| Author |
 Replies: 8 / Views: 1,380 |
|
|
Valued Member
Canada
217 Posts |
How come silver stays silver looking when alloyed with copper, but gold changes colour becoming white, yellow, green, red, rose etc depending on the alloy. I have NCLTs that are 50 Ag and 50 Cu that still are white silver coloured. Howcum?
Please excuse my ignorace of basic chemistry etc.
thanks
|
|
|
|
Bedrock of the Community
United States
17884 Posts |
I can't say why gold tends to take on different colors with different alloys but in the case of silver the cleaning they receive after annealing tends to remove the copper from the surface layer resulting in a "skin" layer on the coin that is of noticeably higher fineness than the overall composition of the coin. Especially if they use an acid pickling of the coin to increase the whiteness of the coin during the cleaning process.
|
|
Rest in Peace
10197 Posts |
Condor101, I have read others as well as noticing myself "residue" for lack of better word and understanding on new Shield cents in the form of milky white spots left that will not come off. Its as if the copper layer has been changed in properties in those areas. Only reasoning is the final wash they go thru? I'd like to find out if the "wash" solution is the same for all coins...this may explain the OP's question as well. Any chemists around?  |
|
Pillar of the Community
Belgium
506 Posts |
As for the color of gold alloys, using several metals (both precious and non-precious): https://en.wikipedia.org/wiki/Colored_goldA silver-copper alloy may have both a silver rich (white phase) and a copper rich (yellowish) phase in which the copper may turn black (Cu-I) or green (Cu-II) when oxidizing. Silver (961.8 °C) and copper (1084.6 °C) melt at different temperature. There is an Eutectic composition Ag(0.72)Cu(0.28) having the lowest melting temperature. Composition differences arising within silver coins increase with the copper content of the alloy. Cooling down abruptly helps inhibiting composition gradients. A salty water bleach may wash away excess copper at the surface of a the copper rich phase. |
|
Bedrock of the Community
United States
17884 Posts |
Silver/copper alloys unless well mixed and then rapidly cooled after pouring the ingots, have a tendency to segregate into regions of different fineness. (See discussion in From Mine to Mint)
|
|
Valued Member
 Canada
217 Posts |
Interesting. So silver stays silver coloured (even in 50/50 alloys) because the final cleaning strips the copper out of the top layer, correct? And I assume that is done on the planchets before striking.
thanks
|
|
Pillar of the Community
Belgium
506 Posts |
The (British) Royal Mint experienced plenty of problems with 50/50 silver copper alloyed coins turning black. (So you can't push your luck that far.) Instead, alloying silver with a white brass (Cu 60, Ni 15, Zn 25), proved a suitable solution. This white brass (sometimes called 'German silver') doesn't tarnish easily; it is used for minting in several countries. |
|
Bedrock of the Community
 United States
12477 Posts |
I have several different fineness silver coins from around the world but the worst looking is an uncirculated 1962 Mexican peso. It's mostly black at this point (it was improperly stored for some time but so were most of the others).
After reading this, I guess it's no coincidence that it's 10% silver, 10% nickel, 10% zinc and 70% copper.
In Memory of Crazyb0 12-26-1951 to 7-27-2020
In Memory of Tootallious 3-31-1964 to 4-15-2020
In Memory of T-BOP 10-12-1949 to 1-19-2024
|
|
Bedrock of the Community
United States
17884 Posts |
Quote:
Interesting. So silver stays silver coloured (even in 50/50 alloys) because the final cleaning strips the copper out of the top layer, correct? And I assume that is done on the planchets before striking. Yes as long as you don't circulate them. (Like your NCLT's) The German states used a lot of low grad esilver alloys for their small denominations, often a 50/50 alloy and after some circulation they would turn dark. And yes the clean/pickling process was done to the planchets after annealing and before striking. |
| |
Replies: 8 / Views: 1,380 |
|
Posted 03/29/2017 12:50 pm
