Home >> Encyclopedic Dictionary Of Photography >> Acetic Acid to For Washed Emulsion >> Asphaltum

Asphaltum

asphalt, light, alcohol and sensitive

ASPHALTUM (Synonyms, asphalt, bitumen, bitumen of Judea, mineral pitch, and Jew's natural product of the decomposition of vegetable substances. It is found on the shores of the Dead Sea, also in a molten state in Trinidad, and as a mineral deposit at Seyssel. Small quantities have been found in Derbyshire, Cornwall and Shropshire.

Commercial asphalt is generally in brownish-black or black lumps, very brittle and nearly odorless. It varies in quality, but it can be purified by boiling in water, when the pure asphalt melts and floats upon the surface, while the impurities subside.

Asphalt melts at between go degs. and zoo degs., and burns with a bright flame. It may be dissolved in oil of turpentine, oil of lavender, benzine, and in solutions of alkalies and alka line carbonates.

Alcohol, ether and chloroform will also dissolve asphalt. By dissolving successively, three different kinds, possessing different properties, can be extracted. The alcohol extract is an oily, odorous substance, yellowish in color, and slightly sensitive to light. The ether extract is a brittle substance, of a brownish-black color, and nearly odorless. It is a little more sensitive to light. The chloroform extract is a black, brittle, odorless residue, containing more sulphur, and much more sensitive to light, than the other extracts.

A method of preparing it for photographic purposes is given by Leaper as follows : The commercial product is finely powdered and digested with alcohol for a couple of hours in a closed vessel at a temperature of 4o degs. C., after which the residue is collected on a filter, washed

with alcohol and left for several days in contact with an excess of ether. The residue is finally air dried and preserved in the dark. Sometimes adulterated with coal tar pitch. This can be detected by breaking a sample, when it will be seen to have a dull instead of a shining fracture.

This property of being sensitive to light makes asphalt exceedingly useful for photo graphic purposes, especially in photo-zincography and photo-lithography. The action of the light upon a coating of asphalt laid upon a zinc plate is to render it insoluble. Those parts not acted upon may be removed by a subsequent operation of washing with a solvent. This action of light is no doubt physical and not chemical, as the solubility can be restored by melting it. It withstands the action of acids, which are then employed to eat away into the uncovered portions of the zinc (see Photo-zincography). In photo-lithography another property of asphalt is made use of, i.e., its attraction for the greasy ink (see Photo-lithography).

Mixed with oil, gutta-percha, or india-rubber, it makes a capital black varnish (see Varnish).