Assaying
Assayers can determine the pure content of precious metals in alloys. EWN assayers use various testing methods.
If the precious metal has the minimum required content, the relevant content mark may be applied to the object.
Laboratorium
Touchstone method
The touchstone method is a fast non-destructive method in which the content of the (precious) metal can be determined. The method can be regarded as a combination of comparison of the colour and the chemical resistance of the precious metal applied to the touchstone with respect to a known alloy. The touchstone method is a testing method that depends on several factors, in which the experience and insight of the assayer are particularly important.
Touchstone Silver
A streak of the silver object is rubbed the touchstone of the silver object. This streak is compared with that of a known alloy (test needle). Determination of the silver content on the test is done by colour comparison only. Adding more or less copper to the pure silver will change the colour of the alloy. In order to be able to see the small differences in colour more clearly, it is possible to use testing acids.
Touchstone Gold
The chosen test needle, which is placed next to the streak of the object, must show the greatest possible similarity in colour. Then a drop of testing acid of the content to be tested (e.g. 14kt testing acid) is applied to the test streaks. When the difference in the action of the acid on the test streaks is greatest, a drop of water is added to the acid on the stone to stop the action. The least affected test streak has the highest content.
XRF
X-ray fluorescence spectrometry (XRF) is a technique of analytical chemistry in which the composition of a sample is determined from chemical elements.
In X-ray fluorescence spectrometry, the object is irradiated with X-rays. The object then emits X-rays at longer wavelengths characteristic of the elements contained in the sample. The technique is non-destructive. In addition to its speed, this is the main advantage of this method of analysis.
Fire Assay
Fire Assay (cupellation) is a process in which base metal and contaminants are separated from the precious metals gold and silver.
This process takes place in a muffle furnace by melting the alloy to be analysed with a certain amount of lead oxidizing on cupels.
The lead becomes lead oxide and will oxidise all base metals and contaminants and these oxides will be absorbed in the cupels (see photo). What’s left is a ball of gold and silver. To determine the gold content, the silver has to be separated from the gold by crushing, rolling and boiling in nitric acid.
Silver titration
The potentiometric titration method is used to determine silver contents very accurately. A precise weight of a sample is dissolved in nitric acid. Then it is titrated with an NaCl solution (table salt solution).
During the titration, an electrode measures the source voltage per quantity of added titrant. Graphically plotted, this results in a curve. From this graph the equivalency point is determined.
The point of equivalence is the point at which, by adding the NaCl solution, all the silver in the solution has been converted to silver chloride. Since the concentration of the NaCl solution and the weight of the sample are known, the exact silver content can be calculated. For this determination of content EWN uses an almost fully automated system.
ICP-OES
The ICP-OES (Inductive Coupled Plasma Optical Emission Spectrometry) is used to determine the precious metal content in a solution. A small amount of sample material is atomized in an argon plasma. Due to the very high temperature of the plasma, spectral lines are emitted by all elements present.
Using a sensitive optical system, these spectral lines can be captured.
Each element has a characteristic number of spectral lines that allow specific determination of the elements present. Comparing the intensity of the spectral line with standardized solutions makes it also possible to determine the precious metal content very accurately.
Accreditation
Assay Office EWN is accredited by the Accreditation Council for
- NEN-EN-ISO/IEC 17020
inspection body type A, registration no. I310 - NEN-EN-ISO/IEC 17025
test and calibration laboratories, registration no. L395
These accreditations guarantee that Assay Office EWN will operate completely independently and has competent laboratory activities.
View the complete scope of Assay Office EWN here
FAQ
- Is the content testing destructive?
No, testing the homogeneity of the objects you offer is not destructive. For this, our assayers use the touchstone method and the XRF (X-ray).
- Do I need to enclose test material?
If you have manufactured the objects yourself, please send test material with the batch for the content test:
- Silver: 0.5 grams per alloy
for potentiometric silver titration - Gold 0.1 grams of gold per alloy
for the purpose of (micro)cupellation
- Silver: 0.5 grams per alloy
- Will I get a certificate
No, you will not receive a certificate or test report. The content mark applied to your object is the actual certificate. You can however ask for a copy of reports on the basis of which a hallmark was applied.
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