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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1306, September 14, 1883
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The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band 27.1883
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588 THE PHOTOGRAPHIC NEWS. ‘[SEPTEMBER 14, 1883. Natural Asphalts with reference to their Photo-ehemical Properties), published at Nuremberg in 1879. The terms asphalt or bitumen include a large group of natural bodies, possessing even outwardly very different properties, and known by different names. Some are hard and compact, others soft and tarry, and some fluid. The only properties possessed by these bodies in common are that they contain carbon and hydrogen, have a brownish black colour, and more or less of a bituminous smell which is peculiar to them. They are divided entirely from different kinds of coal by their behaviour with solvents. The term asphaltum comes from the Greek word for fossil pitch, hh<r<l>a\Tos (and qdaAAoua), and signifies an unchangeable body. The Latin word bitumen is derived from pix tumens. The term asphaltum is generally applied to the solid compact bitumens, such as are used in photography; while the term bitumen, besides being applied to those and the more fluid kinds, is also used in a more general way to signify the whole class of bituminous substances. The supply of bitumen, both solid and fluid, is tolerably abundant, and it is found in all parts of the world, most frequently in volcanic neighbourhoods, accompanying sul phur, sulphuretted hydrogen, and gaseous or liquid hydrocarbons. The principal sources of asphaltum proper—i.e., with a high melting point, brownish black colour, and conchoidal fracture—are, the Dead Sea in Syria, the Island of Trini dad on the coast of Venezuela, Coxitambo in Peru, and the Island of Cuba. Bitumen has usually been considered as formed by the oxidation of petroleum, and, according to the generally accepted analysis by Regnault, Boussingault, and others, is composed of carbon, hydrogen, and oxygen, with, in some cases, a small proportion of nitrogen and ash. In 1877, Helm discovered the presence of sulphur in asphaltum and other bituminous products. As will be seen, Dr. Kayser’s researches show the constant presence of the same element with the entire absence of oxygen, and from experiments made by distilling solutions of sul phur in petroleum, by which he obtained a brownish black, tarry mass, soluble in ether, chloroform, and partly in alcohol, without separation of sulphur. He concludes that the bitumens are produced by the action of sulphur or sulphur compounds, or petroleum, under the influence of high temperature, pressure, and time, possibly with the aid of volcanic action. Some samples of asphaltum examined by the writer give a distinct black tarnish to polished silver when heated on it, and in some cases similar plates coated with solutions of asphaltum in ether have been found tarnished on development. Dr. Kayser examined five different kinds of asphaltum: Syrian, Trinidad, Bechelbronn (Alsace), Maracaibo (Vene zuela), and Barbadoes : but he specially studied the two first, as being the kinds generally met with in commerce, and most suitable for photography. Syrian asphalt comes from the neighbourhood of the Dead Sea, and is found on its banks, or floating on its surface, as well as in beds in conjunction with sulphur and other volcanic products. It has a conchoidal fracture, a peculiar bituminous smell, black colour, and gives a brownish powder. Its hardness is 3 to 4, according to Breithaupt; specific gravity, 1’103 ; and begins to melt at 135° C. (275° F.). It is partly soluble in alcohol, ether ; more in benzole ; completely and easily in chloroform, bisulphide of carbon, turpentine, and the different earth oils. It is insoluble in solutions of caustic potash or soda, weak or strong, hot or cold. With concentrated sulphuric acid, Syrian as phaltum is decomposed only by heat, with evolution of sulphurous acid, and dissolves into a dirty brown fluid. Concentrated nitric acid has very little action on it, even with heat. Syrian asphalt comes into the market in large pieces, which frequently contain small lumps of earthy substances, consisting of carbonate of lime, gypsum, clay, and sand, often moist, and pasty with water. Qualitative analysis showed the presence, besides carbon and hydrogen, of a large proportion of sulphur, as well as a small quantity of nitrogen and mineral substances (iron, manganese, and calcium). Trinidad asphaltum comes from what is known as the Great Pitch Lake on the Island of Trinidad. It forms a basin of unknown depth about half a mile long and 110 yards broad. At the sides, the pitch is hard and cold, but increases in temperature and softens towards the middle, where it is hot and fluid. The whole surrounding country is covered with immeasureable masses of asphalt-like lava. Mud volcanoes are found in the neighbourhood, and, according to some writers, the whole air about is impreg nated with sulphurous fumes. Sulphur and petroleum are also found on the island. Trinidad asphaltum is in most points very similar to the Syrian, only its colour, especially when powdered, is lighter. Its fracture is conchoidal; specific gravity, 1’96; in hard ness, 3 ; it begins to melt at 130° C. (266° F.). Its behaviour with solvents is exactly the same as the Syrian. It comes into commerce in large pieces, which, however, do not show the earthy particles found in the Syrian. It also is composed of carbon and hydrogen, with a con siderable quantity of sulphur, as well as a little nitrogen and ash, the latter an iron compound. Bechelbronn asphaltum is a thick, fluid, tarry substance, of a peculiarly weak butuminous smell, and a brownish black colour. It is found near Beckelbronn in Upper Alsace. It is partly soluble in alcohol, almost enterely iu ether, and entirely iu chloroform and turpentine. Maracaibo asphalt comes from the Lake of Maracaibo in Venezuela, and is also found in connection with volcanic action. Its properties are very similar to those of Trinidad asphaltum. Its specific gravity is 1-091 ; its hardness 3 ; it begins to melt about 130" C. (266° F.). It is very slightly soluble in alcohol; for the greater part soluble in ether and benzole; completely soluble in chloro form, turpentine, and petroleum. Barbadoes asphalt has a light brown colour; specific gravity, 1-041 ; hardness, 2 ; and begins to melt at 110° C. (2308 F.). It is partly soluble in ether and alcohol: completely in chloroform, turpentine, and petroleum. (To be continued.) COLOURING PHOTOGRAPHS ON PAPER AND PORCELAIN. WlIAT shall I paint ? is the first question asked by the tyro. To this the answer is : Let the first essays be made in mono chrome, on the glaze; that is to say, with one colour heightened by one or two others. Photographs of casts on bas-reliefs afford good copies for this purpose. There are also now photographs of flowers to be obtained at many of the best photographers, which are eminently suitable for the beginner’s first lessons. Shall I have a background, or shall I paint on a white surface I is the next inquiry. Backgrounds soften or tone down the colours, while a white ground gives force to the subject simply by heightening its colours by contrast; therefore, if a back ground is desired, paint the subject the more forcibly, in order to overcome the softening effect of the ground; if a white ground, let the treatment of the subject be soft and light, or the work will lack that delicate harmony so much appreciated by all lovers of true art. Good colours for this purpose (monochrome) are red shaded with Brunswick or chocolate browns, or Vandyke shaded with German brown. Any of the colours which wil shade themselves are suitable ; but the learner is recommended o confine himself in his earlier efforts to red and brown, thnse colours, as before mentioned, being more easily used than som , others.
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