Question about iridescence

[awl]

I was recently looking around I denarii when I came across ones that were iridescent. I was wondering if the iridescence can be removed from coins by just siting in a flip or it is a permanent property of a coin like cabinet toning?

[Howard Cole]

The iridescence is caused by the toning of the silver coin.  You get nice purples and blues.  If allowed to continue, it will turn a coin black with time.  This toning is usually caused by sulfur in the air or in contact with the coin.   Modern silver coins that have been put in Whitman folders, or similar holders, the coin tones from the edge toward the center.  The edge is in contact with the cardboard holder which has sulfur in the paper

[Robert_Brenchley]

It's exactly the same reaction as the one which used to make silver egg spoons turn black from contact with the sulphur in the yolks. Presumably it still does if you use silver spoons.

[Ecgþeow]

Is there sulphur in the plastic flips that many of us use in holding our coins?

[Robert_Brenchley]

They still turn black in the flips though. Clean a coin in lemon juice, leave it in a flip for a couple of years, and you may be surprised by what you see.

[Howard Cole]

Robert, the toning does not come from the flip if it is mylar or some other plastic that does not have PVC in it.  Most of the toning is from atmospheric pollution.  In most cities it is hard to avoid.

[Robert_Brenchley]

I never supposed that it did come from the flip! I use that as an example of the inescapability of urban air pollution in my environmental science classes.

[David Atherton]

I suppose I live in a low pollution area. My coins are kept in the mylar 2x2s and have not changed noticebly tone wise in 4 years.

A good thing I imagine, but very problematic for toning coins!