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Up To Scratch: A Very Short History of Gold Assay

History of Gold Assay

Up To Scratch: A Very Short History of Gold Assay

4 minute read

Have you ever wondered where the saying 'up to scratch' originates? A quick Google suggests that it relates to a line drawn on the ground at the start of a boxing match. But what if that explanation is wrong? What if the saying 'up to scratch' originates from a practice that like fighting for fun, began hundreds of years before Christ and continues only slightly altered today. A practice to test that most noble of metals: gold.

Gold is a reasonably soft and malleable in its pure state, too soft for many of its practical uses. It is therefore alloyed with other metals such as a copper and silver altering is hardness, colour and other properties. The proportion of gold contained in this alloy - its carat - is expressed in parts of 24, with pure gold being 24 carat.

From ancient times until modern day, gold has been valued and coveted by humans. And for as long as coins and jewellery and other items have been made from gold, forgeries and counterfeits have been produced. With counterfeiting considered by some as 'the world's second oldest profession', man's requirement to identify not only that an item is gold, but also what proportion of gold, is a long one.

So how do we test gold for purity? Surprisingly, one of the more common methods employed today - acid testing - has its basics in a process that was carried out 2500 years ago called touching.

Touching involves scratching/rubbing the suspected gold item onto the surface of a slightly abrasive black 'touchstone' to produce a thin streak. Where today we would then add drops of nitric acid of various strengths to see if the streak dissolves, in antiquity they would instead make a second scratch with an item of gold of known fineness (referred to by medieval times as a touchneedle). The colours of the two streaks would then be compared for a match.

The earliest archaeological discovery in relation to touching comes from 5th/6th century BC Pakistan. The discovery: a mixture of black pebbles and and siliceous slate cut into strips showed evidence of gold streaks on their surface. Mentions of gold testing stones have also been found in ancient Greek literature from the same period, though it's entirely possible that the process began much earlier still.

How accurate this process was in antiquity is up for debate, and the test does of course rely on a variety of gold alloys being available to test against. But certainly as time went on and as goldsmiths guilds became established across the world, the test became more sophisticated with gold comparison sets of 'touchneedles' growing to include alloys of silver and copper, with varying ratios to facilitate a much broader range of tests.

Records from the Royal Mint in England suggest they held a collection of nearly 650 touchneedles by the middle ages and by the 16th century, touchstones and touchneedles were described in numerous printed books and were a staple bit of kit within the goldsmith's inventory.

touchneedles for testing gold purity

Set of touchneedles from Italy, 1682.
Each needle is numbered to indicate the alloy with which it is tipped.
Source: V&A Museum

Nowadays of course, assay offices test most of their vintage gold jewellery using X-Ray fluorescence machines, and pawn brokers who take a great deal of unknown metals in over the counter will also invest in these expensive advanced testing instruments. For the vast majority of jewellers and hobbyists however, the 2500 year old touchstone scratch test remains the cheapest, simplest and most efficient way of checking their wares.

So while i'm quite the fan of boxing, and while I realise that men have been punching each other for fun for as long as gold coins and gold jewellery has been in existence, I am not certain the sport can categorically claim the saying 'up to scratch' as its own.

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