Through a glass (less) darkly

I made another visit to Emma at the Leeds Museums Discovery Centre to see how the glass from the University of Leeds collection is coming along. It’s certainly much easier to see the underlying condition and colours of the glass, now that the dirt of the last hundred years has been removed! In Emma’s view, the glass had been cleaned when first excavated, so any interesting content residues had probably been lost at that stage. What remained was black, sooty dirt – a reminder of the air quality in industrial cities such as Leeds at the turn of the 19th century, when this collection arrived in Yorkshire.

We can now see clearly the bluey-green glass of this unguent bottle, broken at the neck. One of the qualities of glass is that it’s near-impossible to tell whether breakages are ancient or modern, but it’s quite likely that the neck was broken in antiquity so the contents could be poured.

Unguent bottle after cleaning © University of Leeds

Unguent bottle after cleaning
© University of Leeds

It’s also possible to make out the air bubbles trapped in the base of this candlestick vase, an indication of its method of manufacture. This might be considered a flaw, but I prefer the view expressed by J.H. Middleton in The Engraved Gems of Classical Times:

‘A great deal of the superior beauty of ancient glass is due to the presence of these minute air-bubbles, each of which catches the light and radiates it out from the body of the glass, thus making it internally luminous, not merely transparent.’

Air bubbles in base of candlestick vase © University of Leeds

Air bubbles in base of candlestick vase
© University of Leeds

The objects also show the effects of their long burial in the earth. Surface layers are peeling away from the base of the second candlestick vase (delamination).

Base of candlestick vase showing delamination. © University of Leeds

Base of candlestick vase showing delamination.
© University of Leeds

This produces an iridescent effect, as can be seen in this tubular glass bottle.

Bottle with iridescent surface © University of Leeds

Bottle with iridescent surface
© University of Leeds

This is because alkali are leached from the glass, the rate depending on the warmth and acidity of the soil it is buried in. Silica is also lost, though in lesser quantities than alkali. This means that thin layers of silica are left behind, creating an opalescent appearance and increasing the opacity of the glass. (For a fuller explanation, see S.P. Koob, Conservation and Care of Glass Objects). These layers of weathering form part of the objects’ history, and help to tell the story of their journey from manufacture to use, burial, and eventual excavation.