Flint Glass before Faraday

Although Newton had concluded that refraction of light by glass lenses was always accompanied by dispersion, John Dollond in the 1750s undertook a series of experiments which demonstrated that the dispersion caused by a convex lens of low density crown glass could be corrected by a concave lens of higher density flint glass. In 1758 Dollond was granted a patent for his novel achromatic doublet, which consisted of a convex crown lens cemented to a concave flint lens, and in 1763 his son, Peter, made the first triple achromatic lens, consisting of a concave flint coupled with two convex crowns. Astronomers were delighted with Peter Dollond’s achromatic lenses, and demand for his telescopes quickly spread across Europe.

Dollond’s output of achromatic lenses was limited, however, by the shortage of suitable flint glass blanks. Because flint glass contains a high proportion of lead it is very difficult to obtain large homogeneous blanks. Thus, most lenses of the time were small and of relatively poor optical quality, often containing small bubbles and density gradients, and tints due to metallic impurities. Very rarely were discs larger than 4 inches in diameter suitable for lenses, and those that were, were sold only to privileged customers. So increases in the size of achromatic telescopes and improvements in their optical resolution both awaited the manufacture of high-quality flint glass.

In England, however, the manufacture of all types of glass, even on the smallest scale, was subject to a debilitating excise duty. The excise regulations in Faraday’s time required

John Dollond, for example, had concluded that it would be impossible to carry out experiments on glasses of various refractive indices under the regulations of the Excise Act.

While the British glass industry thus stagnated, a Swiss cabinet-maker named Pierre Guinand taught himself the basics of glass manufacture and began trial melts of flint glass about 1783. After producing some fine 4 and 6 inch discs, Guinand in 1805 began to produce larger discs of exceptional quality, principally due to his use of a fireclay stirrer, which he used to keep the molten glass well mixed until, upon cooling, it became too viscous (5). Later, he developed the method of making large optical blanks by pressing softened homogeneous glass into a circular mould, and discs of 12 inches in diameter became a possibility. Guinand passed his techniques on to Joseph Fraunhofer, a brilliant optician who began to accurately measure the refractive indices of his lenses using the monochromatic yellow light of a sodium lamp (6). Soon Fraunhofer began to design and construct achromatic object-glasses that surpassed the best English specimens, and which still compare well with modern lenses. The Dorpat refractor, which so aroused the envy of the British, was an outstanding example of Fraunhofer’s ability and remained for many years, with its inch object-glass, the largest refractor in the world.

Faraday’s Researches

In response to this threat from abroad, the British government agreed to remove the restrictions on glass and to “bear all the expenses of furnaces, materials, and labour, as long as the investigations offered a reasonable hope of success (7)” and, as mentioned previously, Michael Faraday was appointed to the research. In 1825, the experiments began at the Falcon Glass Works of Apsley Pellat and James Green, but were transferred to the Royal Institution in September 1827, at which time a suitable room with furnaces was provided for Faraday’s convenience (8). Faraday continued his investigation for four years, and the results were reported in the Bakerian lecture of the Royal Society for 1829, published in 1830 (9).

Early in the work, Faraday recognised that the greatest problems in flint glass manufacture were caused by lead oxide, which comprised nearly one-third of the weight of the glass. In the original melt the lead oxide tended to sink to the bottom, leaving an inhomogeneous glass full of striae. Faraday preferred instead a new glass, which he termed

The use of a platinum tray as a vessel for the final stages of the glass manufacture was a unique innovation, one possible in England where malleable platinum was available (although only in small amounts) from first William Hyde Wollaston, and then Percival Norton Johnson. Faraday found that the platinum sheeting served well as a vessel for molten glass.

The largest glass blank Faraday produced was 7 inches square and 0.8 inches thick, and required a platinum sheet 10 inches. square which had been folded into a square tray. He was able to use the platinum trays for more than one glass blank by carefully placing the folds in new positions, or by using the platinum sheet for glass blanks of ever decreasing size. Such platinum vessels maintained the purity of the glass melted in them, and were an improvement over the porcelain or earthenware crucibles generally used.

A second important application of platinum was in the stirrer, which by necessity had to be rigid and strong, and chemically inert to the molten glass. Faraday designed a stirrer with a platinum blade

A third role for platinum was as a centre of nucleation for gas bubbles, with the metal in spongy, powdered form. Faraday had observed that, in unstirred glass, tiny bubbles often flawed the final product. By adding 7 or 8 grains of spongy platinum (carefully prepared by Wollaston’s published procedure of 1828) to each pound of glass, the platinum

In addition to these uses of platinum, Faraday often used a platinum ladle which was simply constructed by riveting a platinum crucible to a platinum wire, which was in turn attached by a screw to an iron rod handle.

Thus, aside from his formulation of a new “silicated borate of lead” glass, Faraday’s major innovations to the techniques developed and held in secrecy by Guinand were the use of platinum vessels for the molten glass, platinum instruments for the stirring and transfer of the glass, and platinum powder for the elimination of bubbles. The importance of platinum to Faraday’s process is such that one suspects he believed from the outset that the inert metal held the key to the production of improved optical glass.

Wollaston’s Contribution

It should not be surprising to find Wollaston’s name connected with the early experiments, for until his death in 1828 he was constantly seeking to broaden the commercial market for platinum. Further he was a member both of the Royal Society’s select committee and the Board of Longitude which jointly chose Faraday as a member of the Glass Committee. It may well have been Wollaston who first thought of using platinum vessels for molten glass, for in his research notebooks is evidence that he supplied the platinum, apparently without charge, for the early experiments.

By the 1820s Wollaston could no longer obtain enough crude platina to fill his outstanding orders for boilers and siphons, and his last production run of malleable platinum was carried out on 2040 oz. of scrap and remnants in 1820/21 (14). Nearly all the malleable metal produced went to William Cary, but three ingots of about 18 ounces each remained in Wollaston’s hands in March 1825. One of these is recorded as being “taken for Pellatt” (15). Elsewhere in the notebooks is a folded sheet of paper detailing the preparation of this platinum ingot for the “Glass Committee” in April 1826 (16). The ingot was flatted to a thickness of 1/55 inch, resulting in a sheet 14.9 inches long and 7.4 inches wide, weighing 18 ounces. From this a strip 2.05 inches wide was cut from the length, “flatted alone to 3 inches [in width], then in copper of oz. per [square] foot till beyond square” (17). From the resulting sheet, Wollaston cut a circle of inches diameter with a calculated thickness of 1/280 inch; this was recorded as the second disc. The first disc, of similar dimensions, had been prepared on 2 March 1826; thus Wollaston was supplying platinum to Pellat for the use of Faraday well before the experiments were shifted to the Royal Institution (in September 1827). In February 1827 Wollaston used the remainder of the flatted ingot to prepare two large circles, each having a diameter of about 13.5 inches and weighing about 6 oz. In March 1827, Wollaston took the remnants from these last two circles, which he characterised as “being shreds & nearly useless”, and rolled them in copper to give five very thin (ranging from 1/410 inch to 1/865 inch in thickness) circles of 4 to 5 inches in diameter (18). It is likely that these thin sheets were placed under the glass-containing platinum vessels, as recommended by Faraday to prevent contamination of the vessels by their support.

There is no evidence that Wollaston continued to supply platinum to the Glass Committee after the one ingot was consumed, and for further work platinum was probably purchased from P. N. Johnson. In his paper on the subject Faraday mentions “the great expense of the material: the value of the plate in question [weighing 3.6 oz.] is about £6 10 s”. (19). This corresponds to a price of about 36 s. per oz., twice Cary’s usual selling price for Wollaston’s platinum.

Further evidence of Wollaston’s close collaboration with Faraday is given by a letter from Wollaston to Faraday dated 27 June 1828:

Although the details of the letter are difficult to interpret, it is clear that Wollaston was taking an active part in the chemical analysis of the optical glasses as late as June 1828, just a few months before a brain tumour caused his death on the 22nd of December.

The importance of platinum to the potential success of Faraday’s research, quite likely foreseen by Wollaston, was recognised by Davies Gilbert, who read, as president of the Royal Society, the citation for Wollaston’s Royal Medal in 1828. The award, given for the process of rendering platinum malleable, was criticised by some who felt that Wollaston was undeserving of a scientific award because he had profited by the process while holding it secret (20). Gilbert, one suspects, was eager to diminish such criticism by emphasising the value to science of Wollaston’s process. His citation read, in part:

Faraday’s Optical Glass

Faraday’s optical glasses were not, unfortunately, to usher in the new “epoch in the history of optics” that Davies Gilbert had anticipated. Although the glasses were quite homogeneous and free from flaws, they performed no better than the glasses produced more cheaply by standard techniques. Even though a report by the Committee for the Improvement in Optical Glass presented in 1831, stated that

Faraday lost enthusiasm for glass-making and turned to his more productive electrical researches. His general opinion on the matter was that “the best step to ensure improvement will be to take off the Excise duty” (23), which the government finally did in April 1845. Only then did Britain, drawing on the expertise of foreign craftsmen, re-establish her pre-eminence in glass manufacture.