Sterling Silver Clay by Lisa Cain

Why I ‘invented’ Sterling Clay and a brief history lesson.


Here are some of my little bent test strips at the Assay Office laboratory sitting in front of some fab bottles waiting to be ground up and tested for all traces.When Mitsubishi brought out PMC Pro in 2010 I was very glad to have a silver clay which was substantially stronger than previous clays but dismayed that it was only 90% silver. This meant that it could not be stamped at Hallmark quality with 925, which is the mark most customers will recognise. Silver made up of 900 particles per thousand (like PMC Pro) is not recognised here in the UK so instead it would need to be marked with the inferior stamp 800.


No one in England would buy something marked 800.


So this presents a big problem to serious metal clay artists here because we are LEGALLY OBLIGED to Hallmark any item of silver for sale to the public if it weighs over 7.8 grams.




However, regardless of the weight, most galleries here will only accept work which is Hallmarked and for very good reason.Lisa's hallmark

You see, a silver Hallmark is not just a stamp saying 925 or 999FS which anyone could put on their work. The Hallmark is an independent verification of purity and usually consists of at least four separate marks, which can only be struck by the Assay Office.


The four marks which normally make up the actual Hallmark are-

1) the Makers Mark

2) the Purity of the metal

3) the Assay Office emblem

4) the Date symbol.


We have four Assay Offices; London, Birmingham, Sheffield and Edinburgh and they each have their own specific city emblem. The artist registers with the office of their choice and their registration lasts for 10 years.


So, I am registered with the London Assay Office whose emblem is the leopard head and my initials are L.C contained within a shield shape.


There are several shapes to choose from (oval, diamond, rectangle etc) but for the 10 years I am registered with London, no one else will have the same initials in the same shaped punch as me. This year (2011) the date symbol is ‘m’.


So when I send a sterling silver ring to be Hallmarked the purity is tested and the ring stamped accordingly with 925 (999 if it is a fine silver ring) along with the other three marks. This photo shows an extra London mark of the lion with raised paw


The Hallmark serves several functions; it acts as a guarantee of purity to the customer and it shows a level of professional commitment which galleries, buyers and stockists all demand. I also like the idea that in 200 years time if that same ring turns up as an antique, it can be traced to me via the makers mark, date symbol and the city emblem. You could see this as making your mark in history!


Fifteen years ago, as a fledgling jeweller, I was very proud to register as a maker and ‘turn professional’ becoming part of this ancient tradition, which stretches back 700 years.


So you can imagine I was pretty nervous when I made a private appointment to visit behind the scenes at the prestigious London Assay Office to discuss the purity of my new Sterling Clay!

Assay office visit


The Assay Office is located at the impressive Goldsmiths Hall, a grand building with an opulent interior that has been on the same site for 700 years, which is tucked away down a back street near St Paul's Cathedral.


It is a somewhat grand and intimidating venue so it was a relief to have a very warm welcome from Dave Merry who is head of Assay Office Training.


A great story teller with over 40 years experience and a passion for his job made Dave the perfect guide. Many anecdotes later, Dave introduced me to Chris Walne, Senior Assayer, who gave me a tour of the laboratories used for precision testing which may be called into play if there are doubts about antiquities or forgeries. The equipment at Chris’s disposal can detect every element contained within a metal sample.







I already knew that I had successfully made Sterling silver out of my clay because I had sent some pieces in to be Hallmarked but I really wanted to Assay office visitknow exactly how accurate my recipe had been, so Chris kindly tested some fired Sterling Clay using a method called Potentiometric Titration.


I am delighted to say that the purity of my Sterling Clay sample came in at a very precise 925.9 particles of silver. So, it’s official: if you follow these recipes you will be on your way to making Hallmark quality Sterling Clay which is also super-strong. Enjoy!


Anyone can take the London Assay Office tour and I strongly recommend it. Goldsmith Hall regularly has exhibitions of outstanding work and the library is truly awesome. Visit if you can! Contact Alison Byne +44(0)207 606 8971 ext 3013


For more info about Hallmarking click here.