CHAPTER II_Glass and Glass Manufacture

THE CHEMISTRY OF GLASS-MAKING AND THE MATERIALS USED

The term “glass,” in a general sense, is applied to the hard, brittle,

non-crystalline, transparent, opaque or translucent vitreous substance

which results from fusing silica with active mineral solvents or fluxes,

such as the alkalies, earthy bases, or metallic oxides. Silica exists in

great abundance, in a free natural state, in the form of flints, quartz,

and sand; and in the latter form it is now most generally used for

glassmaking. When sand alkali and lead oxide are heated together to a

high temperature, the sand is dissolved by the solvent action of the

fused alkali and lead oxide until the whole becomes a molten mass of

glass. The solvent action of the alkalies, soda potash or lead oxide, is

very energetic whilst being heated, and the mass boils with evolution of

gases until, at last, the solution, becoming complete, settles down to a

clear quiescent molten liquid metal, which is quite soft and malleable,

after the nature of treacle. In this condition it is ready for working.

The time and temperature necessary for melting such mixtures vary

according to the proportions and composition of the ingredients.

=Silica=, combined with alumina and other oxides, is freely distributed

in nature in the form of clays, granites, and feldspars, which are also

available for use in glassmaking. Originally glass was made by using

crushed and ground flint stones as the source for the silica: hence is

derived the old name of “flint” glass; but now the large extensive

deposits of white sand present a much more convenient and less expensive

source, and sand has become universally used. Fine white sand is

obtained from Fontainebleau, near Paris; other sources are Lippe, Lynn,

Aylesbury, Isle of Wight, Holland, and Belgium.[2] These are the sources

preferred by crystal glass manufacturers and makers of fine quality

glass, such as chemical ware pressed glass, tube, cane, and medical

bottles, on account of their greater purity. The commoner varieties of

sand from Reigate and Bagshot and even red sand are being used in the

manufacture of the lower grades of glass such as beer bottles and jam

jars, where a greater latitude in the chemical impurities present is

permissible. Only the best and purest silica sands are used for making

cut crystal and optical glasses. In these trades the sand is always

cleaned by washing it in water to clear it from any salt, chalk, or

other impurities which may possibly be present. The sand, after washing,

is heated to redness, or “burnt,” in order to burn off any organic or

vegetable matter, and when cold it is sifted through a fine screen to

take out any coarse grains or lumps. In this prepared state, the sand is

ready for weighing out into the proportions desired for mixing with the

other materials, and is stored for use in covered wooden compartments

situated in or near the mixing rooms, along with the other materials

which may be used in the glass mixtures.

Footnote 2:

_See_ “British Glass Sands” (Boswell), “British Glassmaking Sands”

(Peddle); papers read at the third meeting, Society of Glass

Technology, Sheffield, for further information.

The alkalies, potash or soda, or a mixture of both, are commonly used in

making glass in the form either of carbonates, sulphates, or nitrates.

The soda and potash silicates form very fusible glasses, but they are

not permanent, being soluble in water; therefore they cannot be used

alone. In making glassware for domestic use, other bases, such as lead

oxide, barium, or lime, have to be added to form more insoluble

combinations with the silica or sand.

=Carbonate of Potash= or =Pearlash=, which before the war was imported

into this country by glass makers from Stassfurt, is much prized by

crystal glass makers on account of the colourless silicate it forms when

fused with the best white sand. It is now very expensive and difficult

to get, and is less used on this account. Potash carbonate is very

hygroscopic and absorbs much moisture from the air; therefore it is

necessary to keep it within sealed chests while in store.

Potash and soda each have an influence upon the colour of the resulting

glasses in which they are respectively used. The potash silicate gives

better and clearer glasses than the soda silicate.

=Carbonate of Soda=, or =Soda Ash=, is now more generally used. Being a

less expensive form of alkali, it constitutes a base in most of the

commoner varieties of glassware. Carbonate of soda is manufactured in

England from common salt, of which there are large deposits in the

Midlands. This common salt, or chloride of sodium, is treated chemically

and converted into the carbonate, in which form it is supplied to the

glass manufacturers as soda ash.

=Sulphate of Soda= (=Salt Cake=) is the form of alkali used in window

and bottle glassmaking. In mixtures containing sulphate of soda it is

necessary to use a small proportion of carbon in some form, such as

charcoal or coal, in order to assist the decomposition of the salt and

the formation of the sodium silicate. Sulphate of soda is used in this

class of glassware on account of its cheapness. Glasses made from

sulphate of soda mixtures are not so clear and colourless as those in

which the source of alkali is potash or soda carbonate. On this account,

the best crystal glasses cannot be made from sulphate of soda.

=Potash Nitrate= (=Saltpetre=) is used in glass mixtures to oxidise the

molten metal and improve the colour of the glass. In fusing it

disengages oxygen gas, which purifies the glass while melting, and

assists the decolorizers in their action by keeping up an oxidising

condition within the molten mass.

=Sodium Nitrate=, or =Chili Nitre=, is the corresponding soda salt to

potash nitre. It is much cheaper, but less pure; it has a similar but

not nearly so powerful an oxidising action in the glass as potash nitre.

It is exported from Chili, where it exists naturally in a crude state as

“Caliche,” from which the nitrate is refined by recrystallisation.

=Boric Acid= acts as an acid in glass, as does silicic acid. It renders

glass more fusible and brilliant; it has a searching action upon the

colourising properties of certain metallic oxides when they are

dissolved in the glass. It is an expensive ingredient, but is

considerably used in optical and special chemical glassware in replacing

a portion of the silicates ordinarily used and forming borates. It

cannot be used in large amounts, as an excess produces glass of a less

stable nature.

=Borax=, or =Borate of Soda=, consists of boric acid combined with soda.

It is a very useful glassmaking material and is an active fluxing agent.

If used in excess in glass mixtures it causes considerable ebullition,

or boiling of the metal. In moderate proportions it is used in the

manufacture of enamels for glass, as it helps to dissolve the colorific

oxides and diffuse the colouring throughout the enamel mass.

=Tincal=, and =Borate of Lime=, are other forms in which borates may be

introduced into glass.

=Carbonate of Lime=, =Limespar=, =Limestone=, =Paris White=, or

=Whitening= are all forms of =Calcium Carbonate=. It is an earthy base

and is added to the simple alkaline silicates and borates to form

insoluble combinations or double silicates of soda and lime. By the use

of lime, glasses are rendered more permanent and unchangeable when in

use. Lime forms a very powerful flux at high temperatures. The quantity

used must be carefully regulated according to the proportion of other

bases present; otherwise an inferior or less stable glass may be

produced. In excess it causes glass to assume a devitrified state.

=Dolomite= is a _Magnesium Limestone_, and is a natural stone which is

available for use in making glass in tank furnaces.

=Fluorspar=, or =Fluoride of Lime=, is used in giving opacity and

translucency to glass. It can only be used in small amounts, as the

presence of any large proportion attacks the clay of the pots, causing

serious damage by the sharp cutting chemical action due to the evolution

of fluorine gas.

=Phosphate of Lime= is another material which produces opacity and

translucency, but does not seriously attack the pots. Bone ash is a form

of phosphate of lime, and is procured by calcining bones until all

organic matter is consumed.

=Carbonate of Barium=, or =Witherite=, is a very heavy, white powder,

and is a form of earthy base available for use in glassmaking. It can be

used to replace lime, with similar results. By replacing other elements

in the glass which are of lower density, barium can be used to increase

the density of glass. Like lime it is a very powerful flux in glass at

high temperatures. It gives increased brilliancy and little coloration.

For this reason it is very useful in the manufacture of pressed

glassware, giving a glass which leaves the moulds with better gloss than

is found to be the case with lime glasses.

=Magnesia= and =Strontia= are other bases which are less used in

glassmaking.

=Zinc Oxide= is a base used in the manufacture of many optical glasses.

With boric acid it gives silicates of a low coefficient of expansion and

special optical values. Used with cryolite, it forms a very dense opal

suitable for pressed ware. It is rather more expensive than the other

bases used.

=Cryolite= is a natural opacifying ingredient used in making opal

glasses. It consists of a combination of the fluorides of aluminium and

sodium, and is one of the most active fluxes known to glass and enamel

makers. Its cutting chemical attack on the pots is very intensive. It is

imported from Greenland. An artificially manufactured form of cryolite

is known, which is a little cheaper than the natural variety and gives