Prior to the 1880s, aluminum was a precious metal worth more than gold. During those times aluminum was extremely difficult and costly to refine, and all but impossible to accomplish on a mass production scale.
These factors not only made aluminum more valuable than gold – they also contributed to aluminum’s appeal as a prestigious and luxurious metal. In 1855, aluminum bars were a featured attraction at the then world-famous Exposition Universelle de Paris. During his reign as Emperor, Napoleon III of France allegedly owned a collection of aluminum plates and flatware that were strictly reserved for entertaining only the most esteemed guests. In 1884, a capstone made with 100 ounces of pure aluminum was set on the great Washington Monument.
But today, we use aluminum to wrap up old food. What happened? While aluminum was difficult to refine, it’s not a geological rarity, like gold. Aluminum is actually the most abundant element in the earth’s crust and the third-most common element (after oxygen and silicon) on the entire planet. The development of the Hall-Héroult electrolytic process in 1886 completely solved this problem by making aluminum smelting cheap, fast, and easily adaptable to industrial-scale production. This combined with aluminum’s commonality reduced its value and eliminated its precious metal status.
Originally valued at about $1200 per kg in the 1850s, aluminum dropped all the way to $1 per kg by the 1890s. Talk about a market shake up! While the drop in value may have stung the aluminum investors of yore (if there were any), the boon to humanity cannot be understated. Thanks to aluminum, we have power lines, airplanes, food and beverage containers, and so much more. What would Napoleon think of that?
In this latest entry of our series on non-destructive assays, we’ll be covering two tests that start out the same way, but require vastly different equipment. Both assays require a sample that must be scraped away from the test object. Even though this can mark the object, the “damage” is minor and easily repaired.
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.
In our previous post on non-destructive assays, we covered spectrophotometry – which analyzes the color of an object by simply measuring the light it reflects. Unfortunately, spectrophotometry only works on an object’s outermost layer and usually provides varied results depending on what is being tested. Today, we’ll go over other non-destructive assays that provide more reliable 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.