Re: why are toned Morgans worth more?

On Tue, 02 May 2006 20:49:25 +0100, Tony Clayton <tony.clayton.1962@xxxxxxxxxxxxx> wrote:

In a recent message Vector <vector@xxxxxxxxxx> wrote:

On Mon, 01 May 2006 23:26:00 -0500, Mark Reed <mreed@xxxxxxxxxxxxxxx>
wrote:

what causes it to become toned?

Pollution, humidity, containers which are not inert ... it is tarnish
on the surface of the coin. Sometimes it forms 'pretty' colors, other
times it just looks splotchy and unattractive. I'm pretty sure a
chemist could tell you what compounds produce different colors.

Basically silver tarnishes because of sulphur in the atmosphere or
the paper in the case of rolls. Silver does NOT react directly with
oxygen at room temperature. Indeed, silver oxides can be created,
but are unstable and revert to silver and oxygen.

Silver sulphide is black - heavily tarnished coins go black.

However, if the sulphide layer is very thin it is effectively
transparent. Light reflects off the top and bottom surfaces of the
thin layer, and these two reflected beams combine, a phenomenon
called interference.

If the layer is very thin the two reflected beams are 'in phase'
and reinforce for all wavelengths found in white light.

A little thicker and some wavelengths will be 'out of phase'
and cancel out, removing those wavelengths (and consequently colours),
giving a coloured effect, as seen on a soap bubble.

As the thickness increases the wavelengths that cancel or reinforce change
and so the colour changes. These colour changes are NOT caused by
different chemical compounds, just by different sulphide layer thicknesses.

Eventually as the thickness builds up the 'missing' wavelengths are too
evenly spread across the spectrum and the colour goes, and the blackness
of the sulphide becomes gradually more significant, so your coin greys and
then turns black.

Most Morgans on the market were kept for years in the vaults in the US
still in their original bags, so many survived with little or no sulphide
being formed. However, dipping removes silver sulphide, thus giving
a characteristic appearance which the expert readily observes and
downgrades the value accordingly!

I hope that helps.

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