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Today the standard for fine grit is Alumina and Cerium Oxide. However, in the 1600s, the elements Al and Ce were not known.

What type of fine grit did they use to grind and polish the lenses for telescopes?

I've hunted for this online quite a lot, but cannot find a single website that talks about grit material. They talk a little about grinding machines, but I'm not at all interested in that right now. From what I gather on amateur telescope makers on YouTube, one person can grind out a lens in a day or two, and it's not back-breaking work, just time-consuming.

I understand the basic regiment of using coarse grit, then smaller grit, and eventually to the smallest finest grit possible which today is apparently Cerium Oxide. Remember that visible light is on the order of 400 nm wavelength, so ideally you should grind it down to be smooth at that size, which requires some really fine abrasive grit particles.

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    $\begingroup$ According to the Engineering and Technology History Wiki, in the 17th Century lenses were ground on a lathe and then polished using pitch resin. $\endgroup$
    – nwr
    Commented Jan 10, 2023 at 23:09
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    $\begingroup$ @nwr Pitch is applied to the polishing tool as a base material, not abrasive. Pitch is a substance used to hold the grit. The pitch is slightly soft to conform to the shape of the lens, and abrasive grit is added to the lens, then the polishing tool is rubbed onto it. Pitch does not act as an abrasive and does not do the actual machining away of glass material. $\endgroup$
    – DrZ214
    Commented Jan 10, 2023 at 23:33
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    $\begingroup$ The Corning Museum of Glass video Lens Making in the 1600s may be of some interest. It suggests that the finest polishing was done using "the finest sand or even putty". $\endgroup$
    – nwr
    Commented Jan 11, 2023 at 0:29
  • $\begingroup$ @nwr The finest sand, so what was the finest sand available in the 1600s? That description alone doesn't answer what material it is. To answer that, 2 things are needed: the mineral (e.g. corrundum, silica sand, diamond) and the avg particle size. Also @1:55 "We don't know how many different grits van Leeuwenhoek used." This is the sort of "blank answer" I find a lot, and I'm starting to wonder if these primary sources even wrote down what it was. Edit: not trying to be negative, I'm just trying to illustrate that this mysterious lens making process is extremely elusive and not sure why. $\endgroup$
    – DrZ214
    Commented Jan 11, 2023 at 1:49
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    $\begingroup$ You don't have to know the elements Al and Ce to find some mineral that, ground to a fine powder, makes a good abrasive. Whether or not you determine that mineral to be an oxide of Al or Ce a few centuries later is largely irrelevant. $\endgroup$ Commented Jan 11, 2023 at 10:15

3 Answers 3

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There is an entire article devoted to your query:

From "Lens Making for Scientific Instrumentation in the Seventeenth Century" by Silvio A. Bedini (it is behind pay-wall), Appl. Opt. 5, 687-694 (1966).

The glass blank was fastened to a muller, or handle, and held against the turning pattern. The mullers were made of wood or hollowed stone. One type of glue was made from an infusion of liquified yellow pitch, resin, and finely ground clay to which was added a dram of pulverized glass. Another glue for the same purpose consisted of melted black Spanish pitch, sealing wax, and plaster.

Traditional abrasives were used for lens grinding and polishing. According to Bolantio, the best abrasive was Venetian rottenstone as it could be pulverized and levigated so that gritty particles were reduced to a minimum. Well-purged and minutely ground emery was normally used for polishing; white sand and tripoli were also common. For the final polishing, the pattern was replaced by a wooden form of the same shape. The fine abrasive was dampened and applied to a buckskin fitted tightly over the form. ${ }^{10}$

From there, reference 10, appears to be very relevant "From an Italian 17th-century manuscript account entitled "A Treatise on Optics by Giovanni Christoforo Bolantio" which forms part of a bound manuscript volume with the general title "Trattato Sulla Maniera per Costruire Orologij" in the author's collection.

Perhaps a very serious scholar of science history can get hold of that treatise through major world libraries. I have no idea.

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  • $\begingroup$ Possibly also of interest (I can't check, pay-walled): Giuseppe Molesini, "Telescope Lens-Making in the 17th Century: The Legacy of Vangelista Torricelli," Optics & Photonics News 21 (4), 26-31 (2010): " In the early days of instrumental optics, the know-how of lens making was shrouded in secrecy. A remarkable exception can be found in a letter from 1643, in which the Italian lens-maker Vangelista Torricelli gives his correspondent directions on optical shop practices. " $\endgroup$
    – njuffa
    Commented Jan 11, 2023 at 2:55
  • $\begingroup$ Very nice answer. One thing, For the final polishing, the pattern was replaced by a wooden form of the same shape. This seems a little strange. If it's just a wooden form matching the "negative" of the lens, then I don't see why it wouldn't be used all along. Wood is cheaper than the glass blank itself. And also, what is the "pattern" it is replacing? $\endgroup$
    – DrZ214
    Commented Jan 11, 2023 at 3:04
  • $\begingroup$ Possibly of interest: R. Willach, "The Development of Lens Grinding and Polishing Techniques in the First Half of the 17th Century", Bulletin of the Scientific Instrument Society No. 68, March 2001. $\endgroup$
    – njuffa
    Commented Jan 11, 2023 at 3:42
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    $\begingroup$ @DrZ214 For obtaining a proper shape of a spherical lens, the two glass blanks wear at the same rate, and the symmetric movement translates to spherical symmetry in the lens. At the final stages of polishing, the shape no longer changes much so a wood (or more commonly pitch) tool can be used, which allows for grooves that provide space for the glass dust to settle. While much newer (1938), the book Procedures in Experimental Physics has great descriptions and images about the traditional process of lens making. $\endgroup$
    – jpa
    Commented Jan 11, 2023 at 7:56
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    $\begingroup$ @DrZ214 - I think the key is the buckskin placed over the wooden form in those final polishing stages $\endgroup$
    – davidbak
    Commented Jan 13, 2023 at 16:58
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One of the early publications covering the topic is Pere Cherubin d'Orleans, La Dioptrique Oculaire, ou La Theorique, La Positive, et La Mechanique, de l'Oculaire Dioptrique en Toutes Ses Especes. Paris: Thomas Jolly & Simon Benard, 1671. Polishing materials are discussed on p. 353:

De La Matiere, Du Mordant

Dans le besoin, l'on ne fait pas élection particuliere, de la matiere du mordant; le sable de riviere, peut passablement servir, à former le verre, il ne dure pas neantmoins assez au travail, perdant en peu de temps sa pointe, il est trop lent en son effet. L'émeril, au contraire, es trop dur, pour le verre; & pour les platines de leton: ausquelles, il laisse une mauvaise qualité, incrustant leur superficie, de sa poussiere; qui y retenant long temps sa pointe, gaste ordinairement les verres, que l'on veut adoucir, & conduire au polir: les rayant de traits, que l'on ne peut souvent oster, sans recommencer le travail, ce qui est importun. Le grez mediocrement dur, est donc le plus propre pour ce travail; il se trouve assez communément, chez ceux qui se servent de meules à aiguiser, lesquelles se rompants souvent, leur demeurement inutiles, à autres choses. Ce grez n'a besoin d'autre preparation, que d'estre broyé en poudre, l'on en doit toûjours conserver séparément, de trois, ou quatre degrez de force: pour les employer selon la qualité du travail. On les séparera fort commodément, mettant tout le grez qui a servy, dans un grand vaisseau plein d'eau, & le mouvant bien; car le laissant un peu r'asseoir, tout le plus gros ira au fond; & alors inclinant promptement ce vaisseau, pour remettre toute cette eau dans un autre, tout le plus subtil, s'y écoulera avec l'eau; que l'on laissera entierement r'asseoir, écoulant en suite doucement l'eau, pour avoir le grez, qui sera demeuré auf fond. Ce que reїterant diverses fois, l'on aura séparément, tant de degrez de force, de ce grez; que l'on voudra. On le conservera en des vaisseaux, séparément.

Father Cherubin states that if need be, ordinary river sand could be used to grind glass, but that it is too soft and will not last long. Emery on the other hand is so hard that it causes deep scratches that are hard to polish away and may force one to start the grinding process afresh. He recommends use of a material of medium hardness in the form of broken grindstones (made from sandstone), which have no other uses and are commonly available. These are to be pulverized, and the resulting material is to be separated into three or four different classes by particle size. For this, the pulverized grindstone is vigorously stirred into water. Some time is allowed for the coarsest particles to settle and then the remaining liquid (containing smaller particles still in suspension) is quickly poured into the next vessel, where the settling process is repeated, and so on.

The next section on the same page discusses polishing, for which Father Cherubin recommends tripoli or putty powder:

Du Tripoly

Pour polir les verres déja formez, & adoucis par le mordant; l'on se peut servir de tripoly, ou de potée d'estain. Le meilleur tripoly, est celuy d'Allemagne. Mais de quelque sorte de tripoly, que l'on se serve, le plus leger, est toûjours le meilleur. L'on s'en peut servir en pierre, comme la nature le produit, lors qu'il a les bonnes qualitez; autrement, on le broyera parfaitement avec de l'Eau-de-vie, ou à defaut avec du vin blanc; & l'on en mettra une quantité ensemble, dans un vaisseau de verre, bien fermé; car en cette maniere, il se fermentera, & adoucira excellemment. Apres que ce tripoly, aura demeuré dans son vaisseau, quatre, ou cinq mois, l'on en pourra prendre, & en faire de petites masses, que l'on laissera sécher a l'ombre, pour s'en servir à sec: l'on pourra aussi s'en servir mol, & le prendre immediatement comme il est dans vaisseau.

The best tripoli is the German one, but in any case the lighter the better. If it is of good quality, it can be used as found in nature. Otherwise, it can be ground up and mixed with brandy (or, lacking that, white wine) and is then allowed to "ferment" and soften for fourth to five months inside a closed glass vessel. After that time, it can either be air-dried, or used wet straight from the jar.

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The Herschels used Pomeranian mud. (Though the only citation I have in mind is one of Patrick O'Brian's historical novels!)

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  • $\begingroup$ Can you provide any references to back this up? $\endgroup$
    – mdewey
    Commented May 31, 2023 at 12:23
  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Commented Jun 1, 2023 at 3:26

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