Weighty matters
One of the important questions which we will try to answer is the scale of the glass production at Basinghall and this means that we will have to weigh the entire collection, and for some types of production waste, count every fragment.
I have started this week by looking at the tank metal, the raw molten glass from which vessels were blown. There is no evidence from London that glass was ever made from the raw materials. Instead, it was either imported from the Mediterranean in large blocks, or alternatively broken vessel and window glass, known as cullet, was collected for recycling. This was common practice throughout the Roman world where there appears to have been a trade in broken glass, rather like modern bottle banks, and the evidence suggests that it was the prime source of the basic material for the London glassblowers.
The glass was melted in a furnace, in rectangular tanks, hence the term ‘tank metal’ and at Basinghall some large fragments have straight edges, where they lapped the side of the tank. It looks as if the glass has been allowed to cool in the tank as some of it shows distinctive crystallisation, and this might mean that it was unused – perhaps thrown away when something went wrong with the furnace, or the workshop went out of use – there are so many possibilities and questions!
Most of the fragments are a natural blue-green in colour, but some of the large fragments are a much darker green. There is actually quite a lot of variation in colour, but some is due to the size of the fragments, so I have had to record their general size as well as the weight of the tank metal from each context (archaeological layer in the ground). Later in the project we will be looking at the chemical composition of some of the chunks of glass to see if they may have come from different sources.
After a day or so, I have recorded over 18.5k of tank metal from 125 contexts – that is a lot of glass!