TREATMENTS AND ENHANCEMENTS OF BALTIC AMBER
Amber is a fossilised tree resin. It occurs in deposits world wide and is of varying ages, from the 12 or so million year old amber found in Australia, to the oldest fossilised resin known which is over 300 million years old, but occurs only in hair-fine fibres. (See also 'A Background to Amber'.)
By far the largest deposits in the world occur around the Baltic. The material from this region is aged between 30 – 50 million years old, and is of various origins, though it is considered that the majority is from the tree species which has been named Pinus succinifera.
Amber is made up of carbon, hydrogen and oxygen with other trace elements. Baltic amber contains succinic acid, and is sometimes called ‘succinite’. It occurs naturally in colours ranging from palest cream through golden to dark brown. The palest colours are opaque, due to sub-microscopic gas bubbles in the material – the more bubbles, the paler and more opaque the material. Dark colours can be due to debris inside the material, for example remnants of bark from the tree, or debris from the forest floor.
Natural weathering of amber causes the surface to darken due to oxidation. This can be clearly seen on ancient beads and carvings – for example those from the Stone Age – which may appear almost red on the surface. Even modern items can show signs of weathering in a matter of a few years, depending upon how they are stored. This is most noticeable on the palest colours, which darken to a rich yellow.
Amber has been treated for at least 2000 years. Pliny the Elder (Gaius Plinius Secundus, AD 23 – 79), wrote that amber could be boiled in pig or goat fat, and coloured with organic dyes. Rape seed oil has for hundreds of years been a favourite medium used for clarifying amber (i.e. turning opaque amber transparent).
Attempts have been made to enhance different types of amber, but, with the exception of Baltic amber, this has been without much success. Amber from the Dominican Republic, for example, tends to liquefy when put into an autoclave as it has a much lower melting point than Baltic amber.
About 98% of the amber on the market today is Baltic amber. Of this, an extremely high proportion has been processed or treated to enhance it in some way, for example to clarify it, to darken it, or to induce so-called ‘sun spangles’ inside the material. Baltic amber is also re-constituted by pressing chips and powder together, with or without the addition of artificial fillers.
The amount of amber which is totally re-constituted depends largely on the availability of raw amber. Although there are still huge deposits of the material, shortages can occur due to natural causes such as flooding of the mines. Another reason for re-constituting the material is that Baltic amber is not mined by hand, as are other ambers, but is extracted from the ground by mechanical process or by the use of powerful water jets. This results in much of the material being broken into small pieces, however the wastage is minimal due to the possibility of turning it into pressed amber.
Baltic amber, polished 'rough', showing
variety of colours and opacities.
Baltic amber occurs with opaque, translucent or transparent areas in the same piece. Although natural colours and opacities are available and considered by many to be the most attractive, the most popular variety today remains the clear material containing little spangles that catch the light. This is obtained by placing pieces of amber – either cut or rough -- in an autoclave for several hours, where the combined heat and pressure (sometimes, but not always, in the presence of a gas: nitrogen or argon), softens the material, causing it to clarify. Temperature and pressure can vary according to the effect desired. Amber starts to clarify at 130ºC but is usually treated at between 180ºC - 200ºC. Pressure starts at 25kg/cm², but may be raised to 50kg/cm² when the autoclave has heated up. The whole process takes about twelve hours, including cooling down again.
Left to right: clarified but not heated, heated, heated more, and blackened. Broken opaque bead, not clarified but darkened on surface.
During this process the material shrinks by up to 10% and the surface may darken, though it is usually treated further in an oven to darken it, especially if it was put in the autoclave in its rough form. The longer the amber is heated, the darker the surface becomes, until it is finally black. Temperatures used in the oven are from 100°C - 250°C, and the duration can be anything from one hour to several days. A high temperature gives a faster result but is more risky. It should be noted that it is only the surface of the amber that is darkened, and that longer (or hotter) treatment results only in darker colours and not in the colour penetrating deeper into the amber. All material that is darkened in an oven is already cut and partially polished. It receives only a final, light polish after treatment as anything more would remove the darkened surface, though to produce Baltic amber with a reddish appearance, often called ‘cherry amber’, the material is heated to almost black and then given a good polish. Opaque material can also be darkened to give it an artificial ‘old’ patina.
So-called 'cherry amber'
Today it is popular to darken clarified amber, and then polish away only parts of the surface to give a special effect. (See the illustration of a selection of bracelets). This is also the method used to make amber intaglios. A cabochon of clarified material is darkened, after which the dark surface of the dome is completely polished away, leaving a dark surface only on the back. The carver looks through the dome whilst engraving the back of the piece – a very light touch removes only part of the darkened surface, resulting in mid-tones. Not all of the resulting amber dust is removed as the carver works, as allowing some to remain in the deepest cuts emphasises the three-dimensional effect by appearing pale and opaque. Intaglios are sometimes enhanced by being backed with foil to reflect light through the cabochon. In past times gold leaf was used.
A side-effect of the heating process, either in an autoclave or in an oven, is that it can cause discoidal stress fractures (cracks). These are the ‘sun spangle’ inclusions believed by many to be proof of natural amber. (Note: Discoidal stress fractures can occur naturally in amber, but they are very rare. They can also be induced in plastic amber imitations, though careful examination can usually reveal a different structure.) Very slow heating and cooling may avoid these cracks appearing, but fast heating or cooling down of the material will induce them. If, for example, the material is placed in an oven at 250°C it will crack immediately.
The process of darkening the surface and inducing stress fractures is not limited to natural amber – it can be carried out on amber that has been re-constituted, and stress fractures can also be induced in plastics .
Sun spangles (discoidal stress fractures) in Baltic amber
True green Baltic amber does not exist, but it is possible to buy jewellery containing material that looks green. Usually the amber is mounted in such a way that the back in concealed.
After clarifying the amber and inducing sun spangles, the cabochon is burned on the reverse, blackening the material. The optical illusion resulting from this is a greenish appearance in the amber when viewed from the top. (In some cases it can look almost black, with golden spangles standing out in contrast.) If the item is viewed from the side it can be seen that the body of the amber has not changed colour but is still golden.
Left to right: 'green' amber seen from above, seen from the back showing blackened surface, and seen from the side.
If the burned area is partly polished, thereby leaving a thin film of darkened amber on the base, the illusion gained is that the amber is red.
The green illusion can also be obtained by backing the amber with a different black material – anything from paint to plastic can be used – in which cases the amber should be regarded as ‘backed’ or a ‘doublet’.
Raw amber nuggets that have been burnt on the outer surfaces and sliced without polishing away the rough, darkened edge, have recently come on to the market. The slices are often clear towards the burned surfaces due to the effect of the heat, but retain a cloudy centre. They give no illusion of a different colour.
Darkened and part-polished amber beads.
The photo below illustrates many of the treatment possibilities and their effects on a single pieces of amber that has been cut into seven sections, each treated separately as follows:
1. The natural piece, as it was found.
2. Crust removed, ground and polished.
3. Clarified in an autoclave.
4. Faster heating and cooling in an autoclave, producing ‘sun spangles’.
5. As 4, with gas added in the autoclave. The sliver is backed with a black substance.
6. As 4, plus heated in an oven to more than 200ºC changing the surface colour to a reddish brown.
7. As 3, plus heated in an oven to darken the surface.
Baltic amber begins to soften and stick at 140°C. By heating it further it becomes softer (Note: Baltic amber tends to burn rather than melt.)
There are many methods of pressing Baltic amber in use today, and new and improved methods are constantly being patented. The results can be so convincing that it is virtually impossible to tell the pressed material from natural which has only been enhanced in colour and/or transparency.
Early methods of pressing amber can be easier to distinguish. A method patented in the nineteenth century involved pressing softened amber through tiny holes into a mould, using pressure of about 3000 kg/cm². Called the Trebitsch method, it could produce very clear and homogenous material. The well-documented ‘ambroid’ that was so popular for smoking requisites at the beginning of the twentieth century was produced by this method. Sometimes the resulting mass showed signs of the extrusion process in the form of elongated areas or a feathered pattern of opaque and transparent material. These patterns do not occur in natural amber.
In the Spiller method, patented around the same time, chips of amber were placed in a mould which was submerged in hot paraffin, and pressure of up to 500 kg/cm² was applied. This produced flat discs, rods or blocks of pressed material. In this method, if the raw chips have not been sufficiently cleaned, the outer surfaces of each chip will darken slightly in the process, giving a typical pattern of reddish-brown streaks. These are usually short and straight – another phenomenon that does not occur in natural amber. If, however, the chips have been thoroughly cleaned first, and the material is placed in an autoclave, the chips can fuse together with no visible seams and the result may be totally transparent amber, to which sun spangles may be added and the surface darkened by heat treatment if desired.
Left to right: Feathered pattern in extruded amber, darkened
surfaces of amber chips in pressed amber, and amber chips
pressed together with powdered amber.
A variation on the process uses chips of varying colours and opacities, which are melted together into blocks using powdered amber, without altering their colour or transparency. The result is a distinctive patchy, 'mosaic' pattern. This is used and jewellery, and also sometimes seen in the form of cups and saucers or other drinking vessels, mounted with silver, and selling at high prices. It is not unattractive, but it is not natural amber and to say that it is ‘carved from one solid piece’ is misleading.
Through these processes it is of course possible to mass produce ‘carved’ items at a fraction of the cost of hand made ones. The only actual carving done is the removal of obvious signs of moulding, for example marks from the edges of the moulds.
Pressed amber may be made using only amber chips, or it may be made by welding the chips together with pure powdered amber, which will soften and become adhesive more quickly than the chips. An alternative method of production is to weld the chips together with copal (sub-fossil resin) which melts at a lower temperature and so fills the mould more efficiently, while retaining the colours of the amber. A further possibility is to add a synthetic material – usually a plastic -- so that the result is a composite.
Plastic may also be added to amber chips and dust when producing material for extrusion. In this case the result can be very convincing, especially in the paler versions, as the swirls of colour and opacity can resemble the natural material. Large quantities of beads are produced this way, and tests have shown that some only contain about 20% amber, the rest being plastics and dyes.
Composite amber beads, containing a high percentage of plastic.
The salt-water test (see Tests, below) will normally distinguish between adulterated amber and pure amber, but for the reasons listed it is not always either suitable or reliable. Careful examination will usually give indications that the swirl patterns are unconvincing, especially when viewing the beads end-on.
'Detail of 'polybern' disc, showing clearly the chips
of amber suspended in clear plastic.
‘Polybern’ is an old form of amber imitation, but it is still being made. The name derives from the German for amber, ‘bernstein’, and ‘polymer’ or plastic. Small chips of amber are suspended in a clear plastic which is then moulded. The chips are easily visible to the naked eye and do not resemble the natural patterning in Baltic amber.
If amber had been ground to granules before processing, a colouring agent may be added which will permeate the material. It may also be added as a coating on the outside of clear amber. This was a favoured method used by one company (no longer trading) several years back, of producing ‘red amber’ – supposedly from Burma (now Myanmar). If the surface was scratched the yellow colour beneath became visible.
In recent years amber has appeared on the market which has been artificially coloured, most popular being green or red. The methods used remain a firmly guarded secret. Some are based on thermal treatment, while others have been made from pressed amber granulate to which a pigment has been added. (See 'Green and Red Ambers'.)
There are various tests for amber. The only one that is not in any way destructive is the salt water test. This involves dissolving a large amount of salt in cold water (until the solution is saturated) and dunking the amber in the water. Almost all plastics will sink as they have a higher specific gravity (SG) than salt water. Amber has a lower SG, so floats.
Amber cabochon floating in saturated salt water, while the plastic
imitation amber has sunk to the bottom.
However, this test cannot determine whether amber has been re-constructed, heat treated, or surface dyed. Nor will it differentiate between amber and copal (a sub-fossil resin). Also, it is only suitable for material that is unmounted, and not mixed with other gem materials or metals. Further, inconclusive results may arise with drilled beads, where it is possible that large drill holes may trap air and thus help to keep the material afloat, while heavy thread used to string beads could absorb so much water that it causes amber to sink. These cases, however, are rare, and it remains an excellent method for testing necklaces and bracelets made up exclusively of beads.
Other tests, such as touching the amber with a hot needle to determine its melting point or its smell make little or no differentiation between treated and natural amber either. (NOTE: this test should be avoided if there is any risk that the item is an amber imitation made of celluloid, which is highly combustible),
Amber that has gone through any form of processing that involves heat and pressure is slightly harder than natural amber (giving facetted beads a more vitreous lustre), but hardness is not a particularly useful test either as natural Baltic amber varies in hardness according to where it is found. Similarly, observing a specimen’s reaction to ultra violet light is inconclusive.
It therefore remains that observation is the best way of distinguishing the materials, but when in real doubt an infrared spectroscopy test can be carried out by a specialised laboratory (i.e. one that has a data base with which to compare the results). IR tests show slight changes when amber has been reconstituted or processed in some way.
Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water at 4ºC.
Infrared spectroscopy measures the absorption of light energy, in infrared red light. The results indicate the chemical composition of the material tested.
©2018 Maggie Campbell Pedersen. All rights reserved.