jadeite, fei cui, B-jade, treatments, gems

An acid-droplet test is described as a practical aid for distinguishing untreated fei cui from material that has been acid bleached and impregnated with polymer. The discussion supplements the earlier study by Fritsch and colleagues, which established infrared spectroscopy as a means of positively identifying the impregnating polymer. Three trade categories are outlined: type A, untreated apart from traditional surface waxing; type B, acid bleached and polymer impregnated; and type C, artificially stained. Hodgkinson’s test depends on differences in the accessibility of the material’s microscopic pore and fracture network. On untreated specimens, concentrated hydrochloric acid placed on a clean surface is drawn inward by capillary action along grain boundaries or open fractures and subsequently reappears beyond the original droplet. This may take the form of droplets outlining fractures or a surrounding aureole resembling perspiration. An untreated bead illustrates the emergence of acid along intersecting internal fractures after approximately two to three minutes. In polymer-impregnated material, the acid is reported to remain at the application site until evaporation because the polymer seals the microporous structure. The author discusses initially conflicting observations from the Gemological Institute of America and mixed results in Hong Kong, attributing these partly to fine grain texture, carved surface grooves, and rapid evaporation under warmer or air-conditioned conditions. Enclosing the specimen beneath a glass cover with a small water container is proposed to maintain suitable conditions. Another observation on untreated material was the appearance of brown iron staining on a white tissue used to remove the acid, although no colour had been visible in the droplet itself; such staining would normally be removed during bleaching before polymer impregnation. A stained, more porous type C specimen produced a particularly extensive aureole, while additional polymer impregnation would be expected to suppress this response. Traditional surface waxing reportedly did not prevent the test from working. The article also notes that droplets can magnify dye concentrations and that residual purple colouring may remain from an earlier stage of green staining. The method is presented as an adjunct to the more comprehensive investigation of polymer treatment.