strument used to measure the heat of furnaces. In order to
accomplish this, a piece of clay is dried at a slow red heat,
and fitted accurately into a gunge at the extremity of the py-
rometer. The piece of clay is introduced into the furnace, the
heat of which we wish to ascertain, and exposed for a short
time to the high temperature within. The more the clay
is heated the more it contracts. After a certain period it is
taken out and cooled, and it is again placed in the pyrome-
ter, and slid up the groove, the sides of which gradually ap-
proach each other, until it can be urged no further, when the
degree of temperature marked upon the side of the groove
where the clay remains stationary is that of the furnace.
This instrument is not scientifically accurate, but it is suffi-
ciently so for the purposes of the arts. If we take any of this
pure clay and mould it into the form of a vessel, the result
will be the shrinking of the vessel in such a way as to injure
its form. Not only so, but it will crack in an indefinite
number of directions. In order then to avoid this, the clay
is mixed with silica. At the point where the clay touches
the silica, the latter melts so that the alumina is thus firmly
united with it. The vessel in this case will be smaller, but
it will not crack A remarkable fact connected with the
manufacture of glass is its great expansion by heat. It is
evident that the snore at.liberty a body is left to the action of
its own internal forces, the more permanent will be the con-
dition it will assume, and if these forces be counteracted,
that they will be disturbed by the slightest application of ex-
terior power. Now let us suppose a piece of glass, when hot,
to be dropped into water, and its surface suddenly cooled, we
know that if a bulb of glass be slowly cooled, is very much
lessened in size ; but when the red hot mass of glass is
dropped suddenly into water, the outer coat will Ise at once
solidified without becoming any smaller, the consequence of
which is, that time interior particles are held in a state of sus-
pension with regard to one another. It is natural, therefore,
that if the bulb of glass be subjected to the slightest vibration,
its internal structure will be disturbed. This gives rise to
toys well known, under the name of Rupert's drops, which
are made by allowing drops rd red hot glass to fall into
water. When the point of one of these drops is broken, the
whole mass is resolved into a fine powder by the action of its
own internal forces. violent is the action, that if the
point be broken off under water, the separation of the parti-
cles will break time glass in which it is contained, unless the
vessel be very strong. If we, however, heat one of these
bulbs to redness, and then allow it to cool slowly, no such
action will take place, because the particles have had time to
arrange themselves.
The lecture of to-morrow evening (Saturday) will embrace
the theories of Saccharine, Alcoholic, and Acetous fermen-
tation, &c.; and we should think that it would prove highly
interesting to those ladies who pride themselves upon their
domestic housewifery.