lawsonland
Active member
Why is the gold precipitate black/brown when using SMP?
Just curious
Thanks
Just curious
Thanks
Why Black/brown?
- Thread starter lawsonland
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As I understand it, as the particle size gets smaller, the powder in bulk APPEARS darker, to the human eye. As it decreases in particle size, it goes from metallic gold color to light tan to tan to light brown to med. brown to dark brown to black, with an infinite number of shades in between. When it reaches its smallest size, it is purple. This is called colloidal gold. Colloidal gold is so small, it doesn't settle. I read that a container of colloidal gold was found in an Egyptian tomb and it still hadn't settled.
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Color is generally related to two features. One of them is how finely divided the gold is. If it's fine enough, it's purple in color (think colloidal gold). If it happens to precipitate as reasonably large crystals, it will be a beautiful light golden tan color.
The other issue to be concerned with is how clean the solution is from which it comes.
You are likely to hear an old saying here, the one about garbage in, garbage out. That's one of the many reasons why wise people don't dissolve everything with AR. It works, but the chance of high quality gold is not good, and the risk of losing some of the values is very good, assuming you don't now what you're doing.
My experience in refining filings from the jeweler's bench was consistent. I boiled the filings in nitric, and then dissolved them in AR. The solution was always dirty, as was the resulting gold. I used that gold for my added gold in evaporating, so it actually got refined three times before it was back in the hands of the consumer. Quality, in spite of the dirty process, was excellent.
The very best policy is to remove as much of the base metals as can be removed, before dissolving the values. That way the gold will come down with far less drag-down of contaminants. The other thing you can do to improve the appearance is to precipitate from concentrated solutions, but if they are not clean, you tend to compound the dirty gold yield problem.
Think clean at all times when working with your gold. The benefits far outweigh any of the inconveniences.
Harold
The other issue to be concerned with is how clean the solution is from which it comes.
You are likely to hear an old saying here, the one about garbage in, garbage out. That's one of the many reasons why wise people don't dissolve everything with AR. It works, but the chance of high quality gold is not good, and the risk of losing some of the values is very good, assuming you don't now what you're doing.
My experience in refining filings from the jeweler's bench was consistent. I boiled the filings in nitric, and then dissolved them in AR. The solution was always dirty, as was the resulting gold. I used that gold for my added gold in evaporating, so it actually got refined three times before it was back in the hands of the consumer. Quality, in spite of the dirty process, was excellent.
The very best policy is to remove as much of the base metals as can be removed, before dissolving the values. That way the gold will come down with far less drag-down of contaminants. The other thing you can do to improve the appearance is to precipitate from concentrated solutions, but if they are not clean, you tend to compound the dirty gold yield problem.
Think clean at all times when working with your gold. The benefits far outweigh any of the inconveniences.
Harold
Harold,
Is it dark because it is dirty or is it dirty because it is dark? I think both are possible. I think the finer it is, the more mechanical entrapment of other metals you get and the more difficult (or impossible) it is to clean. I don't think the gold particles are alloyed with other metals. I think the contamination is purely mechanical.
Is dark gold always dirty? Can you re-refine 9999 gold, without contaminating it, and get dark gold? I think you can. I have done it. I once did some experiments in this area. In general, though, I agree. When gold powder is dark, it is almost always dirty. All metals, when very finely divided, can be black. Often, I feel the darkness of the gold can very well be caused by the darkness of the other metals, but not always.
Is it dark because it is dirty or is it dirty because it is dark? I think both are possible. I think the finer it is, the more mechanical entrapment of other metals you get and the more difficult (or impossible) it is to clean. I don't think the gold particles are alloyed with other metals. I think the contamination is purely mechanical.
Is dark gold always dirty? Can you re-refine 9999 gold, without contaminating it, and get dark gold? I think you can. I have done it. I once did some experiments in this area. In general, though, I agree. When gold powder is dark, it is almost always dirty. All metals, when very finely divided, can be black. Often, I feel the darkness of the gold can very well be caused by the darkness of the other metals, but not always.
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I addressed the argument of being finely divided and yielding a dark color, so I fully agree.
One connection I made was the level of concentration. I experienced on only a very few occasions, gold precipitating as those wonderful crystals I spoke of. It happened ONLY from concentrated solutions, and ONLY from re-refined gold (a very high level of purity while in solution). I have no explanation, just a report on what I experienced.
It was unusual for gold to precipitate as anything but finely divided bits from dirty solutions, which is likely a part of the explanation of color. We both agree that finely divided bits appear dark, due to the scattering of light. However, it is also VERY common for the color to improve upon washing. That can be explained by no less than two reasons. One of them is the removal of contaminants that tend to make the gold look dark. The other can be that as the gold is cleansed, it agglomerates in such a way as to reflect light in a more direct path, resulting in a lighter appearing gold. I think both examples are the case.
I agree with finely divided gold that can't be markedly improved by washing alone. It is for that reason I started re-refining my gold. Once precipitated, it can be boiled for hours on end, with no appreciable improvement in quality, but the process of re-dissolving and re-precipitating will liberate contaminants that could not be washed out.
Harold
One connection I made was the level of concentration. I experienced on only a very few occasions, gold precipitating as those wonderful crystals I spoke of. It happened ONLY from concentrated solutions, and ONLY from re-refined gold (a very high level of purity while in solution). I have no explanation, just a report on what I experienced.
It was unusual for gold to precipitate as anything but finely divided bits from dirty solutions, which is likely a part of the explanation of color. We both agree that finely divided bits appear dark, due to the scattering of light. However, it is also VERY common for the color to improve upon washing. That can be explained by no less than two reasons. One of them is the removal of contaminants that tend to make the gold look dark. The other can be that as the gold is cleansed, it agglomerates in such a way as to reflect light in a more direct path, resulting in a lighter appearing gold. I think both examples are the case.
I agree with finely divided gold that can't be markedly improved by washing alone. It is for that reason I started re-refining my gold. Once precipitated, it can be boiled for hours on end, with no appreciable improvement in quality, but the process of re-dissolving and re-precipitating will liberate contaminants that could not be washed out.
Harold
glorycloud
Well-known member
That's interesting Harold. I find that some of the smaller escrap batches that I have done seem to stay very dark, almost
black no matter how much I wash it. The black powder also seems to want to float out when I am pouring off the
solution in between washes.
To re-refine after washing, are you referring to using AR exclusively or can the HCL-CL work as well?
black no matter how much I wash it. The black powder also seems to want to float out when I am pouring off the
solution in between washes.
To re-refine after washing, are you referring to using AR exclusively or can the HCL-CL work as well?
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My experiences were limited to the use of AR, but I can't see a reason why it would make a difference.
The darkness of your gold is most likely related to the level of concentration. When extremely dilute solutions are the norm, I'm having a hard time imagining a heavy precipitate forming.
My method of operation revolved around concentrated solutions, so I rarely ever experienced precipitation from those that were diluted. Near as I can recall, the gold that came from such a lot was generally finely divided and dark, and rarely was greatly improved (in color) by washing. It was definitely improved in quality, however. That's obvious by the change in color of the washing solutions. I'd wash until there was no color change.
Harold
The darkness of your gold is most likely related to the level of concentration. When extremely dilute solutions are the norm, I'm having a hard time imagining a heavy precipitate forming.
My method of operation revolved around concentrated solutions, so I rarely ever experienced precipitation from those that were diluted. Near as I can recall, the gold that came from such a lot was generally finely divided and dark, and rarely was greatly improved (in color) by washing. It was definitely improved in quality, however. That's obvious by the change in color of the washing solutions. I'd wash until there was no color change.
Harold
