A Background to Jet

A BACKGROUND TO JET

Jet is one of the few organic materials of plant origin. It can be loosely described as a form of coal and was highly prized in Roman times, indeed it has been valued as a gem material since the Bronze Age.

Unworked Whitby jet 'rough'.

Jet is the material formed from certain species of tree, which grew in just a few areas where specific geological processes turned it into a homogenous black material that doesn’t fade or lose its polish. In the case of the European jets, they formed in anoxic (i.e. lacking in oxygen), conditions under the sea. Many millions of years ago the land masses looked very different from today, and the area around the town of Whitby -- where the famous English jet is found -- was for a long while under water, with the nearest land probably over 50 km away. Subsequent changes in the geology and the climate altered the landscape further, all of which had an effect of the material and resulted in a very distinctive form of preservation of the wood -- which is why jet is a rare material.

English jet is known to have been formed from at least six different species of tree, though for many years it was believed to derive from the wood of only one species, related to today’s Auracaria (of which the monkey puzzle tree is an example). Recent research has disproved this, but that does not diminish the quality of English jet which is superior to many others, having an exceptionally homogenous, deep velvety-black appearance, and taking a very high polish. Some English jet is about 180 million years old and as such it is the oldest of the organics used for gem or decorative purposes. (Burmite, the amber from Myanmar, is reckoned to be 100 million years old.)

Clockwise from top left, jets from: Ukraine, Whitby (x3) New Mexico, and China

Some Chinese jet found in Fushan province is ‘only’ about 40 million years old. It lacks a deep, vitreous lustre. Other jets have the vitreous lustre but are not as suitable for carving as they tend to crack and cleave. It is therefore reasonable to conclude that, although jet occurs in small areas world-wide, not all material is recognised as the finest gem-quality jet. Today jet of varying qualities from New Mexico, Ukraine, Siberia, Mongolia and China are all found on the market.

Left, Chinese jet carving, and right, Whitby jet carving, showing the difference in lustre.

The best known of the European deposits are from two areas: north-western Spain, and Yorkshire in north-east England. The Spanish jet industry concentrated around Santiago de Capostela (the alleged burial place of the apostle, St James), between the 11th and 17th centuries, where it was carved into souvenirs such as figures of Santiago, the protector of pilgrims, or rosaries, and sold mostly to pilgrims. Other popular motifs included scallop shells (the symbol of St James), and the ‘higa’ (a clenched hand, then used as an amulet to ward against evil, but today regarded as a deprecative gesture in many parts of the world). The English jet industry concentrated at the seaside town of Whitby and the gem became used mainly for mourning jewellery as it was black in colour and could therefore be worn even at times of deepest mourning. It had been used as such in the English Court before Queen Victoria’s time, but it was she who made it so fashionable that a large industry developed in the town. In times past jet was found in France and Germany, but these deposits are no longer yielding any material of significance. Indeed, jet is no longer mined either in Spain, or in England where raw material is found solely by beach combing when the material has been exposed by natural erosion of the brittle, shale cliffs. A small amount of the local material is still worked in both Whitby and in Santiago de Capostela, but much that is found in the tourist shops in those cities is imported from other countries.

A scallop shell carving in Spanish jet: the emblem

of St James the apostle.

English jet is found in thin seams in limestone or shale. It is made up of 75% or so carbon, plus hydrogen, oxygen and sulphur. There are traces of other elements such as nitrogen, iron and copper.

As with amber, no-one knew in ancient times where jet came from. It had certain similarities to amber, for example it was light and warm to the touch. It was found on a beach or near to the sea, and when rubbed against a piece of cloth it became sufficiently electrically charged to lift an object such as a feather. So, like amber, jet was thought to contain magical properties – indeed some people believed that it was a black version of amber that had lain in the ground for longer.

JET AND ITS IMITATIONS

Jet is not a versatile material. It is totally rigid and it cannot be shaped by heat or pressure. It cannot be dyed or bleached but can only be carved and polished, however it can be carved with fairly intricate detail, and the best jet can be polished to a mirror-like shine.

When jet became so fashionable in late 19th century England, other materials were used to imitate it. Some were hand carved, such as cannel coal (a dull black material), and anthracite (another form of coal which is black and shiny but brittle). Kimmeridge shale has a similar appearance to jet though its lustre dulls with time. It has been found in burial mounds. Bog oak – a semi-fossilised wood found in peat bogs in Ireland – could be partially pressed into shape but also required carving. It is easily recognisable as it has a dull, rough, grey-black surface which does not take a polish.

Bog oak brooch and detail, showing rough

grey surface.

One of the most common simulants for jet was dyed or lacquered horn. It had certain advantages over jet – for example it could be thermally moulded and pressed into shape, which meant that hand carving was not necessary and the jewellery was a lot less expensive to produce. Jet necklaces often have carved links, but because jet is rigid the links cannot be eased into one another to form a chain. Therefore jet links – or at least every second link -- must be cut in half and glued back together. Horn becomes slightly flexible when heated so links could be joined without the need to cut them in half and glue them. Horn may sometimes be identified by the fact that it can appear translucent in thin areas. It can also show signs of the fibrous striations present in the natural material. (See ‘A Background to Horn’ and 'Early Plastics as Imitations of Organics'.)

From left to right: jet link with 2 joins, horn link with 1 join, and vulcanite link with no central join.

As it made economic sense to copy jet in materials that could be moulded and therefore mass-produced, early plastics were an obvious answer. Thus another very common jet simulant was one of the earliest ‘plastics’: a semi-synthetic called vulcanite (also known as ebonite or ‘hard rubber’). It was made of natural rubber cured with sulphur and dyed black. Vulcanite fades with time to a khaki colour and loses its high gloss. When rubbed hard between the fingers it may exude a slight smell of rubber and sulphur, however both these identifying features can be disguised if the item has been lacquered. When half-cured vulcanite was a little flexible and links could be bent for joining.

Vulcanite pendant, and detail of same showing how

the surface and colour degrade.

Other early plastic materials occasionally used to copy jet included cast phenolic – often called ‘Bakelite’ – and celluloid, though examples of these are rare, and undoubtedly more materials than those mentioned were experimented with in an effort to copy jet, with varying success. Gutta percha (also a natural rubber), is one such example, but contrary to popular belief little was used, and very little remains today as it proved less durable than vulcanite. It was used mostly for encasing underwater cables.

Two semi-synthetic materials that were moulded have sometimes been incorrectly labelled jet. Shellac – a plastic material made from the secretion of the lac insect, mixed with a filler -- is sometimes confused with jet as it is most commonly dyed black. However shellac was used for a few specific items, such as hair brush backs and Union cases (i.e. folding photo frames for Daguerreotypes, which were the earliest photographs using silver nitrate coated copper film. They were unstable and needed to be kept in the dark.) Bois durci – made from albumen and filler and dyed black – was usually made into large, decorative items such as plaques and was not used for jewellery.

Left, Bois durci plaque, and right, shellac union case.

‘French jet’ or ‘Vauxhall jet’ are two names given to black glass made in the late 19th century, and imitating jet. Although deep black and with a high lustre, glass is cold to the touch and much heavier than jet, so there should be no reason to confuse the two. If viewed by transmitted light thin areas look slightly transparent. Jet is opaque.

Left to right: 'French jet' brooch, horn brooch from front, horn brooch from back showing pin screwed into place, and jet brooch back with glued pin.

The best method of identifying jet is by handling and examining it. Old jet imitations are seldom totally convincing, as they lose their lustre and many discolour. Modern imitations are few and far between. As already stated, the links in a jet chain will be cut through and glued together in order to make up the chain. Further, fittings such as brooch pins are usually glued into place on a jet item, while they are normally screwed into place in horn, vulcanite and other simulants.

It can more difficult to discern exactly which type of simulant has been used, for example horn and vulcanite have often been confused, as have vulcanite and the geologically younger jets such as that from China, which lacks the high lustre of the jet from England or Spain.

TESTS

The so-called ‘streak test’ -- where the item to be tested is drawn across an abrasive surface of neutral colour, and the colour of the resulting streak of powder is examined – is not entirely reliable but is useful as it does give an indication. Whitby jet leaves a brown residue while most other materials leave a black one. Unfortunately jet is not the only member of the ‘coal family’ that will give a brown streak. It is less easy to obtain a ‘streak’ from several other materials, and if successful, they may give a brown ‘streak’ due to surface dye. Vulcanite also produces brown residue.

The powdered brown residue from left Siberian jet,

and right, Whitby jet.

Care should be taken if this test is undertaken, as it is slightly destructive because it removes a small amount of the surface of the item, so cannot be used on a visible area. The test is well suited for beachcombed material, which must be tested when totally dry. A variation of the streak test that gives the same indications is to scratch a tiny amount of the material with a very sharp blade. This is, of course, also destructive but can be useful when testing items such as beads, where a minute sample can be taken from inside the drill hole.

Testing for fluorescence under ultra violet light is not possible, and black colours are notoriously difficult to test with FTIR photospectrometry (in a gem-testing laboratory). Over the years there has not been a great deal of research into jet, but that is now being rectified and today research is being undertaken by advanced laboratory methods.