A paper property that measures a paper's resistance to changes in its chemical, structural, or optical properties over time. Permanence includes such things as resistance to yellowing and fading upon exposure to light (lightfastness) over time, and the paper's ability to retain its strength over time. Paper permanence is measured using accelerated aging conditions, and describes its brightness loss and yellowing. A paper's retention of its folding endurance over time is also a useful test for determining how well a paper will retain its strength. Some of the factors that affect a paper's deterioration over time is the deterioration of the cellulose fibers themselves (which moisture accelerates), and the use of acid-rosin sizing. Acid paper tends to be less permanent than alkaline paper.
The term permanence also refers to the ability of a printing ink to resist fading or changing color upon exposure to light or weather. The permanence of an ink is generally governed by the lightfastness of the pigment used. (See Pigment.)
A substance's permanence is also known as age resistance.