# Assaying silver



## PreciousMexpert (Jun 26, 2009)

Percentage of pure silver in unknown silver metal


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## markqf1 (Jun 26, 2009)

Fire assay.
You could also cupel it. Grind and mix a small amount with some powdered lead or lead wool and cupel it in the furnace at 1750 degrees F for about 45 minutes. The resulting bead will contain your ag as well as any other pm's that your sample may have contained. Just make sure the lead you use is "silver free".

Mark


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## PreciousMexpert (Jun 26, 2009)

How can I find out if silver is 700,800,950.etc


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## Oz (Jun 26, 2009)

There is a simple solution to this, buy them at a set percentage of silver spot value dependant on assay. If this is for the bars you have talked about in another thread they are 8’’ X 3’’ X 3’’ that is 1,180 cc and if pure silver would weigh 12,378 grams or 398 troy ounces (if your measurements are accurate). At $14.00 spot silver that is $5,572.00. 

That is enough money that if you are just learning to determine purity you should not trust your numbers as it could cost you dearly. Offer them 70% of spot on 1 bar based on an assay (pay for one) citing high costs of refining silver (very true). Once you know what you are dealing with you can both then negotiate better deals on future bars but unless you trust them greatly I would still use an assay for each and every bar. Lead alloys wonderfully with silver and is cheap.


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## PreciousMexpert (Jun 26, 2009)

HI Oz
Thanks for your help
My question is partly related to my other thread .
Also if someone came up to me to have some silver assayed 
I have to tell them the exact purity amount in their sample
Thanks


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## goldsilverpro (Jun 26, 2009)

A silver assay is your top priority. Were it me, I would buy on silver content alone. You need to sample the bar properly and you need to find a reliable lab to do the assay. Cupellation will give answers about 1% to 2% low. This is compensated for by cupelling a sample of pure silver at the same time and then adjusting the numbers accordingly. The Volhard titration method can be quite accurate. One of the most accurate methods is gravimetric analysis, but a lab would probably charge more for that. You could run the assays yourself but this would entail buying equipment and going through a fairly long learning curve. Were I you, I would find a lab.

There are several ways of pulling a sample, but here's one way I would sample a bar. I would use about a 1/8" drill bit and drill 5 holes. On one side, drill 2 holes near 2 of the corners about 3/4" in from the corner. On the other side, get the other 2 corners plus 1 in the center of the bar. For best results, drill each hole halfway through the bar. Before drilling, clean the drill areas on the surface of the bar. You don't want slag or any other junk in your sample. I would use a small grinder or wire brush, such as those for a Dremel tool, to clean the area. Get some type of clean snips to cut the drillings into small pieces and mix everything together. From this, the sample will be taken. The assays take about .25 to .50 grams each. I would have duplicate assays run.

If the bar is from photo solutions, you have 3 situations. First, you need to know an approximate silver purity in order to know how much to pay. For this, you don't need to go through the sampling rigmarole that I outlined above. Just drill a couple of holes in clean areas. The second situation is that the bar is pure to start with, even though it doesn't look like it. If so, a silver cell is not necessary as I explained before. This can only be determined by having an excellent sample. Thirdly is that the bar isn't pure but the impurities are only iron and/or sulfur, both of which can be removed in a melt, without having to use a silver cell. This could only be determined by flux melting some or all of it. This is important because, if it can be purified by melting, it makes the refining much less expensive. 

If the bar is not from photo solutions, it could be anything and it could possibly have gold, etc., in it. In this case, cupellation and subsequent parting will show everything. If, however, the silver content is less than 90%, the cost of refining will be much more. The silver will first require being dissolved in nitric and then cemented, in order to upgrade it so it can be run through a silver cell. If there is enough gold, it could offset these costs. If not, I would probably not buy it (if it were less than 90%) unless I could buy it very cheap.

A simple way to get an initial idea of purity would be to clean an area and apply a drop of Schwerters solution. The redder the color, the higher the silver. However, if the bar is impure, the types of impurities can influence the color. If it is pure red, I would also put a drop on a pure silver coin, at the same time, and compare the colors. I might buy based on this test if the color showed pure and the price was right. However, considering the money involved, I would much prefer a true quantitative assay.

To help zero in on the best refining options, I need to ask if this is a one-time deal? If so, how many bars are involved? If not, how many would you get per given time period.

Looking back over what I have written, I realize I didn't make things very clear. Maybe you can get something out of it.

Be careful!. May the buyer beware! The more knowledge you have about this bar, the more likely you'll come out on top.


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## PreciousMexpert (Jun 26, 2009)

Hi GSP
I do have to be careful 
Thanks


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## peter i (Jun 26, 2009)

Silver can be assayed gravimetrically with reasonable accuracy.

Dissolve a weighed sample in nitric acid, dilute with hot water and precipitate as chloride with HCl. Stir to make the AgCl clump up nicely.

Filter in an pre-weighed Gooch crucible, wash with water, dry to constant weight at 110°C, weigh and calculate the amount of silver in the original sample.


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## PreciousMexpert (Jun 26, 2009)

Hi peter i 
I really appreciate your help


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## goldsilverpro (Jun 26, 2009)

Peter said:


> Silver can be assayed gravimetrically with reasonable accuracy.
> 
> Dissolve a weighed sample in nitric acid, dilute with hot water and precipitate as chloride with HCl. Stir to make the AgCl clump up nicely.
> 
> Filter in an pre-weighed Gooch crucible, wash with water, dry to constant weight at 110°C, weigh and calculate the amount of silver in the original sample.



Peter,

I thought of that, but was concerned as to how he would pull it off with probably very little equipment. I doubt if he has a Gooch crucible. I haven't used them for many years. What do they use in them now, instead of asbestos fiber? What if he were to use a tared filter paper instead? It would have to be a very retentive paper. Or, what about just keeping it in a tared beaker and rinsing, stirring, settling, and decanting it several times with quite a bit of distilled water - and, finally, drying and weighing it in the beaker. Would the amount of AgCl converted to silver by the light and heat be enough to seriously affect the results? And, shouldn't the amount of HCl or NaCl (solution) be only slightly more than what is needed? Also, I doubt if he has a 3 or 4 place balance. A large sample could be used to offset this, if he only has a 1 or 2 place balance. Finally, shouldn't the amount of nitric used not be in excess? 

I also thought about titrating it with NaCl solution. Sort of a simplified Guy Lussac method. Instead of using a buret, maybe the S.G. of the NaCl could first be determined simply. This could be put into a tared dropper bottle. The amount of titrant used could then be determined by weight. Of course, this would be slow, especially near the end. A large amount could be added at first, but then it would be drop by drop. After each drop, the solution would have to be stirred and allowed to settle. The end point should be fairly easy to see (actually, not see). 

I'm not a chemist but I have spent quite a bit of my life doing analytical and assay work. I have occasional thoughts about coming up with a simple detailed step-by-step method for analyzing silver that the amateur could use, without a lot of equipment or knowledge. I would appreciate it if you and the other chemists could give me your (their) thoughts about this.

Chris


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## Oz (Jun 27, 2009)

PreciousMexpert,

After the above commentary perhaps you see why I suggested not trying to accurately assay bars of such value for purchase if you had not done so before. Start small for limited risk if you make a mistake, or get an assay. 

GSP and Peter,

I greatly enjoy hearing your dialogue on silver assaying, I’m sure I will learn much.


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## peter i (Jul 3, 2009)

goldsilverpro said:


> Peter said:
> 
> 
> > Silver can be assayed gravimetrically with reasonable accuracy.
> ...



I've use fine gooch'es without filter, it worked nicely, and the AgCl dried up to a small pellet that would drop out (and be nearly impossible to reduce to silver again).
An oven dried paper filter might be OK, but I've never tried it.
Decanting should work, but it would be terribly laborious (and have a significant risk of failure)

I got the method from a big ole' chemistry book (Scotts ?... I'll have to find the copy again), and it claimed that light was not a serious problem, but should it happen, moistening with dilute nitric- and hydrochloric acid would convert the silver back to chloride. Then wash with hot water again.

It takes a significant excess of strong HCl to dissolve any silver chloride, I would not worry about it.


Titration could be done by dissolving the silver in excess nitric (the nitric will allways be in excess when dissolving, otherwise the last sliver of silver would take indefinitely long to dissolve), then titrate with NaCl solution while hot and stirred well. But you would still have to weigh out the pure NaCl with high precision, and then dissolve it in a volumetric flask (not just a graduated cylinder).

In the gravimetric approach, the balance is the only precision instrument needed.

I'm afraid there is no way around having as a minimum a good balance for doing quantitative analysis, as the precision will be just as good (or bad :mrgreen: ) as the measuring.


The simplest method is to dissolve in nitric, cement with copper, wash carefully, dry and weigh.
When I do this with coin silver or sterling, I'm normally accurate within a percent.
8)


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## Refiner232121 (Jul 6, 2009)

> 4.foil in lead
> Use about 10 grams of .005", silver-free, lead foil. Wrap the foil securely around the metals and form it into a package so it's about 1/2" square, or less.



Can someone tell me how to get rid of the lead


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## lazersteve (Jul 6, 2009)

Heat the lead and sample in a cupel with a furnace that has a vent for air to get in.

The lead absorbs into the cupel wall along with the impurities and leaves the PMs behind. 

This is called cupellation.

Steve


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## Refiner232121 (Jul 6, 2009)

Thanks very much Steve


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