The Monster in your Basement
Mould (fungi) is present everywhere in our world, with more than 100,000 recorded species of mould. But what a person does not want to see, is mould on an artifact of any sort. Mould prefers to grow in damp areas with limited air flow such as basements and other areas of high humidity. As mentioned in the humidity section, mould is not likely to appear until the humidity passes approximately 66%.

When handling artifacts, the oil on your fingers that remains behind will also promote mould growth. Headgear, especially those made of leather, are susceptible to mould formation due to sweaty fingers having touched that area during handling. If mould is discovered, vacuum it off first, then attempt to brush off as much as possible and vacuum again. Having done that, clean the area with medical grade rubbing alcohol. This will usually kill the mould and should remove what remains. After it dries, a small application of Lysol mould disinfectant to the area might be good insurance against further episodes. Once mould starts on leather, it is extremely persistent and could reappear. Re-application of rubbing alcohol and Lysol will help stabilize the area and prevent growth. Once again, mould growth is directly related to humidity level. The best prevention, is to ensure a well-ventilated area, control the humidity, and to wear cotton gloves while handling leather.
Mould growth on leather due to high humidity. The mould is visible as white flecking on the leather.