Screwpiles: The Forgotten Lighthouses

Fresnel Theory Meets Glass-Making Reality

Saint-Gobain produced the glass that was poured into molds to form the individual prisms and lenses. The components were then sent to the Paris factory for final grinding and polishing. Mechanical grinding processes were developed fairly early, but the final polishing was done by hand to precise specifications. English red rouge was used by local women—contracted by Saint-Gobain—to polish the glass elements to their finished dimensions.

Optical-quality glass making in the early 1800s was in its infancy. There were several problems that had to be overcome in order to produce Fresnel’s lenses. First, much trial and error was needed to settle on the precise mix of components in the glass mixture. Slight changes in the quality of the ingredients and variations in the mixture caused the poured glass to be inconsistent. Second, the molds had to be accurately made and the pouring of the molten glass into the molds had to be

Thomas Tag, U.S.L.H.S.

done while avoiding the creation of bubbles and other distortions in the glass. There were many rejects of molded prisms that had inconsistent dimensions. Third, tedious grinding and polishing had to be done by hand on each prism to bring the glass to its final dimension. Finally, the individual prisms had to be assembled and then kept in place with absolute accuracy or the entire lens would be ineffective. 16 Saint-Gobain was the only factory with a process capable of consistently making the high-quality glass required for the Fresnel lenses. Their formula produced a very hard, crown glass with a slight green tint.

Saint-Gobain grinding process, circa 1820 Thomas Tagg, U.S.L.H.S.

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