| Composition of Metallic Mixture. | |||||
| Lead. | Tin. | Melting Point. | Temperature. | ||
| Lancets | 7 | 4 | 220° | C | Hardly pale yellow. |
| Razors | 8 | 4 | 228° | ” | Pale yellow to straw yellow. |
| Penknives | 81⁄2 | 4 | 232° | ” | Straw yellow. |
| Pairs of scissors | 14 | 4 | 254° | ” | Brown. |
| Clasp-knives, joiners’ and carpenters’ tools | 19 | 4 | 265° | ” | Purplish colour. |
| Swords, cutlasses, watch-springs | 48 | 4 | 288° | ” | Bright blue. |
| Stilettos, boring tools, and fine saws | 50 | 2 | 292° | ” | Deep blue. |
| Ordinary saws | in boiling linseed oil | 316° | ” | Blackish blue. | |
Steel is of a greyish-white colour, and has a sp. gr. varying from 7·66 to 7·93. During hardening the sp. gr. becomes reduced from 7·93 to 7·66, whilst it experiences a slight
increase of volume. The property (already pointed out) that steel possesses of becoming hard after being heated to redness, and suddenly chilled, does not belong to pure iron, such as may be obtained by electrolysis. Unlike pure iron, too, steel presents a granular instead of a fibrous structure when broken; the best samples closely resembling silver in this respect. The chemical difference between hard and soft steel appears to consist in the much more intimate combination of the carbon with the iron in the hard variety than in the soft. In this latter kind the carbon seems to be only mechanically mixed, for if it be immersed in hydrochloric acid the iron is dissolved, and leaves the carbon behind. Steel is the most tenacious of all the metals, being greater than that of either cast or malleable iron.
What is termed case-hardening (which see) is a process by which small articles of iron, such as keys, gun-locks, &c., are superficially converted into steel. It is performed by heating the articles in contact with iron filings or powdered charcoal. Another method is to make the iron substance red hot, and then to sprinkle powdered potassic ferrocyanide all over it.
STEREOCHROMY. This is a branch of the pictorial art confined to the embellishment of walls and monuments. In the operations by which it is accomplished it will be seen that the soluble silicates (water glass) play an important part.
The foundation for the future picture or coloured design must be of some durable stone or imperishable cement. Over this is first placed a layer of lime mortar, to which is applied when it is dry and has become sufficiently hard, a solution of water glass, by which all the interstices of the mortar are filled up. Another coating of mortar made of sharp sand and a lye of chalk is next laid on, and this, after it has been carefully smoothed, properly levelled on the surface, and become quite dry, is washed over and thoroughly impregnated with water-glass solution. When this last layer has become dry it is ready to receive the painting, which must be executed in water colours. After laying on these colours may be permanently fixed by covering them with water glass. The following are the colours used:—Zinc white, chrome green, chrome oxide, cobalt green, chrome red, zinc yellow, oxide of iron, sulphide of cadmium, ultramarine, ochre, &c. Vermillion is inadmissible, since, in fixing, it turns from red to brown. Cobalt ultramarine, on the contrary, increases greatly in brilliancy upon the application of the fixing solution. Stereochromatic paintings are found to be very durable, and impervious to damp, smoke, or variations of temperature.
STE′REOTYPE METAL. See Type metal.
STER′LING. The truth of the old proverb, that “all is not gold which glitters,” is often
painfully experienced by the purchaser of modern jewelry. The following table will, therefore, prove highly useful to the reader in determining the value of articles in gold, provided he ascertain the ‘fineness’ of the metal, either by examination or written warranty:—
Sterling value of Gold of different degrees of ‘Fineness,’