What is the cause of dark spots that sometimes appear on the paper of drawings and paintings? Reddish-brown spots are known as "foxing", art impressionism caused by mold or the deterioration of the metallic impurities left in the paper from the manufacturing process. other colors of spots may be one of many types of mold damage. mold spores are everywhere in the environment, and mold thrives on cellulosic materials, art especially in conditions of high humidity (above 65% of relative impressionism humidity). keeping artworks out of high humidity areas like bathrooms or exterior walls, can help reduce the development of such stains. if i have a work of art on paper art that appears to not be flat, should i be concerned? paper is hygroscopic, reacting to changes in climate by expanding when it is humid and contracting when it is dry. a gentle undulation in the paper called "cockling" occurs under these conditions, especially with larger works. this may impair the work aesthetically as well as cause mediums such as gouache to crack or flake as it is unable to expand and contract as the paper itself does.

do not store or display works of art in areas of potentially high humidity or water leakage, e.g. basement, bathroom, outside walls, under pipes. avoid areas where temperature and humidity fluctuate, or where there is inadequate air circulation, e.g. attic impressionism and places listed above. do not hang artworks over or under radiators, heating and cooling vents, active fireplaces, humidifiers, and vaporizersa. the hygroscopic nature of wood means that it will take water from the atmosphere and expand, but it will contract as the humidity lessens. the direction of shrinkage is almost always around the circumference, which causes a solid piece of art wood to crack vertically. keeping it in a steady relative humidity can stabilize the sculpture; if the wood does not absorb or release moisture, it will no longer expand or contract.