Up until the mid-17 th century, the fabrication of high-quality lenses was the technical and practical prerogative of a few individuals, who sometimes signed their masterpieces like artists. The processes of fabrication - often kept secret - are written in the lenses themselves, and their history can be traced through accurate scientific analysis. Obviously, the great value of historic lenses calls for non-invasive methods, that is, functional tests in which light is used as a probe. The shape of a lens can be measured, its surface micro-roughness determined and minute details observed under the microscope.
The composition of the glass determines the optical characteristics and contains information on the production techniques and origin of the lenses. Basically, two analysis methods are employed:
- spectrophotometry is used to measure the transparency of the glass to various colours and to recognise some of substances present in the vitreous mass;
- X-ray fluorescence is used instead to determine the constituent atomic elements more directly and to identify the basic vitreous matrix.
With the data acquired, the recipes used for the different varieties of glass can be determined.
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