Assaying

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Assaying is an important part of the hallmarking process. To make sure it is done carefully, WaarborgHolland has, besides its assaying department, a modern, accredited laboratory. This laboratory meets strict standards and is accredited under ISO/IEC 17025 by the Dutch Accreditation Council (Raad voor Accreditatie, see L210). There are several methods for testing whether a particular item contains precious metal (of a particular fineness).

Testing on the touchstone is a method that shows whether a metal is precious or base. The fineness of the precious metal can also be determined with the test, without damaging the item. A simple form of this method was already used in antiquity, by the Egyptians among others, to expose forgeries. The method can be seen as a combination of comparing colour and chemical resistance to attack by acid. Precious metal of unknown fineness applied to the touchstone is compared with precious metal of known fineness. From the difference in colour and in attack, a conclusion about the fineness can be drawn. Testing is simple to carry out, but drawing a correct conclusion about the fineness of an item takes a great deal of experience. What is needed for testing: touch needles (whose fineness is known precisely), a touchstone (lydite), testing acids and sufficient light, preferably north light. Before a test can be carried out, the item to be tested has to be filed lightly in one spot to remove any surface layers. These can strongly affect the test result.

The touchstone method is a fast, non-destructive way of testing. The kind of precious metal and its fineness are determined by comparing colour and chemical resistance. Touchstones, testing solutions (a mixture of acids) and touch needles (alloys of known precious metal fineness) are used for this.

The advantage of the touchstone method is that, in principle, every part of an item can be tested.

A test streak of the locally filed item is made on the touchstone by rubbing the item firmly across the stone. Next to it, a streak is made with the touch needle whose colour and (presumed) fineness match the streak of the item. The two test streaks are then compared closely for colour. If the colour does not match, a new streak is made next to it with a different touch needle. The streak of the item is now in the middle. Then, with a glass rod, a drop of testing acid of the right strength (matching the presumed fineness) is applied to the streaks. Immediately after applying the testing acid, a glass rod with a drop of water is picked up. At the moment the difference in the acid's effect on the test streaks is greatest, the chemical reaction is stopped by adding the drop of water to the testing acid. The test streaks are carefully wiped off with a paper tissue. The result can now be examined thoroughly. The test streak that the acid has attacked least has the highest fineness.

The silver test has two parts. First, acid is used to determine whether the item contains silver, and then the silver fineness is determined by comparing colours. For the first part, the item has to be filed well to remove any surface layer. A test streak exactly the size of the filed spot is made on the touchstone. If silver testing solution is now applied to the streak, a bluish-white precipitate forms if the item contains silver. If the item is base metal, the test streak disappears completely. To determine the fineness, colour comparison is used. The test streak of the (filed) item is enclosed by test streaks of the silver needle of the presumed fineness. By comparing the colours closely, the fineness can now be determined. The yellower the test streak, the lower the silver fineness. This is caused by the copper added to the silver. Often more than one test streak is needed to determine the correct fineness.

The sample is irradiated with low-energy X-rays. As a result, the irradiated atoms in the sample emit fluorescent radiation that has a different energy level for each element. This radiation is picked up by a detector that converts it into an electrical signal.

Using known signals from standards with which the instrument has been calibrated, a PC can calculate the concentration in the sample. The number of pulses of a particular energy level picked up by the detector is a measure of the concentration of an element in the sample. XRF can therefore be used for both qualitative (which element is present) and quantitative (how much of the element is present) analysis.

• The great advantage of XRF is that it is completely non-destructive;
• Because a collimator shapes the X-rays into a very narrow beam (0.5 micron), even the smallest parts of a sample can be examined (such as the spot where it was tested on the touchstone);

XRF needs almost no sample preparation. Because XRF is a surface technique, however, any surface layer first has to be removed from the item at the measuring point.

For several centuries, cupellation has been the standard method for determining the gold fineness of an alloy. This method, laid down in an international standard, is highly accurate. Cupellation is used to check gold alloys submitted for assaying. The assay office can also carry out fineness determinations on, for example, bars and drillings.

The sample material is weighed precisely. The amount depends on the stated fineness: an amount is weighed out that contains about 150 mg of fine gold (for 14 carat gold, about 256 mg). The sample material is wrapped in a measured amount of lead. This is then placed on a block of magnesium oxide (the cupel) in a furnace at a temperature of 1050°C. At this temperature, the base alloying metals such as copper and zinc oxidise out of the alloy, fuse with the lead and are absorbed into the cupel. The precious metals gold and silver do not oxidise and remain on the cupel as a small bead. This bead is called the prill. After 17 minutes, cupelling, as the process in the furnace is called, is complete and the cupel is taken out of the furnace. The prill is rolled into a flat strip and then rolled up into a small coil, called a cornet. The silver is then separated from the gold by boiling the cornet in nitric acid. The silver dissolves in it, and the fine gold remains in the shape of the cornet. The cornet is weighed precisely. This amount of fine gold is related to the weight of the sample material. From these two weights, the gold fineness is calculated.

Because cupellation is a destructive method, the assay office developed microcupellation. Here the method described above has been adapted so that only 15-20 mg of sample material is needed. This makes microcupellation an almost non-destructive method, with a measurement uncertainty only slightly less than that of ordinary cupellation. Because less sample material is used, the total treatment time is also shorter.

At WaarborgHolland, as at many international laboratories in the gold and silver industry, the silver fineness of alloys is analysed by potentiometric silver titration. Because of its great accuracy, speed and simplicity, this is the international standard method for determining silver fineness. As such, the method is also described in an international standard. It is used on silver alloys used for jewellery and utensils. In an adapted form, however, fixing and developing baths can also be analysed.

The precisely weighed sample material (90-200 mg) is dissolved in nitric acid. The silver ions now free in the solution are bound into silver chloride by adding a solution of table salt (NaCl). This precipitates as a solid. The decrease in free silver ions in the solution goes together with a decrease in the potential of the solution. This is measured with an electrode. The decrease in potential is greatest at the moment exactly all the silver ions have been bound. This is called the equivalence point, and it is also the end point of the analysis. The amount of NaCl solution added at the equivalence point determines the amount of silver originally present in the solution. Because the concentration of the NaCl solution is known precisely, the amount of silver present in the sample solution can be calculated. This amount of silver is related to the weight of the sample material. From these two weights, the silver fineness is calculated.

Under the Hallmarking Act, items that contain base metals such as copper, brass and titanium must carry the word METAAL (metal) or a comparable indication. This indication must be on the base metal part. WaarborgHolland will strike the indication metal on this part.

If assaying shows that the fineness of items does not meet the lowest legal fineness, hallmarking is not possible and these items may not be sold as "gold", "silver" or "platinum". Any misleading or incorrect indications of fineness must be removed. WaarborgHolland can do this on request. If you do not want this, a declaration must be signed that these items will not be placed on the market.

Once an item has been approved, it may be given a hallmark.