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The photographic news
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- 27.1883
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- Bandzählung
- No. 1316, November 23, 1883
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November 23, 1883.1 THE PHOTOGRAPHIC NEWS. 743 sensitiveness of asphaltum by preliminary treatment with ether seems to have been known and acted upon long before Dr. Kayser’s investigations. In one of his papers read before the French Academy of Sciences in May, 1853, Niepce de St. Victor mentions a method of making his varnish sufficiently sensitive to operate in the camera in fifteen minutes, by pouring over the plate anhydrous sulphuric ether, containing a few drops of rectified oil of lavender. In this case, no doubt, the ether removed the more insensitive parts of the as phaltum. Scamoni, in his “ Hand-buch der Heliographie," recom mends the asphaltum to be broken up into small pieces, and well washed with ether ; the coloured fluid is then poured off, and the residue dissolved in turpentine or other solvent. Choice of solvent.—As regards the choice of the solvent to be used in making the sensitive solution, there seems to be a great variety of procedure. Benzole is generally recom mended, either alone or with the addition of 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 also said to increase the sensitiveness. The benzole must be pure and quite free from water, otherwise the coating will be uneven and streaky. A few fragments of chloride of calcium, placed in the bottle with the benzole, will remove water, if present. Husnik makes a distinction between benzole from coal tar, and benzoline from petroleum, saying the former should be used in pre ference to the latter. Working in India at high temperatures, I have generally preferred turpentine, with the addition of a little oil of lemon peel, as more manageable in coating than benzole. A mixture of one part of Huanik’s solution with one or two parts of turpentine has given me good results. The tur pentine should be the rectified oil, which dries quickly, and not ordinary turpentine, which remains tacky for some time. Some kinds of asphaltum, and according to Niepce de St. Victor and Herr Morch, some of those most suitable for photographic use, are insoluble in turpentine, or soluble with difficulty, i n which case some other solvent must be chosen. The addition of thymol will, in some cases, especially with the sensitive product extracted by ether, aid the solution in turpentine. Ether was used in the early photo-lithographic processes by Barreswil and Macpherson; but it is unsuitable because, as shown by Dr. Kayser, it does not dissolve the most sensitive part of the asphaltum. It is also unsuitable from its very volatile nature, and the readiness with which it attracts moisture from the atmosphere, which makes it very difficult to get an even coating. The solution in ether has the peculiarity of giving a “ grain” to the coating, which may be useful in half-tone work, and may probably have been one reason of its selection in the early processes. With some specimens of asphaltum, tho writer finds that the solu tion in ether gives exceedingly tacky films, quite unfit for use. In any case, tho first ether, which generally contains the oily, insensitive, constituent soluble in alcohol, should be poured off, and the solution in the second ether used. Chloroform is one of tho best solvents of asphaltum, either in its original state or after extraction with ether ; but it is expensive, and obviously undesirable to use, especially for coating large plates. The elder Niepce dissolved his asphaltum in oil of laven der, and it has always been usual to add some essential oil to the sensitive solution. According to Niepce de St. Victor, benzine and the essential oils acquire sensitiveness by expo sure to light, and he recommends the addition of about 10 per cent, of an essential oil in order to render the varnish more sensitive to light, and make it tougher and more viscous. Many essential oils will do, but he gives the pre ference to pure oil of lemon peel, which forms a homo geneous varnish, drying better, and more sensitive than that prepared with oil of spike. It is also harder, and gives sharper lines. With very volatile solvents, the addition of an essential oil is useful in retarding the evaporation of the solvent and enabling an even coating to be given. Preparation of the sensitive solution.—This is very simple. The selected asphaltum, or the residue after washing with ether, is finely powdered, and placed in a bottle with the solvent, and shaken till dissolved. The proportion of asphalt to solvent varies according to the object required. Some kinds of asphaltum give a thicker or darker solution than others, though used in the same proportions. According to Scamoni, half-tone work requires a thinner solution than line work. For engraving, typographic etching, and all work in which the asphaltum coating is to resist the action of strong acids or etching fluids, the coating must be thicker than is necessary for zineographic or lithographic printing. It must be borne in mind, that the thicker the coating, tho longer will be the exposure required to produce complete insolubility when developed. From 5 to 10 per cent, of asphaltum makes a suitable varnish for etching purposes ; while for surface printing on stone or zinc, solutions containiag from 2 to 5 per cent, will be sufficiently strong. The following formul given by various operators will serve for guidance :— Kruger, for gillotype— Syrian asphaltum 1 part Rectified turpentine... ... ... 9 parts Pure oil of lemons ... ... ... 4 parts Liehert, also Fortier, for photo-zincography— Syrian asphalt ... ... ... 3 parts Anhydrous benzine 100 ,, Oil of lemons ... ... ... a few drops Rodriguez, for gillotype— Asphaltum ... ... ... ... 8 parts Essence of lemon ... 3 „ Anhydrous benzole ... ... ... 100 parts Fichtner— Asphalt ... ... ... ... 5 „ Benzine ... ... ... ... 90 „ Oil of lavender ... ... ... 10 ,, Morch— Sensitiveasphaltum extract... ... 10 „ Benzole from coal tar ... ... 150 „ Balsam of Peru 5 ,, The object of the balsam of Peru is to enable the develop ment to be controlled ; otherwise it is too rapid. Despaquis uses as a solvent in summer one part of chloroform to three parts benzine, and in winter one part of chloroform to two parts benzine. According to Kruger, asphalt solutions should not be used too old, and therefore should not be made up in large quantities. By age they become insensitive, and give a powdery coating which gives ragged lines on development. Solutions of asphalt must be carefully filtered or allowed to settle well before use, so as to give a perfectly even coat ing, free from specks of dust, &c. Botts. The Manchester Photographic Seciety will hold a soiree and exhibition on Tuesday, December 18th. The German Photographic Society have decided to hold their next annual meeting in Berlin. Our photograph of Madame Sarah Bernhardt this week is by Nadar, of Paris, and it is probably the finest specimen of sun-painting that has ever issued from a type press. That the long-looked-for coming of pictorial newspapers illus-
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