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What is an Assay?

When collecting or selling bullion or coins, you may have noticed that some items come in an assay card, or are accompanied by a certificate of authenticity. In the bullion industry, the definition of assaying has two meanings. In precious metal refining, an assay process is a test procedure for measuring and analyzing the purity of a metal. This procedure includes determining its composition in ores, alloys, mineral deposits and precious metals. This way mints assure that the bullion meets its precious metal standards of content and purity.

Methods of Assaying

Assaying processes depend on the precious metal type and the particular piece that is being analyzed. Each assay starts with taking a rock or molten metal sample and continues with one of the following methods:

Destructive methods:

  • Fire Assaying – the oldest, most accurate, and destructive assay method. The sample is placed in a crucible and heated to find out the amounts of non-precious and precious metals. It’s a standard method for testing precious metal bullion in mints and refineries.
  • Dry Assaying – heating fluxes, special chemical cleaning agents, together with the sample, to let the precious metals separate, cool down and settle at the bottom.
  • Wet Assaying – dissolving the sample by using solvents and chemically analyzing the solution.

Non-Destructive methods:

  • Spectrograph Assaying – analyzing the intensity and strength of the light emitted by the sample when passing an electrical discharge through it.
  • X-Ray Fluorescence Assaying – identifying the precious metal and its purity by irradiating the sample with an X-ray beam. Analyzing the level of intensity of the X-rays emitted by the sample.

Other Assaying Methodssilver assaying through titration, gold assay by cupellation. Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) for assaying platinum.

Assay Types

Bullion pieces mostly arrive in assays as a special proof. Small bullion pieces are often sealed in assay cards which bear their date of creation and assay info. Larger bullion pieces usually come with an assay certificate which has a similar design and acts as a proof of authenticity and purity. Both assay cards and assay certificates assure the sellers and buyers that the bullion product is pure and legit.

What is a Certificate of Authenticity?

A COA (Certificate of Authenticity) is a sticker or a seal placed on a piece of paper or certificate. This Certificate of Authenticity seal is a warranty that the bullion coin or piece is authentic. Though, unlike an assay, a COA doesn’t guarantee a thorough process which certifies the exact purity of the piece. The COA also doesn’t seal the bullion like the assay card does.

However, the COA displays essential details, such as the approval and authenticity stamp, the mint mark, and purity, serving as a proof that the bullion item is genuine. Some precious metal pieces don’t come with COAs, like bullion offered in bulk, or they may come accompanied with an assay instead.

Assay products vs. COA products

Bullion bars ordinarily arrive with an assay proof because they are large, valuable, and have high purity. Limited or special coin releases, on the other hand, arrive with a certificate of authenticity which proves their genuineness. Sometimes, limited edition coins and special bar releases also come with an assay.  

Precious metal assaying is fundamental both for producing and selling bullion. Both assays and certificates of authenticity assure the buyer or owner that the precious metal item was minted according to the mint’s standards and requirements of precious metal purity and content.

How To Test If Gold Is Real

A true symbol of high status, success, and prosperity, gold is a luxurious, highly desired precious metal. However, even if you own a stamped gold piece, its hallmark isn’t a guarantee of its real gold content. This article explores the best testing methods on how to perform a proper gold test. Remember that you can always get a free top-notch appraisal from our expert NYC Bullion team.

Gold Properties

Before proceeding to any testing, you should know that gold is considered a long-lasting metal since it:

  • Doesn’t tarnish or rust
  • Is non-magnetic
  • Conducts heat and electricity
  • Is dissolved only by nitro-hydrochloric acid (1:3)
  • Is malleable, ductile, and sectile – easy to hammer, press and cut into different shapes

Since gold is sectile, it also needs to be alloyed with other metals. Its alloys are measured using either the Karat system or the Millesimal Fineness System. One Karat unit is equal to a 1/24 part of pure gold in an alloy while the millesimal purity is indicated by parts per thousand of genuine gold by mass in an alloy. Take a look at the below gold purity chart for more details.

Karat System % of Gold Content Millesimal Fineness System Gold or not gold

(US standards)

8K 33.3 333 Not gold
9K 37.5 375 Not gold
10K 41.7 417 Gold alloy
14K 58.3 583 (585) Gold alloy
15K 62.5 625 Gold alloy
18K 75.0 750 Gold alloy
20K 83.3 833 Gold alloy
21K 87.5 875 Gold alloy
22K 91.6 916 Gold alloy
23K 95.8 958 Gold alloy
24K 99.9 999 Pure Gold

To find out whether your gold jewelry or bullion piece is fake or genuine, use the following gold testing methods. Home methods will help you easily test for real gold while professional tests will reveal the exact fineness of your piece.

The Magnifying Glass Test

the magnifying glass test

The first and simplest home test for gold requires just a magnifying glass and your gold bullion or jewelry. Carefully inspect your piece with the magnifying glass for these visual clues:

  • any signs of discoloration – gold doesn’t react to any environmental factors. So, if your piece has discoloration marks, it’s either not gold or gold plated.
  • color and shininess – pure gold is not very shiny and has a nice soft yellow color. Any shiny, yellow or toned(reddish) item is definitely not pure gold.

Hallmark/Stamp/Jewelry Markings Test

hall mark.stamp test

The second step is the hallmark test, for which you may also need a magnifying glass. Look for any gold hallmarks or jewelry hallmarks (inside the surface or near the clasp). These markings must show at least the gold’s purity (in millesimal fineness or Karats) and, if selling jewelry, it can also be accompanied by the manufacturer’s stamp.

Beware of “HGP,” “GF,” “HEG,” “GP,” “HGE,” “RGP,” “GEP,” or ”GP” hallmarks because they all mean that the item is gold plated. Also, avoid fake “800,” “925” or “950” marks on gold pieces since these are all silver hallmarks. The hallmark test isn’t 100% accurate and shall be used only to identify if the gold is not genuine or pure. If your item is marked as gold, but you’re still in doubt, proceed with further testing.

The Skin Test

skin test

The skin discoloration test is one of the easiest ways to test gold at home. Just hold your gold piece in your hand for a few minutes. Real gold doesn’t leave any spots or stains on skin, so if you notice a black or green chemical reaction with your perspiration, then it’s certainly an alloy.

However, keep in mind that if you apply powder or liquid foundation on your tested skin area, a real gold item might leave a black streak on it, so your testing won’t be precise. Therefore, before performing the gold test, please ensure that you aren’t wearing any makeup and your skin is clean.

The Float Test

The Float Test

Another easy method of testing gold jewelry or bullion at home is the float test, for which you need to place your gold piece in a cup of water. Due to gold’s high density (19.32 g/ml), pure gold will immediately sink to the cup’s bottom. Fake or plated gold will hover or float above, while gold alloys will tilt up halfway. Also, if your piece will rust or discolor, then it’s fake.

However, please consider that a fake item could consist of other heavy metals which also sink. So, use this test to spot fake gold that floats or hovers. If your item sank and you still have doubts, take another test.

Porcelain Tile Test (or the Scratch Test)

Porcelain Tile Test (or the Scratch Test)

The porcelain tile test, also known as the gold scratch test, requires a ceramic plate or a piece of unglazed porcelain tile. Scratch or rub your gold across the tile/plate and take a look at the color of the streak. A golden, yellow streak indicates real gold while a black streak shows that your item is fake or it’s pyrite. If you don’t want to damage your gold piece, consider using other harmless testing methods.

The Magnet Test

The Magnet Test

One of the easiest and most convenient home gold tests is the magnet gold test for which you need a high strength magnet. This solution is cheap and portable since a magnet is accessible anywhere and anytime. Because genuine gold is not magnetic, it won’t be attracted by the magnet. Gold allows or fake items will instantly attract the magnet while slightly attracted pieces are more likely gold plated. Also, keep in mind that jewelry pieces usually have strength elements (like wires, clasps) which aren’t made from gold and can be magnetic.

Thus, if the magnet attracted your item, you can skip the following tests. Otherwise, it’s likely pure gold, and it may need further testing for a higher accuracy. Also, remember that other precious metals (like silver) are also non-magnetic, so make sure to perform additional testing to certify that your item is real gold.

The Acid Test

The Acid Test

If you don’t mind damaging or scratching the samples, you can apply the gold acid test. You need to purchase a gold testing acid kit (for each Karat), a black gold testing stone, gold Karat needles and 2 glass beakers. One beaker with baking soda/water mix and other just with water. Before testing gold with acid, be sure to choose a well-ventilated area and to take all the safety measures for protecting yourself and your working area from any contact with the acid.

Each bottle from the acid kit is a distinct mixture of nitric acid and other components. Take your gold sample and scratch a slight line on the black stone, making sure you’ve reached over any potential plating. Mark your line and make similar parallel lines with the other gold Karat needles. Then take the lowest karat acid and apply its content across or on each line and wait for 30 seconds. Wash the stone in the baking soda/water mixture then rinse it in water and pat it with a paper towel.

A reaction (dissolved line) shows that your sample has a lower purity, a slight reaction means that you’ve matched the Karat while no reaction indicates that you have a higher Karat gold. If your sample didn’t react, repeat the test steps with the next acids until you match the Karat. If the last test acid in the kit is for 22K and you need to know whether your piece is 23K or 24K, you should use gold testing machines.

Gold Testing Machines – Electronic and XRF Thermo Gold Test

Electronic and XRF Thermo Gold Testers

The most harmless and accurate testing methods are the XRF thermo and electronic gold testing machines, such as the Sigma Metalytics Precious Metal Verifier and XRF Thermo Scientific Analyzer.

Sigma metalytics machine

This device performs gold test for bullion and jewelry without using chemicals or damaging them. It uses electromagnetic waves to determine the Karat of your item based on gold’s electrical characteristics. The waves penetrate deeply into the tested piece, ignoring its surface and assessing its body, then displaying the result in a few seconds.

The Sigma Metalytics electronic gold tester comes with various sized sensors for each gold sample. Moreover, if you want to sell gold coins or any other gold bullion that is encased, you don’t have to remove them from their protective holders. Just place your sample on the corresponding sensor, select the expected purity, wait for the “Ready” message and press “RUN/CAL.” If the result is between the brackets, your tested piece matches the selected fineness while an up arrow indicates that your piece is not gold.

XRF Thermo Scientific™ Gold Test

It’s the most expensive precious metal tester, yet the most accurate and nondestructive one. This gold analyzer uses X-ray fluorescence (XRF) spectrometers to test the gold piece. The device sends X-rays through the sample and makes atoms get momentarily excited, moving them into a higher energy state. When the atoms return to their ground state, they release radiation which is detected and read by the machine. The machine returns the exact Karat or metal composition of the tested piece. It also comes with a unique AuDIT™ gold-plating detection technology which alerts you if the tested item is gold plated.

Free 100% Reliable Test – Free Professional Appraisal

The best and easiest testing method when selling gold is to get analysis and evaluation by professional appraisers, like NYCBullion. Our team uses high-class digital gold testers, including the Sigma Metalytics Precious Metal Verifier and XRF Thermo Scientific Analyzer. Along with our top-notch expertise, we guarantee a free, fully transparent appraisal of your goods and the most generous quote on the market. You can get your free evaluation from NYCBullion either by sending your gold item or by visiting our store. For any questions, just call us at (212)-354-5557, message us, or, come to our retail office at 30 West 47th Street, New York, NY 10036.