Week 10 of 12
Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation
We continued on from yesterday (see Tuesday 14th June 2011; Desalination) on the topic of desalination today, with a lecture and then a follow-up session in the ICCROM laboratory with Veronique Verges-Belmin. We were shown how Mercury Intrusion Porosimetry can be used for identifying pore sizes in poultices although it cannot measure pore sizes over 200μm, or those that are significantly small. Tests on some poultices were carried out in a study (reference can be located if requested) that showed how vastly different the pore sizes in different poultice compositions can be and therefore the potential options for desalination of stones with particular porosities. The pore size range was from fume silica with pores 0.005-0.05μm in size to pores c.180μm in size in a 0.5-1mm sand.
We discussed the difference in water content possible in different types of poultice and looked at a graph that identified how there is generally an optimum water content in different poultice compositions. Once you cross the optimum water content you soon find that you have an unusable slurry and adhesion to your substrate is no longer possible. The water content in a poultice can also affect the level of shrinkage that occurs upon drying which is important because this is related to adhesion. If a poultice partially detaches from the stone substrate then the danger is that water migrating back towards the surface will deposit as efflorescence and subfluorescence within the stone rather than within the poultice. Although it may seem logical to have a very wet poultice, or to pre-wet your stone prior to poulticing, the danger is that you will drive the salts deeper into the stone and then will not be able to remove them with your poultice. A poultice that works by advection will really only remove the salts from the outer 20-40mm of the stone and will never result in a complete extraction of salt from the stone substrate.
We revisited our poultice samples made up yesterday in the second half of this morning, and the findings that came from the tests concurred with the theories we were taught yesterday and this morning. These include that thinner poultices result in lower levels of shrinkage, that increasing the sand content in a poultice results in a lower amount of shrinkage (same mechanism as in a mortar) and that the cellulose poultice with the longer fibres resulted in a lower level of shrinkage than the one with short fibres. Many new poultice materials are coming onto the market and each needs to be carefully assessed before it's applied for its suitability for a substrate and to identify if it may already contain salts- clearly not a desirable quality for this purpose!
The poultice samples that Rutger (Dutch participant) and I made using two types of cellulose poultices on sandstone, brick and volcanic tuff.
This afternoon we were joined by Barbara Davidde, Director of the Underwater Archaeology Operations Unit at the Instituto Superiore per la Conservazione ed il Restauro. The underwater archaeology unit was set up in 1997 and is an office of the Italian Ministry of Culture. Although historically it was common to remove archaeological finds from underwater, the current aim is always to preserve them in situ wherever possible. Lime mortars with a hydraulic set (either naturally or those which have a pozzolanic content) can be applied underwater directly onto the stonework, although different application techniques are required. The biggest problem that underwater stone objects or structures face is biological colonisation, although there are also human issues such as boat anchors damaging sites and unmonitored underwater tourists. These objects do not however, have issues with water or salts because these stay in a constant state- the salts are always in solution. Biological growth needs to be removed mechanically underwater as a biocide could not be constrained to a specific object. It is possible to protect certain stone objects from deterioration in similar ways to those above water level such as covering mosaics that have been conserved with geotextile sheeting. Despite the aim to keep as many objects in situ underwater as possible, it is common for limestone or marble sculptures to be removed because they are much more prone to serious macro-biological attack by organisms that bore into their surfaces.
Simon Warrack, ICCROM Stone Conservation Course Coordinator, gave us the last lecture of the day on 'Ta Reach- Sculpture Repair'. Simon talked us through the long and interesting history of Cambodia and the use of the Angkor Wat temple since its initial construction in the 12th Century. It was constructed as a Hindu temple but through the centuries fluctuated between use as a Hindu and as a Buddhist temple depending on the rule of the country. The change from Buddhist to Hindu use in the period 1243-1295 saw the destruction of all Buddhist symbols present at that time, or their adaptation to Hindu symbols. When the French arrived in 1860 the temple was Buddhist, but due to the conservation principles followed at the time, the hundreds (or possibly thousands) of Buddha sculptures or religious donations or offerings were removed from the temple to return it as closely as possible to its original Hindu structure. No thought seems to have been given to the thousands of Buddhists who used this temple for worship at that time.
The focus of Simon's lecture was on a single statue which many believed to be the Hindu god Vishnu (probably why it was never removed at the end of the 19th Century), but it is now generally accepted that it is a manifestation of a Buddha. This sculpture is a very important symbol for the Buddhists who visit Angkor Wat for worship. In the 20th Century the sculpture was restored and new cement arms and head were added where they had been missing. Simon was involved in the conservation of this sculpture and due to the 'living' nature of this piece of cultural heritage, community consultation was seen to be an essential part of this project. Through community consultation it was agreed that it was essential for the sculpture to retain its full set of arms and head, but that new arms were to be carved from stone and following three years of research and discussion the head was located, which had previously been removed, and it was returned to site and put in place. This is a very simplified version of the story Simon told and it was a really nice case study of the need for us to recognise 'living' heritage. It was an example of a project where it was necessary to bend some of the rules laid out in the various conservation charters (the 'restoration' of missing parts) and to appreciate the current use of the site as one of its most significant values. Simon made a video about this project that includes all of the details that I have missed out (and may correct any details that I have confused), which can be seen on youtube.com by following these links; part 1, part 2, part 3, part 4.