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chemistry people, Why does sulphur tone coins?
psxchelly
Posts: 568 ✭✭
Of is it the way sulphur is used in certain products?
thanks
michelle
thanks
michelle
0
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Jeremy
Don't forget the 10% copper, which is a much more reactive metal than silver.
The colors have to do with the thickness of the toning and the way it reflects or absorbs the different wavelengths of light. We're talking layers of molecules thickness here.
Sulfur is chemically analogous to oxygen. When reactive sulfur combines with silver in large quantities there is a tendency to get the black coating of silver sulfide. Try putting egg yolk on an old silver coin. When the exposure is of low concentration and over a long period of time the layers build up slowly giving the sometimes brilliant colors. Its all in how the surface layers are oriented and how they reflect light. The longer the sulfides take to form the more structured the surface layers are.
Wayne Miller's book on silver dollars provides an excellent discussion of how silver dollars tone and why the human eye perceives different colors. Basically, as sulfur molecules react with silver molecules, a thin layer of silver sulfide forms on the coin's surface.
This layer of silver sulfide then results in a phenomenon known as thin film interference whereby reflected light is 'distorted'. As stated by Miller, "color is perceived as a difference in the length of light waves which radiate or are reflected from an object. Short wave lengths are perceived by the eye as blue; medium lenghth waves as green, and long waves as red."
Hope this helps.
GSAGUY
The main reason i asked is because i noticed on EBay that there are similar coins, that were placed in holders and whatnot but the same date coins reacted differently to similar situations.
Defects are the wrong photopigments or lack of. The most common defect is called Protanopia, defective red cones, the reason it is the most common is because half of the cones in your eye are red. To the Protanope, red looks much darker than green.
The firing of your Ganglion cells also contributes to the colors you see. I will see if I can post a couple of pictures that will help everyone test their Color Vision!
Bajjerfan, I'm sure you're correct in that surface prep also plays a role in the 'final product'. However also keep in mind that minute differences in the thickness of the silver oxide layers can have significant impact on the colors perceived even for coins stored side by side. This can explain why coins from a single bag can have dramatically different color patterns. In fact, the few two-sided mint bag-toned coins I've seen (GSA's specifically) have different colors and patterns from one side of the coin to the next!
I believe that natural toning takes place on the molecular level and over a long period of time. Imagine sulfur molecules disengaging from the medium in which they're bound and slowly drifting onto the surface of the coin. Over time, these molecules pile up in the low points and spill over or cascade down onto other exposed surfaces of the coins. Microscopic differences in the thickness of the silver sulfide layer are then evident on the coin's surface....in the form of differing colors perceived by the human eye.
I've always imagined that monochromatically toned pieces (a single, evenly-laid color) must result from the coin's surface receiving a very uniform deposit of sulfur molecules.....perhaps when a coin is perfectly flat in the bag relative to the main source of the sulfur molecules?
Whatever the correct answers, I do find the topic of toning fascinating.
GSAGUY
I fail the test...............but I don't feel badly.
All I see when I look to the right is the blue radish with the hot pink leaves.
GSAGUY
Just took me a minute to figure out what I was supposed to be looking for.
GSAGUY
Try this one, it will show you what I mean about light! Go into a really dark place like a closet, hit yourself on the back of the head, you will see flashes of light when there isnt suppose to be any!
chuckle.
GSAGUY
Regarding color blindness, or more correctly, color deficiency, as WSM stated the gene for it is carried on the X chromosome. In the real world, about 10% of the male population has a red-green defect to some degree. In the female population, both X chromosomes must carry the defect to manifest itself, so square the 10%, resulting in 1%.
WSM, ganglion cells have no direct relationship to color vision. They are "summation" cells, connected to a group of photoreceptors, and sum their input. Their axons form the optic nerve, which contains 6 million individual nerve fibers, and travel to the brain. Ganglion cells die as we age, and die prematurely in glaucoma. Those with glaucoma do not exhibit any real-world color defects, though in research testing, it's been show that there is some loss of sensitivity to blue.
Barry (ophthalmologist)
What has amazed me is the number of collectors of toned coins who've told me that they are, to some degree, color-blind as it relates to reds and greens!
On fellow, who lives in the Houston area, told me that when looking at a tree fully leafed out, he can't tell if it's dead or alive.
Since I'm a big fan of both reds and greens on my Morgans, it's hard to image what they'd look like if I couldn't fully see perceive those colors.
GSAGUY