The yellow ink used for the orange 6¢ contained ferric oxyhydroxide (the chemical compound of Iron), and as everyone knows iron rusts. Normally the sulphurization occurs in the ink drying process. However as the Bureau had, supposedly, a tight control on this process, it was doubtful that this was the cause of the ink sulphurising. Another potential source of sulphurization is a reaction to acid. And here is where the mailbags come into focus. The mailbags at the time frequently got wet, they were made out of canvas and without some kind of preventive action they would mould. And mouldy mailbags is not what you would want your mail to be carried in. The mailbags were treated with a special chemical (probably ethylene oxide), one that prevented mould and also prevented the breakdown of the canvas material. The chemical would be allowed to absorb into the bag, and after spraying, would be available for use 24 hours later. But, if the bag was used before then, the chemical would not have absorbed fully and would have reacted to the iron compounds used in the ink on the stamp.

Dipping a used stamp in hydrogen peroxide will change the stamp back to the original color. Using a q-tip with hydrogen peroxide and gently rubbing the top of the stamp without soaking the stamp will correct the color on mint stamps.