Yellowing is the process where white paper turns yellow or brown. It is usually caused by a slow chemical reaction between the fibers in paper and acid residue left behind from when the paper was manufactured. Acid is used to break down the fiber in wood and only papers made with this acidic chemical pulp age this way, though long term surface contact with such a paper can also transfer damaging residue. This can often be found in the form of album marks left on postcards. Exposure to heat, light, moisture, smoke, and other elements can also yellow any paper. Usually a slow process, the speed of yellowing is dependent on acidic content or length of adverse exposure.