Volltext Seite (XML)
For many years manganese alone was the doctoring substance employed in decolourizing glass made with iron- contaminated sands, if we except the method of employing a large excess of alkali. Glass made with a large excess of alkali, although fairly pure in colour, has the fatal demerit of being very soft (approaching soluble glass, indeed), but, still worse, that it sweats (to use a vulgar but significant term) to such an extent that it is by no means an uncommon occurrence to see the surface of such glass bedewed as though by steam, and to see tears running down it; and if the finger be touched on one of those tears and then applied to the tongue, the unmistakable alkaline flavour will soon indicate the nature of the glass whence they exude. Manganese as a corrector or doctorer of colour possesses,•unfortunately, the property of turning the glass made by its admixture into a pink or puce colour when the glass is exposed to sunlight or broad daylight; indeed, heat alone will develop the pink tinge when an excess of manganese has been used in the formation of the glass. If anyone takes a stroll up Portland Place they will there see some very fine houses, the windows of which appear to be glazed with a puce or purple tinted glass. If a lady is our companion she will probably say, “ What a beautiful colour those windows are! ” She little. Enows that when the houses were built the glass in those windows was as bright as crystal, and that years of sun light have actually turned the glass pink. But so it is; doubtless those windows were glazed with plate some fifty or more years ago, when the precise action of manganese was not quite so well understood as now. Arsenic has no such action—at least, not so far as our ex perience warrants us in saying so—for arsenic is a more recent introduction in glass-making. Red-lead has a similar decolourizing action on glass ; but it is inadmissible in what is known as crown glass. Crown glass metal is that which is known as separate and distinct from flint glass. The term crown glass is retained because crown glass was the only usual form in which window glass was made ; but seeing that crown glass is almost an obsolete article, the term window glass metal would be much more appropriate. It might be difficult to upset the existing terms of crown glass and flint glass ; but that they are decidedly inconsistent will at once be seen when it is pointed out that the term crown glass arose from the fact of the glass being blown in the form of a crown, or table, as it is sometimes termed, a name entirely derived from the shape in which the glass was formed ; whilst flint glass takes its name truly and correctly from its. composition, as flints were actually employed in its production. Yet we maintain that the terms are most misleading, and we should much prefer to see such terms as window glass, lead glass, bottle glass, which would carry their meaning to those outside the limit of the technically initiated. Manganese in excess in the mixing of any glass produces a pink colour at once when the glass is blown, although a very high temperature will burn it out, or the colour will sink and the molten mass will be paler on the top than when the bottom of the pots are blown from. Seeing that many manganese ores contain large quantities of iron, it will easily be understood that a glass manufacturer may keep on increasing the dose of manganese until he produces metal of such a dingy, dirty colour, that nothing will bring it clear, not even a big dose of arsenic. Arsenic is likely to be much more free from iron, indeed may be quite free from it; hence it is much superior to manganese, but re quires judicious care not to overdo it in the mixture. Red-lead, except in lead glass (erroneously called flint glass), is inadmissible, because, although it corrects colour and renders the metal brilliant, it at the same time renders the glass so soft as to be easily scratched sometimes even with the finger-nail; and, moreover, the metal will not blow with the same amount of ductility, and it cannot be vitrified in open pots, for the simple reason that the flame of the furnace reduces the lead to the metallic state, and it burns away. The cost of red-lead also is against its employment, except in lead glass, or optical glass, or fine heavy glass, or in the manufacture of the more ex pensive kinds of coloured glass (called antique), and is quite prohibitory in any kind of ordinary window glass. There are other substances used in the manufacture of common window glass, such as chalk, limestone, pearl ash, cryolite, and common salt, as well as charcoal, cullet, or broken glass, kaolin, and even common clay, all of which in special cases find employment in the mixture where particular results are desired. For the present we leave out of the question the ingredients used to colour glass; we are treating now of window glass metal, such as is made into crown, sheet, plate, or rolled plate; what we may term the elementary glass as distinguished from the more complex silicates, and where the object in view is either extreme ductility or great fluidity, together with good colour, moderate working temperature, fineness of texture (freedom from seeds, knots, bubbles), and, above all, facility of being cut by the diamond; for, however much a given glass may possess all the other qualities, if it cannot be cut its value is depreciated enormously, as the manufacturer knows to his cost. Probably few industries call forth more real skill and high chemical knowledge than that of glass-making. According to the mixture so should the result come out to the satisfaction of the mixer; but it is not always so, for the chemicals play strange pranks—yea, the sand itself, and the coals used in the furnace, the alkali, and the doctor- ings—may all or any of them upset the calculations of the glass-maker. Instances are on record of where the men have actually stolen salt-cake from the mixing-room during the night, and added it to the metal in the pots, in order to reduce the time required for their attendance on the furnace until the metal should be founded, and fined, and ready for the blowers. Immense fortunes have been sunk by English glass manufacturers in experimenting with furnaces and pro cesses, which of course has had the result of placing the industry upon a remarkably firm basis, so far as the com prehension of all the conditions necessary to success can do; but perhaps the most deplorable fact is,. that for several years past, and until a very recent date, the busi ness has had to be carried on without profit, if not, indeed, in several cases, at an actual loss. The art of glass-making is deserving of better things. —o- Bz-the-3Bye. ' POISONOUS CHEMICALS. The subject of poisons is attracting the attention of the legislature once more, and we shall scarcely be surprised if photographers are not in the future a little more hampered in respect to their supplies of such chemicals as are notoriously of a poisonous character. There are three classes who suffer from poisoning: those who swallow poison inadvertently, those who swallow it designedly, and those who swallow it because it is administered to them. The second class can hardly be protected by the law, and in the case of the third, legislation also is of slight avail if the would-be poisoner is a person of intelligence ; all that can be done is to make the acquisition of poison a difficult and circuitous proceeding, which may reduce casualties, but cannot prevent them. In the photographic world, and indeed if we take the gross number of poisonings, it will be found that by far the majority of deaths are those of suicides. A man or woman does not commit self-destruction because it is easy to obtain cyanide of potassium or other noxious drug, but because he or she is tired of life, and desires to quit it. The instrument by which the deed is committed is of no importance when once the deed has been resolved upon. And yet, to hear some people talk, it is due simply to the ready way in which poisons are to be purchased that we
