PERMANENCY The stability of a photographic image is deter mined primarily by the process by which it was produced, but it is limited by the durability of the medium on which the image is supported. If the image will endure without serious loss of quality as long as the medium will last, it may be regarded as permanent. In some cases the image is almost indestructible, and therefore has much greater durability than its support. Deteri oration is far more frequently the result of defec tive or careless working than an inherent weak ness of the process. Gelatine, thoroughly hard ened, is an enduring substance. Paper of good quality and purity will last for hundreds of years without other change than a mellowing of its colour. In silver processes, imperfect fixing is a frequent cause of fading in negatives and prints, even the washing being of secondary im portance. A properly fixed and washed nega tive should be permanent, especially if varnished to protect it from the atmosphere. Properly made carbon and platinotype prints are quite permanent ; imperfect adhesion of the carbon tissue to its final support, or incomplete removal of the iron salts from platinotype, will cause rapid deterioration, although metallic platinum is absolutely permanent. A sulphide-toned bro
mide print should be as enduring as any photo graphic print, silver sulphide being a most stable substance. An ordinary bromide print should be as enduring as a negative, but all prints on gelatine papers should have the gelatine coating hardened ; in its soft condition it will absorb moisture from the air, and this is prejudicial to its stability. Silver prints on the various printing-out papers are generally considered to be far from permanent ; but the trouble is invari ably due to imperfect work, and is not inherent in the process. Silver prints have been kept for nearly twenty years without showing any change beyond the mellowing of the paper.