Assaying and Hallmarking
H H D Ajith L Siriwardena
Senior Gem Appraiser- Superintendent of Customs
M.SC.(Gemmology),B.Com(Special),FGA,IGA,MGA,Assay & Hallmarking London, Pearl grading GIA, Dip in Marketing, ANPS
Assaying is the determination of the precious metal contents in an artical and there are three main
methods for accurate determination of composition of the articles. Cupellation for gold ,titration for
silver and inductively coupled plasma optical spectrometry (ICP OES) for platinum and palladium.
Oldest and easiest basic fineness resolving method is touch tests by articles. Therefore samples are
lightly rubbed onto a “touch stone “leaving in a smear of material on the stone. Chemicals are applied
on the smear and fineness is determined.
Constituents of “Touch Acids” used for testing Gold and Silver
9 Karat gold
14 karat gold
18 karat gold
22 karat gold
Silver
Nitric Acid
De-ionized water
Nitric Acid
De-ionized water
Nitric Acid
De-ionized water
Sodium Chloride
Nitric Acid
De-ionized water
Sodium Chloride
Silver Sulphate
Sulphuric Acid
De-ionized water
160ml
300ml
250ml
160ml
250ml
160ml
0.5g
250ml
160ml
4g
4.7g
5.7ml
500ml
In addition to touch test modern techniques are also commonly used in most of the laboratories, such as
an X-ray Fluorescence (XRF) spectrometer used to determine the fineness of the article. Therefore the
XRF machine is used to detect the composition of the article. This technique can be identified as nondistractive, speedy and very accurate for the finished articles. This method is widely used in most of the
assay offices around the world. One of the disadvantages of this method is, if an article is plated with
thickness of more than 2 microns of another alloy it may cause a false result of real composition of the
article. Therefore sometimes a very small abrasive window may be applied to overcome this problem.
Cupellation or fire assaying method can be traced to pre-Roman times. The gold sample is weighed very
accurately and a known amount of silver is added in a process called “inquartation” .The gold and silver
mixture is wrapped in lead foil and shaped in to a ball. The ball is placed in a “cupel” which is a porous
refractory material. The cupel placed in a furnace at 1100 C and in the furnace the lead ball and all the
content melt. All metals except gold and silver are absorbed by the cupel. The remaining sphere of gold
and silver is rolled into a cornet shape and placed into the nitric acid where the silver is dissolved out
from the gold. The remaining pure gold is reweighed to allow the fineness of the alloy to be
determined.
Titration,in this process the silver sample is weighed accurately and examined with a reaction with
sodium chloride with silver nitrate solution to form silver chloride and the reaction is monitored using an
electrode connected to the computer. .
X-ray Fluorescence (XRF) spectrometer (above right ) and Inductive Coupled Plasma Optical Emission
Spectrometry (ICP OES) (above left).
Inductive Coupled Plasma Optical Emission Spectrometry (ICP OES) ,in this process platinum or
palladium is weighed accurately and dissolved in aqua regia which is a mixture of hydrochloric and nitric
acids. The solution is diluted and injected into the flame of (10,000C) of an ICP spectrometer. The
sample ionizes and emits radiations which are analyzed, and with the finding the amount of platinum or
palladium contents can be calculated.
Hallmarking once assayed successfully they can be marked according to the composition of the sample.
Sampling and accuracy may be change according to the legislation of the assaying office or the country.
Sri Lankan assay office hallmarkings;
Sri Lanka NGJA assay office Legal standards
Gold
24 Kt
22Kt
21Kt
18Kt
14Kt
10Kt
9Kt
8Kt
Fineness
Hallmark
999
916
875
750
585
417
375
333
Silver
Fineness
Hallmark
999
999
Sterling 925
800
Platinum
999
950
500
Fineness
Hallmark
999
950
900
850
In gold there are three main methods of hallmarking: Hand marking, Press marking and Laser marking.
Handmarking or stamping is the most traditional method of marking.It is where the article is manually
punched by a highly skilled operative. As shown above.
Presmarking is essentially an automated version of handmarking.
Lasermarking is the most safe and less destructive system in this process .Material is etched away using
a very fine but high powered laser beam as shown above.