COLOURED GLASS AND ARTIFICIAL GEMS
In colouring glass, either or several of the following colorific oxides
may be used. They are added to the batch before fusion. Varying
proportions are added, according to the depth of the colour desired.
Occasionally the colour is influenced by the nature and composition of
the rest of the batch. In some instances several colouring oxides are
used. In this way many delicate tints are obtained; in fact, there are
but few colours that cannot be produced in glass.
=For Green Glasses= the following oxides may be used: Chromium oxide, 2
to 6 per cent. of the batch; black oxide of copper, ·5 to 3 per cent.;
red iron oxide, ·5 to 1 per cent.; or a mixture of two or three of the
above oxides in less proportions. Salts of chromium, copper, or iron may
be used as the carbonates, sulphates, and chromates.
=For Blue Glasses=, cobalt oxide, ·1 to 1 per cent. of the batch; zaffre
blue or smalts, 1 to 3 per cent.; nickel oxide, 2 to 4 per cent.; iron
oxide, 1 to 2 per cent.; black oxide of copper, 2 per cent.
=For Violet and Purple=, manganese oxide, 2 to 4 per cent. of the batch.
=For Rubies=, red oxide of copper, gold chloride, purple of cassius,
antimony oxysulphide, selenium metal in small proportions.
=For Yellows=, uranium yellow, 4 to 6 per cent. of the batch; potassium
antimoniate, 10 per cent.; carbon, 6 per cent.; sulphur, 5 per cent.;
ferric oxide, 2 to 4 per cent. Silver nitrate and cadmium sulphide are
also used.
=Black Glass= is obtained from mixtures of cobalt oxide, nickel oxide,
iron oxide, platinum and iridium. Many very dark or black bottle glasses
are obtained by using basalt, iron ores, or greenstone in a powdered
form, added to the batch ingredients.
=White Glasses= or =Opal= are obtained by using phosphate of lime, talc,
cryolite, alumina, zinc oxide, calcium fluoride, either singly or in
double replacements of the bases present in the glass batches.
Many of the colouring oxides give distinctive colours to glass of
different compositions; also the resulting colours may vary with the
same colouring ingredient, according to reducing or oxidising meltings.
Thus, in a batch of reducing composition, red copper oxide gives ruby
glass, but in oxidising compositions the colour given is green or
bluish-green. Iron oxide in an oxidising batch gives a yellow. In the
reducing batch it gives bluish or green results. Manganese is similarly
affected.
Many colouring oxides give more brilliant tints with glasses made from
the silicates of potash and lime than if used in glasses composed from
silicates of lead and soda. For many colours the lead glasses are
preferred. In colouring the batches, the colouring oxides must be
intimately mixed with the batch materials before fusion, more especially
in the preparation of the pale tints, where only small quantities of
colouring are necessary. It is a well-known fact that careful mixings
give good meltings, for then the materials are more evenly distributed
and uniformly attacked during the melting. Careful and exact weighings
are necessary when using colorific oxides, and a pot is kept for each
respective colour melted, so that the different colours and crystal
glasses do not get contaminated with each other. When open pots are used
for colours, the colour pots should be kept together in one section of
the furnace, so that whilst melting, especially during the boiling up of
the batches, the colours do not splash over into the other pots
containing crystal metal.
As a rule, smaller pots are used for coloured glass; generally they are
only a third of the size of crystal melting pots. When this is so, they
are set together under one arch of the furnace, and the workman informed
which pots contain the respective colours. All colour cuttings and
scraps should be kept separate from other cullet for re-use. Coloured
glasses are expensive, and no waste of glass should be permitted.
=Artificial Gems.= In the manufacture of the glasses for imitation
“paste” jewels, every effort is made to procure pure materials and
colorific oxides. The base for making artificial gems is a very heavy
lead crystal glass termed “=Strass paste=,” which gives great brilliancy
and refraction. The composition of such a paste would be: Best white
sand 100 parts, pure red oxide of lead 150 parts, dry potash carbonate
30 parts. These should be thoroughly well melted until clear and free
from seed, and the molten mass ladled out of the pot and quenched in
cold water, or “de-graded.” This assists in making the paste
homogeneous. After repeated melting and de-grading, the paste or cullet
is collected, dried, and crushed for use in making the coloured pastes.
Usually, this strass metal is melted in small, white porcelain crucible
pots holding about 5 to 10 kilogrammes of the metal and heated in a
properly regulated gas and air injector furnace. The coloured paste is
kept in fusion for a whole day, after which it is slowly cooled and
annealed within the pot, and the gems cut from the lumps of glass thus
obtained. The following are some of the compositions used in the
preparation of the respective gems.
=Opal.= Powdered strass paste, 1,000 parts; white calcium phosphate, 200
parts; uranium yellow, 5 parts; pure manganese oxide, 3 parts; antimony
oxide, 8 parts.
=Ruby.= Powdered strass paste, 1,000 parts; purple of cassius, 1 part;
white oxide of tin, 5 parts; antimony oxide, 10 parts.
=Beryl.= Powdered strass, 1,000 parts; antimony oxysulphide, 10 parts;
cobalt oxide, ·25 parts.
=Amethyst.= Powdered strass glass, 1,000 parts; purest manganese oxide,
8 parts; pure cobalt oxide, 2 parts.
=Emerald.= Powdered strass glass, 1,000 parts; green chrome oxide, 1
part; black copper oxide, 8 parts.
=Sapphire.= Powdered strass glass, 1,000 parts; pure cobalt oxide, 15
parts.
=Topaz.= Powdered strass glass, 1,000 parts; antimony oxide, 50 parts;
uranium yellow, 10 parts.
=Garnet.= Powdered strass glass, 1,000 parts; antimony oxysulphide, 100
parts; gold chloride in solution, 1 part; pure manganese oxide, 4 parts.
=Turquoise.= Powdered strass glass, 1,000 parts; cobalt oxide, ·5 parts;
black copper oxide, 10 parts; white opal glass, made with tin oxide, 200
parts.
After suitable pieces of glass of the requisite tints are obtained, they
are cut and ground on a Lapidary’s wheel, then polished, engraved, and
set as gems.
=Artificial Pearls= are now cleverly made in glass. A tube of the
requisite size made of translucent or opal glass is cut into small
sections, which are heated on a tray to softening point whilst set in a
rotatory movement. As the heat increases they gradually melt in and seal
at the openings, when they are removed from the tray and sorted.
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