How to Test the Purity of Gold Nuggets & Placer Gold Dust

Photo of author
Written By Monica

Have you ever wondered how pure your gold nugget is? How about how pure your placer gold dust is?

We know that gold found in nature isn’t 100% pure. But the good news is testing the purity of your gold can be cheap, easy and fun. And you can do it right from your home!

So in this guide we’re going to give you four ways you can test the purity of your gold. Two of these methods can be done at home, and two can be done with the help of a professional in the jewelry or gold refining industry.

Gold nuggets and gold dust need to be treated a little differently. The goal of testing a gold nugget is to not melt it in the process, because it’s actually more valuable in its original form than melted down, even if it has a “low” purity.

But fine gold dust doesn’t have the same premium value as nuggets, and if you want the most accurate purity test then melting it is a good option.

Specific Gravity Approximation for Gold Nuggets

Testing gold purity by specific gravity principles is one of the oldest known methods of assay.

Archimedes of Syracuse was a famous Greek mathematician, engineer, inventor, astronomer, and councilor of kings who was born in 287 B.C.E.

His most famous story goes like this. The King of Sicily, King Hiero II, had given a goldsmith a block of gold from which to make a crown. When Hiero got the crown back he suspected the goldsmith mixed in some silver and kept some gold for himself. He asked Archimedes if he could prove it.

But there was a catch: Archimedes could not damage the crown in any way.

Archimedes, his brilliant 22-year-old advisor, was given the task of determining the purity of the gold crown.

He stumbled upon the answer sitting in a full bathtub one day when he realized the volume of water displaced by his body overflowed out of the tub and onto the floor. He famously jumped out of the the tub, yelling “Eureka!” which means “I found it!” in Greek.

He knew he would need to compare the density of the crown to the density of pure gold. If the crown was less dense than pure gold, but the same volume, it would indicate the presence of silver or other metals.

The problem is – how do you measure the “volume” or “mass” of an irregularly shaped object like a crown or, say, a gold nugget? The solution was to give the crown a bath, and measure the volume of water displaced. He could then measure the density of the volume of that crown versus the density of pure gold.

Here’s a great video about it.

Thus Archimedes’ Principle was born.

Specific Gravity uses Archimedes’ Principle to find the ratio of the density of an object to a fluid (usually water).

If you know the specific gravity of pure gold, and the specific gravity of a gold nugget, then you can approximate the purity of that gold nugget, give or take a few % depending on what other kinds of minerals or metals are present.

This is the cheapest and fastest way to determine the purity of your gold nuggets at home without melting them down. It also works for solid gold jewelry without gemstones.

It’s also the least-damaging method on this list to approximate the fineness, or purity, of a gold object. Thus it’s the most popular way to measure gold purity in items with great artistic or historical value that cannot be damaged in any way, even for micro-analysis.

Unfortunately, it’s not always incredibly accurate, especially when the gold content is low. According to this study, for gold nuggets that were 87%+ pure, multiple specific gravity tests determined gold purities that were off by 3-4% in either direction. For gold that was less-pure, that margin of error could be even higher.

That’s why it’s called an “approximation.”

This also doesn’t work for fine placer gold dust, or flour gold, as there isn’t a large-enough piece of gold to tie a string to and submerse in water.

All that said, it’s still a great place to start if you want to get a ballpark of how pure your gold nugget is. Here’s what you’ll need:

  • thin thread, fishing wire or dental floss to tie around the object you want to test
  • cup or bowl
  • distilled water
  • precise scale (we use this one because it’s cheap and accurate to .001 grams)
  • density chart
  • calculator
Real Bering Sea Gold Paydirt from Emily Riedel Eroica

Learn to pan gold at home with our 2 Gram bag of Eroica Paydirt!

Buy real gold paydirt from the Bering Sea and prospect at home!

We know where it’s from, because we sucked it up the nozzle ourselves.

Here’s a video on how to approximate the purity of a gold nugget using specific gravity.

The first step is to weigh your gold nugget or jewelry on the scale. In the video example the first nugget weighs 4.53 grams, that’s your “dry weight”.

Now get your glass and fill it with enough distilled water to fully submerge your nugget without it touching the sides or the bottom. Put that on your scale and zero it out. You then tie your thread around the nugget and dip it into the water completely, careful not to touch the sides or the bottom. Shake it a little to get rid of any air bubbles that might be in the tiny crevices of your gold nugget. Wait for the scale to settle on the weight. In the example video the weight of the nugget submerged in water is .32g, that’s your “wet weight”.

Now you have your nugget weight of 4.53 grams (dry weight) and the weight of the water after gold displacement of .32g (wet weight). It’s time for a little math.

Divide your nugget weight by the weight of the water, in this case 4.53/.32 = 14.15625. Then check the density table, it shows that it’s 14k gold. 

Gold Density Chart Specific Gravity

Then check our gold purity chart to find the %.

Gold Purity Chart for Nuggets & Jewelry - Karats & Fineness
Gold Purity Chart

“Eureka!” Now you have a good approximation of the purity of your gold nugget!

Gold Acid Testing

Checking your gold purity with the acid test is a relatively easy and inexpensive way to test your gold. There are numerous kits available on Amazon like this Acid Testing Kit. These kits come with different acid strengths to test 10k, 14k, 18k and 22k gold along with additional acids to test silver and platinum and a testing stone/tile.

In short, you scratch your gold on a dark slate and use acids to see if it dissolves. The stronger the acid required to dissolve your gold, the higher the purity. It’s cheap and simple, but it’s a bit of a process and, like the specific gravity test, isn’t precise.

It will also do a little bit of damage to your nugget during the scratching process. But most prospectors find it’s negligible.

To begin the process you will need:

  • a testing stone
  • acid
  • gold nugget

Place the testing stone on a flat surface, such as a tabletop or countertop. Then, take your gold item and scratch it hard on the stone to make a solid line a couple inches long. Next, take your acid testing kit and apply a drop of acid, starting at the acid for the lowest karat.

Once the acid has been applied look to see if the gold line is still there after about 10 seconds. If it is, try the next level up of acid on the line of gold to see if it stays. If your line of gold has disappeared or gotten really faint then its purity is less than that acid you used. 

For example if I was testing a gold nugget I found panning in one of the 19 Places to Pan for Gold in Alaska I’d start with the 10k acid first. If I could still see the the line I’d test it with the 14k acid solution and so on until the line disappeared, telling me the approximate purity of my gold nugget.

You can use this method with placer gold dust, too. You just might have to get creative to figure out how to smear a small gold flake on your testing stone.

Dan Hurd’s video below does a great job demonstrating how to do this.

Acid testing is a quick, easy, and cost-effective way to determine the purity of gold. Like Specific Gravity, it’s not the most accurate way to determine the purity of your gold but it will get you in the ballpark within a few karats for just a small investment and minimal damage to your gold nugget.

X-Ray Fluorescence Gold Assaying (XRF Gold Assay): The Jeweler’s Test

XRF stands for X-ray fluorescence. It’s a great non-destructive way to find out how pure your gold is.

Unfortunately, these machines cost thousands of dollars so they’re usually too expensive for the average recreational gold miner. But most jewelers and precious metal dealers have one and can help you quickly check the purity of your gold nugget. 

The XRF process starts by shining X-rays on the gold. When the X-rays hit the gold, they make the atoms inside give off their own light, called secondary X-rays. These secondary X-rays are different for each type of element, like gold or silver. A special machine then collects these X-rays and sends them to a computer. The computer looks at the different X-rays and figures out what elements are in the gold.

XRF is a fast and accurate way to check gold content in a nugget. It will also check levels of other metals like silver and copper without destroying or damaging your nugget.

However, XRF is not perfect. It might not be able to find very small amounts of other elements in gold. And other heavy elements can sometimes make the XRF results inaccurate.

Overall, XRF is a good way to check how pure gold is. It is fast, accurate, and can check for many elements at the same time without having to destroy your nugget or melt it down. You just need to find a jeweler or refiner willing to help you.

Gold Fire Assay: The Most Accurate Gold Purity Test

Fire assay is a way to figure out how pure gold is by using fire. This is an old method that has been used for centuries to check the purity of gold and the most accurate by far.

The process starts by taking a small sample of the gold and mixing it with other substances called fluxes. These fluxes help to melt the gold and make it easier to separate the pure gold from any impurities. The mixture is then put into a small clay cup called a crucible.

The crucible is placed into a very hot furnace. The heat from the furnace melts the gold and the fluxes, and the impurities are left behind. The melted gold is then poured into a mold to make a small bar called a button.

Buy gold from your favorite (obviously) Bering Sea Gold cast member!

Buy real gold straight from the Bering Sea via the Eroica’s sluice box.

We know where it’s from, because we sucked it up the nozzle ourselves.

Buy Gold from Bering Sea

The button is then cooled and taken out of the mold. The impurities that were left behind in the crucible are called slag. The slag is removed and the button is weighed. The weight of the button is used to figure out how much pure gold is in the sample.

Want to see Diver Dan melt some gold dust into a mini gold bar with a TIG Welder? Check out the article below!

The button is then tested chemically to confirm the presence of gold and other precious metals, like silver. 

The assayer then puts together a report for you show you the breakdown of you gold and any other metals in it. Here’s an example of an assay report Emily Riedel got back from Oxford, with some important numbers redacted.

Fire Assay Gold Purity Example
Fire Assay Gold Purity Example from Emily Riedel & Oxford Refining

Here’s a cool video that explains the fire assay process.

Fire assay is a very accurate way to check the purity of gold, but it is also a time-consuming and labor-intensive process. It can take almost a full day to complete the whole process.

Thus, it’s going to cost you a fee to send a sample to a refiner to get fire assayed. I’ve seen quotes for $25-$30 after a quick Google search. Or, if you send enough gold to a refiner with the intention of selling it, they’ll assay it for free but take a small % of the gold. We’ve seen 1.5% most commonly.

This is a great way to test the purity of placer gold dust, as you can send in a small or large sample depending on what you want to melt down. However, it’s not a good option for gold nuggets, because you don’t want to melt them down or damage them and decrease the value.

Gold purity is an interesting subject, but the real fun is in the finding. Why don’t you check out Bering Sea Paydirt’s treasure map, and see where regular people are finding gold all over the world!

Real Gold by Emily Riedel from Bering Sea Gold

Sign Up for our Newsletter and Get 10% off Your First Purchase in our Shop, Including Gold and Paydirt!

Leave a Comment

Share to...