LENSES.
Optical glasses have engaged the attention and investigation of scientific men for centuries. We read of the wonderful exploits of the burning lens of Archimedes, and find the remains of lenses thousands of years old in the ruins of Nineveh, Babylon, and Pompeii. They are of the utmost importance in the science of astronomy. The slow progress made in perfecting them shows the inherent difficulties that exist in obtaining glass of the required purity. One of these is the different specific gravities of the material used. Hence the lower part of a pot of melted glass is of greater specific gravity than the top, causing a tendency to cords or threads, an evil which science has yet to learn to overcome. Not even the large bounty offered by the English Government and the Board of Longitude has been successful in effecting any important improvement in this branch of manufacture. Munich enjoys the reputation of producing the best lenses, and consequently the finest telescopes. Sir Isaac Newton, Gregory, Dolland, Keir, and others adopted lenses made from flint- and from crown-glass, it being necessary to use both in the construction of achromatic telescopes, one possessing as small and the other as great dispersive powers relative to the mean refractive powers as can be procured. But the inherent defect of the lenses still remained. M. Macquer remarks, "The correction of this fault appears therefore to be very difficult." He had tried in vain to remove it by very long fusion and fierce fire. Others have found this by experience not to correct, but to augment the evil. Mr. Keir is of opinion that some new composition must be discovered, which, along with a sufficient refractive power, shall possess a greater uniformity of texture.
Since then, it is certain some improvement has been made in the composition for lenses. In an English paper we find the following:—"One of the most remarkable optical lenses of modern manufacture is that produced by Messrs. Chance, English manufacturers, being an attempt by them to improve the manufacture of glass for optical purposes. The diameter is twenty-nine inches, and it is two inches and a quarter thick. It is really not a lens, but a plain disk intended for a lens, should its quality be sufficiently fine. The weight is about two hundred pounds. This piece of glass was inspected, on its first public exhibition, by eminent scientific judges. It was by them examined edgewise, transversely, and obliquely; it was viewed by daylight and by candle-light; it was tested by the polariscope and by other means; and after having been thus subjected to a severe ordeal, it was pronounced to be the largest and finest known specimen of the kind."
The promise held out by the foregoing we fear has failed, as in very many previous cases, or the world ere this time would have heard of its success. An achromatic object-glass for telescopes consists of at least two lenses, the one made of flint-glass, and the other of crown-glass. The former, possessing least power of dispersing the colored rays relative to its mean refractive power, must be of greater value than the latter. It is upon this principle that the achromatism of the image is produced, the different colored rays being united into one focus. Flint-glass, to be fit for this delicate purpose, must be perfectly homogeneous, of uniform density throughout its substance, and free from wavy veins or cords.
From the foregoing, the reader will see that, as has been said, the chief difficulty which exists in making telescopic lenses arises from want of pure glass. Every attempt to correct this evil has failed; it is well known our best telescopes and like optical instruments have always achromatic lenses, and for photographic purposes achromatic lenses are indispensable. If philosophers and astronomers have with so imperfect lenses attained so much, what may not the astronomer look for when science gives him lenses made from pure glass? If the heavens, by imperfect instruments, have so far been unveiled, to what extent may he not then be able to penetrate the pure ether, and reveal planets and heavenly bodies as yet unknown?
We close our reminiscences of Glass and its manufacture, by presenting to our readers a view of an American model glass factory of the present day.[4] By comparing this view with the sketches heretofore given of the early Venetian and French factories, they will perceive the very great improvement which is apparent over the ancient plans, an improvement conducing alike to the health and comfort of the workmen. Thirty years have passed in its development, during which many difficulties arose from the conflicting opinions of the English and German glass-makers; and, in fact, it was not until the proprietors boldly separated themselves from the current and influence of old, and almost fixed opinions, that any decided progress was shown in the development of manufacturing efficiency, or any plan contributing to the health and comfort of the workmen employed.
[4] ] See drawing No. 4, at end of book.
It is to be borne in mind that the first glass works in this country were established by the Germans, who used no other fuel than wood, the furnaces for window-glass constructed under their directions being for that fuel only; on the other hand, the English workmen who introduced the making of flint-glass had made use of no other fuel than coal, and the English were therefore obliged to adopt (for the want of coal) the German plan for furnaces, and adapt the same to the making of flint-glass. The house was like the furnace, half English and half German, and from the year 1812, for thirty years, little or no improvement was made in this particular. Year after year the old plan was followed, until necessity paved the way for new plans in the effort to secure a less expensive mode of melting glass.
The result has been highly favorable. More than one half has been saved in the melt, annealing leers, and working places, yielding the workmen greater space and facilities in performing their work, and no longer exposing them to the discomfort of extra heat, smoke, and unhealthy gases. These improvements have enabled the American manufacturer to sustain his business in the severe and trying competition with foreign manufacturers, who forced their glass into this country through their agents a few years since, in such quantities, and at such reduced prices, as seriously to affect the prosperity of our artisans; yet, aided as they have been by a tariff directly promoting foreign interest, and by the very low rates of wages paid on the Continent, they have been successfully contended with, and now a home competition has sprung up, reducing prices below a fair standard,—a competition, the result of enterprise, which will, erelong, regulate itself, for we fully hold to the maxim, that competition, honest and well sustained, is the soul and life of business:—
"No horse so swift that he needs not another
To keep up his speed."
There is no mechanical employment in this country yielding so good returns to the industrious as a good worker in glass, of the present day, can secure in the exercise of his skill. And we may still further say that there is no mechanical branch of industry offering such advantages for the full manifestation of a workman's real skill and industry, if the conventional usages which restrict the work could but be abrogated,—usages tending to a limited amount of work, and consequently making the workman to realize but a limited amount per week. Such workmen, of all others, should be allowed the inherent and inalienable right to work as long, and at such times, as the individual may deem for his comfort and interest.
We have expressed the opinion that the manufacture of glass is as yet but in its infancy. The experience of every day confirms the assertion, and illustrates the maxim that "life is short, art is long."
The time is not far distant when this country will become, we think, the largest exporter of glass, and the manufacture compose a most important item in every assorted export cargo. In this connection a hint to ship-owners may not be amiss. It is well known that in England, when a ship is put up for a foreign port, it is the custom to rate the freight according to the value of the merchandise,—dry goods paying the highest freight, hardware the next highest, earthen and glass ware the lowest. If our merchants would adopt this plan, very many of our bulky manufactures would find a market abroad; when, however, the same rate is required for a cask of glass ware as for a case of silks or prints, it taxes the latter a small percentage, but practically vetoes the export of the glass.
Our task is now ended; our object has been to give a simple and succinct outline of the characteristics and progress of the Glass Manufacture, to suggest such hints as might bear upon the further advance of the art, and the preservation of those practically identified with the manufacture, and, if possible, to attract the attention of those hitherto unacquainted with its nature and history. If we have neglected the maxim that "those who live in glass houses," &c., it has not been from the want of honest endeavors to remember it; and if we have contributed either to the instruction or the pleasure of any reader, (and this is our hope,) we shall not regret the hours spent in the preparation of this little work.
APPENDIX.
RECEIPTS, ETC.
There are plenty of receipts for the composition of flint or crystal glass, but no mixture that we know can secure a uniform shade in each pot. The component parts of glass are well known, and the mixer's sure guide is to watch the effect of heat on each pot, for he soon finds the mixture that gives good color in one pot will in another in the same furnace prove bad. If he possesses sufficient knowledge of the chemical causes, he can correct the evil.
Among the valuable receipts for rich colors is the following, for RUBY GLASS, which takes lead both in cost and richness:—
Take one ounce of pure gold; dissolve in a glass vessel two ounces pure sal ammoniac acid, and five ounces of pure nitric acid, which will take six to seven days; drop in at a time say one twentieth part of the gold. When the first piece is dissolved, drop in another twentieth portion of the gold, and so on until the ounce of gold is all dissolved. This will require twenty-four hours. Evaporate the solution to dryness. Then prepare in a glass vessel six ounces pure nitric acid, two ounces muriatic acid, and one ounce of highest proof alcohol; mix them well together, and drop in pure grained tin a bit at a time, but beware of the fumes. Stir it well with a glass rod; dilute the solution with eighty times its bulk of distilled water; then take the prepared gold, dissolved in a quart of distilled water, and pour it steadily into the solution of tin as above prepared, stirring all the while. Let it settle twenty-four to thirty hours; pour off the water, leave the settlings, pour in two thirds of a quart of water. Stir it thoroughly; let it settle thirty hours; pour off as before, and filter the precipitate through filtering paper. The result is the purple of Crassus. The ounce of gold thus prepared must be well incorporated with the following batch: say thirty-two pounds fine silex, thirty-six pounds oxide of lead, sixteen pounds refined nitre; melt the same in a clean pot, one little used, and smooth inside; when filled in, put the stopper to the pot loose, leaving it slightly open; leave it five or six hours, or time to settle, then a back stopper can be put up. In the usual time it will be ready to be worked out in solid, egg-shaped balls, and exposed to the air to be partially cooled; they are then to be placed in the leer under a strong fire, which will in two or three hours turn them to a red color; then the pans may be drawn slowly to anneal the balls.
It is well known to mixers that colored glass is derived from metallic oxides. To obtain the proper color depends on the purity and strength of the metallic oxides. The following receipts have with success been used:—
ALABASTER.
| To | 500 | lbs. | of batch add |
| 30 | " | phosphate of soda, | |
| 10 | " | allumine,—i.e. calcined alum, | |
| 3 | " | calcined magnesia. | |
BLACK.
| To | 1400 | lbs. | of batch add |
| 180 | " | manganese, | |
| 100 | " | calcined iron scales, made fine, | |
| 20 | " | powdered charcoal, | |
| 10 | " | arsenic. | |
CANARY.
| To | 100 | lbs. | of batch add |
| 8 | ounces | best oxide of uranium, | |
| 1 | dr. | oxide of copper. | |
The common colors of purple, blue, emerald, or green, are too well known to require to be repeated here.
The following receipt for crystal glass is on the European standard, viz.:—
| 1200 | lbs. | silex, |
| 800 | " | red lead, |
| 440 | " | pearlash, |
| 50 | " | nitre, |
| 10 | " | phosphate of lime, |
| 10 | oz. | white oxide of antimony, |
| 24 | " | manganese, |
| 32 | " | arsenic, |
| 20 | " | borax. |
GERMAN SHEET GLASS.
| 400 | lbs. | silex, |
| 130 | " | soda, |
| 126 | " | hydrate of lime, |
| 4 | " | charcoal, |
| 7 | " | nitrate of soda, |
| 4 | " | arsenic, |
| 1 | " | manganese. |
Gold-colored spangles may be diffused through the glass by mixing gold-colored talcs in the batch.
AGATE.
| To | 150 | lbs. | flint batch add |
| 10 | " | phosphate of lime, | |
| 6 | " | arsenic. | |
BLACK.
| 600 | lbs. | flint batch, |
| 40 | " | manganese, |
| 46 | " | oxide of iron. |
LIGHT EMERALD GREEN.
| 200 | lbs. | flint batch, | |
| 2 | ½ | " | iron filings, calcined, |
| ½ | " | antimony. |
ORIENTAL GREEN.
| 110 | lbs. | flint batch, |
| 1 | " | oxide of uranium, |
| 2 | oz. | carbonate of copper. |
OPAL.
| 500 | lbs. | batch, |
| 60 | " | phosphate of lime, |
| 4 | " | arsenic, |
| 20 | " | nitrate of soda. |
Said to turn without cooling.
William Gillender, of England, gives the following receipt for Bohemian Red, or Ruby:—
| Sand, | 62 | lbs. |
| Lead, | 76 | " |
| Nitre, | 22 | " |
| Antimony, | 8 | oz. |
| Manganese, | 3 | " |
Add one ounce of purple of Crassus to every eighty pounds of the above batch.
WAX RED.
| To | 15 | lbs. | flint batch add | |
| 1 | " | raw brass, | ||
| ¾ | " | crocus martus. | ||
This he says is very good.
TURQUOISE.
| To | 1100 | lbs. | flint batch add |
| 90 | " | phosphate of lime, | |
| 15 | " | arsenic, | |
| 15 | " | calcined brass dust. | |
VIOLET.
| To | 100 | lbs. | flint batch add | |
| 1 | " | calcined brass, | ||
| 1 | ½ | " | zaffre. | |
Receipts for window-glass are as numerous as for flint. The following are in general use in England, so says Gillender:—
CROWN GLASS.
| Sand, | 1400 | lbs. |
| Quick lime, | 480 | " |
| Sulphate of soda, | 560 | " |
| Charcoal, | 25 | " |
PLATE GLASS.
| Sand, | 300 | lbs. |
| Sulphate of soda, | 450 | " |
| Quick lime, | 100 | " |
| Nitre, | 25 | " |
| Charcoal, | 5 | " |
DIAMOND GLASS.
Four pounds of borax, one pound of fine sand; reduce both to a subtile powder, and melt them together in a closed crucible set in an air furnace, under a strong fire, till fusion is perfect. Let it cool in the crucible, and a pure, hard glass, capable of cutting common glass like a diamond, which it rivals in brilliancy, is produced.