Assaying and Hallmarking
H H D AjithL 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 samplesare 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 / Nitric AcidDe-ionized water / 160ml
300ml
14 karat gold / Nitric Acid
De-ionized water / 250ml
160ml
18 karat gold / Nitric Acid
De-ionized water
Sodium Chloride / 250ml
160ml
0.5g
22 karat gold / Nitric Acid
De-ionized water
Sodium Chloride / 250ml
160ml
4g
Silver / Silver Sulphate
Sulphuric Acid
De-ionized water / 4.7g
5.7ml
500ml
In addition to touch testmodern 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 non-distractive, 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 micronsof another alloyit may cause a false result of real composition of the article. Therefore sometimes avery small abrasive window may be applied to overcome thisproblem.
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 theamount 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 / Fineness Hallmark / Silver / Fineness Hallmark / Platinum / Fineness Hallmark24 Kt / 999 / 999 / 999 / 999 / 999
22Kt / 916 / Sterling / 925 / 950 / 950
21Kt / 875 / 800 / 500 / 900
18Kt / 750 / 850
14Kt / 585
10Kt / 417
9Kt / 375
8Kt / 333
In goldthere 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.