| Abstract: | SUMMARIES. Ivory is richer in magnesium than other bone or dental material. In the older mineralogical litera¬ture there are a few reports of the occurrence of the magnesium phosphates bobierrite [Mg3(PO4)2 . 8H2O], newbervite ( MgHPO43H2O) and struvite ( MgNH4PO4 . 6H2O) in excavated mammoth or elephant tusks. Newesely found newbervite deposits on ivory artifacts, which is confirmed in this study. As bleaching with ammoniacal hydrogen peroxide damages ivory and leads to struvite crystals, it should be abandoned. The roles of chemical and microbiological factors in the formation of magnesium phosphates from ivory are discussed.
CONCLUSION. Because, unlike bone and other teeth, ivory is rich in magnesium, the magnesium phosphates new¬beryite, struvite and bobierrite can be found as alteration products. Whitaker [21] and Ribbe [22] have found that struvite samples in mineralogical collections are transformed in the open air to new¬beryite over the course of some months. Slow evap¬oration of ammonia and water could occur according to the formal equation: Whether this reaction also occurs with synthetic struvite is not fully clear from the literature. Newberyite found on ivory objects could thus cither be produced by precipitation from a magne¬sium- and phosphate-containing solution, or be the end product of a struvite intermediate; further investigations are needed to clarify this. Long periods in high humidity seem essential for both the chemical and the biological formation, that is, for the leaching of magnesium and phosphate ions or for the growth of organisms. If ivory objects are permanently stored at a relative humidity not exceeding 55%, it is hoped that no new alteration products will form. Because bleaching with ammoniacal peroxide visibly attacks the material, it is no longer acceptable in ivory restoration. Although conservators do not use such drastic conditions (concentration, exposure time) as in the tests described here, they have no effective control of how much damage is done dur¬ing application. If there really is a need for bleaching for aesthetic reasons, this has to be discussed criti¬cally in the light of modern conservation ethics and the principles of minimal and reversible intervention. The experience of conservators that ivory on display is "readily bleached by light' [23] (and, therefore, yellowing occurs only in the dark) is of potential use as an alternative to chemical bleaching, but has so far not been studied as a restoration method for ivory under controlled conditions. Perhaps the best idea in respect of bleaching is to wait until the yellowing is better understood at the molecular level. |
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