The darker grey quadrant abcd defines the physically safe range of temperature and RH for photographs. Outside the quadrant, the dotted line shows the glass transition temperature (Tg) of gelatin, which varies according to RH. This is the temperature at which rapid image degradation is likely to occur. The diagonal contour lines (1–500) map relative chemical stability. If a photograph is stored in the environment defined by line 10 it will take 10 years longer to reach the same stage of chemical deterioration as a photograph stored in the environment defined by line 1. Lowering the RH within the quadrant at a fixed temperature will only increase the chemical stability by a factor of 2–3. However, lowering the temperature at a fixed RH can increase the chemical stability by a factor of more than 100. Therefore, the beneficial effect of dropping the temperature within the quadrant, as opposed to the relative humidity, even by a small amount, can clearly be seen. There is considerable advantage to be gained by cold storage for more unstable material. Conservation heating The above graph has implications for the effectiveness of expenditure in designing storage. Initially it can be used to select a suitable level of chemical stability for a collection. This is dependent on what is already known about the stability of the particular photographic materials in the collection and how long the photographs need to be kept in an acceptable condition. Within the physically safe quadrant it is easier and more cost effective to achieve the desired chemical stability by allowing a relatively broad range of relative humidity and narrow range of temperature (within the physically safe quadrant). However, some historic buildings without air conditioning tend to be cool but damp, particularly in winter. In such buildings