Hallmarking

Every piece of handmade jewellery is hallmarked with my own personal maker's mark which is 'AJB', this simply stands for Ashley James Brooker. As well as the makers mark there is a date letter and a mark depicting the percentage of gold, plus where the hallmarking took place. If you have ever watched the Antiques Road Show you will already know how they were marked. Hallmarking also acts as your guarantee of purity of gold. You will notice in our gallery that all our pieces are no less than 18ct. My way of thinking is that this is the best you can get - 9ct being being a lot paler and harder, 22ct being a lot softer and more of an orange yellow.

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).

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.

The material removed is collected into one or more 'assay papers' and sent to the laboratory for assaying.

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.

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.

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.

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.

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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