Published on JTV.com: August 2012
Maggie Campbell Pedersen and Bear Williams investigate the infrared and optical characteristics of treated and natural ambers and copals. Originally printed in Gem & Jewellery News.
The correct identification of ambers and copals can be extremely difficult. Not only are we expected to be able to differentiate between them, but we are also expected to be able to recognize and identify all of the treatments given to them. Unfortunately these are constantly increasing in number and complexity. One treatment that is appearing more and more is autoclavetreated copal (including the greened variety), yet still many misunderstandings remain about this material.
When identifying ambers by sight we have a few basic rules from which to start. Fortunately, we know that green is not a natural amber colour (unless it is the result of fluorescence), and so when we come across green resin we know that it is either plastic or a natural resin that has been treated in some way.
Baltic amber is probably the most visually recognizable of all the well-known ambers in its natural form, but it is also the most versatile when it comes to treatments. With heat and pressure it can be clarified or totally re-constituted from powder or chips. Simple heat-treatment can darken the surface or produce discoidal stress fractures (the so-called ‘sun spangles’) inside the material. Like many other resins it can be turned green through a series of treatments in an autoclave. Greened Baltic amber may retain the amber’s characteristic swirls of opaque and clear material, making it easy to recognize.
Discoidal stress fractures rarely occur naturally and so their presence suggestsheat-treated Baltic amber — other ambers are seldom, if ever, treated this way. This is because they are too rare and expensive, and, secondly, the younger ambers such as those from Mexico or the Dominican Republic have a lower melting point than Baltic amber and would melt if heated to the high temperatures necessary to produce the fractures. However, nowadays we are seeing copals treated in an autoclave to produce sun spangles.
It is becoming more and more necessary to carry out tests on the resins we come across, and not just to rely on recognition by sight. The question of whether a resin is natural or treated, and whether it is amber or copal, can have a big influence on the value of the finished item. For example;greened, 40 million year-old Baltic amber is more valuable than greened, 300 year-old Colombian copal, and clarified and heattreated Baltic amber would be more valuable than heat-treated copal used in imitation of Baltic amber.
The normal gemmological tests are not very useful for resins. We know that all ambers float in saturated salt water, but so do natural or treated copals, as well as pressed amber. In general, specific gravity tests can therefore only differentiate between natural resins and their simulants, but not identify them.
Gemmological tests that are reserved more for organics can give us some clues, although they too are only rough guides.m For example, the hot point test may give us some idea of the nature of the resin,but it is vague at best. As a rule the older and more ‘mature’ the resin (i.e. the more stable it becomes through the evaporation of the volatiles, and the cross-linking and polymerizing of the chemical components), the lower its melting point and the more rapidly a hot point will melt the surface of the resin. In addition, the more fragrant the smell emitted by melting, the younger the resin is likely to be. However, when copal is treated and ‘artificially aged’ in an autoclave, the hot point test gives an incorrect result as the surface has been ‘matured’.
The same principle applies to testing with solvents. While Baltic amber is almost inert to solvents, some of the younger ones such as Dominican amber will soften slightly. Untreated copals will soften very quickly, but autoclave-treated ones will again give misleading results due to the artificial aging of the material.
Natural resins from various localities will react very similarly under crossed polars, although there is evidence that treated ones may react differently (see part 2 xx).
Under UV light, ambers from different localities may fluoresce different colours; for example, Baltic amber fluoresces a paler, chalkier colour than many others. Treated materials tend to fluoresce less strongly. However, fluorescence is always strongest in freshly cut material and often tends to fade or change colour with time, so the results are not clear enough to draw anything but vague conclusions.
However, we still need to know what the materials are, as well as which treatments they may have undergone, both for academic purposes and to ensure that the general public are not being cheated. It used to be simple; Baltic amber could be identified by sight — the treatments used on it were well-known and easily recognized — whilst other ambers were rare, looked different and were never treated. Copals were pale yellow and never treated. Today, while we may be able to recognize some of the resins by sight and have a ‘gut feeling’ about others, we often need the help of more sophisticated equipment found in specialist laboratories.
Over the past couple of years I have constantly come up against resins that need verification; most recently these include examples of greened materials that were thought to be Colombian copal and Baltic amber. To confirm my identifications, they, along with other samples, have been tested with great patience by Bear and Cara Williams at Stone Group Laboratories in Missouri, USA.
Image 1: Treated Colombian copal. 2: Pendant with greened Baltic amber. 3: Typical Baltic amber rough (left), and Colombian copal rough (right). Photos © Maggie Campbell Pedersen.Read Part 2