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12 C O N T E X T 1 7 9 : M A R C H 2 0 2 4 Technology Bulletin (Vol LIII, No 4, 2023) notwithstanding that by being published in the USA it is not always easily accessible in the UK. However, its technical content is at the cutting edge of historic environment management and often essential reading. A paper by Morgan Matheson and Michael Duric looks at the use of advanced documentation and analysis technologies, where there is a need to accurately represent and analyse historic building structures using digital technologies. Such survey methods can play a central role in documenting both the tangible and intangible qualities: architectural, historical, physical and material. This has been put to particularly good use in the conservation of a greenhouse with a warped, cast- iron structure at the Shaughnessy House in Montreal, Quebec. The paper examines how advances in point-cloud scans, BIM software and THERM (two-dimensional building heat-transfer modelling) and fundamental heritage and material considerations have formed the basis of a successful architectural- heritage conservation project. The house by William T Thomas was completed between 1874 and 1875. The conservatory was added in 1910 with an exposed cast-iron structure, featuring curved glazing and decorative cast-iron elements. The approach to conservation and the use of THERM as the preferred standard software is well explained by the authors and the eight-page feature is particularly well illustrated in colour. Those interested in the latest developments in software as a conservation management tool will find this article informative, as the use of point clouds and H-BIM methodologies and their numerous advantages for projects, both in the indexing and representation of collected data, are well explained. An article by Robert Kornfeld and Charles Van Winckle deals with another cast-iron structure, a fire watchtower in Harlem, New York. The tower was built in 1856 using both cast- and wrought- iron components: the spandrel beams, girders, columns and stair treads were cast iron, while the rails, tie-rods and connectors were wrought iron. At some point between 1878 and the early 1900s, the watchman’s booth on the fourth level was decommissioned and the watchtower remained standing, principally due to community advocacy. The tower was designated as a New York City Landmark in 1967 and added to the National Register of Historic Places in 1976. The watchtower deteriorated throughout the mid- to-late 20th century, culminating in a partial collapse of the fourth- level spandrel beams. By 1994, the tower was braced with an external frame to prevent its collapse. The article illustrates the effects of past deterioration and ingenious structural-repair options, ranging from post-tensioning of the existing columns and concealing a structural reinforcing frame to full replacement, with the new steel structure resembling the original design. Ultimately, these options were determined not to be appropriate and were not assessed in full. The approach taken forward aimed to have the minimal aesthetic impact that could be detailed in a manner that would be reversible. Most important, they would have been appropriate and true to the original design concept. The way the works to all the individual components were carried out is thoroughly explained. For Context readers, it is perhaps the general principles of authenticity and reversibility that will provide some useful object lessons, so the article may have a wider resonance. APT Bulletin The APT Bulletin, published by the Association for Preservation Technology International, is particularly distinguished for the clarity of its graphic presentation of projects. A paper by Edmund P Meade demonstrates this in dealing with the seismic evaluation and retrofit of unreinforced masonry buildings in Italy. Axonometric drawings of loadbearing masonry buildings that have suffered structural damage are especially well illustrated. Given the need to justify structural reinforcement and repair options in heritage impact statements, the presentation of this work is particularly notable. The author provides a shortlist of further reading and a number of informative footnotes. The bulletin also publishes, from time to time, a centre-section set of Practice Points. In the present issue, No 25 deals with a nine- page update on metals for historic roofing by Jeffrey S Levine, Remo R Capolino and Julie M Palmer. This deals with both the commonly encountered and more unusual roofing materials, concentrating in particular on copper, lead- coated copper, tin-zinc-alloy- coated copper, tin matte, stainless steel, zinc and lead. The authors describe the composition of these materials, important installation requirements, the significance of water run-off on appearance and the general, gradually emergent weathering characteristics. For readers dealing with metal roofs, some of the materials will be relatively unfamiliar. The Practice Point notes that lead roofs on historic buildings are relatively common in the USA where the material was more commonly used in smaller, more decorative elements such as turrets and eyebrow dormers.