ASPHALTUM (Fr., Asphalte, Bitume de Judge ; Ger., Asphalt, Judenpech) Synonyms, asphalt, bitumen, bitumen of Judea, mineral pitch, and Jew's pitch. A natural product of the decomposition of vege table substances.
The term asphaltum comes from the Greek word for fossil pitch, harrtpakros (and •cintAXoitat) and signifies an unchangeable body. The Latin word bitumen is derived from piz lumens. Geologically, asphalt is, as stated above, a natural product of the decomposition of vege table substances ; and it is found in all parts of the world, most frequently in volcanic neigh bourhoods. The principal sources are on the shores of the Dead Sea in Syria (whence comes the name Syrian asphaltum), and in the Great Pitch Lake of the Island of Trinidad ; but it is also found in the Lake of Maracaibo (in Vene zuela), at Coxitambo (in Peru), at Barbadoes, and in the island of Cuba ; whilst in Europe there are deposits at Seyssel and at Bechel bronn (in Alsace). Small deposits have been found in Cornwall, Derbyshire, and Shropshire. For photographic processes, however, only the Syrian and the Trinidad asphaltum are con sidered, these having the characteristic pro perties of asphaltum—namely, brownish-black colour, high melting point, and conchoidal frac ture. Bitumen has usually been considered to be formed by the oxidation of petroleum, and according to the generally accepted analysis is composed of carbon, hydrogen and oxygen, with a small quantity of nitrogen, together with sulphur and mineral substances (iron, man ganese and calcium). Syrian asphalt comes into the market in large pieces, which frequently con tain small lumps of earthy substances consisting of carbonate of lime, gypsum, clay, and sand. Trinidad asphalt also comes into commerce in large pieces, which, however, do not contain the earthy particles found in the Syrian. The commercial asphalt can be purified by boiling in water, when the pure asphalt melts and floats upon the surface while the impurities subside. The Syrian asphalt begins to melt at 295° P. (135° C.), and the Trinidad at 266° P. (13o° C.). The specific gravity of the former is r•ro3, and of the latter i•96 ; both kinds behave the same in relation to solvents. They are partly soluble in alcohol and ether, more so in benzole, com pletely and easily soluble in chloroform, bisul phide of carbon, tetrachloride of carbon, tur pentine and the various mineral oils. They are insoluble in solutions of caustic potash or soda, weak or strong, hot or cold. With concentrated sulphuric acid the Syrian asphaltum is decom posed, but only by the heat, with evolution of sulphurous acid, and it dissolves into a dirty brown fluid. Concentrated nitric acid has but
very little action on it, even with heat.
The results of analysis show that the Syrian and Trinidad varieties are similar in com position. Each is found to contain fo per cent. of sulphur, and this constitutes an important factor in regard to photographic sensitiveness. By successive treatment with boiling alcohol, boiling ether and chloroform, it is found that both kinds may be separated into three com ponents, differing in their chemical composition and photographic properties. The portion (a) insoluble in ether shows the highest sensitive ness, (b) the substance soluble in ether is less sensitive, and (c) an oily substance extracted by alcohol is quite insensitive. The part that is insoluble in ether is, as a rule, easily soluble in chloroform and turpentine, and less so in benzole and petroleum. Syrian asphaltum contains 52 per cent. of the sensitive component, while Trinidad contains only 38 per cent.
Spectroscopically examined, solutions of Syrian asphaltum of the most sensitive kinds show weak absorption bands, whilst the less sensitive kinds show the bands more strongly.
As obtained from dealers, asphaltum is gener ally fit to be used at once ; but, if necessary, it may be purified by powdering it and digesting it with dilute hydrochloric acid, which dissolves the earthy particles. Some authorities recom mend a treatment with boiling water, by which soluble and earthy particles may be separated out. In order to obtain the most rapid results, it is desirable to extract the least sensitive con stituent of the asphaltum with ether, and use the residue in making the sensitive solution. The simplest way of doing this is to digest the powdered asphaltum in a bottle with an excess of ether, shaking it up from time to time, and, if necessary, stirring it with a glass or wooden rod. The ether is changed at intervals of a few hours, till all or nearly all the soluble constituents are removed. The last ether is then poured off, and the residue thoroughly dried. Husnik's asphaltum, which is believed to be prepared in some such way, is obtainable as an article of commerce in the dry powder form. The solvent used for making up the sensitive solution is generally benzole, which should be quite free from water. Sometimes chloroform is added, and generally some essential oil, such as lavender or lemon, which prevents the too rapid drying of the coating, and so keeps it uniform ; the addition of oil is said to increase sensitiveness.