Characteristics of Amber with Irradiation Treatment
The organic gem amber is particularly popular in China, Europe, the Middle East, and South Asia. With strong demand for rarer colors, some enterprises use electron accelerator–charged particles or 60Co gamma-ray radiation to irradiate amber and enhance its color from light yellow to orange-yellow and orange-red.
As early as 2012, foreign suppliers tried to introduce irradiation-treated amber into the Chinese market. Due to the booming demand for natural golden and beeswax amber in the Chinese market, amber manufacturers had little interest in the irradiated products. After the market cooled in late 2015, irradiated amber emerged as a new variety (figure 1). Some irradiated amber (containing microstructure defects in the original material before treatment) was accompanied by dendritic inclusions with color enhancement or change (figure 1B). The amber materials used for irradiation treatment were selected from Baltic amber. After irradiation, the red color was usually internally uniform when the weathered skin was polished. Due to differences in original bodycolor and flow pattern, the irradiated amber with the superimposed red color can be red, orange-red, orange-yellow, or yellow-brown, or have a red-white alternating pattern (figure 1C, far right). Early irradiated amber from the end of 2015 was easy to identify due to the presence of dendritic inclusions. Recently, the generation of dendritic inclusions has been suppressed and the quality of the samples has improved (figure 1C). The dendritic inclusions are basically inhibited, and the irradiated amber will not produce them as long as there are no cracks or defects on the surface that can be broken down by the irradiation source. Moreover, the enhanced color via irradiation treatment is much more natural (figure 1D).
The authors selected 13 Baltic rough amber samples for irradiation treatment via an IBA Rhodotron TT200 electron accelerator to investigate the effects of different irradiation doses on the color enhancement of amber (figure 2A), considering that the utilization of high-energy gamma-ray could change its color into red, possibly accompanied by the introduction of dendritic inclusions.
Since the irradiated amber on the market consists of Baltic amber, 12 of the 13 sample groups were Baltic amber (including transparent, translucent, and opaque yellow polished double-sided wafers and round beads), and one group was Burmese amber. The irradiation dose ranged from 10 to 200 kGy (1 kGy = 1000 J/kg), accumulated multiple times, and the samples began to display a red color at 150 kGy. When the irradiation dose reached 200 kGy, the samples turned obvious red, and the irradiation effect showed most clearly. The color changes of representative samples (1 and 2) under different radiation doses were selected; only sample 1 with surface cracks produced dendritic inclusions. When the irradiation dose exceeded 200 kGy, the samples turned varying degrees of red (figure 2B); sample 2 most clearly showed the effect (figure 2C).
The dendritic inclusions are triggered by microstructure defects (physically weak points) and extend like microfractures (figure 3). The roots usually start from the micro defects of amber and form a structural deterioration zone with microcracks. Under the high-energy electron beam, the microcracks in the degradation zone expand rapidly and connect to form a submicroscopic “tree.” The continuous development of the submicroscopic tree forms the root-like whisker region with a macro fractal structure. The growth of dendrites can be regarded as a random growth process of fractal clusters composed of discontinuous microcracks.
If the irradiated amber undergoes heat treatment at 100ºC for 10 minutes in an oven, its original color will be restored (figure 4A and figure 1, C-4); the treated color can also fade and return to its original yellow after heating in a pressure furnace. But once the dendritic inclusion is formed, it is irreversible (figure 4B). After about a year in a natural environment, the induced color from irradiation will automatically fade (figure 4C). Along with a significant reduction in brightness, the color will no longer be uniform (figure 4D) and color spots will appear (figure 4E). Even the original irradiation points are clearly displayed as color spots after fading (figure 4F), indicating that the irradiated color is not stable.
Some irradiated ambers with bright color and high transparency that lack the dendritic inclusions can be distinguished using a conventional 365 nm ultraviolet lamp, but the treatment of lighter-colored amber is not as easy to distinguish. The non-irradiated natural Baltic amber showed a strong absorption peak at 521 nm. The irradiated Baltic amber (orange-red) showed three characteristic peaks at 535, 578, and 728 nm (figure 5A). Dendritic inclusions are common in the earlier versions of irradiated amber. In addition, as the irradiation dose increases, the fluorescence intensity of the amber weakens. Under the 365 nm UV lamp, irradiated amber (samples C-3 and C-5 with irradiation doses higher than 150 kGy) has weaker fluorescence intensity than natural amber. Furthermore, the weaker florescence is superimposed on the darker bodycolor of orange-red (C-3, C-5). The fluorescence of amber with heat treatment (C-4) is the same as that of unirradiated natural sample (C-1).
