Context 159 - May 2019

C O N T E X T 1 5 9 : M A Y 2 0 1 9 25 Building services unsurprising given the historic requirements, but it reinforced the idea of a standardised design, and showed that all the lanterns had been of a standard size and shape. However, the entrance paths which the overthrows spanned varied in width from just over 2 metres to more than 3.5 metres. One of the major challenges of the project was working out how this variation could be accommodated in a standardised design. Archival research Archival and historical background research was carried out to determine if there were any record drawings and or images of the street prior to the introduction of municipal gas lighting the 1870s. Little information came to light other than some sketches that gave an impression of the overthrows’ general shape and confirmed one of the findings of the fabric analysis, that originally there were lanterns to every doorway. More surprisingly, no usable photographs have been discovered to date that show the lanterns in place. Great Pulteney Street tends to be photographed along its grand axis rather than in elevation, obscuring any details that may have been present at the time. Technical design The only complete original overthrow left in situ was minutely measured and the setting out ana- lysed. After discussions with a blacksmith about how they set out work, it became apparent that their training still teaches the use of the golden section for making scrolls, a very Georgian pro- portioning system. This perfectly matched the analysis of the existing metalwork and a generic central cradle was designed for mass produc- tion. To accommodate the differing widths of entrances, the support arms were designed as a gentle cyma curve which is infinitely scalable. Each part of the overthrow was scrutinised and measured using imperial dimensions. Simple things such as the tapering of the main support arms from 2⅛ x ¾ inches down to 1 x ½ inches needed to be replicated exactly, as this is what gave the metalwork its elegance and strength. As the lanterns would be heavy, the strength of the support arms needed to be capable of carrying the loads. This in turn governed how the metal was processed. Unlike off-the-shelf steel that has regular sections and uniform properties, traditional blacksmithing techniques of taper- ing, jumping up, quenching or tempering allow the blacksmith to modify metal’s sections and properties to make it a bit stiffer here, or a bit more springy there. Subtle but critical details. Lantern design What stood out from the fabric analysis and archival drawings was the circular form of the lantern, with a conical metal cowl and glass bowl. Unlike the more standard square lanterns, these are complex shapes to make. It quickly became apparent that in order to finalise the design, more knowledge was required about the manufacturing processes of the time. It was fairly obvious that the glass bowl would have been hand blown and designed to mirror the curvature of the cradle that it sat in. The bowls are particularly large, which could have posed a problem as most glassblowing is now predominantly for small items. Fortunately, the Liquid Glass Centre, a business set up for teaching glass blowing, was within a matter of miles of Bath and one of its former owners, Tom Atherton, was able to modify his kilns for blowing glass of the size required. Several trials were carried out before the correct profile and thickness were achieved. The nature and colour of the glass was deemed vitally important. A special Georgian mix was researched and prepared using ground flint to give the glass a slightly grey rather than a more ferrous green tinge. Seed (small air bubbles), ripples and other imperfections were deemed acceptable as this confirmed the nature of the manufacture. The first batch of 10 lantern bowls was produced in this way before a more com- mercial firm of scientific instrument glass blowers was approached to mass-produce the bowls to the same design using the same glass mix. Forming the lantern cowl and ventilator ring was an equally complex process. The shape of the cowl as a curved cone was unusual but appeared consistent in all the archive drawings. Tell-tale holes found in the wrought iron top rail indicate that every house on Bath’s Pulteney estate originally had overthrow lanterns. The most complete of the original Georgian overthrows in Great Pulteney Street