Assaying Basics

Understanding a refiner’s metal sampling and analysis processes

Preparing the Sample

How refiners obtain a sample will depend on the lot. For most metal lots, refiners typically use one of three methods: drill, pin, or dip. To obtain a drill sample, the refining lot is melted down and formed into a bar. A small section is then drilled out of each end of the bar and used as the sample. For a pin sample, the lot is melted down and a glass tube is dipped into the molten metal to draw the sample, which then cools into a pin shape. For a dip sample, the lot is melted down and a small amount is collected and poured into water to create a granulated sample.

Refiners and independent assayers may have their own preferences for which type of sampling to use, but the consensus is that the pin and dip samples tend to be more representative, as they gather a more thorough mix of metals while the sample is still molten. Since lower-grade materials (such as gold under 10k) contain more elements that could stratify, the refiner has to take special care to ensure it captures all of the layers. Drill samples should be done with only higher-grade materials, although the pin and dip options are still preferable. Some companies may take a drill sample from both the top and the bottom of the bar to improve accuracy.

If you are turning in sweeps or plating solutions, the refiner may use other methods. Sweeps are often incinerated, with the remains pulverized to ensure a fine powder is produced. The powder is poured into a drum, where it may be blended further, and the samples are then drawn through the use of a sampling rod that captures powder from different areas of the drum. Similar rods are also used to draw out samples from plating solutions, but be aware: With some (primarily electroless) solutions, the precious metal may leach out during storage and transport, and actually adhere to the inside of the drum. Make sure your refiner checks the drum’s interior during preparation, otherwise you may be losing metal—and money.

Analyzing the Sample

Once the sample is prepared, it’s ready to be analyzed for content. A variety of factors go into determining which analysis method to use on a particular lot-everything from the metals present in the sample and their concentration to the preferred timetable to what instrumentation refiners have available. In most cases, refiners will have a preferred method or methods of analysis that they employ faithfully. Below are summaries of the most common metals analysis methods used.

Fire assay. The fire assay method is widely considered the most accurate technique for gold assay. The refiner will take a sample of about 250 mg and add silver to dilute the gold content to 25 percent or less. The sample is then wrapped in lead foil, placed in a crucible (called a “cupel”), and heated in a furnace to oxidize the lead and any other base metals into a slag, leaving behind a gold-silver alloy. The alloy is placed in nitric acid to dissolve out the silver, and the residue of pure gold is weighed to calculate the gold content of the sample. This “cupellation” method is also commonly used with silver.

Gravimetric wet chemical analysis. This method is used for a variety of materials that have a mixture of elements, including base metals and Platinum Group Metals (PGMs). The analysis will, through a series of chemical reactions, isolate the metals in a sample so they can be weighed individually in their pure, elemental form. It can provide very accurate values when performed by skilled chemists. Often a single sample may be put through a series of such tests.

Note on both fire assay and gravimetric wet chemical analysis: Sometimes a sample may be put through a combination of these two methods, depending on how many metals (precious or base) are thought to be present. Both fire assay and wet chemical analysis can be time- and labor-intensive, taking up to a week at times because of the various steps involved.

Inductively Coupled Plasma (ICP). The ICP method involves converting the sample into a mist form and injecting it into a plasma flame. Any solids in the sample break down into atoms, which are analyzed with a mass spectrometer or optical detector to determine the amount of each element present. The test is faster than gravimetric analysis and can determine all of the elements present in a sample in one shot, as opposed to isolating one element at a time. Industry observers note that, when operated properly, with a set of standards that has been carefully prepared and verified for accuracy, ICP approaches the accuracy of gravimetric analysis in many instances. One of the main challenges of using ICP is that results are derived by comparing them to a known standard, as opposed to having an isolated amount of pure material to weigh, such as in gravimetric analysis. That means the instruments need to be calibrated regularly to ensure they haven’t drifted.

X-ray Fluorescence (XRF). In this method, the surface of a sample is bombarded with X-rays, which causes the components to give off their own radiation in particular wavelengths. The XRF machine detects the radiation and identifies the metals and amounts present based on the spectrum of emitted radiation. Because its accuracy is limited when working with samples of differing alloys, and results can be affected by factors such as the surface of the sample, XRF is generally used as an initial step to discern which elements are present as opposed to determining the value of a settlement. At some refineries or metal testing facilities, samples will be analyzed first with XRF for a quick ballpark estimate, and then re-tested with another, more precise measurement. The main challenges of using ICP and XRF is that results are derived by comparing them to a known standard, as opposed to having an isolated amount of pure material to weigh, such as in gravimetric analysis. As with ICP, because results are being compared to a known standard, the instruments need to be calibrated regularly.

Some refiners also have the capability to conduct Atomic Absorption Spectrometry (AAS) tests, although most concur that these tests are not accurate enough for their results to be used as official assays. Since AAS can test for only one element at a time, they are often used to gauge how much precious metal may be found in plating solutions, which typically do not contain multiple metals.

Witnessing the Lot’s Processing

Given the importance of the sampling procedure, you’ll want to make sure the results are as accurate as possible. The best way to accomplish this, of course, is to send your lot to a reputable company. When choosing a refiner, get recommendations from colleagues, check with the Jewelers Board of Trade (jewelersboard.com) to ensure a refiner has no financial issues, or ask trusted suppliers for advice.

Once you’ve made your choice and have a refiner you trust, there’s nothing wrong with asking to check their work. In fact, many refiners encourage it, and have carefully outlined procedures for ensuring that both they and their customers are satisfied with the assay’s results. There are two primary ways for you to conduct such checks:

1. Witness the processing of the lot. Many companies will, upon request, allow the preparation process for the sampling to be viewed firsthand: Customers can watch the melt, then take with them their own samples to have tested independently. Some companies may charge a fee for this service, since it involves dedicating more staff to give a customer’s materials prompt attention, or require a minimum lot size. But, as the saying goes, seeing is believing—and seeing how a refiner handles your material will give you invaluable insight into the company’s capabilities and quality of work.

2. Ask for a sample to be sent for independent analysis.  Customers who can’t witness the processing firsthand can ask to have a sample sent either to them directly or to an independent assayer of their choice. Taking multiple samples from a lot is standard procedure for many refiners, and typically there is no extra fee or minimum requirement for this service (although it’s always best to check your specific refiner’s policies prior to sending in a lot). The cost of an independent test depends on the type of testing and metal—generally, a fire assay for gold may cost between $50 and $175, for instance, while an ICP analysis for various precious metals could run from $200 to $400.

If the results of the refiner and the independent test lab differ by a certain margin, the refiner and customer will usually turn to an “umpire”—an independent assayer that both parties have agreed upon, usually in advance. Most refineries are open to accommodating their customers in this way, and often keep a list of possible umpire laboratories. Typically, the party whose results are furthest away from the umpire’s assay will pay the fee for this service.

In the end, it all comes down to making sure that the return on your refining lot is fair and accurate. After all, your lot is not just a pile of metal waste; it represents your hard-earned dollars. And by entering the refining relationship with a better understanding of the process, you can ensure you keep as many of those dollars as possible.