Week 9 of 12
Module 5: Conservation Interventions and Treatments; Criteria for Selection and Implementation
This morning we had a lab session with Gottfried Hauff, a professor and head of the Stone Conservation Course at the Department of Restoration, University of Applied Sciences Potsdam, investigating the effect of porosity on the use of consolidants. We carried out tests on different types of stone using Karsten tubes (Rilem tubes) and identified the wide variations in capillarity between different types of stone and between three stones of the same group (i.e. three sandstones). Gottfried highlighted the importance of always recording the size of the tube opening that you are using because you will get quite different results with a 2cm opening and with a 4cm opening. If you want your work to be repeatable or to be used by others in future for comparisons it is essential to record this information. There are also likely to be slight deviations in the opening size where the putty used for fixing the tube blocks part of the opening; a clever suggestion for accurately recording this size is by taking an imprint of the putty with an ink pad following the test and printing it beside your results from which the opening area can be calculated. Ideally you would always use the tube with the largest opening because it is supposed to be more accurate, but this is not always possible on curved surfaces. As with other test methods you need to be careful which area you select for carrying out the test because the putty tends to leave a slight mark on the stone due to its oil content.
The Karsten tube (Rilem tube) test for capillarity on a cube of sandstone.
We also discussed another technique for identifying the capillarity of a stone, which is a simple 'drop test' where a drop of water is put onto the surface of a stone and the time it takes (in seconds) for this to be absorbed is recorded. This test is good in that it allows you to identify how much the capillarity varies across the face of a stone. This test is most useful as a comparative test rather than as strict quantitative data. To increase the accuracy it is possible to use a 'norm pipette', which produces water droplets of fixed sizes- this is a simple but very effective piece of equipment I'd like to get for my work at the Scottish Lime Centre Trust.
The 'norm pipette'.
The second test we carried out was to trial several different alkoxysilane consolidants on cubes of a single type of sandstone. The cubes of stone were placed into small pools of each consolidant and the rate of uptake timed. These tests demonstrated the point that lower concentration alkoxysilane consolidants have a higher absorption rate, that they all have higher absorption rates than water, and that 'old' consolidant (one that was a couple of years old) is no longer as effective as it should be when new.
Gottfried Hauff comparing the different levels of consolidant absorption in the cubes of sandstone.
This afternoon George Wheeler spoke to us on the topic 'Evaluating Performance of Consolidants; Lab Methods and Protocols'. We covered all of the main techniques for analysing stone which can be used prior to carrying out a consolidation to ensure that you fully understand your substrate. These were; Mercury Intrusion Porosymmetry, Water Vapour Transmission, Flexural Strength Tests (both '3 Point Bend Tests' and Biaxial tests), Ultrasonic Velocity, Micro-abrasion, Colourimetry, X-Ray Diffraction, X-Ray Fluorescence, Scanning Electron Microscopy, Polarising Light Microscopy, Micro-chemical testing and Conductivity tests.
Three additional basic factors that need to be assessed are the presence of biology, salts and pollution/gypsum crusts. It is important to identify the types of biology present on your stone if you wish to carry out a consolidation because removal of some or all of this growth may be necessary as an initial phase; it is necessary before this occurs to also identify the 'value' of the presence of this growth. As mentioned yesterday, you need to assess the presence of soluble salts and if it is possible (or necessary) to remove them prior to consolidation. If you do wish to remove the salts it is essential to select your method carefully; we were told of an example when a stone from a dry climate was repeatedly submerged in a water bath to remove the salts (a common technique), but the stone was not properly understood and it contained a high content of expandable clay resulting in the stone experiencing a high level of deterioration.
Gottfried Hauff finished the day with discussion on a few different consolidant-based research projects that have been carried out to understand the durability of consolidants with time and the possibility for re-treating previously consolidated areas. This session proved to be an interesting critique on research methods and the reliability of data.