I've recently noticed that a number of the silver coins in my collection have started to tone but, weirdly enough, only on one side. I keep them in blue-lined Linder trays and only a few of the coins seem to have affected - 1902, 1911 and 1928 shillings (all Uncirculated) and a 1902 Maundy set, all of which I've had for years. The toning is a sort of gold colour and isn't unattractive but, as it's only on the obverse of each, it looks odd. The reverses show no sign of toning and neither do any of my other coins (so far, anyway). Does anyone have an explanation, please? Is it anything I should worry about and is there any way I can cure it if, that is, a cure's needed?