Ferric chloride stains
In electrical engineering, ferric chloride is used as an etchant for printed circuit boards. On textiles the stuff leaves yellow spots that are usually not get out. The solution: Vitamin C.
You get vitamin C (ascorbic acid) in the supermarket. Simply moisten textiles and sprinkle with Vitamin-C. Must work longer, at five-year-old spots about a weekend :-) The yellow spots of vitamin C then go out in the normal laundry. Better to wash separately!
