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What is Toning? by Steve Davies

Originally published on April 10 and 11 2018

In Gerry's absence, I'll be writing a blog on toning, what causes it, and some things to look out for. Most of you would only know me from my consignments (I am the Fort Lauderdale consigner), but in addition to being an avid collector, I also work full time as a chemist. This gives me some insight to the toning process, and I'd like to share some things I've learned in my journey through this great hobby of ours. Some of this will be very simple review for advanced collectors, but I'm hoping everyone will be able to find something interesting.

Let's start with the basics. What is toning? It is oxidation of the original metal of the coin. Whether the oxidation is caused by oxygen, sulfur, chlorine, etc., is specific to the metal and the conditions it was exposed to. This is an important point. Oxidation does not necessarily mean that Oxygen was involved. In chemistry, it simply means that the metal atom has lost one or more electrons. It's also important to remember that all the metals behave differently. Copper will react much differently than Silver. For the purposes of this entry, let's confine ourselves to Silver.

Silver's primary oxidation method is to be converted to the sulfide, Ag2S. A single Sulfur atom is bonded to two Silver atoms. This is somewhat of a simplification, given that most silver coins are an alloy, but let's run with it. What does this look like?

Pure Silver Sulfide Powder
Not very appetizing is it? You may have seen some coins that look similar to this. It is often called 'terminal toning', and is the end state when a coin has been exposed to a lot of sulfur. You may find coins like this that were in a fire, or exposed to high hydrogen sulfide levels. Looking at the coins in a person's pocket to see if they are black is an old diagnostic for determining whether that person died of hydrogen sulfide exposure.

So, great, this is what causes Silver toning. Where do all the reds, greens, blues, and other great colors come from? While Silver Sulfide itself is black, it is translucent at very small thicknesses, as are all materials. Believe it or not, at 10 nanometers, even gold is translucent! When we have very thin materials, we get something called thin-film interference. This causes certain wavelengths of light to be strengthened, and other wavelengths are weakened, aka destructive interference. This causes certain colors to appear, and the color that appears is based on the thickness of the material! In our case, this would be the silver sulfide layer. Let's take a look at this awesome Half Dime:

1834 LM-1 Capped Bust Half Dime

This dime shows some of the classic diagnostics of old album toning. The actual album backing was made of cardboard, but the material that held the coins often contained high quantities of sulfur. You will see similar, yet slightly different characteristics from coins stored in an old envelope or tissue wrapping. Let's take a look:
  1. Color progression. Extremely important. Be very caution of any coin that has iridiscent coloring that is all one color on both sides. It is ok for non-iridescent toning to be more monolithic, but vivid colors should show at least some variation. If it the coin is all the same color, this indicates the film thickness is the same all over the coin. Given that this coin sat in an album, the sulfur diffused in from the edges, towards the center of the coin. If it were all one color, would that make sense? Not at all! This coin, on the other hand, has concentric bands of color, just as it should. It also has the colors in the correct order. The silver sulfide film thickness decreases as we travel towards the center of the coin.

  2. Thin film based coloration should change based on light source, viewing angle, light angle, and the refractive index of the material above it (air in most cases). The underlying luster can also affect how we view it. A properly album toned piece should shift its colors based on how you hold it.

  3. Devices show different levels/colors of toning than surrounding fields. Again, an indication of album storage. the devices may have been touching the cardboard, and diffusion of sulfur gas into these areas would have been more difficult. As this is a circulated coin, there may also be differences in luster which can produce differences. And lastly there is the effect of residual skin oil from circulation. Of course, skin oil has its own compounds that will tone silver. That leads us another example:

The high end circulated look. I love this example from the Oregon Beaver collection as it perfectly illustrates 'haloing.' This is when the devices will be ringed with what is typically lighter colored toning. We can easily imagine this happening as someone grasps a coin. The oils are transferred to the fields and devices, but not to the fields immediately adjacent to the devices, particularlly the stars. This, in combination with the variable colored surfaces and color difference between high points and low points shows us this is a strictly original coin. Of course, beware of cleaning halos, which happen when someone shined a coin up but didn't get the crannies. These can be deceptive, especially in online photographs.

Note that not all toning colors come from thin film interference. Grey or brown typically are caused by light amounts of silver sulfide. This coin is shades of brown, grey, and very small dark patches, which gives it its pleasing, circulated appearance.

Since this blog is getting a little long we'll end it for here. Let's cover some quick things to look out for, both positive and negative:
  1. Halos around devices - a good thing, just make sure it's not due to cleaning (which would be evident through luster differences). This sometimes manifests as 'pull away' toning on uncirculated coins.

  2. Perfectly solid color coins - be cautious and look for other diagnostics. Be especially concerned about purplish solid coloring, it could be the effect of a coin doctor.

  3. Subtle variations in the toning - mottling or variegation. Most coins should have these to some extent.

  4. The high points and low points color variation - most circulated coins should look different in these areas to at least some extent. Liberty's gown in the seated series is an ideal area to see this.

  5. Liquid witness marks - it's ok for a coin to be a little splotchy, but be especially on the look out for round circles of coloring or non-coloring. Usually the sign of an improper dip or the intentional addition of a toning substance.
I always find it helpful to look at a coin and ask myself how this toning came to be. If I can't explain it easily through natural means, or have never seen the type before, that's a sign to be cautious. Of course, there is no substitute for expertise, which we can gain by looking at lots of coins. It doesn't hurt that this is the most fun to be had in the hobby!