Desert amethyst colour

This object, a telephone pole insulator made by Australian Glass Manufacturers and embossed with "AGEE", has been identified as a "CD 490, type II" ( www.cyberbeach.net/~dknetzke/aussie_manufacturers.htm ). Trying to understand why a moulded piece of glass can become such a beautiful and iconic object has occupied me, on-and-off, for more than 35 years. Some of its properties remain a puzzle.

The first of its remarkable attributes is its beautiful 'amethyst' purple colour. Glass which has taken on this colour has indeed become known as 'Desert Amethyst'. It did not start its life with this beauty: it is something that must be acquired steadily over years of exposure to the harsh Australian Sun, rich in ultraviolet rays - glass sunburn. As well as its principal constituents, silicon dioxide, sodium oxide, lime, magnesia and alumina, utilitarian glass always contains impurities, the most abundant of which is usually iron. Iron makes glass green: hence the ubiquitous green bottle. The glass melt for this "CD 490" probably contained some manganese oxide in order to 'whiten' the green tint; though why this was necessary for a glass insulator I don't know. Anyway, the manganese in its divalent form (Mn2+) responds to Solar ultraviolet radiation by losing another electron and becoming trivalent (Mn3+). This makes what is called a 'colour center' or 'F-center' which absorbs green and yellow light to make purple. A similar process occurs in natural quartz amethyst except that it is the iron in this case that makes the colour center: trivalent iron (Fe3+) in the crystal is irradiated to make [FeO_4]^4- and an electron.

The transmission spectra of both desert and quartz amethysts are shown in the top panel. While they are clearly similar, showing a dip in the green/yellow, the desert amethyst is a lot more complicated, both in terms of the number of absorbing components and the remarkably high transmission around 405nm: an unusual spectrum.

The fluorescence spectra of the insulator are shown in the previous post. I am still working on figuring these out.

The lower panel here illustrates a rather esoteric effect that I noticed while making the transmission measurements. To get enough light through the glass, I used a roughly focussed incandescent filament and bounced the light around the dome at the top of the insulator. This produced high signal/noise spectra which are very 'rough' (the three middle spectra) - quite unlike the usual transmission spectra I get when I use a diffuser after the lamp. Light shining directly through the plane glass at the bottom of the insulator (the bottom spectrum of the four) is much smoother. It seemed to me that the absence of the diffuser resulted in multiple paths of transmission through the glass (which has smooth but 'bobbly' surfaces) of light with a significant degree of coherence that they all interfered with one-another. To test this, I took a pair of spectra with and without the diffuer at the top of the insulator with the same positioning of the lamp and the spectrometer fibre (the top and the middle of the five spectra). Indeed, it seems that the diffuser destroys the coherence and produces a smooth spectrum - albeit with lower s/n.

To be continued...

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