What Happens at the Assay Office?

Every piece of gold or silver I make that meets the weight threshold gets sent to the London Assay Office before it goes anywhere near a customer. I mention this on the site because it matters, but I realise most people have never thought much about what an Assay Office actually does. Here's the full picture.

What an Assay Office is

An Assay Office is an independent body authorised by law to test and hallmark precious metals. There are four in the UK: London, Birmingham, Edinburgh, and Sheffield. London is the oldest, operating continuously since the 14th century. The leopard's head mark it stamps has been in use since 1300.

The offices exist because precious metal jewellery has always been vulnerable to fraud. A dishonest maker could claim a piece is 18ct gold when it's mostly copper. The Assay Office's job is to make that impossible, or at least, to make it detectable.

What I send and why

I send finished pieces to London, which are tested and stamped, and come back to me.

UK law requires hallmarking on gold items over 1 gram and silver items over 7.78 grams. Anything above those thresholds that I describe as gold or silver gets hallmarked. Below those thresholds, pieces are exempt, but the metal is the same either way.

How the testing works

The Assay Office uses several methods depending on the piece. The traditional method is touchstone testing: rubbing the metal against a stone and comparing the mark to known standards. For more precise work, they use X-ray fluorescence, which can analyse metal composition without damaging the surface. For definitive analysis, they use fire assay - a process of melting a small sample and separating the gold from the alloys to measure the pure gold content precisely.

The result of all this is a confirmed purity reading. If my piece is 9ct gold, the test confirms it is 9ct gold. Then the stamp goes on.

What the stamp contains

A full UK hallmark has four components: the maker's mark (my personal punch, registered with the Assay Office), the metal and purity mark (375 for 9ct gold, 750 for 18ct, 925 for sterling silver), the Assay Office mark (London's leopard's head), and sometimes a date letter, though this is now optional rather than compulsory.

These are either struck into the metal using hardened steel punches, or lasered on for more delicate items. That's why they last as long as the piece does.

Why London specifically

I use London because it's where I've always sent work. There's also something I find satisfying about the continuity of it - the leopard's head on a piece I've made by hand in England in 2024 is the same mark that's been used on London gold since the 1300s. That's a long tradition to be part of, even in a small way.

From a practical standpoint, all four UK Assay Offices are equally authoritative. A hallmark from any of them carries the same legal weight. The leopard's head is just the one on my pieces.

What it means for you

It means that when I say something is 9ct gold or sterling silver, you don't have to take my word for it. An independent body has tested it and stamped it. That stamp has been the standard in this country for seven hundred years because it works.

It's one of the things I'm most proud of in how I run this business.