Article by Mark Milevski
While many heritage conservation practices have remained consistent over the past 20 years, there are some key areas where the industry and governing bodies have learnt smarter ways. And with new knowledge and understanding, clients and stakeholders have a growing preference for more sensitive approaches that honour the stories, design intent and materials of the original structure.
Heritage building conservation has evolved over the decades, reflecting changing attitudes towards preservation, sustainability and cultural significance. Since Stone Initiatives was established in 2003, we’ve seen these changes reflected in the heritage and conservation-related testing we are engaged to undertake. This includes the need for project managers and architects to adhere to more stringent regulations, an increased appreciation for heritage value by both built environment professionals and their clients, and the discovery of new restoration methods and materials.
Our founding year, 2003, was also significant for heritage conservation in Australia as it saw the creation of the National Heritage List. The Australian National Heritage List was established with the aim of recognising heritage locations of outstanding significance to the country. The heritage list includes various categories such as architectural and cultural sites, natural landscapes, and areas of significance to the Indigenous people of Australia. The National Heritage List was brought into existence through a modification to the Environment Protection and Biodiversity Conservation Act of 1999, effectively replacing the former Register of the National Estate.
Impact on conservation practices
One of the more significant impacts of the establishment of the National Heritage List is its influence on heritage building conservation practices. Buildings and structures listed on the National Heritage List are subject to stringent conservation guidelines and regulations. This has played an important role in shaping the way heritage conservation is approached in Australia, emphasising the importance of preserving the cultural and historical significance of these places.
The Burra Charter was adopted by Australia ICOMOS in 1979 and is named after the historic town of Burra in South Australia, where the charter was first implemented. The Burra Charter is a set of principles and guidelines for the conservation and management of cultural heritage places that predates the National Heritage List. The Burra Charter establishes a practice benchmark for individuals involved in offering guidance, making choices regarding, or executing projects related to culturally significant places. The Burra Charter has an insightful philosophy: “…do as much as necessary to care for the place and to make it useable, but otherwise change it as little as possible so that its cultural significance is retained…”. In regard to existing fabric and materials the philosophy also applies: “changing as much as necessary, but as little as possible”. Any heritage work carried out by Stone Initiatives also follows these guidelines.
The influence of the Burra Charter and the presence of heritage registers at various government levels, along with the regulations and obligations they entail, have resulted in renewed commitment to historical accuracy.
Mortar specifications for heritage buildings
The change in mortar specifications for historic and heritage buildings is an example of how the Burra Charter and the presence of heritage registers at various government levels have had a positive impact on the approach to conservation. Twenty to forty years ago, it was common to use cement in the restoration of masonry that previously had lime-based mortars. It was common to specify a 1:2:9 or 1:1:6 mortar mix (cement:lime:sand) for the repair of historic masonry, with the intent to strengthen the structure and delay any future repairs. Fast forward a couple of decades and we are now seeing the negative implications of this practice, with preferential decay targeting weaker masonry elements such as soft ashlar, while the modern hard cementitious repairs remain intact and reasonably unaffected.
After observing the negative effects of using modern incompatible materials on heritage and historic structures, the use of traditional materials such as lime-based mortars have experienced a resurgence in heritage restoration, particularly naturally hydraulic lime mortars, known for their compatibility with historic masonry and an excellent alternative to the ‘touch of cement’ approach, common in the 1980s, 90s and early 2000s. Lime-based mortars offer flexibility and breathability, and can serve as a sacrificial element, making them ideal for preserving older buildings that require accommodation for rising damp and salt damp management, a common issue in the Australian heritage built environment.
|Incorrect mortar||Correct mortar|
|1. Cement mortar, moisture and salts are forced though softer element (stone in this case), therefore the stone decays before the mortar.|
2. It is difficult and expensive to cut out decayed stone and replace it with new stone (this comes with its own set of difficulties, e.g. it is a heritage building and the stone type is no longer able to be quarried, so an alternative stone must be sourced).
3. It is difficult to remove cement mortar without damaging the stone.
4. The stone is in very poor condition.
|1. Lime mortar, moisture and salts are forced though softer element (mortar in this case), therefore the mortar decays before the stone (mortar is the sacrificial element).|
2. It is easy and relatively cheap to rake out decayed mortar and replace with fresh lime mortar.
3. It is easy to remove lime mortar without damaging the stone, so there is no damage to the stone throughout the repointing process.
4. The stone is in very good condition.
At Stone Initiatives we test a wide range of mortars, from mud-based to modern cementitious mortars with metal oxide pigments and chemical additives. Over the past two decades we have observed a growing enthusiasm for the removal of modern cementitious repairs in favour of substituting them with lime-based mortars that are complementary to the period in which the original structure was completed. This progressive shift is a step in the right direction and likely a practice that will increase into the future.