October 28, 2010 #099 - Producing Lens Blanks in the 1800's

During my reading of historical journals about lens manufacture, I found information recorded on how lenses were ground and polished but very little on how the glass was manufactured to produce the raw blanks. I found an article in the 1887, "Publications of the Lick Observatory Vol. I", giving a good explanation on the blank making procedures of the period as it related to the construction of the Lick Observatory telescope. Below is an interesting explanation of the process, very time and labor intensive, but it worked.

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During the summer of 1874, Mr. D. O. Mills visited Washington and New York and had frequent consultations with astronomers and others, specially with Professors Newcomb and Holden in Washington, and with Dr. Henry Draper in New York.

It was decided that Professor Newcomb should go to Europe to investigate the matter of procuring the glass necessary for a large reflector or a large refractor...

Professor Newcomb's report (dated March 4, 1875) is as follows:


REPORT OF PROFESSOR NEWCOMB

"I have the honor to report to the Lick Trustees, that in accordance with the request conveyed in your letter of December 3 to Mr. D. O. Mills, I have made a journey to Europe for the purpose of collecting such information as might enable the Trustees to decide upon the course to be adopted in order to secure for the State of California a telescope ' larger and more powerful than any yet made.' For this purpose I communicated with such of the leading men of science of England, France, and Germany as were most likely to give me either suggestions or information, having especially in view to learn (1) what is likely to be the limit of available size of a refractor, and (2) what optical firms could prudently be entrusted with the construction of such an instrument as that required by the Trustees. Beginning with the latter question, the result of my inquiries was, as might have been expected, that the majority of the opticians did not come within the range of the requirement. Not deeming it necessary to specify each firm by name, I may say in a general way that most of them, while of high character as men of business, had never successfully undertaken the production of a telescope of any but the smallest size, and there was no good reason to believe that an attempt on their part to make one of the size required would end in anything but failure.

Special details are given by Professor Newcomb of his negotiations with four several firms, viz.: G. & S. Merz, of Munich; Thomas Cooke's Sons, of York; Eichens & Martin, of Paris, and Howard Grubb, of Dublin; and copies of a set of provisional specifications drawn up by Professor Newcomb, and by him transmitted to these gentlemen, together with their replies and estimates, are appended to his report. As circumstances brought these negotiations to an end, it is not necessary to give this matter here. Professor Newcomb's report on the method of glass founding adopted by the two most celebrated makers has a permanent value, and is accordingly given below.

The Glass Discs.

"There are but two parties to choose from: Messrs. Chance Bros. & Co., of Birmingham, and Ch. Feil, of Paris. I regret to say that I could collect no certain evidence that either of these parties can really be relied on to furnish discs of much more than 30 inches in diameter within any reasonable length of time, and the claims of the two are so nearly balanced that I must request the Trustees to delay deciding between them until I have investigated the specimens of glass which each has furnished, in order to learn which is the best adapted to a great objective.

"The state of the case will be better understood by explaining the process of making such discs, as I learned it from the parties themselves and from others. The materials are mixed and melted in a clay pot holding from 500 pounds to a ton, and are constantly stirred with an iron rod until the proper combination is obtained. The heat is then slowly diminished until the glass becomes too stiff to be stirred longer, when the mass, pot and all, is placed in the annealing furnace. Here it must remain undisturbed for a period of a month or more, when it is taken out; the pot and the outside parts of the glass are broken away to find whether a lump suitable for the required disc can be found in the interior.

"If the interior were perfectly solid and homogeneous, there would be no further difficulty; the lumps would be softened by heat, pressed into a flat disc, and re-annealed, when the work would be complete. But in practice the interior is always found to be crossed in every direction by veins of unequal density, which will injure the performance of the glass; and the great mechanical difficulty in the production of the disc is to cut these veins out and still leave a mass which can be pressed into a disc without any folding of the original surface. Two parts or surfaces of the glass can never be joined or welded closely enough to serve the purpose; every change of shape in pressing the glass must therefore be made in the interior mass, as it were, without breaking the surface.

"The Messrs. Chance found it necessary to try a large number of pots of glass before they succeeded in producing one from which a disc of 26 inches clear aperture could be made. The time required for each trial depends on the stage of the process at which the failure is discovered. If on first breaking the pot, the interior mass is found too full of veins, only one or two months will have been occupied. But it may happen that after the disc has all been put into shape, annealed, and ground, the final trial may show a failure, and then several months will have been occupied in the trial. Practically the Messrs. Chance were four years in producing two pairs of 26-inch discs for Messrs. Alvan Clark & Sons.

"I could find only two points in which Feil's process differed from that of the Messrs. Chance:

"(1) His pots are much larger, about twice the size of Chance's, which, it seems to me, must be an advantage.

"(2) His system of cutting out the veins is more slow, careful, and elaborate.

"He reheats the glass a number of times and moulds it into shape gradually, cutting out all the veins he can reach after (each) cooling. As the glass must be re-annealed after every heating, each trial takes a much longer time than in the case of Chance, but final success is far more probable.

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"In the matter of finding a combination of glasses which shall be perfectly achromatic, I regret to say that my conferences with the highest authorities in Europe resulted in nothing satisfactory. Nothing can be done but to select from the specimens of glass furnished by the competing bidders that combination which gives the best results. According to Professor Stokes no improvement can now be made on the usual combination of hard crown and dense flint glass. Should this be so, I do not think any advantage will be gained by attempting more than 34 inches clear aperture."

Clear Sky - Rich