THE CHEMICAL AND PHYSICAL PROPERTIES OF GLASS
The main essential and peculiar property of glass is its transparency.
When subjected to a gradually increasing temperature, glass becomes
softened, and whilst hot it is plastic, ductile, and malleable, in which
state it can be cut, welded, drawn, or pressed. A thread of glass can be
drawn so thin and fine that it can be twisted and bent to a remarkable
extent, showing that glass is flexible.
The above properties shown by glass while softened under heat permit it
to be shaped and formed by a variety of methods, so that in the
manufacture of the different kinds of glass we find goods pressed,
blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling,
the form given to them is retained permanently.
Another property of glass is its conchoidal fracture and liability to
crack under any sudden change of temperature. Advantage is taken of this
peculiarity in dividing or cracking apart glass when necessary, during
the stages of the manufacture of any glass article.
[Illustration:
_By permission of_
_Melin & Co._
HORIZONTAL CRACKING-OFF MACHINE
]
If a glass worker, in making an article of glass, desires to detach or
cut apart certain sections, he applies a cold wet substance, such as an
iron file wetted with water, to any portion of the hot glass, which
causes it to fracture at the point of contact with the cold metal, and a
slight jar is then sufficient to break the two portions apart. This
method of chilling heated glassware to divide it is applied in the
mechanical process of cutting up the long cylindrical tubes of glass
into short sections for use as miners’ safety lamp chimneys. Wherever it
is desired to cut them through, a narrow section or line round the
cylinder is first heated by a sharp, hot pencil of flame projected from
a burner against the rotating cylindrical tube of glass at equidistant
short sections, and the divisions chilled by contact with a cold, steel
point, or the heated area may be gently scratched with a diamond point,
when a clean, sharp fracture results exactly where the chill or scratch
has been applied and spreads round the whole circumference in a circle,
giving neat, clean-cut divisions. In cutting narrow tube and cane, the
fracture caused in the structure of the glass by scratching its surface
with a steel file or diamond is sufficient to cause it to break apart
without the application of heat.
A piece of hot glass will weld on to another piece of hot glass of
similar composition. The glass maker uses this method of welding for
sticking handles on to jugs, etc., during the process of making table
glassware.
The density of glass varies according to its composition. Certain
classes of lead and thallium glass for optical work are of very high
density. The specific gravities of such glasses may vary from 3·0 to
well over 4·0. In soda-lime glasses the density is less and approaches
2·4. Ordinary crystal glass approximates to a specific gravity of 3·1.
The elasticity and thermal coefficient of expansion of glass can be
regulated within normal limits. Glasses are now manufactured which can
be perfectly sealed to copper, iron, nickel, and platinum wires.
Glass, if kept heated for any length of time at a temperature just short
of its softening or deformation point, becomes devitrified and loses its
transparency, becoming opaque and crystalline. In this state it has much
of the nature of vitreous porcelain and is totally different to
manipulate, being tough and viscid on further heating. This devitrified
state may occur during glassmaking, where the metal is allowed to remain
in the pot or tank furnace for a considerable time under low
temperature. Small stars or crystals first develop throughout the glass
and continue to grow until it becomes a stony, white, opaque, vitreous
mass. “Réaumur’s Porcelain” is a glass in a devitrified state, and is
used for pestles and mortars, devitrified glass being less brittle than
ordinary glass and similar to vitrified porcelain.
Glass can be toughened to an extent which is surprising. Bastie’s
process consists of plunging the finished glass article whilst hot into
a bath of boiling oil, which toughens the glass so much as to make it
extremely hard and resistant to shocks, losing most of its brittle
nature. Strong plates of glass are produced by a process of toughening
under pressure. These plates of glass are used for ship porthole lights
and in positions where great strength is required. Toughened or hardened
glass is of great value in the production of miner’s lamp glasses and
steam-gauge tubing. Glass, when hardened, is difficult to cut even with
the diamond, and difficulty is experienced in finding suitable means to
cut it into shapes to suit commercial requirements.
“_Prince Rupert drops_,” or tears, exhibit the state in which unannealed
glass physically exists. These are made as a curiosity by dropping a
small quantity of hot metal from the gathering-iron into a bath of water
and then taking the pear-shaped drops out quickly. These pear-shaped
drops of glass will stand a hard blow on the head or thicker portion
without breaking, but, if the tail is pinched off or broken, the whole
mass crumbles and falls to powder. This well illustrates the latent
stresses or strains apparently in a state of tension and thrust within
the structure of unannealed glass.
Glass is not a good conductor of heat. This accounts for the necessity
of slow cooling or annealing glassware, and also applies when re-heating
glass, which must be done slowly and evenly to allow time for the
conduction of the heat through the mass gradually. Glass is a
non-conductor of electricity, and is used to a considerable extent in
the electrical trades for insulation purposes. Most glasses are attacked
slightly, but not readily, by water and dilute mineral acids. Continued
exposure to a moist, humid atmosphere causes slight superficial
decomposition, according to the stability and chemical composition of
the glass. Old antique specimens of glass show the superficial
decomposition caused by long continuous exposure to atmospheric
moisture. Many antique specimens have been known to collapse instantly
upon being unearthed. The first change in antique glass is exhibited by
a slight iridescence forming on the surface, gradually increasing
towards opacity afterward disintegration sets in, until it finally
collapses or crumbles to powder. Glasses high in lead are readily
attacked by the acid vapours met with in the atmosphere, but the harder
soda-lime glasses are more resistant. An excess of boric acid, soda, or
potash also renders glass subject to disintegration and decay.
Hydrofluoric acid attacks all silicate glasses, liberating silicon
fluoride. Use is made of this acid reaction in decorating glasswares in
“Etching,” by exposing the surface of glass to the fumes of hydrofluoric
acid gas in some form.
The most permanent glasses are those containing the highest proportion
of silica in solution, but the available heat necessary to decompose
such highly silicious mixtures is limited by the present known
refractory materials which can be procured for constructing the
furnaces. Quartz glassware is a highly silicious glass. It is now made
and used in the manufacture of special chemical apparatus and laboratory
ware such as crucibles, muffles, etc., which have to withstand severe
physical and chemical tests. This quartz glass possesses remarkable
features in its low coefficient of expansion and resistance to heat
changes. It is highly refractory. Articles made of this glass can be
heated to red heat and plunged directly into cold water several times
without fracturing. Several varieties of quartz glass are now
manufactured, and a new field for investigation is presented in applying
the features and properties of this glass for use in chemical processes.
In a purely physical sense glass is a supercooled liquid, the silicates
are only in mutual solution with each other, and they appear to be
constantly changing. Glass cannot be described as a homogeneous or
definite chemical compound. Many of the after effects and changes which
occur in glass, and the formation of crystals in the devitrification of
glass tend to prove the above assertion. The colour changes which take
place when ruby and opalescent glass is re-heated, and even the change
in colour of glass going through the lehr, cannot be explained unless in
the above sense of viewing these remarkable changes. Glasses with an
excess of lime in their composition are more subject to devitrification
than lead glasses or those of moderate lime content constructed from
more complex formulas. The presence of a small proportion of alumina in
glass prevents this tendency to devitrification and ensures permanency.
Those glasses which have the highest silica content, and which have been
produced at the highest temperatures, show the greatest stability in
use. Bohemian glasses of this type contain as much as 75 per cent.
silica, and are produced in gas-fired regenerative or recuperative
furnaces, where the heat approaches 1,500° Centigrade. Such glass is
much sought after for enamelling on, being harder and less easily
softened by the muffle heat firing on the enamels used. Taking two
corresponding glasses of the same basicity, or proportion of silicic
acid to the bases present, those formulae which have the greater
complexity of bases produce the more fusible glasses. A multiple of
bases constituting a more active flux than a single base content, it
follows that a compound mixture of silicates fuses or melts at a lower
temperature than the respective simple silicates would. These facts are
useful in constructing commercial formulae for glasses.
Glasses containing lead oxide as an ingredient are subject to reduction
when exposed to flames of a carbonaceous nature. The carbon partially
reduces the lead oxide to its metallic state, forming a black deposit.
On this account, lead glasses cannot be used in blow-pipe working with
the ease with which soda-lime glasses can be worked, without reduction
taking place. English crystal glass, which contains a high percentage of
lead, is usually melted in hooded or covered pots to prevent the
carbonaceous flames of the furnace reducing the lead and otherwise
destroying the clearness of the glassware. Soda-lime glass and others
without the presence of lead can be melted in open pots without any fear
of reduction. Modern gas-fired recuperative furnaces, in which more
complete combustion of the carbon takes place, can now be used for
melting lead glasses in open pots, thus presenting a great saving in the
fuel required to melt and produce such glass, besides permitting the use
of a cheaper form of pot. This cannot be done with the ordinary English
coal-fired furnaces.
Advantage is taken of the reducing action of the coal-gas flame when
producing lustre and iridescent glassware. A small proportion of easily
reducible metal, such as silver or bismuth, is introduced into the glass
and first melted under oxidising conditions. It is then reduced in
after-working by flaming, which deposits the metal in a thin sheen upon
the surface of the glass, where it comes in contact with the reducing
flames. An example of this effect is shown in Tiffany lustre ware, in
which silver chloride is used and reduced within the glass, giving a
pretty coloured iridescence on the surface, due to the reflection of
light from the particles of metal deposited under the surface.
“Aventurine” is a form of glass in which copper and iron oxides are
introduced under reducing conditions during melting. The glass is then
allowed to cool slowly. The metallic copper tends to separate out in
small spangled crystals, which give a pretty sparkling effect. The use
of strong reducing agents with very slow annealing is necessary to
produce this effect. Copper and gold ruby-coloured glass presents other
instances of partial precipitation of the metal by reduction within the
glass. According to the extent of reduction, the glass ranges in colour
from yellow, ruby, to brown.
The manganese silicate is readily affected by oxidising or reducing
conditions, the purple colour being present under oxidising influences
and a greenish-grey colour under reducing conditions. In using manganese
as a decolorizer, the glass maker may have added too much of it to his
glass, in which case it shows too prominent a purple colour. To destroy
this excess of colour he pushes either a little strip of green willow
wood or a clean potato to the bottom of the pot of metal. The reducing
action of the carbonaceous gas involved takes out the excess of purple
colour by partially reducing the manganese present to a colourless
state.
The colour of glass is gradually affected in course of time by sunlight.
This change in colour is often noticeable in old windows, the glass
having developed a yellowish green tint in course of time from the
action of the solar rays.
Glass which has been incompletely fused or not sufficiently melted to
give a complete solution of the materials present is in a weakened state
of cohesion and is liable to disintegration. The presence of
undecomposed sulphates, chlorides, or borates in the glass also tends to
early disintegration. A continual exudation and crystallisation of salt
takes place upon the surface until the glass wholly disintegrates away
to a white powdered salt.
Glass is a poor conductor of heat. When a piece of glass has been
expanded under the influence of heat, and is rapidly cooled, the
superficial outer portions become intensely strained and contracted upon
the interior portions, which retain the heat longer. Under these
conditions of cooling, glass is apt to “fly,” or collapse and fall to
pieces, owing to the outer portions giving way under the great strain.
These stresses or strains are relieved in the process of annealing,
under which they are gradually eased by a slow and regular cooling from
the heated condition. Certain glasses, the composition of which shows
considerable differences in the density of the respective bases present,
are more subject to this defect than those in which the bases are of
more even density and homogeneous in character. Such glasses should be
“de-graded” and re-melted in order more thoroughly to diffuse and
distribute the denser portions throughout the mass. In de-grading glass,
the hot glass is ladled out and quenched in cold water, dried, and
re-used as “cullet.”
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