Track 2: Materials over Time: Points of Change
Finding Paskapoo: A Lesson in Stone Sourcing – Finding a suitable match when the original is no longer an option
Tuesday, September 25
8:30 AM - 10:00 AM
Location: BNCC- 101BG
In response to a requirement for fire resistant building following the Calgary, Alberta Fire of 1886, distinctive yellowish-buff sandstone, Paskapoo, was locally quarried from outcrops in the Calgary area. The sandstone was used in many landmark buildings, including Calgary City Hall, and many buildings along Calgary’s main street, Stephen Avenue; Calgary became known as Sandstone City. However, by the early 1900s Paskapoo sandstone quickly fell out of use due to functional and operational problems with quarrying, the fragile quality of the stone, and competition from other stone supply, particularly the Manitoba Tyndall and Indiana limestone.
Calgary City Hall, a national historic site was completed in 1911. This monumental Romanesque Revival Style building clad in Paskapoo sandstone continues to house the offices of the Mayor and city Councillors. As part of the Calgary City Hall rehabilitation project EVOQ Architecture was mandated by Lemay + Toker Architects to act as Heritage Architect. Large expanses of the masonry facades suffered severe deterioration due to weathering, longstanding and repeated saturation, and the fragile quality of the stone itself (The cement is of a calcareous nature and the stone is markedly bibulous – William Parks Report on the Building and Ornamental Stones of Canada, 1915). In fact, testing revealed that the stone is not very resistant to salts and frost. It was clear from the outset that the rehabilitation strategy would consist of preserving and protecting the surviving stone by introducing a water shedding strategy and that the stone that was heavily deteriorated and subject to continued high exposure to water and salts would have to be replaced with a stone, perhaps with better performance but still compatible with the existing Paskapoo. With Paskapoo no longer available, it was critical to find suitable replacement sandstone for the work. As a first step, the outcrops that were originally quarried and survived urbanization were visited and evaluated for reactivation. The second was to conduct an international tender call for stone supply that yielded suppliers from Germany, Spain and the USA. This presentation will briefly present the Calgary City Hall project and the masonry rehabilitation strategy and describe the extensive stone sourcing process which included visits to several quarry sites and buildings. Topics will include: Considerations and challenges in reactivating dormant quarry sites; Establishing compatibility criteria to prepare a call for tender for replacement stone supply; Evaluating candidate quarry sites, capacity and operations; Evaluating chemical, physical and visual compatibility between original and replacement stones including laboratory testing, assessment of field performance in existing buildings, short and long term weathering characteristics; tying it all into a sensible and clear preservation strategy.
- Appreciate the challenges in rehabilitating the Paskapoo sandstone cladding of Calgary City Hall. (Strategies for protecting masonry faces and repair techniques)
- Understand the process in evaluating the potential of reactivating existing quarry sites (Environmental, Legislative, Property titles, operational issues)
- Understand how to develop and integrate selection criteria in a call for tender for stone replacement when the original stone is no longer available.
- Appreciate the process of evaluating and selecting the best replacement stone, based on visual, empirical, functional, performance, weathering and operational criteria.