technology Portable sPectroscoPy
archaeology Optical
researchers at the University of Michigan are measuring plant spectral reflectance and absorbance, to understand how plants grow and acclimatise to indoor lighting. Image courtesy of Mojtaba Navvab
optical spectroscopy and the sensing parameters required to do this.
From archaeologists characterising ancient glass to architectural lighting design, portable spectrometers are finding uses in a host of applications, as greg blackman discovers
T
he optical characteristics of glass can tell you a lot about its provenance. Glass originating from Roman times will have a different optical signature than that
from medieval times, say. A full chemical analysis might be required to date it precisely and trace it back to a particular time and place in history, but examination with an optical spectrometer is a good starting point. What’s more, with the modern portable spectroscopy equipment now available, these readings can be made quickly and easily on site rather than having to gather samples and take them back to a laboratory. Archaeologists at the department of Art
History and Archaeology at Vrije Universiteit Brussel in Brussels, Belgium, have teamed up with the university’s department of Applied Physics and Photonics to characterise ancient glass optically from sites within Belgium and abroad. The researchers are using lab-based spectroscopy techniques as well as portable equipment from Avantes to make measurements on site. The aim is to understand what type of archaeological questions can be answered with
20 electro oPtics l May 2011
Ancient glass, like modern glass, consists of a network former, which in most cases is silica (a 3D lattice of silicon and oxygen bonds), and network modifiers. Network modifiers, or fluxing agents, are introduced to lower the melting temperature of silica, as ancient furnaces in particular were not hot enough to melt pure silica. The two main network modifiers used throughout history have been sodium and potassium. However, introducing fluxing agents has the unwanted effect of making the glass more porous. Therefore, a second class of network modifiers (elements like calcium and magnesium, among others) were added to strengthen the glass. In addition, small amounts of metal oxides, such as copper, chromium, cobalt, iron or manganese were used to colour the glass. ‘Chemical additives are in some cases useful indicators to date glass, as there are chronological differences in the chemical composition of glass, especially in terms of the network formers and colouring agents,’ comments Dr Wendy Meulebroeck, a researcher involved in the work at the department of applied physics and photonics. Meulebroeck explains that one technique to date glass is to measure the concentration of iron impurities based on absorption bands in the UV and blue part of the spectrum. Iron concentrations are linearly related to the so-called UV absorption edge – the wavelength at which the transmission value decreases by 50 per cent of its maximum. The technique was used to analyse glass window fragments from the excavation of the Castle of Bladelin in Middelburg-in-Flanders, which dates from the 15th century. ‘Optically, we were able to see differences in the older lozenge [window] fragments (15th to 16th century) and the younger Flemish renaissance-type [glass] (16th to 17th century),’ notes Meulebroeck. ‘For example, the earlier dated lozenge windows have a lower value for the UV absorption edge than the Flemish renaissance parts originating from a later period.’
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