on The Chemistry of Silver: Toning, the Thin-Film Interference Phenomenon and Rainbow Colors, Cleani

Baley · February 16, 2004 5:44AM

There have been several questions pertaining to toning and dipping of coins in recent days, so maybe a thread about silver chemistry is called for. Copper reactions will also come into play, as that metal represents about 10% of most US silver coins, and gold coins also tone a bit due to their copper and silver content, but when people talk about toning and dipping, they're usually talking about silver coins.

Now I'm not an expert in this subject, but do know a little bit and hope others will respond and flesh out this basic explanation, and maybe get some question and answers going about silver compounds.

Silver in its pure form is a silvery white metal, malleable, ductile, an excellent conductor of heat and electricity, is pretty stable, that is to say, not very reactive. (it doesn't "rust" as much as iron, which forms the brown iron oxides called rust, and it doesn't corrode like copper does) Silver is fairly scarce but by no means rare, and has been a decent store of value since antiquity, hence the metal's use in coinage, basically since the first coins.

Silver does not readily react with elements in normal pure air (Nitrogen 79%, Oxygen 20%, remainder trace Carbon Dioxide, Argon, etc.), at normal temperatures, but the metal will oxidize at high temperatures, forming dark compounds. But it is usually sulfur and halide containing compounds that make silver tone black and in the various colors sometimes seen.

In chemical reactions Silver forms Ag+ ions, which are proton donors in in reactions and combine with -1 elements and compounds (proton acceptors) such as the halides Fl-, Cl-, Br-, I- (Fluorine, Chlorine, Bromine, Iodine) and the HS- (hydrogen sulfides) and H2SO3- and H2SO4- (hydrogen sulfoxides). These compounds ("silver salts") are darker, more optically dense than silver, and absorb, reflect and refract light differently than pure silver.

When a silver coin is newly minted, the force of the press forces the silver into the details of the die and collar, creating a "fresh" silver surface. As soon as it leaves the dies, the coin is exposed to different environments. If hermetically sealed, for example in inert plastic, and left in a very stable place, such as a temperature and humidity controlled vault containing inert gas (or vacuum) , it will theoretically stay bright and white forever, since it will have no elements to react with.

On the other hand, drop the coin into a bag of sulfur powder or cup of sulfur containing liquid, it will turn black in minutes, maybe seconds. All of the surface silver will have combined with the free sulfur, forming a dark, thick silver sulfide coating. This coating will be difficult to remove (dip off, more on how this works later) and if the coating is removed, a dull surface will remain, since the dip removes the Silver and sulfur both, leaving behind the less lusterous silver that was underneath the top layer.

We are most concerned with storage conditions in between these extremes, the kind which produce light to medium toning and possibly some attractive colors, patterns, framing effect, highlights, etc. Such toning is a very delicate process, and takes a long time, month to years to decades...

Baley · February 16, 2004 5:45AM

Imagine a bag of 1000 silver dollars, coined in, say, 1885. They are all fresh and white and untoned.

Imagine the bag being placed into a vault in some government building somewhere, and left undisturbed for many many years, maybe a generation or two.

These bags cointains some sulfur compounds, because the material they were made of was impregnated with these compounds during manufacture.

What happens is the sulfur compounds are released from the bag slowly over time, and fall onto the top coins exposed to the bag, or are tranferred by contact with the bag.

Imagine the tiny sulfur atoms and compounds falling, tumbling down the surface of a tilted coin underneath them, each one piling atop the others before it like snow falling on your windshield, and as each particle settles on the coin's surface, it bonds to a silver atom, or to the silver compound already formed beneath it, forming a layer of graduated, varying thickness.

Now there are a lot of ways for a coin to get a layer of compound on it's surface, of varying thickness, the above bag example is a classic, but paper products like coin albums and envelopes, the paper in rolls, the felt bag in the dresser, the velvet lining in a coin case or cabinet, whatever, it's usually textiles in contact with the coins that causes the toning....

Baley · February 16, 2004 5:59AM

So what happens when a film of varying thickness is deposited upon a coin's surface?

Well as light from the light source bounces off the layer, it is reflected and refracted differentially, because the light rays travel through the layer and interfere, on the way back out, with incoming rays. This prism effect is similar to the "rainbows" one sees in a thin film of oil in a rainy parking lot.

The compounds formed when silver reacts with various contaminants upon its surface sometimes have colors of their own, and this effect can contribute to the colors of toning seen when viewed by an observer. Without going to much into the optics involved, basically colors are because the object absorbs all the spectrum except the color seen, and it's that wavelength that enters the eyes.

Other factors are the other metals in the coin alloy, primarily copper but there are frequently trace impurities, and the composition and extent of these can affect the way a coin tones.

In general, the longer the process of the thin film formation takes, the more smooth it's appearance will be, and the more narrow and tight color transitions are possible.

1jester · February 16, 2004 6:02AM

I like this thread already!

JrGMan2004 · February 16, 2004 6:06AM

<< In chemical reactions Silver forms Ag+ ions, which are proton donors in in reactions and combine with -1 elements and compounds (proton acceptors) such as the halides Fl-, Cl-, Br-, I- (Fluorine, Chlorine, Bromine, Iodine) and the HS- (hydrogen sulfides) and H2SO3- and H2SO4- (hydrogen sulfoxides). These compounds ("silver salts") are darker, more optically dense than silver, and absorb, reflect and refract light differently than pure silver. >>

Just to point out something. Ag+ ions are actually are 1 less electron than protons. And an Ag- has 1 more electron than protons. Electrons can move, not protons. If the number of protons is changed, then you have a completely different element. Thought I'd point this out, a minor difference, but a technical detail you had wrong.

Baley · February 16, 2004 6:08AM

ah, you're right of course, I didn't mean to imply that the proton moved, just that the plus charge was available for bonding with a corresponding negative charge, provided by the electron.

Good catch!

Newmismatist · February 16, 2004 6:09AM

We're waiting!

Isn't there something about visible light and wavelengths that range from about 4,000 to 7,000 angstroms and dispersion involved here? (Something I remember from physics a long time ago) - also something like it resembles an oil slick on water?

topstuf · February 16, 2004 6:21AM

Baley, you forgot to mention that the mint is a government facility and that further study is needed. If, as you say, sulfur ions are loose in the coin bags, it seems that an environmental impact study is needed at the very least.

And after the results are in, an appropriation for the hiring and administration of sulfur disposal specialists should be immediately funded.

No coins should tone during my administration. Read my lips.

Baley · February 16, 2004 6:32AM

LOL, I'm not going to carry this whole discussion! Please, expound on your ideas, and possibly someone with a library or some internet search "gumption" will post some quotes and links.

For now, I think most "dip" solutions contain "thiourea" or "thiouric acid" or some such, and remove tarnish by ionizing the silver compounds from the surface of the coin (both the silver atom and the negatively bonded "mate") competitively into the solution. Does anyone know more about how "dip" solutions work?

The other, older, more common method of silver cleaning is the Abrasive Soap method, basically grandma's silverware polish.

gsaguy · February 16, 2004 6:44AM

Excellent post Baley.

GSAGUY

Barry · February 16, 2004 6:47AM

The dip is an acid, or proton donor. It is more reactive than the silver, therefore the sulfur in the Silver sulfide layer reacts with the dip, causing a chemical reaction.

SportsModerator · February 16, 2004 6:48AM

OUTSTANDING THREAD!!!

Thank You

kyleknap · February 16, 2004 6:52AM

An excellent post Baley!

For more on this topic, I recommend those of you who have not already read TomB's essays on this topic take a look at Ivy league Coin.

Kyle

gsaguy · February 16, 2004 6:54AM

Good link KyleKnap.....I also recommend Wayne Miller's book which has an excellent discussion of the topic.

GSAGUY

JrGMan2004 · February 16, 2004 6:57AM

Proton Donor? Hmmm... I think he means electrons... but otherwise is correct. And the reason why you can't undo a dip, is b/c to undo a chemical reaction, you would have to use another chemical reaction that would act in the opposite direction of the dip, as opposed to a physical reaction, which is like sawing the coin in half

which can be put back together by physical means, or like mixing gas and water, they will mix at first, but given time will seperate, the water on bottom, the gas on top... ok, rambling on...

Shamika · February 16, 2004 7:00AM

"On the other hand, drop the coin into a bag of sulfur powder or cup of sulfur containing liquid, it will turn black in minutes, maybe seconds."

Damn, I knew I should've kept that bag of sulfur out of my office.

LucyBop · February 16, 2004 7:01AM

<< Silver in its pure form is a silvery white metal, malleable, ductile, an excellent conductor of heat and electricity, is pretty stable, that is to say, not very reactive. >>

incorrect, silver is highly reactive, which is why the bullion coins will tone quicker, due to their higher concentration of Ag, as opposed to
the .900 Silver coins......

<< Silver does not readily react with elements in normal pure air >>

a tough find in todays world or peoples homes......

<< On the other hand, drop the coin into a bag of sulfur powder or cup of sulfur containing liquid, it will turn black in minutes, maybe seconds. All of the surface silver will have combined with the free sulfur, forming a dark, thick silver sulfide coating. This coating will be difficult to remove (dip off, more on how this works later) and if the coating is removed, a dull surface will remain, since the dip removes the Silver and sulfur both, leaving behind the less lusterous silver that was underneath the top layer.
>>

The chemical reaction causes a thin oxide to grow..... Sulphur, O2, H20, sodium from human sweat reacts with the Silver and a oxide
is grown. When a oxide grows it also consumes a little bit of the surface it is growing on, the intial rate of growth can be quick but then it will slow down as a oxide doesn't like to grow on itself........ The thincker the oxide, it will reflect light in a different spectrum resulting in the different colors that the human eye interprets.

<< So what happens when a film of varying thickness is deposited upon a coin's surface? >>

to deposit a film and a reaction on the coins surface resulting in the growing of oxide are two different things....

Barry · February 16, 2004 7:01AM

George,
Are you referring to my post? An acid is a proton donor, aka a Hydrogen ion. If/when the hydrogen combines with the sulfur, you get Hydrogen sulfide, which is the same stuff that comes bubbling out of the electorlye in a car battery when you charge it. Not good to sniff.

LucyBop · February 16, 2004 7:07AM

<< the same stuff that comes bubbling out of the electorlye in a car battery when you charge it. >>

huh?

that would be sulphuric acid.....

Baley · February 16, 2004 7:12AM

yes, an acid has free protons (H+), which will compete (successfully) for negatively charged ions from the surface of the coin.

This is a chemical reaction, as opposed to the mechanical process of "scrubbing" the tarnish off of a coin. (which would occur, for example, with a fine-grit silver polish, steel woll, brillo, and the like.

Either way, a thin film of silver is lost from the surface of the coin when the coin is cleaned.

Barry · February 16, 2004 7:21AM

<< 

<< the same stuff that comes bubbling out of the electorlye in a car battery when you charge it. >>

huh?

that would be sulphuric acid..... >>

No. That would be Hydrogen sulfide gas. The sulfuric acid is in solution, in the water.

LucyBop · February 16, 2004 7:24AM

i had some stuff bubble up on me during a recent battery change and not outgas........

it was sulphric acid.....

Baley · February 16, 2004 7:29AM

could we "steer" the convo back to coins? what would happen if you dropped a coin into battery acid?

what would happen if some of the HS gas from the battery, just a little bit, drifted over and got on the coin?

JrGMan2004 · February 16, 2004 7:44AM

Ok, you guys keep saying that it's sharing Protons, which they do NOT! Change the number of protons, and you've changed the element. If Hydrogen gave away 1 proton, how many protons would it have left? Hmm? 0, zippo... so how could a Hydrogen atom give a proton away? Protons and Neutrons are tightly packed in the nucleus and are not traded amount atoms. Electrons on the other hand are flying around in the electron cloud and can be exchanged with other atoms. And, since Acids are Negatives, they need to give AWAY electrons to become Neutral, stable... the protons and electrons need to be equal to balance the charge. So... it would follow, that if an atom is negative, it has more electrons than protons, correct? Thus, to become neutral (which is what the atoms are seeking when they bond with other atoms), it needs to give away an electron, to an atom which needs an electron. Or, in some cases, it shares the electrons that it needs to become neutral or stable (the outer electron level to be filled), Oxygen, for example, has 8 protons and 8 electrons. Since, to become stable, it needs 2 more electrons in the outer level (2 in the first level, 8 in the next level) to become stable, it often will combine with another Oxygen atom, sharing 2 of their electrons with each other, an Ionic bond I believe it is. Anyways... the bottom line is that atoms share electrons, not protons or neutrons...

Newmismatist · February 16, 2004 7:53AM

<< could we "steer" the convo back to coins? what would happen if you dropped a coin into battery acid?

what would happen if some of the HS gas from the battery, just a little bit, drifted over and got on the coin? >>

Well the battery acid would do "bad" things to a nice toned coin!

It would remove the silver sulfide "tarnish" and release Hydrogen sulfide gas - which incidently smells like rotten eggs.

You might be able to "re-tone" the coin by passing the H2S if the silver would react with the sulfur ions and again form a silver sulfide on the coin - haven't "waved" any Morgans over hydrogen sulfide since chem class in HS, but if memory serves me correctly it'll turn the coin black.

BTW H2S is deadly: here's a web article I found:

<< Hydrogen sulfide (H2S) gas is normally heavier than air, but when agitated, it can erupt from the confines of [a sewer] pipe in levels of toxicity which paralyze the lungs.

OSHA warns that levels of 300 ppm cause the olfactory nerve to lose sensitivity. At first a "rotten egg" odor is detected but on the second or third breath, the odor is no longer noticed. At 600 ppm, breathing is inhibited, as the lungs fill with the gas.

At higher levels, paralysis of the lungs occursÑonly very fast action could save the victim by removal from the gas area and immediate application of oxygen. >>

So just in case any burgeoning "coin doctors" want to start a "coin toning factory" using H2S - just keep in mind what happened to John Sanford Saltus when he was in the process of cleaning coins!

(but that's a topic for a different thread)

nolimitz · February 16, 2004 7:54AM

Sorry but I believe Light wave are measured in Nano meters, not angstroms
Pete

LucyBop · February 16, 2004 8:03AM

angstrom is a unit of measurent used to determine thickness, such as oxide growth or depostion. Some of the anayltical tools at Intel use a light beam to determine the thickness but angstrom is a measure ment of thickness.... we have used the term nano=meter in relation to depth of a device or structure on a given region of a silicon wafer...

mgoodm3 · February 16, 2004 8:08AM

Here's my contribution.

"Imagine the tiny sulfur atoms and compounds falling, tumbling down the surface of a tilted coin underneath them, each one piling atop the others before it like snow falling on your windshield, and as each particle settles on the coin's surface, it bonds to a silver atom, or to the silver compound already formed beneath it, forming a layer of graduated, varying thickness."

I'm not buying the theory of sulfur falling off of the bag onto the coins theory. It likely happens in the gaseous phase with some sulfur compounds in the air combining with the silver. varying thickness is more likely related to proximity and availability from the source.

The thin film interactions I believe are directly related to the thickness of the layer in relation to the wavelength of light reflecting. I think that a portion of the light is reflected off of the surface of the film and a portion will go through the film to the next boundary, the coin. The distance traveled from the edge of the film and back out is the key. If that distance is 1 wavelength of the light, that light will interfere constructively with the light leaving making that wavelength stronger. Other wavelengths will destructively interfere and there will be less of those wavelengths leaving the surface.

So say the wavelength of green light is 520 nm, you'll see green toning off of a film that is ~260 nm thick, or potentially multiples of 260. If it gets too thick you won't get enough light penetration to show nicely.

edsondl · February 16, 2004 8:16AM

<< Ok, you guys keep saying that it's sharing Protons, which they do NOT! Change the number of protons, and you've changed the element. If Hydrogen gave away 1 proton, how many protons would it have left? Hmm? 0, zippo... so how could a Hydrogen atom give a proton away? Protons and Neutrons are tightly packed in the nucleus and are not traded amount atoms. Electrons on the other hand are flying around in the electron cloud and can be exchanged with other atoms. And, since Acids are Negatives, they need to give AWAY electrons to become Neutral, stable... the protons and electrons need to be equal to balance the charge. So... it would follow, that if an atom is negative, it has more electrons than protons, correct? Thus, to become neutral (which is what the atoms are seeking when they bond with other atoms), it needs to give away an electron, to an atom which needs an electron. Or, in some cases, it shares the electrons that it needs to become neutral or stable (the outer electron level to be filled), Oxygen, for example, has 8 protons and 8 electrons. Since, to become stable, it needs 2 more electrons in the outer level (2 in the first level, 8 in the next level) to become stable, it often will combine with another Oxygen atom, sharing 2 of their electrons with each other, an Ionic bond I believe it is. Anyways... the bottom line is that atoms share electrons, not protons or neutrons... >>

JrGMan2004:

Point 1:
What is a hydrogen atom that loses an electron? Give up? It is a proton. Acids in aqueous solution have pH's of less than 7, which corresponds to an excess of hydrogen ions (i.e. protons) in solution. A hydrogen doesn't give away a proton, it is a proton. Now, normally in acidic solutions, protons do not hang out by themselves, but instead bond to water molecules and form hydronium ions (H3O+). But when given an energetically favorable proposition by other ions in the solution, the protons (hydrogen ions) will either bond to some other ion(for instance if you evaporate the solution, the solid acid (HCl for example) precipitates since protons are too reactive to be a gas (usually) and hydronium ions don't make a good gas either, since they are charged (charged gases are plasmas and do not exist well on earth under non-exotic conditions), anyway, I digress. If a basic solution is added to the acidic solution, the proton can bond with the hydroxyl ions (OH-) in the base to form water. Here the hydronium has transferred a proton to the hydroxyl. If a metal is added to the solution, the metal atom can give an electron to the proton to form a hydrogen atom. Two hydrogen atoms find each other (they are very lonely) and form hydrogen gas which then bubbles out of the solution. Meanwhile the metal ion enters the solution and may form a precipitate with other components in solution.

When chemists talk of proton movemement they do not mean between nuclei. They mean hydrogen ions in solution.

Point 2:
Two oxygen atoms sharing electrons form a purely covalent bond, not ionic. In order to form an ionic bond, one needs two opoositely charged ions.

Point 3:
Electrons in atoms do not "fly around". Please consult quantum wave theory for more information.

<< Sorry but I believe Light wave are measured in Nano meters, not angstroms >>

Nolimitz, one can measure light in any unit of measurement that they desire. A nanometer is just 10 angstroms.

nolimitz · February 16, 2004 8:17AM

Lucy
both Angstroms and Nm are units of measure. Light how ever is generally expresed in nanometers or 100's of angstroms
A penny for a dollar thought from a Micron employee to an Intel one. Just curious what did/do you do at Intel?
Pete

link

LucyBop · February 16, 2004 8:26AM

<< Light how ever is generally expresed in nanometers or 100's of angstroms >>

I was at one time a Group leader for the Diffusion Enginering group...

and if you ever apply at Intel, don't use this answer for a question about units of thickness....

edsondl · February 16, 2004 8:27AM

I agree with mgoodm3.

The sulfur atoms do not come from physical contact with particles, but from dissolved atoms in the air. Since sulfur and silver are quite reactive, the proximity of the silver surface (coin) to the sulfur source (bag) is very important. The same thing in albums which contain sulfur in the paper. Vapor pressure of sulfur and diffusion also play an important role.

Another thing to consider is the permeability of the surface layer. If it is easier for the sulfur to move over the top of the coin to an unreacted area, the toned area will grow larger, but if it is easier for the sulfur to diffuse through the already reacted layer, the color will change (as the layer grows thicker) and move toward opaque.

tjkillian · February 16, 2004 8:29AM

Excellent topic! Metallurgy is cool. Why can't silver oxide be removed through electrolysis? If one could attach a DC current to a coin, could the silver oxide (toning) be removed by simply reversing the chemical reaction by adding energy?

Tom

LucyBop · February 16, 2004 8:34AM

<< Excellent topic! Metallurgy is cool. Why can't silver oxide be removed through electrolysis? If one could attach a DC current to a coin, could the silver oxide (toning) be removed by simply reversing the chemical reaction by adding energy?
>>

the only way a oxide can be removed is with a acid dip such as hf that will attack and eat the oxide, since a oxide consumes while it is growing, if it is completely removed from a coin the silver surface that was consumed during the growth will be removed, the question is to what extent and will it really be visable to the naked eye......

the more I think about we have used plasma (charged gas) etchers to remove films, though i don't believe grown oxides....

grown oxides have always been removed in a HF dip of either 5:1, 10:1 or 100:1 hf to water mixture....

edsondl · February 16, 2004 8:35AM

Tom,

Yes. Electrolysis is a solution to remove toning.

You can also do an electrolytic reaction (in solution) to move electrons to the silver ions (turning them back into metal atoms) from aluminum atoms (which then form ions and bond with the sulfur ions) Sulfur prefers aluminum to silver.

However, the silver surface will then form an oxide anyway for passification (sp?)

-Dan

LucyBop · February 16, 2004 8:39AM

<< but if it is easier for the sulfur to diffuse through the already reacted layer, >>

maybe with heat in the 400 - 600 c range to diffuse it throw a oxide layer that has previuosly grown, even then its tough....

this is why with some layers on a computer chip we deposited a film rather then a grown oxide, because it isnt easy to diffuse as said above.

the oxide becomes a protective barrier, and diffusing thru it isnt easy at all. At Intel we would often add dopant Implants to certion regions of the wafer with high energy Implant tools, then they would goto a diffusion furnace so we could drive/diffuse those implants to a desired depth, as well as activated them, then we would do a high temp anneal to repair damage done to the silicon via the implanting...

edsondl · February 16, 2004 8:40AM

<< the only way a oxide can be removed is with a acid dip such as hf that will attack and eat the oxide, since a oxide consumes while it is growing, if it is completely removed from a coin the silver surface that was consumed during the growth will be removed, the question is to what extent and will it really be visable to the naked eye...... >>

Lucy,

I will assume that since you worked with intel that the films of which you speak were Silicon Dioxide (silica, or glass). HF etches glass very, very well, but does not react with most other oxides. The silicon ion froms (SiF6)2+, or a similar ion complex that is very soluble in water.

In theory, electrolysis will leave the silver atoms on the surface of the coin, if the layer is not so thick that the whole sulfide layer falls off in a chunk during the process. However, atomic diffusion during the toning process may still hinder luster.

LucyBop · February 16, 2004 8:42AM

not so, the age of the oxide doesn't matter in semiconductor circles, the integrity of it does......

HF will eat thru it........ regardless of age.....

we would also dip in a sc1 and sc2 solution, which the exact chemicals used slips my mind, a megasonic clean of some sort, to remove organics from the slilicon.....

edsondl · February 16, 2004 8:47AM

<< the oxide becomes a protective barrier, and diffusing thru it isnt easy at all. >>

Not all oxide layers are protective.

Rust on iron is not protective. However, zinc oxide is (this is why we coat steel and iron with zinc (galvinized steel) to prevent oxidation and rust.

The oxide layer on aluminum and silver are also protective. They form immediately on exposure to air, but since oxygen can not diffuse through these oxide layers, the layer does not grow very thick and most of the metallic luster is still observable.

Based on the layer thicknesses observed with silver sulfide, I doubt if AgS is a completely protective barrier. It depends on the relative size of the sulfur atoms to the interstices of the AgS crystal structure and the strength of the bonds.

nolimitz · February 16, 2004 8:52AM

Hf is used to etch Si02 (glass) not all oxides.

LucyBop · February 16, 2004 8:56AM

when you talk rust, your talking a corrosive..... (how do you spell that)

edsondl · February 16, 2004 8:56AM

<< not so, the age of the oxide doesn't matter in semiconductor circles, the integrity of it does...... >>

I'm sorry, I don't see where I mentioned "age". I don't see how age would matter either, unless the processing produces high-energy (strained) bonds that later relax. And the integrity of an oxide layer is vital for it to fulfill its role as an electronic restistor.

I merely said that HF is not an universal etchant for oxides. HF dissolves SiO2 very well, (which forms on the surface of any Silicon wafer immediately in the presence of air) but not B2O3 (boric oxide), for instance. HF also is a real beast with organics, especially those containing calcium (which is why in small doses of skin contact, HF can maime or kill a person, nasty stuff)

BAJJERFAN · February 16, 2004 8:58AM

First off copper and silver are chemically similar and and copper reacts faster with all species except sulfur. If you were to put a Morgan dollar or a new Edison commem (both are 90% silver, 10% copper) into battery acid the sulfuric acid would react preferentially with the copper. Silver is not generally very reactive if at all with sulfuric acid. The gas that comes off of a car battery when it charges is hydrogen.
Surface atoms on a coin are oxidized over time by pollutants in the air and in the materials with which they come into contact. Those of you who took high school chemistry probly remember doing the experiment where you took elemental iron powder and heated it with elemental sulfer to form iron sulfide. Electrons are transferred from the iron to the sulfur so now both elements are in an ionic state. Treatment of this compound with a mineral acid like sulfuric or hydrochloric releases the rotten egg smelling hydrogen sulfide. I don't believe that this is how coins are cleaned.
Dips based on thiorea work by reducing (or adding electrons to) the oxidized silver atoms on the surface of the coin there by converting them back to the silver atoms that they started out as. Electrochemical methods work similarly by using other metals such as zinc or aluminum.

looky here folks

LucyBop · February 16, 2004 8:59AM

<< Hf is used to etch Si02 (glass) not all oxides. >>

again incorrect....

some of the things I speak of are being done as we speak.......

we did have older technologies were we did deposit a glass and not grow one, and then we would do a high temperature reflow of the glass.... but this is different from a grown oxide...... wether its a dry oxide or a wet oxide......

Newmismatist · February 16, 2004 9:03AM

Here's some information that is necessary to understand some of what has been discussed regarding toning:

<< Visible light runs from the familiar blue to green to yellow to orange to red. Red light is the least energetic of the colors of visible light, and blue is the most energetic. Beyond the red end of the visible part of the spectrum lies the infrared and radio radiation. Infrared "light" is familiar to us as heat, while radio waves are used for TV and radio broadcasts. Beyond the blue end of the visible spectrum lies ultraviolet light, X-rays and gamma-rays.
What is an Angstrom?
Radiation travels in waves. Scientists use the length of the wave (the distance between peaks) to define the energy of the radiation. Astronomers measure this length in "angstroms," a unit of measure equal to 1 hundred-millionth of a centimeter. It's a convenient shorthand to avoid writing lots of zeroes when talking about the wavelengths of light. In everyday terms, a sheet of paper is approximately 1,000,000 angstroms thick. . . . Visible light, on the other hand, covers the range from 4,000 to 8,000 angstroms. >>

and then you get this:

It is very common to see rainbow patterns when you look at the surface of a CD, and also when you look at soap bubbles or a thin film of oil floating on still water. All three of these effects are created by constructive and destructive interference of the light hitting the film.

Think about a very thin film of oil floating on water as an example. When white light strikes the film, most of it passes through it, but some is reflected off of the top and bottom layers of the film. In the same way, when you stand in front of a window in your house, you can see a reflected image in the window. Most of the light is passing through the window and out of the house, but some is reflected back at you. The light that reflects off the top layer of a film of oil travels a slightly shorter distance than the light reflecting off the bottom layer, due to the thickness of the oil [for toning on coins substitute thickness of the layer of silver sulfide, or whatever subsatance forms the thin layer on the surface of the coin)]. The incoming ray of light enters the thin film. Part of the light passes straight through the thin film, part of it reflects off the top layer of the film and part of it reflects off the bottom layer of the film. If the film thickness is on the order of the wavelength of light, then interference can occur. Violet light has a wavelength of about 4,000 angstroms (hundred-millionths of a centimeter), while red light has a wavelength of about 7,600 angstroms, so we are talking about an extremely thin film. As we know, white light is made up of all different colors of light.

If the film is just the right thickness, a red light wave (or blue, yellow or green) will bounce off the top and bottom layers in perfect alignment, and the two resulting sine waves will combine to double the amount of red light seen. Or, they may reflect exactly out of phase, and red will be eliminated.

Think of a CD: The surface is mirrored so the rainbow colors are much more intense. In a CD, the angle that the CD is held at controls the colors you see -- the angle controls the path difference of the wave length of light - which is the same reason why the colors of some toned coins will change colors as you rotate them in the light.

The above edited from an article on the web dealing with light and what's called "thin film interference". I understand it, but I can't take full credit for the nice explanation. Hopefully it's undestandable.

LucyBop · February 16, 2004 9:03AM

<< HF also is a real beast with organics, especially those containing calcium (which is why in small doses of skin contact, HF can maime or kill a person, nasty stuff)
>>

HF is not used to remove organics and never has been at Intel.

HF does attacks calcium so if a person came into contact with it they would immediately jump into a very very cold shower which would cause the skin pores to close, then the would get calcium shots and treatments... and those shots hurt....

It is really escaping what we used for organics.... I wanna say Nitric Acid was in the SC1 tank..... but I honestly don't remember......

edsondl · February 16, 2004 9:29AM

<< << Hf is used to etch Si02 (glass) not all oxides. >>

again incorrect....

some of the things I speak of are being done as we speak.......

we did have older technologies were we did deposit a glass and not grow one, and then we would do a high temperature reflow of the glass.... but this is different from a grown oxide...... wether its a dry oxide or a wet oxide......
>>

<< 

Lucy,

Why do you continue to argue about HF as being used to etch all oxides?
It works very well at dissolving silicon and silicon forms (regardless of method of manufacture) which include:

SiO2-quartz
SiO2-silica
SiO2-glass
SiO2-crystobalite
SiO2-tridymite
SiO2*xH2O-opal
SiO2-lechatelierite
SiO2-amorphous but non-glassy
Si-silicon (crystalline)
Si-silicon (amorphous)
etc.

However, many other oxides, sulfides, carbides, halides (all ceramic materials) are insoluble, or only slightly soluble in HF. Hf is not a universal etchant. It works very well for silicon by forming soluble silicon-fluoride ionic complexes. Otherwise, many more things would be listed as soluble in HF in the CRC.

edsondl · February 16, 2004 9:30AM

Sorry. I should specify.

CRC Handbook of Chemistry and Physics

LucyBop · February 16, 2004 9:38AM

ok, in my field, I am limited to oxides used in developing semiconductors, and they are very simlilair to what a toned coins is...

at Intel we are/were not limited to any text book.....

we were in the buisness of developing future technologys and process's to maintian market leardership.....

we were in the buisness of developing science... innovation.....

DHeath · February 16, 2004 9:47AM

Have any of you good chemists had any experience with urea monohydrochloride? It appears to be fully soluble, biodegradable, not carcinogenic, only mildly acidic, and far easier to fully rinse and neutralize.

BAJJERFAN · February 16, 2004 9:49AM

What on earth does HF or hydrogen fluoride have to do with cleaning coins? I have never dipped a coin or used a dip in any other way except maybe for some french onion dip on some Lays tater chips. An acid based dip would clean a coin if the surface atoms were oxidized to the point that the silver was no longer as atoms but as ions. Say we have some silver oxide on the surface. Reacting this would give silver sulfate which is washed away resulting in a net loss of silver from the coin. The other reaction product would be of course water, the two hydrogens of the sulfuric acid combining with the oxygen from the silver oxide.
I'm also pretty sure that toning is NOT caused by little specks or atoms of sulfur floating around in bags of Morgans or tumbling down the mountain of Morgans in a bag like a boulder.

on The Chemistry of Silver: Toning, the Thin-Film Interference Phenomenon and Rainbow Colors, Cleani

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