Dr David Penney and Dr David Green (UK)
This is the second in a series of articles concerning fossils in amber. In the first, we focused on the biodiversity of organisms in the major deposits of the world, including the techniques available for distinguishing genuine fossils from fakes (see Issue 26, Biodiversity of fossils in amber). When the first fossil amber specimens were examined back in the 1600s, only very basic microscopy was available to examine the inclusions. In recent years, great progress has been made in amber preparation procedures, photomicroscopy and advanced imaging techniques, which can all now be employed in the study of fossils in amber.
Optical properties of amber
To understand the rationale for the preparation techniques described below, it is worth reviewing the way light passes through amber and the way that images are formed. Amber is usually transparent or translucent. The more transparent it is, the less the light is absorbed as it travels through the specimen. As opacity increases, more light is absorbed and inclusions become more difficult to see. Arthropod inclusions are visible because they have differing opacities and refractive indices to the enclosing amber.
When light rays cross boundaries between media with different refractive indices (such as amber and air) they bend. If the surface is flat and polished, the light rays bend in a predictable manner and it is easy to see what lies within. If a surface is curved, irregular, undulating or scratched, the light rays bend in different directions, depending on where they come out, and the image is distorted. For maximum visibility, a specimen should be prepared with a flat, optically polished surface in the minimum depth of amber. A minimum depth is important, as variations in refractive index in the body of the amber produce optical distortions. Flaws, dust and other foreign bodies between an inclusion and the specimen surface scatter the light, reduce contrast and also make the inclusion more difficult to see.
Many collectors and curators value large, intact amber specimens and it is not always possible, or even desirable, to cut and polish a specimen. Fortunately, there is a simple technique that can eliminate the distortions produced by a curved, irregular or scratched surface. Immersing a specimen in a fluid with a similar refractive index to amber dramatically increases its visibility. This technique reduces light scattering at the surface and eliminates the distortions produced by irregularities and curvature.
Preparation of amber inclusions
Once raw amber has been washed and cleaned, it is often possible to determine whether or not it contains inclusions by coating it with a thin smear of oil and holding it up to the light. The oil fills scratches and flaws on the surface of the amber, increasing the visibility of any inclusions that are present. If this is not practical, for example, due to the external surface being particularly crusty, a ‘window’ is ground and polished into the specimen. When something of interest is discovered, further preparation is usually required. This typically involves cutting or grinding, and then polishing the amber. The first of these processes requires a circular trim saw (ideally a faceting saw) with a thin diamond blade. Chipping of the amber is minimized by rotating the specimen during the cutting process. Water is used as a coolant and lubricant to protect the specimen from overheating. For smaller pieces, a fine handheld jeweller’s saw can also be used.
Once it has been trimmed to size, the surface of the amber should be ground to remove saw marks and then polished. Amber is relatively soft and the best results are achieved by careful grinding by hand, or on a flat lap (that is, a large horizontal wheel similar to a record turntable or potter’s wheel), using successively finer grades of good quality abrasive paper. Check the specimen regularly under a microscope to avoid grinding it too much and damaging the inclusion. Saw marks are easily removed using a medium 360-grade paper. Specimen and hands must be cleaned before transfer to 600, 1,200 and 2,500-grades. At each stage, a careful inspection of the surface is required, as even the tiniest scratches cannot be removed by polishing. A fine napped polishing pad, charged with a one micron diamond compound (14,000 mesh equivalent) or 200nm alumina (which is probably better, but takes a bit longer), can be used to produce a highly polished surface. The polishing compound is made up as a paste in water, which acts as a coolant to protect the amber and its inclusion from overheating. If the surface remains dull after a few minutes of gentle polishing using a figure of eight motion, the specimen should be returned to 600-grade abrasive paper and the process repeated.
When mechanical equipment is used, it is important to follow the safety instructions. The specimen should be rotated during grinding and polishing to get an even finish, because the outside of the lap rotates faster than the inside. With an anti-clockwise spinning wheel, it is easier to manipulate the amber with the wheel turning away from the operator, that is, the amber should be held on the right-hand-side of the wheel. Particular care should be taken to maintain a firm hold of the specimen. Should it slip, a fast-spinning wheel can transport it a great distance in a random direction.
Check the amber carefully for internal fractures. The forces generated during cutting, grinding and polishing can cause specimens to break. In some cases, they may be glued back together. It may then be possible to resume the preparation process without further damage to the specimen. However, this is not always the case.
The clarity of most Tertiary amber, in conjunction with the excellent preservation of the fossil inclusions, means that cutting to an appropriate size, followed by grinding and polishing (Fig. 1) is sufficient for most scientific study. More elaborate techniques, using immersion fluids and embedding media of similar refractive index to amber have been developed by some researchers. Specimens can be immersed in water white oil of cedar wood (or similar) once they have been trimmed to the appropriate size. The refractive index of the oil is very similar to amber, so, using this technique, inclusions can be viewed from multiple angles by rotating the specimen in the oil.
A variation on the above technique is particularly useful for brittle Cretaceous ambers, such as those from Lebanon. It is difficult to extract large specimens as they commonly shatter, so most inclusions are recovered from fragmentary material. The brittle nature and small specimen size makes cutting and grinding the raw amber impractical. Therefore, to prepare inclusions for study, it is best to shave slivers of amber from the specimen using a razor blade, getting as close as possible to the inclusion dorsally and ventrally. This is then placed in a deep fluid mount, made by gluing a shallow plastic ring to a circular microscope slide cover-slip, which has been filled slowly – so as not to create air bubbles – with Canada balsam. The amber is gently eased into the cell, taking care not generate bubbles, which, if formed, can be removed using a fine needle. Canada balsam has the same refractive index as amber and it seeps into any surface cracks, significantly increasing the clarity. A second cover slip is fixed onto the preparation using plastic cement. Once it has begun to set, the excess cement can be shaved off around the edge to create an aesthetically pleasing finish (Fig. 1).
An innovative modification of the above technique was employed by scientists working on Cretaceous amber from France. The amber was shaved as close to the inclusion as possible, but from all sides rather than just dorsally and ventrally. The specimen was then glued to the blunt end of a thin pin. Four small glass panes were glued to a microscope slide to make a small cell structure. One of the panes had a hole in the centre, through which the pin was inserted with the inclusion on the inside. The pin fitted snugly but was free to rotate. This cell was then filled with Canada balsam and a thin glass pane was glued on top. When the specimen was viewed under the microscope, the pin could be rotated to view it from many different angles.
Brittle ambers can also be mounted in blocks of synthetic transparent plastic (Fig. 1), which is fluid when mixed and solid once set. This can be done to facilitate study as well as to preserve the specimen. This technique makes the amber easier to handle and prevents it from shattering during grinding and polishing. The block can be reset in fresh plastic as many times as desired to polish it from various angles. However, care must be taken to avoid unwanted air bubbles, but these can be minimised by allowing the plastic to set in a vacuum (if possible). The result is a tiny, inclusion-bearing piece of amber, highly polished on many sides, set in the centre of a hard transparent plastic block. This is a time consuming process, but worth the effort for some specimens. It has the bonus of protecting the amber from accidental damage and oxidation through exposure to air. Hoffeins (2001) provides a short explanation and practical advice for undertaking this embedding process without access to specialised technical equipment. Collectors should find his paper particularly useful. It is important to choose a synthetic plastic that is as hard as the amber. If the plastic is softer, it grinds more quickly than the amber and can pull at the surface causing it to shatter.
Attempts to dissolve Lebanese amber in chloroform to extract the inclusions have been successfully carried out. Articulated insect fragments, which retained their softness in a manner similar to freshly collected entomological material, were recovered. This process may seem absurd to some, as much of the beauty of amber inclusions is in the amber itself. However, to a palaeontologist, the inclusion is more interesting than the surrounding amber matrix and a technique that can separate the two has scientific merit. Indeed, it can be frustrating when museum curators will not permit further preparation of specimens for scientific study simply because of aesthetic concerns. Readers are cautioned that any attempt to dissolve out specimens may lead to loss of both the amber and the inclusion.
Light microscopy and photography
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