Does the annealing process effect toning?

I have a theory. I want to see if it has any scientific basis or if I am just dreaming stuff up.
I have always wondered why you see far less attractively toned peace dollars then Morgan dollars. I know Morgans spent a lot more time in bank bags and in vaults but there were quite a few 1922 and 1923 peace dollars that sat around as well. It gets me thinking... why so few rainbow peace dollars? I begun thinking of the things that had changed at the US mint between 1904 and 1921... and one of the things that changed was the annealing process. I believe that between 1904 and 1921 the US mint went from wood fired annealing furnaces to gas fired annealing furnaces... but this information could be incorrect (anyone?). Could this change in the annealing process be the explanation for why peace dollars essentially don't rainbow tone?

Thoughts?

 

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I suspect yes, and this is not the first time I've heard it suggested.

 

Annealing changes the crystalline structure of the metal. That will absolutely affect the tonability (if that is a word) of the coin. However, annealing is only driven by the time and temperature. If the wood to gas change altered the time / temperature profile of the annealing, it definitely could affect the tonability. The fact that wood is used versus gas would have no affect providing they followed the same time / temperature profile. As for the rest of your post, I have no information to validate or contradict any of it.

 

You are making an assumption that may or may not be valid - that all silver sheets undergo the same annealing process. I could easily see a thicker sheet using a different annealing temperature/time profile.

 

I go along with GDJMSP. I think it is the processing of the dies that make the big difference. I suspect the pressure of striking would neutralize most of the crystallization created by the annealing process. IMO.

 

If annealing does nothing, then why do it? Also, you said "most of the crystallization". How much does it take to affect the toning process?

 

Annealing , as I understand the use in the mint" is to soften the metal, and now that I reflect on my previous answer, I should say

as you did, as I am not sure if it increases or decreases the amount of crystallization. The info I can google on the process says that annealing softens and strengthens the metal. I still think that the striking process of the dies,and the shape, height of the flow lines, the processing of the dies are responsible for, are the most important factors for the excessive ( rainbow) type of toning.

I know you have more metallurgy experience than I, so I will defer to your explanation of the crystalline structure.

Jim

 

I think the rainbow toning that is more common with Morgans is really an issue of their storage. With the Peace Dollars, I thought it was the finishing process that they screwed up.

Ruben

 

My experience with metal processing is minimal. What we need here is Lehigh.

 

I would think that a gas fired furnace is much easier to control as far as temperature is concerned... Making proper annealing harder...

 

Was there any difference in the material used in the $1000 bags to store the coins? Perhaps there was a chemical component which preferentially attracted/reacted with sulfur dioxide and thus prevented the Peace Dollars from much exposure.

 

Your (great?) grandparents used to bake bread in a wood fired oven. If you know what it is, I would think you could control it. There may have been a problem accurately knowing what it is. However, I will bet gas goes hotter than wood. Since I don't know what the process is, I have no idea if that makes a difference or not.

 

What are you talking about? First, the wood stoves aren't anealing coinages on an industrial scale. Secondly, metal isn't bread...aside which the bread sucked back then compared to what you can do in a modern oven.

Ruben

 

Consider the notion that different fuel sources leave different trace elements in the exhaust gas, and that difference can account for different behavior.

For instance, coal is much dirtier than natural gas; coal tends to have more sulfur. There is some conjecture that trace sulfur from coal combustion might conceivably impart toning.

As to whether that actually happened or not is a different issue; I'm only mentioning it as a possibility and perhaps part of the OP.

 

I made my living as a process control chemical engineer for 26 years. I can assure you that it is much easier to control temperature with natural gas than with coal. There are many reasons.

 

Using direct heat, you are correct about the gas exposure. The best ways to control it would be indirect heat. Also, if memory serves me, aren't they washed AFTER the annealing?

Secondly, the exposure to sulfur and other such gasses would be minimal when compared to the days, weeks or months of exposure to the air pollution from that era. I have seen pictures from +-1900 where the street lights were turned on at noon because you had no sun due to the smog.

 

Doug will probably come back and say : Morgan surface ^^^^^^^^^^^^^^

Peace ~~~~~~~~~~ ( or upside down "uuuuuu" )

 

I can only help a little as I have not worked in the metallurgical field for many years. The need to anneal silver prior to the coining process is actually caused by earlier processes within the coining process. In order to make the planchets, the silver must be rolled in order to get the sheet of metal to the desired thickness before the planchets can be punched. This rolling process is a form of cold working which has the effect of increasing hardness while simultaneously decreasing ductility. The annealing stage allows for the softening of the metal by raising the temperature of the metal to the point that it recrystallizes. Once this is done and the metal is air cooled, it will be sufficiently ductile for the coin process.

Now here is where my absence from the metallurgical field hurts me. Most of my experience with annealing was applicable to engineering metals such as steel and aluminum. I have very little experience with precious metals such as silver. However, I do know that controlling the annealing process is important with all metals and the greater measure of control with regards to time and temperature will yield better results. In that regard, I must assume that the gas furnaces were capable of producing a much more consistent product.

Having said all that, I am not sure if the annealing process has anything at all to do with the corrosion of the metal. Martensitic steel will corrode just as easily as ferritic steel, or even pearlitic steel. Given that knowledge, I don't know what effect (if any) the annealing process would have on the corrosion process of silver. The crystalline lattice structure of silver is FCC (Face Centered Cubic). Annealing affects the mechanical properties of the metal (ductility & hardness) by changing grain size and orientation but the crystal structure remains unchanged. The effect of grain size on the corrosion of silver is beyond the scope of my metallurgical background.

 

Not a metallurgist here, just someone who likes to ask questions.

Given that annealing changes the hardness of the metal, could differently annealed planchets wind up with differently sized and shaped flow lines after being struck?

Have I understood right from Doug's posts that flow lines affect toning (and that's why dipped coins re-tone in a way that's not the same as original toning)?

So, whether annealing was the relevant variable or not, it's at least a possible hypothesis? A hypothesis disproven by rainbow toning on other coins that went through the same furnaces, but a possible one?