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The Hallmarking Process
Overview
When a packet containing articles for hallmarking is sent to
the Assay Office, it is first weighed and the contents checked
to ensure that the number and weights of the enclosed articles
correspond with the hallnote which should accompany the packet.
The articles are then sampled and assayed before hallmarking
is carried out. In recent years, Assay Office London has
received over 69,000 packets containing over 6 million articles
per year. The Assay Office is registered to ISO 9001:2000.
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Sampling
Sampling is the removal of a representative quantity of material
from the articles contained in the packet to be given a full
assay. It is a requirement that all articles contained in the
packet should be grouped according to fineness. This homogeneity
is checked by carrying out a series of 'touch tests'on
the articles.
In the touch testing process, the articles are lightly rubbed
onto a 'touchstone', leaving a thin smear of material on the
stone. Chemicals are applied to the smear and from the reaction
that takes place, an initial indication of the fineness is determined.
Once a sampler is satisfied that the articles in the packet
are the same, thus the homogeneity, a small quantity of material
is removed. This can be achieved by cutting, eg,
by removal of a casting sprue, or by 'drawing' (scraping).
The material removed is collected into one or more 'assay
papers' and sent to the laboratory for assaying. |
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In addition to the touch test, the modern
sampler can also use an X-ray fluorescence (XRF) spectrometer
to determine the fineness of an article. The machine works by firing
an X-ray beam at the article. The beam interacts with the article
which gives out its own characteristic X-rays. These can detected
and used to determine the composition of the article. The technique
is very accurate and non-destructive, making it useful for finished
articles.
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| X-ray Fluorescence Spectrometer |
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| X-ray Fluorescence Spectrometer
(close up) |
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Assaying
Assaying is the accurate determination of
the precious metal content in an alloy. There are three main methods
of assaying: cupellation (for gold), titration (for silver) and inductively
coupled plasma optical emission spectrometry ICP OES (for platinum).
Cupellation
This process can be traced to pre-Roman times. The gold sample
is weighed very accurately. A known amount of silver is added
in a process called 'inquartation'. The gold and silver
mixture is wrapped in lead foil and shaped into a ball. The
ball is placed in a 'cupel' which is essentially a porous
refractory material.
The cupel is placed in a furnace at 1100°C and left
for 20 minutes. In the furnace, the lead ball and all of its
contents melt. All metals, save for the gold and silver, are
absorbed into the material of the cupel. When the cupel is removed
from the furnace, a sphere of gold and silver is thus produced.
The sphere is rolled into a cornet shape and placed into nitric
acid where the silver is dissolved out from the gold in process
known as 'parting'. A sample of pure gold is
created which is reweighed to allow the fineness of the alloy
to be determined. Cupellation produces an accuracy of about
1 part per thousand.
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Titration
In this process a silver sample is weighed accurately, placed
in a beaker and a fixed quantity of nitric acid added to form
a silver nitrate solution. The beaker is placed into an autotitrator
where a sodium chloride (common salt) solution is added in known
quantities. The sodium chloride reacts with the silver nitrate
solution to form silver chloride and the reaction is monitored
using an electrode connected to a computer. From the electrode
response, it is possible to tell when all of the silver has
reacted and thus how much of it must have been present. A weight
of pure silver can thus be calculated. Titration also produces
an accuracy of about 1 part per thousand. |
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ICP OES
A plasma is a high energy gas of ions and electrons, created
by electrically vibrating the gas atoms using an alternating
electric field. In the ICP OES technique a controlled plasma
flame from the gas argon is created. The temperature of the
flame reaches up to 10,000°C. A platinum sample is weighed
accurately, placed in a beaker and dissolved in aqua regia which
is a mixture of hydrochloric and nitric acids. The solution
is diluted and injected into the flame of an ICP spectrometer.
The platinum ionises and emits radiation which is analysed with
a spectrometer. From the intensities of the radiation, the amount
of platinum in the solution can be calculated. The technique
produces an accuracy of about 10 parts per thousand. |
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Failure of Assay
If the initial assay of a packet and its articles indicates that the
fineness is not to the level indicated on the hallnote, a retest is
carried out. If the same result occurs, the offending articles are
usually marked at a lower standard. If this is impossible, then the
customer is contacted to find a mutual way forward. As a last resort,
the Assay Office is empowered to destroy the articles.
Hallmarking
Once a packet and its articles have been
assayed successfully, the articles can be marked. There are three
main methods of marking: handmarking, pressmarking and lasermarking.
The selection of the method of marking used depends on the quantity
and nature of the articles.
Handmarking
The most traditional method of marking is handmarking where
the article is struck using a punch powered by an operative
with large muscles and a hammer. A key part of handmarking,
and indeed pressmarking, is the support tool. This not only
helps hold the article during marking but helps limit the amount
of bruising and thus setting back required. Handmarking is used
for low volume marking, display marking and when pressmarking
is difficult.
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Pressmarking
This is essentially an automated version of handmarking. It
allows both hands to be used, making it ideal for smaller articles
such as chains and bracelets. It is also good for long runs
of similar articles such as rings. |
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Lasermarking
The most recent method of marking is lasermarking.
In this process, material is essential etched away using a very
fine but high power laser beam. Because it is an etching process,
it is ideal for use on delicate or hollow items. It is also
useful for marking logos or signatures. |
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