Reviews
Part of Bullion authentication guide: evidence and limits
Bullion test methods compared: a qualitative overview
Bullion test methods compared as a qualitative overview of what each measures, their practical limits, and why results should be combined.
What to take away
- The bullion test methods compared here each measure a different property and rule out different substitutions.
- Weight and dimensions are strong first checks but can be engineered to match.
- Magnet response is not a universal precious-metal test.
- Conductivity depends on instrument, geometry, alloy, thickness, and calibration.
- XRF is fast elemental analysis near the surface, not a guaranteed view through a bar.
No single ranking applies to every coin, bar, package, or suspected alteration. Choose a test sequence from the item's official specification and the failure mode under review.
Method table
Bullion test methods
Weight
- Measures
- Mass
- Strength
- Fast, cheap
- Limit
- Mass can be engineered
Magnet
- Measures
- Magnetic behavior
- Strength
- Finds magnetic cores
- Limit
- Nonmagnetic fakes pass
Conductivity
- Measures
- Electrical response
- Strength
- Below-surface response
- Limit
- Geometry and calibration
XRF
- Measures
- Elemental fluorescence
- Strength
- Rapid multi-element
- Limit
- Surface only
Measures
- Weight
- Mass
- Dimensions
- Geometry
- Magnet response
- Magnetic behavior
- Conductivity or eddy current
- Electrical response
- Specific gravity
- Mass divided by volume
- Ultrasound
- Sound travel or echoes
- XRF
- Elemental fluorescence
Main strength
- Weight
- Fast, cheap specification check
- Dimensions
- Tests diameter, thickness, and shape together
- Magnet response
- Quickly finds some magnetic cores or alloys
- Conductivity or eddy current
- Can compare metal response below immediate surface
- Specific gravity
- Links density with measured mass and displacement
- Ultrasound
- Can reveal internal interfaces or thickness
- XRF
- Rapid multi-element surface characterization
Important limit
- Weight
- Correct mass can be engineered
- Dimensions
- Soft metal and relief complicate contact
- Magnet response
- Many non-genuine materials are also nonmagnetic
- Conductivity or eddy current
- Geometry, thickness, alloy, package, and calibration matter
- Specific gravity
- Air, attachments, cavities, surface tension, and error distort result
- Ultrasound
- Requires suitable geometry, coupling, reference, and skill
- XRF
- Surface layer may not represent the interior
Weight and dimensions
Use the issuer's gross weight, not fine-metal weight, when weighing an alloyed product. Measure a clean item without forcing caliper jaws against relief. Repeat readings and log instrument resolution.
NIST's table of element densities used in X-ray attenuation data lists gold at 19.32 and platinum at 21.45 grams per cubic centimeter, along with values for other elements. Real bullion may be alloyed, struck, shaped, packaged, or contain allowable production variation, so elemental density is not automatically the expected measured density of every finished product.
Magnet response
A strong attraction can reject many claimed gold or silver products, but a lack of attraction proves little. Use the test without scraping surfaces. Record magnet type, distance, motion, package, and response. Sliding or pendulum demonstrations depend on conductivity and geometry as well as magnetism.
Conductivity and eddy current
These instruments apply a field and interpret electrical response. Use the correct metal mode, sample thickness, sensor size, and calibration standard. A reading through plastic may differ from direct contact. Irregular shapes, edge proximity, temperature, plating, and alloy composition can alter results.
Specific gravity
Specific-gravity measurement requires accurate dry mass and submerged volume or buoyancy. Remove bubbles, avoid contact with the vessel, control attachments, and record water temperature if precision matters. Sealed cards, cavities, porous objects, and wettable surface features make the method unsuitable or misleading.
Ultrasound
Ultrasonic testing may compare sound velocity or observe echoes from interfaces. Couplant, surface shape, thickness, grain structure, calibration, and probe frequency matter. Do not apply an instrument preset for one metal and thickness to another without validation.
XRF
The Getty Conservation Institute's comparison of XRF advantages and limits describes rapid multi-element characterization without sampling, while warning that geometry affects readings and that only a thin surface layer is analyzed for most materials. It recommends using XRF with other methods. A plated bar can therefore require a depth-sensitive method or qualified assay beyond a clean surface spectrum.
Combine orthogonal checks
Prefer tests that examine different properties. Weight plus dimensions plus visual design plus a material-response method is more informative than repeating three versions of surface observation. When results disagree, check reference, instrument, package, calibration, and operator before deciding the item caused the conflict.
Common questions
Which test should come first?
Visual comparison, weight, and dimensions usually provide low-risk baseline evidence.
Does a magnet test prove silver?
No. Many metals and alloys are not strongly attracted to a magnet.
Can XRF see through packaging?
Instrument capability and packaging affect the result. Follow the validated method and record whether packaging remained.
Are multiple pass readings conclusive?
They strengthen consistency only to the limits of the selected methods and references.
