acid solutions for assaying on a touchstone

[autumnwillow]

Hi!

I'm pretty much new here. My father owns a jewelry manufacturing business and I am currently handling it.

What my craftsman does for assaying is simply rubbing the jewelry with the touchstone and pouring a few drops of nitric acid into it, then he judges the karat of the jewelry depending on the color. He adds 'ashes' to the touchstone and a few drops more of nitric acid to check whether it goes above 14k.

I think that this method might not be accurate so I'm investing my time in other assaying techniques especially with non-standard karat like 13k 15k 16k 17k etc. If anybody can guide me on how to make the acid solutions properly for 10-24k gold I would really appreciate it.

Cheers!

 

[Harold_V]

The best advice I can offer is for you to buy one of the test kits available through jewelry supply stores. Vigor used to sell one which came with test needles, touch stone, three small vials with a dauber stopper for each, some potassium dichromate (for making Schwerter's solution--tests silver), plus instructions. Acid must be procured independent of the kit.

You don't need multiple concentrations of test solutions. By making a mark with a known karat (using the test needles) and a mark from the unknown piece, you can observe how quickly the mark is dissolved. It's as simple as that. It's not painfully precise, but will tell you if you have 10 K instead of 12 K. With experience, I understand it can be quite accurate.

There are few methods of non-destructive testing. This one comes as close as you can get without spending a huge amount of money for equipment.

Harold

 

[autumnwillow]

Thank you for your reply.

Unfortunately I live in Asia, I don't think customs would allow shipping of acids overseas. Not unless companied with expensive documents and/or permits.

I have been thinking about creating my own 'known' karat pins but the problem that crosses my mind is the gold color involved, yellow gold jewelry can contain 50% gold and 25% copper & 25% silver or 30% silver 20% copper and other mixtures. This goes the same with rose/red gold and white gold. Thus, a standard 14kt white gold will contain 58.5% gold and 41.5% palladium, but other 14kt white gold jewelry will contain 58.5% gold and 41.5% nickel/zinc/manganese/silver/copper. The marks would be different.

If I were to make all possible known karat pins with their color, it would be around 50 or more pins.

I need a fast solution for assaying, unfortunately I can not afford to buy an XRF machine yet.

 

[4metals]

The test kits come without acids, you get them locally and mix the solutions, so shipping internationally should not be a problem. Making your own test needles will work but you will need to have a sample of every color alloy as they will react differently. Unfortunately if you make a lot of different alloys, you need a lot of test standards.

 

[autumnwillow]

I was thinking that there could be a formula for acids to assay properly. Say an 18k acid will not change anything to the mark, same goes with 14k, 12k, 22k, etc. Probably proper combination of nitric acid, HCL acid and/or distilled water.

Anyway, I think I'll go invest in the pins.

Just one question though, how does a household refiner do a proper inquartation? How do they assay their gold? I'm assuming most are not using the fire assay method.

Again. Thank You.

 

[4metals]

You're assuming the test acids react in a specific karat range which is not true, the mixture of acids dissolve the gold and the rate of the reaction is the indication of karat. The test needles act as your standards.

The household refiner can inquart by knowing the approximate karat without assay as the 3:1 ratio is not an exact number, too much silver is not an issue and as long as you're a bit over 2 to 1 silver to gold the parting will still be complete. The 3:1 number is just a good target number.

 

[lazersteve]

.. how does a household refiner do a proper inquartation? How do they assay their gold? I'm assuming most are not using the fire assay method.

Again. Thank You.

You can watch a video on my website of the process.

http://goldrecovery.us

Welcome to the forum,

Steve

 

[goldsilverpro]

I was thinking that there could be a formula for acids to assay properly. Say an 18k acid will not change anything to the mark, same goes with 14k, 12k, 22k, etc. Probably proper combination of nitric acid, HCL acid and/or distilled water.

Not that I know of. This is a comparison method. Ideally, you would have a wide collection of alloys of known composition with which to compare the acid reaction (rate of reaction, colors produced, etc.) with that of the unknown, side by side.

Just one question though, how does a household refiner do a proper inquartation? How do they assay their gold? I'm assuming most are not using the fire assay method.

In order to calculate the amount of silver needed, only the gold content is important. People use markings, touchstone, etc. There is some guesswork, also.

With a pile of marked, mixed scrap, you can estimate the gold content well enough to do this. Just separate it into piles of the same karat, weigh each pile, and make the inquartation calculations. One thing you know for sure is that the gold will never be marked too high. In the worst case, the gold will be marked so low that gold settling problems, due to the production of colloidal gold, will arise when parting with nitric. This can mostly be alleviated by parting the silver with weak warm nitric, say, 12-15%, by volume, diluted with distilled water, instead of the normal 50/50 nitric. When fire assaying, I often add as much as 6:1, silver:gold, in order to assure that the parting is complete. I first part to completion with a hot 7:1, H2O:HNO3, pour this off, and then finish with hot 2:3, H2O:HNO3, to remove the last traces of silver. Using this 2 step nitric method, I have never, not once, produced any colloidal gold.

If you have a lot of unmarked gold, such as sprues or clippings from a jeweler, you have a different problem. You could test every little piece on the touchstone and go from there. You could melt everything together, sample it, and test the sample(s). If you use a touchstone on the sample(s), it should work fine if the gold was yellow to start with. With white or other colored golds, it would be nice to be able to fire assay.

However, after saying all of this, it's not rocket science. It is far, far better to have too much silver than to have too little. With too little, you can get incomplete parting. In my experience, colloidal gold isn't nearly as much of a problem if you first use weak nitric. There's a lot of flexibility in this and good weighings, a little touchstone work, and some educated guessing, you can get good results.