How Accurate is Portable XRF Compared to Fire Assay for Gold Analysis?

A very common question we receive from folks in the precious metals business is about the accuracy of portable XRF technology.

Pawn shops, jewelers, and cash-for-gold operations need to accurately assess if a piece of jewelry is made of real gold and precious metals or if it’s alloyed with other metals, and more importantly, the percentage of the precious metal content so value can be determined.

You cannot always tell the exact metal, or the percentage of precious metals in a piece, just by looking at it.

What is Fire Assay

Fire assay is a widely used method for determining the concentration of precious metals, such as gold and silver, in various materials. It is a highly accurate and reliable technique that has been used for centuries.

The fire assay process involves several steps. Firstly, a representative sample of the material is taken and mixed with various reagents, including lead, fluxes, and collectors. This mixture is then heated in a furnace at high temperatures, typically around 1,100 to 1,200 degrees Celsius.

During this heating process, the precious metals in the sample form a molten alloy with the added lead, while impurities are oxidized and absorbed by the fluxes. The molten alloy is then poured into a mold and allowed to cool, forming a solid button or bead.

Next, the solid button is typically subjected to a process called cupellation. Cupellation involves heating the button in a furnace at a temperature of around 950 to 1,000 degrees Celsius in the presence of oxygen. As the button is heated, the lead in the alloy oxidizes and is absorbed by the cupel, leaving behind a bead of precious metals.

The final step involves weighing the precious metal bead and calculating its concentration based on the weight of the original sample. This concentration is typically reported as a percentage or in parts per million (ppm).

Fire assay is the most accurate technique, and is considered the legal method for gold hallmarking, but it is a destructive method because you have to melt the piece in order to determine the gold or other metal concentration.

What is XRF Technology

Portable X-Ray Fluorescence (XRF) technology is another technique commonly used in pawn shops, cash-for-gold operations, and jewelry stores.  With XRF precious metals analyzers the shop workers can measure the content of all gold and precious metals, as well as determine the presence and concentration of other trace, alloying elements, and dangerous heavy elements, which could impact health and the valuation of the pieces. Most importantly, it is a non-destructive technology, so the piece does not lose any value during testing. This is an especially important factor if you’re dealing with high value items.

XRF is an acronym for X-ray fluorescence spectroscopy. XRF is a non-destructive analytical technique used to determine the elemental composition of materials. Handheld XRF analyzers work by measuring the fluorescent (or secondary) X-rays emitted from a sample when excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays, or “unique fingerprints”. These “fingerprints” are distinct for each element, making handheld XRF analysis an excellent tool for quantitative and qualitative measurements.

Fire Assay vs XRF

To alleviate concerns, we conducted a study, tested samples, and compared XRF precious metals analysis with fire assay. We analyzed six large and flat items made of different types of gold with various grades and various compositions, and we found that accuracy was typically within 0.1 or 0.2% of the fire assay results.

We then analyzed small and irregular samples and found out that the accuracy was better than 0.5 weight percent. Here’s a chart so you can see at a glance the results for the various pieces of gold.

There are a few exceptions to this study.  We do not recommend using portable XRF analysis on gold-filled jewelry, gold bars, and bullions. If it is used, we strongly suggest that a secondary analysis is done just to ensure the absence of thick plating or any adulteration.

We’ve gone into more details about precious metals, alloys, and their testing methods in our webinar Not All that Glitters is Gold. You can download the recording anytime and see a demo of the analyzers in action.

Additional Resources

• Download the recording: Not All that Glitters is Gold
• How does handheld XRF work?
• ebook: XRF in the Field: XRF Technology for the Non-Scientist

Editor’s Note:  This article was originally published October 13, 2020, but has been updated, refreshed with new information, and broken links repaired.