Gold Assay Techniques: A Comprehensive Guide
Accurate testing of gold is crucial for determining its purity and value. Here, we explore 12 key gold assay techniques, highlighting their features, costs, disadvantages, and best-use cases.
1. X-Ray Fluorescence (XRF)
Devices:
- Niton XL3Gold+
- HITACHI X-MET8000
Features:
XRF analyzes the top 20 microns of a sample, offering high accuracy and quick results.
Cost: $20,000–$30,000
Accuracy: High
Destructive? No
Main Disadvantage:
Only assays the surface, missing deeper impurities.
2. Eddy Current Testing
Device:
Features:
Evaluates both surface and bulk properties, useful for detecting counterfeit coins and bars.
Cost: $2,000–$3,000
Accuracy: Moderate
Destructive? No
Main Disadvantage:
Can be fooled by alloys with similar conductivity to gold.
3. Fire Assay
Devices/Services:
- Certified labs (e.g., Sheffield Assay Office)
Features:
The gold standard for accuracy, fire assay is destructive and best suited for professional lab analyses.
Cost: $50 per test
Accuracy: Very High
Destructive? Yes
Main Disadvantage:
Destructive and requires specialized skills.
4. Resistance Measurement
Devices:
Features:
Measures electrical resistance, offering moderate accuracy.
Cost: $400–$600
Accuracy: Moderate
Destructive? No
Main Disadvantage:
Only tests surface resistance and may yield inaccurate results for some alloys.
5. Acid Test
Device:
Features:
Quick and simple, often used for jewelry and bullion.
Cost: $20–$40
Accuracy: Moderate
Destructive? No (Minor Damage)
Main Disadvantage:
Should only be used as a secondary confirmation test.
6. Ping Test (Sound Test)
Device:
Features:
Relies on the distinct sound of genuine gold when tapped.
Cost: Very Low
Accuracy: Low
Destructive? No
Main Disadvantage:
Limited to certain coin types and not highly reliable.
7. Laser-Induced Breakdown Spectroscopy (LIBS)
Device:
- HITACHI VULCAN
Features:
Highly accurate for surface analysis, providing detailed elemental composition.
Cost: $20,000–$30,000
Accuracy: High
Destructive? No
Main Disadvantage:
Only assays the surface.
8. Density Test
Device:
Features:
Measures density through water displacement, offering moderate accuracy.
Cost: $200–$500
Accuracy: Moderate
Destructive? No
Main Disadvantage:
Cannot detect tungsten-core fakes or hollow parts since it only measures density.
9. Spectrometer
Device:
- Optical Emission Spectrometer
Features:
Highly accurate for surface elemental analysis, commonly used in advanced labs.
Cost: $10,000–$50,000
Accuracy: High
Destructive? No
Main Disadvantage:
Expensive and limited to lab use.
10. ICP (Inductively Coupled Plasma)
Devices/Services:
- Certified labs (e.g., ALS in the UK)
Features:
Extremely accurate and ideal for precise elemental analysis.
Cost: $60–$150 per assay
Accuracy: Very High
Destructive? Yes
Main Disadvantage:
Expensive, lab-exclusive, and destructive.
11. Magnetism Detection
Device:
- MagneticScreenScale
Features:
Detects magnetic impurities, helping verify bullion purity.
Cost: $1,000
Accuracy: Limited
Destructive? No
Main Disadvantage:
Limited to certain types of bullion.
12. Ultrasonic Testing
Devices:
- Ultrasonic Gold Tester
Features:
Measures internal structures to detect voids or non-metallic inclusions.
Cost: Varies
Accuracy: High
Destructive? No
Main Disadvantage:
Limited primarily to bullion testing.
Conclusion
Each gold assay technique has its advantages and limitations. Choosing the right method depends on your goals, budget, and the level of accuracy required. For surface testing, XRF and LIBS are excellent choices, while fire assay and ICP provide the highest accuracy for professional evaluations.
When assaying gold through non-destructive methods, it is recommended to follow A. G. Metals’ BSDC-ND 4-Step Process for Bullion Authenticity. This method ensures reliable results by combining the strengths of various testing techniques. For more details, visit the A. G. Metals website.