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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1285, April 20, 1883
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Zeitschrift
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
Band 27.1883
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APRIL 20, 1883.j THE PHOTOGRAPHIC NEWS. is used instead of gas, and at a lower pressure. Turn on the ether tap gradually, and work the oxygen up to it until the force is full on, and the latter partially so. The flame should be slightly tinged with red, and a dark cone of about one-eighth of an inch long should be seen in the centre. The hottest part is immediately beyond this dark cone, and if you bring the lime within it there will be a blank nucleus in the centre of the spot, and a loss of light. When once the proportions of the two gases have been properly adjusted, the flame will need very little further attention during the lecture ; but towards the close the amount of ether vapour will decrease somewhat, and the oxygen tap of the jet will require altering accordingly. I conclude my paper with a word on the subject of chlorate of potash, as used for the manufacture of oxygen gas. Some two or three years since, Mr. F. York sent to me a sample of the crude crystals which are sold at a low price for the preparation of oxygen, and I found them to answer sufficiently well for lantern purposes. Further experience, however, has shown me that this first crystallisation is not of reliable quality, and that it often yields oxygen so highly contaminated with chlorine as hardly to admit of purification. It requires also a stronger heat, and more oxide of manganese to decompose it. I do not observe any appreciable difference in the light when oxygen from the crude chloral is used; but, taking into account the destruction of the retorts, and the corrosion of the taps, to say nothing of the inj ury caused to the bag, I am not sure that there is much economy in employing it, although the recrystal lised is exactly double the price. SECURING A DEAD BLACK SURFACE ON OPTICAL BRASS WORK. BY WILLIAM FORGAN.* If an article has been lacquered before, that lacquer must, in the first place, be removed, and the article afterwards carefully cleaned and polished. When the brass has not been lacquered before, it must receive before lacquering the highest possible finish and polish if it is desired to make a fine job. This is done by taking out the marks of the file with finer and finer sorts of emery paper or cloth, then polishing with rotten stone or oil, and giving the article, after this has been cleaned off, a final touch with a buff-stick and crocus powder. The article must be carefully wiped clean, and care taken that it is not touched with the fingers after this has been done, as these would leave a greasy mark. Care must be taken at every step to invariably lay the successive strokes of the emery cloth or paper and polishers in the same direction. When the desired degree of polish has been attained, a quantity of lacquer is poured into (say) an egg-cup, which is a very convenient receptacle for the purpose. A fine, flat camel’s hair-brush is taken, and the article being gently warmed and held on the left hand, a small quantity of the lacquer is taken up on the brush, and then the brush is drawn over the brass with straight strokes, always, if possible, in the same direction. The article may generally be held by screwing a piece of wire into some hole in it, and holding the wire with a small hand-vice. As many coats of the lacquer may be given as desired by keeping the brass hot; the degree of heat is an important element in the success of the operation. Holtzappel says it must not be warmer than boiling water, but so far as my experience goes, I should say the heat of boiling water would be too great. Considerable skill is required in lacquering well, and that skill can only be attained after a good deal of experience. The great secret of lacquering for beginners, at all events, is to take as little as possible of the lacquer at a time on the brush, have the article perfectly clean, a good brush with no loose hairs in it, aud clean, and not make the article too hot. Now as to lacquers. There are a great variety of them. As a rule, English brass-work is covered with a very pale lacquer containing almost no colour; while, on the contrary, Continental lacquers contain too much colour. I show you two microscope object glass boxes, one of which is finely polished and lacquered by Mr. Wray, and the other is by Dr. Carl Zeiss, of Jena. Mr. Wray’s is a very fine specimen of lacquering. It has, however, in it a fugitive colouring material, and when a certain time elapses, the brightness leaves it. Zeiss’, again, has a great amount of colour in it. Wray’s looks well when new ; but it very soon fades, and the colour becomes bad. I show you • Read before the Edinburgh Photographic Society. another box of his, which I have had for some years, in which the colour has changed very much. The simplest and best pale lacquer, Holtzappel says, is made of shellac and spirits of wine only, in the proportions of about 4 lb. of the best pale shellac to one gallon of spirit. It is, he says, required to be as clear and bright as possible, aud is always made without heat by continu ous agitation for five or six hours. If not clear it may be filtered, and should be kept out of the influence of light. It may be coloured for yellow tints with turmeric, cape-aloes, saffron, or gamboge; and for red tints with annatto or dragon’s blood. What I have described is the process adopted for lacquering the outside of photographic lens mounts, or all those portions of the brass work where the light does not pass. The inside of the mounts, however, are treated in a totally different way. We must have, where light passes, a surface as nearly dead blank as can be got. This is obtained in the inside of the tubes, by mixing finely-triturated lamp-black with the lacquer used for the outside, and applying the black lacquer in one or more coats with heat to the inside of the tube. The result is a finely- grained black surface which reflects no light. As soon as the surface has received one or two coats, no more must be given, as the repeated application of the lacquer would make the surface glossy—the very thing which it is wished to avoid. This method of blackening the brass does excellently for all portions which are not to come into contact with the fingers ; but wherever the brass requires to be handled, we must have recourse to something different from lamp-black. One of the modes adopted for that purpose is to bronze the articles. There are various ways of doing this. Everyone must be familiar with the ordinary greenish colour of gas-fittings which are bronzed. The article is first thoroughly cleaned from all grease, and then dipped in vinegar or a strong solution of sal ammoniac, or sal ammoniac and vinegar mixed in the proportion of one to three ounces of the sal ammoniac to one pint of vinegar. Holtzappel says a quick bronze is made with one ounce of corrosive-sublimate dissolved in one pint of vinegar. The best aud most rapid, however, of all the bronzing liquids, is the nitro-muriate of platinum, called “ chemical-bronze.” It is known in the shops as the ter-chloride of platinum. This produces the colour very readily. All these methods, however, merely, so far as I am aware, give a bronze tint, and not the black surface we should like to get. I show you here a few lens stops of Mr. Grubb’s, which have seen some use, and you will see that the bronze has nearly all disappeared. If I take a piece of clean brass, and touch it with the platinum solution, the bronze effect is almost instantly produced; but it does not, in my hands, produce black. The bronzing process is invariably used with all articles put together with soft solder. The method I am about to describe requiring a considerable amount of heat, the articles must be without any soldered joints. When I first began to “ work in brass,” a great many years ago, at the lathe, I experi enced much difficulty, sometimes not being able to give some parts of the articles I produced a sufficiently dead-black surface, such as the settings of lenses, lens stops, and such like. Merely bronzing in such a case will not do, and lacquer and lamp-black is worse. I became acquainted with an Edinburgh optician who had been taught his knowledge of brass work finishing for philosophical instruments in the work-shop of the late Mr. John Adie ; he knew no method except bronzing. At that time he had a large business in the sale of the student’s Natchet and Hartnack microscopes, the brass stages of which are, perhaps, the most beautiful specimens of blackened brass which can be produced. We wrote to Natchet, and asked him how it was done, and, I think, he replied that it was done by nitrate of silver. We tried that, and failed. There was nothing for it but to fall back upon the bronze again. Some time after, when the Rev. J. B. Reade described his microscope kettle-drum condenser in the Proceedings of the Royal Microscopical Society, he incidentally mentioned that those portions of the mount which it was necessary to blacken were blackened by nitrate of copper. My friend and I tried it almost as soon as I had read it, and we succeeded at once, to our great delight, in producing a black surface which was everything we wished. I prepare the solu tion by dissolving copper wire in nitric acid, weakened by adding, say, three or four parts of water to one of acid. The article to be blackened is heated to pretty hot, and then dipped into the solution ; it is then taken out, and heated over a Bunsen burner or spirit lamp. When the article is heated to the proper temperature, the green colour of the copper first appears, and as the heat is increased, the article becomes of a fine dead black.
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