Foxing is very common on older notes from tropical regions, e.g., the Caribbean, Malaya/Malaysia, Thailand, Hong Kong, Singapore, etc. It is seldom seen on Canadian notes. It's due to the continually high humidity and high temperature in those countries which encourages mould and related organic problems to develop over a long period of time. Foxing usually appears as light yellow-brown marks (typically 1 mm to 5 mm or so in diameter) which are visible on both sides of the note. When holding the note up to a bright light the foxing spots often appear darker. I've also seen blackish foxing spots (and soft spongy paper) which constitute a more advanced and much worse form of foxing.
Take a look at eBay Item 190202015392 which is a rare Malaysian high-denomination $1000 note with many small brown FOXING blotches and spots, but otherwise in EF condition. The term "FOXING" is actually used in the auction description. I would have been interested in this note myself if it wasn't for the foxing. I've attached a high-resolution image of the note taken from the eBay site. If you download it and then blow it up you will see the annoying foxing spots very clearly. As far as I'm aware, foxing can only be removed by bleaching in water and that would ruin the note.
{http://img.villagephotos.com/p/2008-1/1293040/MALAYSIA$1000-IMAGE.jpg}
« Last Edit: May 01, 2008, 08:09:03 am by Ottawa »
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" Buy the very best notes that you can afford and keep them for at least 10 years. " (Richard D. Lockwood, private communication, 1978).