Digging up the Glass Works

What follows is based on a recent visit to the Thornbury Society for Archaeology and Local History by Mr Reg Jackson. The Society meets on the second Tuesday of each month at St Mary's Church Hall, commencing at 7.30pm. There is a small charge of £1.50 for visitors who are always most welcome. Full details of our programme can be found at Mythornbury.com

If you lived in an eighteen century city where there was a need for containers of various types, sizes and colours and where you had easy access to sand, limestone and coal, what kind of industry were you likely to develop? The answer is glass making and Bristol's influence in this industry was considerable and far reaching with some of the product ending up as far afield as Ireland and the West Indies.

In 1760 there were fifteen glass factories in Bristol and Mr Reg Jackson came to the Thornbury Society in March to enlighten us and illustrate what he and his team found when they excavated the glass works in Portwall Lane, close to St Mary Redcliffe Church.

Portwall Lane is exactly in the location its name describes. Medieval Bristol sat in a loop of the River Avon which provided boundaries on two sides. On the third, there was a port wall breached by two gates, one at the end of Redcliffe Street and the other at the end of St Thomas Street. Portwall Lane itself had no access through the wall and was, therefore, a quieter place to live. Mr Jackson was able to say with some certainty that prior to the glass works, the site had been occupied by some fine two-storey, slate roofed houses which were very unusual in the Bristol area. These, of course, were demolished for the construction of the glass works some time before 1768.

Between March and June last year, Mr Jackson and his team carried out excavation works at the site beginning with exploration trenches which revealed that the Portwall Lane works had operated two large glass smelting cones. These structures were enormous chimneys with fires, furnaces and flues where the ingredients of the glass were melted and workers formed the product into the required shapes. The height of the cones was critical if sufficient draught was to be provided for the fires as high temperatures were needed to work the glass.

Although no examples of the product were found in Portwall Lane, Mr Jackson's team was able to establish that the glass being made was mainly for window use. The glass was spun into a circular disk with what we would recognise as a bullseye in the centre. This part of the glass was considered a blemish and cut away leaving the outer glass for use. Examples of this glass can still be found and are easily identifiable by the circular ridge marks.

So profitable were the glass works in Portwall Lane that in 1785 the owners added a second cone. What remains at ground level of this structure, Mr Jackson's team found to be much more complete. The archaeological team was able to distinguish many of the tunnels which ran under the furnaces enabling the fires to be fed, as well as the flues used to control the working temperatures. Furthermore, around the outside of the cone, were the remains of a series of small rooms identifiable as coal yards, mixing rooms, sand houses and kelp storage rooms.

Kelp (essentially large seaweed imported from Ireland) became an important part of the process during the rein of King James 1 who was concerned that British forests were being consumed at an unsustainable rate. Although coal was plentiful in the local area, coal-fired furnaces were unable to achieve the very high temperatures necessary in the manufacture of glass. However, by adding kelp ash and sand to the glass mixture, the working temperatures could be reduced.

Towards the end of the eighteenth century glass making in Bristol started to go into decline following the introduction of heavy taxes and excise duties on glass. As you may be aware, some property owners started to board up windows in their homes to avoid the duties giving rise to the phrase "day light robbery". Fewer windows would have had an inevitable effect on Bristol's glass making industry so that by 1833, only four glass houses remained.

Only twelve weeks were allowed for the excavation of the site. At the end of this time the site was carefully backfilled and the developers moved in. Careful mapping by the archaeological team and considerable and detailed liaison with the architects has ensured that the foundations and lift shafts of the new building are sensitively placed to preserve the history that lies beneath. So that it will be possible for future generations also to be digging up the glass works.

The Thornbury Local History and Archaeology Society always welcomes new and occasional members. Details of our programme can be found on this website, the library or the Town Hall. Our meetings are on the second Tuesday of the month, held at St Mary's Church Hall beginning at 7.30. There is a small charge, which includes a hot drink during the interval.