There are several ways to measure the precious metal content of items. In many cases, the assays that give the most accurate results also damage the item. For example, an X-RF machine requires a core sample to give accurate results for a gold bar. Likewise, fire assays are extremely accurate, but the item is melted down in the process.
If the end goal is to sell the item based on the melt value of the precious metals it contains, then any damage caused by an assay is inconsequential. But what if you want to authenticate an item without causing any damage? There are assays for items like antiques, museum acquisitions, archeological finds, and other delicate items that must remain in pristine condition. In the next few blog posts, we’ll explain the testing parameters, equipment, methodology, and overall accuracy of non-destructive assays, such as:
Spectrophotometry
More often used to check consistency in manufacturing, spectrophotometry objectively measures colors with extreme accuracy. A spectrophotometer analyzes the light reflected by a sample to determine its spectral reflectance curve (SRC). Usually expressed as a three-digit number, the SRC is different for every shade of color. This makes spectrophotometry useful for checking the gold content of an item without causing damage.
A spectrophotometer is sensitive enough to tell the difference between plated or solid gold at all karat levels. The caveat is that they can only really show that the color of the object you are testing is consistent with the known value of an authentic piece. For example, let’s say you test a ring you know for a fact is 14k solid gold, and you get a SRC of 21.4. A different 14k ring may contain the same amount of gold, but mixed with different impurities. Because the different impurities change how the ring reflects light ever so slightly, it’s unlikely to get the exact same SRC score.
Continuing with the above example, all a spectrophotometer can tell you is whether the ring’s color falls within the acceptable range for 14k solid gold. It’s a useful test for confirming an item’s gold content, but doesn’t provide detailed information. In addition, the spectrophotometer only measures reflected light, so it can only test the outer surface of an item. In our next post, we’ll cover other assays that provide more thorough results.
Ultrasonic Testing
This assay uses ultrasound to measure changes in the consistency of the material being tested. To conduct the test, an ultrasonic flaw detector (pictured) generates an ultrasonic pulse and emits it via a stethoscope-like transducer that is pressed against the test subject. Ultrasonic waves travel through any medium in the same direction until they meet a different material, which causes them to reflect back to their source. The transducer detects any changes in the ultrasonic pulse to find out if the outside and inside of an object are made of the same material.
Ultrasound assays are a service we offer here at MGS. It’s the ideal test for quickly catching counterfeit bullion (e.g. tungsten-plated bars) or sorting out precious metal plated items from solid ones.
Thermal Conductivity Testing
As the name suggests, this test measures how well a material conducts thermal energy. A specialized heat-generating sensor is attached to the object. The heat from the sensor dissipates into the object. The sensor records the temperature and time elapsed to calculate the thermal transport properties of the material. Since every metal has unique thermal conductivity properties, you can use the test results to narrow down exactly which metal makes up the object.
Even though thermal conductivity testing is extremely accurate, keep in mind that the results only apply areas where the heat reached. You’ll want to test in multiple spots to check if the amount of precious metals are consistent throughout a piece.
Acid testing
At MGS, we use acid assays every day to evaluate gold purity so we can pay a fair price for it. First, a sample is collected by scraping the object in question against a touchstone (usually made of basalt, slate, or another dark-colored stone with a fine-grained surface) – leaving a visible streak. By applying different concentrations of nitric acid to the streak and measuring the reaction, we can determine the presence of precious metals and accurately estimate their purity. For example, stronger concentrations of acid will dissolve silver and lower-karat gold, while pure gold and platinum will be unaffected.
While the acid assay is a quick, easy, and accurate method for confirming the presence or absence of precious metals, it doesn’t tell you much else about a sample. For a detailed analysis without damage, you’ll need…
Energy-dispersive X-ray spectroscopy (EDS)
A scanning electron microscope with EDS capabilities can measure the chemical and physical characteristics of a sample at the particle level. In principle, EDS is very similar to the X-RF assay we offer our clients. The essential difference is that the X-RF assay exposes a sample to x-rays and measures the fluorescence emitted, while EDS focuses an electron beam on a sample and measures the x-rays emitted. Each element’s emission spectrum is unique to its atomic structure, so you can determine the exact composition of a sample by analyzing its emissions. In other words, not only can EDS tell you if a sample contains gold, it can tell you what impurities are also present and in what ratios. For example, the EDS results for a piece of gold jewelry would look something like this:
- 29.6% Copper
- 2.48% Zinc
- 8.17% Silver
- 59.74% Gold
- 100% Total Mass
It’s also worth noting that the size of the sample can be extremely small – even smaller than what an acid assay requires. Unfortunately, the extremely detailed test results are not worth the cost for most people.
