To assay a metal is to subject it ( such as silver or gold)
to chemical analysis, to determine the amount of alloy.
most accurate and widely used method, with an accuracy of 2-3 parts
per ten thousand (0.02%), is the Fire Assay (Cupellation) method. This
involves taking a small scraping from the article, typically about 250
milligrammes, weighing it accurately, wrapping it in lead foil with
some added silver, cupelling it in in a furnace at about 1100°C
to remove all base metals and then placing the resulting gold-silver
alloy button in nitric acid to dissolve out the silver (known as 'parting')
and re-weighing the resulting pure gold. This is the standard reference
technique used by the national Assay laboratories worldwide for Hallmarking
and is covered in the International Standard, ISO 11426:1993.
A simplification of this technique involves omitting the initial cupellation
stage and just melting the sample with added silver and/or copper, rolling
to a thin sheet and then dissolving out the silver and base metals with
nitric acid. This is satisfactory only when there are no other impurities
present, is less accurate, but generally sufficient for most small workshops.
||Gold content, %
||Two nines, Minimum allowed
||Standard in Portugal
||Dental Gold Minimum
||583/58.3% in USA
||Minimum in USA
Hallmarking is the
application of a quality control mark to an article of precious metal.
It is also called an assay or standard mark. They are usually applied
after accurate independent testing by one of four UK assay Offices which
are legally empowered to test precious metals and apply a hallmark to
them. These offices are Birmingham, Edinburgh, London and Sheffield.
A hallmark indicates that an article has been independently tested
at an Assay Office and guarantees that it conforms to the legal standards
of precious metal content, known as the fineness.
A hallmark indicates at least three facts:
Who made the article (makers mark)- the initials of the maker What
the metal is, and its purity - the fineness mark Where it was tested
(Assayed) and marked - the Assay Office mark
The Leopard's Head is the mark of the London Assay Office and has
been in continuous use since 1300, when the Wardens of the Company were
given responsibility for marking gold and silver wares, which passed
assay, with the King's mark of the Leopard's Head. A handy guide to
hallmarks from the different Assay Offices can be purchased quite inexpensively
from your local bookstore.
The fineness of the precious metal content of jewellery and silverware
in parts per thousand. Sterling silver is indicated by 925, which
means it is 92.5% silver or 925 parts of silver in every 1000 of the
silver alloy. The current legislation is the Hallmarking Act 1973.
Following amendments to the Act in 1998 and January 1999, the sponsor's
mark, fineness mark and Assay Office mark remain compulsory. But the
Date Letter indicating the year of hallmarking, which had been in use
from 1478, is now a voluntary mark, as are the traditional fineness
symbols, the Lion Passant for 925 Silver, Britannia for 958 Silver
and the Orb for 950 platinum. Platinum was only recognised as a hallmarkable
precious metal since 1976.
The standards of fineness are:
|99% pure gold
|99.9% pure gold
|800 grade silver
|99.9% pure silver
|95% (UK standard) Platinum
|99.9% pure Platinum
An article cannot be legally described as being of gold, silver or
platinum unless it is hallmarked, or the article weighs less than 0.5
grams in platinum, 1 gram in gold or 7.78 grams in silver.
The history of gold and silver quality standards goes back to the
early uses of these metals as money. Legal regulations governing the
marking of jewellery began here in the UK in 1239 and in one form or
another have existed throughout the civilised world since.
Penalties for violation of these laws have varied. In 1397 a report was made
on the false counterfeit stamps of two goldsmiths who were sentenced to be
placed in the pillory at Westminster with their ears nailed to it and with
a ticket over their heads upon which their offences were written. They each
later had one ear cut off, were imprisoned and fined 10 marks.