GLASS.— Artificial Gems.

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fusion, with oxide of tin and the solution of gold;the mixture then appears like moist grey sand. Thesimple addition of a solution of gold to the flux,without any oxide of tin, is likewise capable of pro-ducing rose and carmine-coloured glass.

Blue is produced by oxide of cobalt. The colouringpower of the oxide of cobalt is so intense, that purewhite glass is rendered sensibly blue by the additionof xnVirth part of the oxide.

Green may be produced either by protoxide ofiron, protoxide of copper, or oxide of chromium.The tint produced by the first of these substances haslittle brilliancy. The oxide of copper yields a beau-tiful emerald. For this purpose the glass is mixedwith the product obtained by heating copper toredness with access of air, or with powdered verdi-gris, which is then decomposed in the fire, and con-verted into oxide by oxidizing agents. Care must betaken to prevent the protoxide of iron from beingconverted into sesquioxide, and the oxide of copperfrom being reduced to suboxide. The oxide ofchromium, which occurs as a pigment in commerce,yields the purest and most brilliant grass-green hue.

The following are the approved recipes for imitat-ing precious stones:—

Diamond .—Colourless strass, without any addition.

Topaz —

Very white strass. 1000

Clear yellowish orange-red glass of antimony,. 40Purple of Cassius,. 1

A very good imitation of topaz is obtained from—

Strass , ..:. 1000

Oxide of iron,. 10

Ruby .—The preparation of topaz furnishes a meansof obtaining constantly and at pleasure very finerubies. The mixture for topaz often gives an opaquemass translucid at the edges, and presenting in itsthin plates a colour red by transparency: 1 part ofthis opaque topaz-matter, and 8 parts of strass meltedin a Hessian crucible, when left thirty hours in thefire of a potter’s furnace, give a fine yellowish crystalsimilar to strass. Remelted with the blowpipe, thisproduces rubies of the finest water.

A ruby not so fine, and of a different tint, may bemade by employing the following proportions:—

Colourless strass, . 1000

Oxide of manganese,. 25

Emerald .—The emerald is very easy to imitate.That which succeeds best results from the mixtureof the oxide of copper with colourless strass. Ifoxide of cobalt be added, the green obtained presentsblue reflections. The composition which gives thebest imitation of natural emerald is as follows:—

Colourless strass,. 1000

Pure oxide of copper,. 8

Oxide of chromium,. 0-2

By increasing the proportion of chromium or oxideof copper, and adding oxide of iron to the mixture,one may vary the green shade, and imitate peridotor dark emerald.

Sapphire .—To produce a fine oriental blue colour,employ very white strass, and very pure oxide of

cobalt. This composition, put into a luted Hessiancrucible, should remain thirty hours in the fire. Theproportions are:—

Colourless strass,. 1000

Oxide of cobalt,. 15

Amethyst —

Colourless strass,. 1000

Oxide of manganese,. 8

Oxide of cobalt,. 5

Purple of Cassius,. 0-2

Syrian Garnet. —The artificial garnet is a kind ofdark ruby, and is made after the following formula:—

Colourless strass,. 1000

Glass of antimony,. 500

Purple of Cassius. 4

Oxide of manganese,. 4

In the fabrication of artificial stones, the materialsmust be pulverized with great care. The mixturescan only be well made by repeated sifting. Thesame sieve must not be used for sifting differentcompositions, whatever care be taken to clean itafter each operation. Lastly, to obtain masses wellmelted, quite homogeneous, without striae or bubbles,it is necessary to employ substances very pure, andmixed in a state of extreme division; to obtain thebest crucibles, to melt with a graduated heat, in afurnace which is quite equal in its maximum tempera-ture ; to leave the metal in the furnace duringtwenty-four to thirty hours, and to cool the cruciblesvery slowly, that their contents may undergo a kindof annealing.

Coloured Flint-Glass.—A ssuming as the com-position of the glass to be coloured the commoningredients and proportions, and remembering thatthese materials, when mixed together, are technicallytermed “batch,” and when used with the usualprecautions produce a highly pellucid flint-glass,this may be regarded as the basis of the followingcoloured glasses.

Soft White Enamel. —To 6 cwts. of batch add 24lbs. of arsenic, 6 lbs. of antimony.

Hard White Enamel. —To C cwts. of batch add 200lbs. of putty prepared from tin and lead.

Blue Transparent Glass. —6 cwts. of batch colouredwith 2 lbs. of oxide of cobalt.

Azure Blue. —6 cwts. of batch with about 6 lbs. ofoxide of copper.

Ruby Red. —0 cwts. of batch with about 4 ozs. ofoxide of gold.

Amethyst or Purple. —6 cwts. of batch with about20 lbs. of oxide of manganese.

Common Orange. —To 0 cwts. of batch add 12 lbs.of iron ore and 4 lbs. of manganese.

Emerald Green. —To 6 cwts. of batch add 12 lbs.of copper scales and 12 lbs. of iron ore.

Gold Topaz Colour. —6 cwts. of batch with 3 lbs.of oxide of uranium.

Bontemps points out that the colouring propertiesof metallic oxides are greatly modified by the degreeof heat to which the glass is subjected, and by othercircumstances.

Iron. —It is generally agreed that oxide of- irongives a greenish colour to glass, to the mixture of