The oxide of lead is an essential constituent of the denser glasses, and may be regarded as replacing the lime, so as to form with the quartz-sand a silicate of lead. It assimilates best with purified pearl ash, on account of the freedom of this alkali from iron, which is present in most sodas.
Its atomic constitution may be represented as follows:—
| Computation. | Analysis. | ||||||
|---|---|---|---|---|---|---|---|
| Silicic acid | 5 | atoms | = | 2877· | 59·19 | 59·20 | |
| Oxide of lead | 1 | = | 1394· | 5 | 28·68 | 28·20 | |
| Potash | 1 | = | 590· | 0 | 12·13 | 9·00 | |
| Oxides of iron and manganese | — | — | 1·40 | ||||
| 4861· | 5 | 100·00 | 100·00 | ||||
The above analysis by Berthier relates to a specimen of the best English crystal glass, perfectly colourless and free from air-bubbles. This kind of glass may however take several different proportions of potash and silica to the oxide of lead.
The composition of mirror plate, as made on the Continent, is as follows:—
| White quartz-sand | 300 | pounds |
| Dry carbonate of soda | 100 | |
| Lime slaked in the air | 43 | |
| Cullet, or old glass | 300 |
The manganese should not exceed one half per cent. of the weight of the soda.
Optical glass requires to be made with very peculiar care. It is of two different kinds; namely, crown glass and flint glass. The latter contains a considerable proportion of lead, in order to give it an increased dispersive power upon the rays of light, in proportion to its mean refractive power.
Optical crown glass should be perfectly limpid, and have so little colour, that a pretty thick piece of it may give no appreciable tinge to the rays of light. It should be exempt from striæ or veins as well as air-bubbles, and have not the slightest degree of milkiness. It should moreover preserve these qualities when worked in considerable quantities. Potash is preferable to soda for making optical crown glass, because the latter alkali is apt to make a glass which devitrifies and becomes opalescent, by long exposure to heat in the annealing process. A simple potash silicate would be free from this defect, but it would be too attractive of moisture, and apt to decompose eventually by the humidity of the atmosphere. It should therefore contain a small quantity of lime, and as little potash as suffices for making a perfect glass at a pretty high temperature. It is probably owing to the high heats used in the English crown glass works, and the moderate quantity of alkali (soda) which is employed, that our crown glass has been found to answer so well for optical purposes.
Practical details of the Manufacture of Glass.