Paper, leather and parchment are hygroscopic materials and take up moist from the air. Paper for example contains always some 10% water. They are also susceptible to light. Unlimited exposure to heat, moist and light will cause damage. With climate and light level control we can limit the damage. Climate When the environment is too dry, materials dry out and become brittle. In a too moist climate mould spores, which are always present, start developing on the material. Various moulds and bacteria live from the cellulose in paper, and the proteins and collagen in parchment, leather and the animal glues used in older books. Mould growth is recognized by the typical mouldy and stale smell. Paper weakens and some moulds can be recognized by stains in the paper and hyphal threads. The air humidity is measured with a hygrometer in x% RH. The Relative Humidity is the amount of moist the air contains: 0% being bone-dry and 100% being saturated. For paper based materials we strive for a balance between 48% and 55% RH. If the relative humidity drops below the 48% humidification becomes necessary, above the 55% we need to humidify. Temperature and humidity are correlated. The moist intake capacity of the air is temperature dependant. Cold air is more quickly saturated than warm air. When the air temperature drops, the RH increases and when this reaches saturation the moist condenses, which is best seen on a single glazed window. We must be aware of 'cold spots' in which neighbourhood the RH will be over the 55% mark. An outside wall can be a cold spot and it is advisable not to hang artwork on or place a bookcase against an outside wall, unless it is very well insulated. Little temperature changes causes considerable changes in humidity. The fibres in paper, leather and parchment expand in humid conditions and contract in dry conditions. Fluctuations in temperature cause fluctuations in RH, which causes strains in the fibre structure of the objects thus in the long term weakening them. This is especially the case with items that are made of a combination of materials as is the case with books. Differences in contractions may cause extra tension on where these materials meet. | 
