One of the most important aspects of conservation in an historic property is the concept of ‘Relative Humidity’ or RH for short. This was and still is a slightly tricky concept for me to get my head round, as maths and physics were never my strong subjects, so I’ll try my best to explain. (Mentor Katy D took great delight in firing off pop-quiz style questions, and watching me squirm…) Humidity is a fairly destructive mistress and can have a myriad of effects on a collection. A simple way to dive in to the subject is with a definition: ‘‘Warm air, holds more moisture than cold air’’. We keep an eye on this with percentages, and the dictionary definition of Relative Humidity is the ‘moisture content of the air, divided by the maximum amount of moisture the air can hold at that temperature’. 0 = very dry, and 100 = very damp. The target range for historic properties is between 40-65%. This safe zone is to fend off mould and corrosion when the air is too damp, and if too dry, the air can draw moisture from organic objects, and cause cracking in wood. So the temperature alone is not an issue but its effect on RH is. During winter months, when heating gets switched on, the air get’s drier and stuffy and it tries to absorb moisture from objects. Then when the heating is off, the air gets damper and the materials try to absorb the extra moisture. But, the RH can also increase when the temperature remains the same, such as on a rainy day. Of which we have many to due to our ‘unique intermittent weather system’.
February 24, 2012
‘It’s all Relative…’