THE AGE OF BLOODSTAINS CANNOT BE ESTABLISHED WITH ANY DEGREE OF CERTAINTY. The drying time may be altered by many circumstances of influences. Generally, bloodstains dry in 10 to 20 minutes. By observing the drying action, the investigator may be able to ESTIMATE the period of time which has elapsed since the stain was made. Drying is more rapid on a rough surface. Oily substances may not only increase the drying time but may alter the appearance of the blood. Perculiarities of an individual's blood may affect the drying time. A single drop of blood that falls on a dry surface (i.e. table or wooden floor), usually will dry completely in about an hour at room temperature. Blood which has collected in a pool dries slowly. Blood drying depends upon the size and depth of the pool formed and the temperature and humidity to which it is subjected.