I am losing gold in the process

B

brainless

New Member
#1
Hi guys, I am new here, and to be honest I am pretty ignorant to the topic of fire assays. I have been taking karat gold scrap and melting them down into bars and taking samples off those bars. I have been selling my gold to another party who is using a fire assay. Their assay is consistently coming out about 2 to 4 percent purity above what I am showing in my fire assay. This other party has been doing it a while now and they are reputable so I believe it's me losing gold somewhere in the assay and that they are doing theirs correctly. How could I be losing gold and showing less purity than what is there? Does anyone have any information that could help me? I appreciate all help because I am truly lost right now.
 
fireguy

fireguy

Supermoderator
#2
Hi guys, I am new here, and to be honest I am pretty ignorant to the topic of fire assays. I have been taking karat gold scrap and melting them down into bars and taking samples off those bars. I have been selling my gold to another party who is using a fire assay. Their assay is consistently coming out about 2 to 4 percent purity above what I am showing in my fire assay. This other party has been doing it a while now and they are reputable so I believe it's me losing gold somewhere in the assay and that they are doing theirs correctly. How could I be losing gold and showing less purity than what is there? Does anyone have any information that could help me? I appreciate all help because I am truly lost right now.
Hi:
The type of bullion analysis you are describing, where you are comparing results with another party, is somewhat difficult. There will be differences in procedure, furnaces and other equipment, temperature, products that will cause variation.

With that said, you (and the other party) should be doing what is called a "corrected bullion analysis". Along with your samples you run a similar sample made up of high purity gold, silver, and copper to match your material. Then you can calculate the losses (and gains), and correct the results.

The SMA Methods Manual Volume 2 presents the complete procedure:
www.lmine.com/product/9051x.html

2-4 percent difference sounds like a lot, so you may want to look for some other problem in your assay first. What sample size are you using and what procedure are you following? With that information we may be able to identify a problem.
 
B

brainless

New Member
#3
Hey fireguy, sorry it took me so long to get back to you. The process I am following goes like this. I am taking karat gold (evenly melted scrap) in sample weights of .25 to .5 grams and wrapping it up in a one inch by one inch lead foil with more than enough silver to inquart it. This is placed in a small bone ash cuppel and placed in the oven. I let the temperature get from 1700 to 1800 degrees Fahrenheit. I have noticed It drops to about 1600 degrees when I open the door to check the cuppels. After it has melted into a bead I pull it out and let it cool. Then I place it in a porcelain crucible and begin the nitric dissolve. I start with a three parts water one part nitric (The nitric acid is 70% pure) then my second run I go to three parts nitric and one part water. I have no problem getting it to dissolve and I am very careful in not losing gold during the filtering. I wash my gold and dry it then I weight it. I am careful not to leave any in the crucible. It is always coming out about two to four percent low, sometimes more. I have been refining for years and have the process down very well. I have refined many batches that the samples came from and my refining yields is always higher than the fire assay shows. My XRF also shows there to be more purity there, and the other party's assay is always coming out higher. So I have no doubt that it is something I am doing wrong in this. I have noticed a problem of getting the metals to mix together in the bone ash crucible while in the oven. Maybe I am leaving some in the crucible? I am just stumped on what is causing me to be off on my readings. I really appreciate you helping me, I am lost on this and ignorant to the topic of fire assays. Thank you very much.
 
fireguy

fireguy

Supermoderator
#4
Hey fireguy, sorry it took me so long to get back to you. The process I am following goes like this. I am taking karat gold (evenly melted scrap) in sample weights of .25 to .5 grams and wrapping it up in a one inch by one inch lead foil with more than enough silver to inquart it. This is placed in a small bone ash cuppel and placed in the oven. I let the temperature get from 1700 to 1800 degrees Fahrenheit. I have noticed It drops to about 1600 degrees when I open the door to check the cuppels. After it has melted into a bead I pull it out and let it cool. Then I place it in a porcelain crucible and begin the nitric dissolve. I start with a three parts water one part nitric (The nitric acid is 70% pure) then my second run I go to three parts nitric and one part water. I have no problem getting it to dissolve and I am very careful in not losing gold during the filtering. I wash my gold and dry it then I weight it. I am careful not to leave any in the crucible. It is always coming out about two to four percent low, sometimes more. I have been refining for years and have the process down very well. I have refined many batches that the samples came from and my refining yields is always higher than the fire assay shows. My XRF also shows there to be more purity there, and the other party's assay is always coming out higher. So I have no doubt that it is something I am doing wrong in this. I have noticed a problem of getting the metals to mix together in the bone ash crucible while in the oven. Maybe I am leaving some in the crucible? I am just stumped on what is causing me to be off on my readings. I really appreciate you helping me, I am lost on this and ignorant to the topic of fire assays. Thank you very much.
OK. While the process you describe would be OK to obtain a rough assay, it is missing some important steps.

1st, it sounds like you are cold loading the cupel (putting the lead on a cold cupel and putting into the oven). This is not good assay technique, especially for this type of bullion analysis. Preheat the cupel up to full temperature and let it soak at temperature for about 15 minutes. Then place the lead button onto the cupel in the furnace.

2nd. It sounds like you are cupelling too hot. I would not exceed 1700F.

3rd. Your first analysis would be a "preliminary", which is done to determine the approximate gold and silver composition of the sample.

4th. Your final analysis should have the proper amount of silver added (not an excess) as determined by the preliminary. This amount of silver is typically 2.5-3 times the amount of gold (don't forget to include the silver in the sample). You will also use the result from the preliminary analysis to determine the composition of your proof sample, which will be made up of high purity gold, silver, and copper. The results of the proof will be used to correct for any gains or losses in the analysis.

The SMA Methods Manual II reference I gave earlier is the procedure that was ultimately adopted by ASTM.

A few other important factors:
-Parting acid is typically performed in two steps. The first uses 33% HNO3, for 45 minutes. The second, 66% for 45 minutes.
-Don't boil the parting acid.
-Wash at least 3 times with DI water
-After drying, anneal the gold (1100-1300F)
-Allow to cool before weighting on microbalance

Hopefully this gets you on the right track to obtain the good results you are looking for.
 
B

brainless

New Member
#5
I really appreciate this advice. I am going to try it this week and I will get back to you on how it goes. Thanks again.
 
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