160 THE BUILDING CONSERVATION DIRECTORY 2025 CATHEDRAL COMMUNICATIONS one or two levels within the main built structure are usually less likely to suffer from temperature fluctuations or solar gain. WIND (AIR) SOURCE The microclimatic conditions relevant to the organ are not simply those to which the instrument is exposed in the building, but also those in the area from which wind (air) is provided by the blower. If the blower is sourcing external air, even indirectly through an externally ventilated blower room, this is often significantly different and more unstable than the air within the building and can result in the issues outlined above. In fact, because the air is being delivered directly into the working parts of the organ, the effect can be more significant. Instruments drawing their air from within a building can often enjoy more stable tuning conditions, especially if their location is neither disparate nor close to large windows. BUILDING HEATING Many church organs were built in the Victorian period at a time when churches could not be heated to the levels they are today. Timbers which have been seasoned for many years prior to the construction of an instrument can quite suddenly be subjected to thermal shock, especially when new heating systems are introduced. Heating is generally used in historic buildings to provide comfort for those using the building, rather than for the conservation of the fabric or sensitive content, such as paintings, timber furnishing, pianos and organs. The temperature fluctuations will cause variations in relative humidity with a 1°C change in air temperature resulting in an approximate variation in RH of 3%. Low levels of background heating may merely suppress the RH over time with the timber parts of the organ equilibrating with the resultant conditions. Sudden increases in temperature for church services or concerts will cause the RH to fall, resulting in dimensional change causing potential damage and loss of tuning. If this happens regularly, long-term physical deterioration of the organ can occur. The effect of heating on an organ can vary depending on the type of heating, with the impact likely to be greater if comfort heat is provided by a convective (warm air) system, rather than a radiant (local infrared) system. Swift changes in air temperature are also likely to be more deleterious than slow progressive changes. Direct gas-fired systems generate Thermal images of a college chapel (top), where heating causes limited stratigraphic temperature variations, and of a medieval church (below), where the variations are sharper due to the convective heating used. The effects of sunlight on this organ case are illustrated in the thermal image below. a further problem, sometimes causing the condensation of water vapour (a part of the gas combustion process) on the cold metal organ pipes. The water can then run down the pipe into the soundboard and onto the pallet mechanisms; the part of the instrument that is most sensitive to the effects of water. Warm air is buoyant and therefore convective heating can also cause a significant stratigraphic variation between the floor of the building, where people are located, and the upper part of the building. This approach is generally inefficient in terms of energy use but for organs the effect can be significant with the upper part of the organ at a higher temperature than the lower sections. Conversely, if the heaters are at the base of the organ, it may in fact cause the lower part of the instrument to be warmer (and thus experience lower RH). HUMIDIFICATION AND AIR CONDITIONING To counter the issue of wind being drawn from locations where air can be particularly dry and cold, such as external winter air or even dry internal air (over-warmed by the heating system), mechanical humidifiers have often been used in organs. These can be effective tools, especially for organs with pneumatic key-actions (highly complex by their nature) where their impact is well understood and their use is accurately managed. However, they can also generate raised moisture content in unexpected parts of the organ resulting in issues with elevated humidity and condensation. A humidifier is a mechanical unit producing water vapour which is directed into the organ’s working components (via its internal windways). Several types of humidifiers can be found in organ installations. The simplest is one where water is dripped over a pumice basket and the humid air is then blown gently through the organ’s wind system (but only when the organ is switched off). A second, more intrusive type of humidifier boils water and produces steam which is then blown into the instrument. This type is not recommended as it can be highly destructive, loosening animal glues and stripping calico finishes from components. Both types of humidifier produce water vapour in the wind path which is then drawn into the main organ mechanism. The introduced water molecules do not simply disappear. Some of the vapour will be expelled in the air passing through exhaust valves specifically provided in the mechanism, or through the pipes, and dispersed into the surrounding air space. Some will be absorbed into the porous materials (for example, wood and leather) and will increase the cellular moisture content (MC). If the increase in MC is significant, this can lead to expansion and contraction of wood and leather causing splitting or encourage microbiological growth (mildew). Depending on the temperature, some
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