Colors can darken if attacked by dust or by certain dyes and mordants. Unlike cotton, wool is highly receptive toward mordants. This can apply not only to dyes but to mordants as well. Fustic is a bright yellow dye that is very color-fast when used with mordants. Uranium salts are mordants of silk or wool. Iron mordants "sadden" colors, while alum and tin mordants brighten colors. By using different mordants, a variety of yellows, oranges and browns could be obtained. Iron, chrome and tin mordants contribute to fabric deterioration, referred to as "dye rot". Some colours, especially dark brown, require iron mordants, which can damage or fade the fabric. Aluminium triacetate mordants have been used with cotton, other cellulose-based fibres, and silk. In histology, mordants are indispensable in fixing dyes to tissues for microscopic examination.