State casket, the shrine for Friedrich III of Austria, which was made in the second half of the 17th century, from the parish church of Mauerbach, currently located in the Archdiocese of Vienna. Credit: Archdiocese of Vienna’s Department of Art and Preservation of Historical Monuments. ©Stephan Doleschal.
Original ivory capital removed from the shrine (left) and the 3D printed Digory capital (right).
The original ivory and Digory capitols in place on the state casket, the shrine for Friedrich III of Austria.Ivory trade is banned to protect declining elephant populations from illegal poaching for their tusks. This presents a problem for the restoration of historic ivory artefacts. Natural materials such as bone, shell, or Tagua palm tree nuts or artificial substitutes based on polymer composites mixed with inorganic particles, casein, or ivory dust can be used for repairs. But these bulk materials are not ideal for intricate, delicate artefacts and can be costly.
Now researchers have developed a substitute material closely resembling ivory that can be printed into complex three-dimensional structures using additive manufacturing [Rath et al., Applied Materials Today 23 (2021) 101016, https://doi.org/10.1016/j.apmt.2021.101016 ]. The interdisciplinary team from the Institute of Material Science and Technology at TU Wien, Cubicure GmbH, Addison KG, and the Archdiocese of Vienna’s Department of Art and Preservation of Historical Monuments developed the alternative from a mixture of a dimethylacrylic resin and calcium phosphate particles. The organic component of the mixture, bisphenol-A ethoxylated dimethylacrylate, once combined with the desired proportion of calcium phosphate particles is polymerized with a UV laser. Since the artificial ivory can be produced using a digital technique, the researchers dub the new material ‘Digory’.
“We wanted to develop a high-quality, animal-friendly replacement, with comparable aesthetics that can be used for cost-effective restoration of art pieces,” explains Thaddäa Rath, first author of the study.
The proportion of solid particles can be adjusted to give the right level of opacity, which varies in natural ivory. A loading of around 30 vol.% is optimal for creating an artificial material with both the appearance and mechanical character of natural ivory. To match the natural material’s creamy hues, yellow and red pigments are added to the composite, and a process of scratching and staining is used to imitate characteristic Schreger lines.
“When replicating ivory, it is important to mimic its aesthetic characteristics, such as the color, translucency, and surface gloss,” says Rath. “We also wanted to make sure that Digory could be processed with similar handcraft techniques as ivory, so the mechanical properties need to be comparable.”
Digory can be polished, shaped, or glued just like the real material using traditional techniques. Importantly, though, Digory can also be distinguished from real ivory using non-destructive methods like UV opacity tests. The researchers believe Digory could make the repair, restoration, or replication of historical cultural or religious objects easier, more accurate and more economical.
“Since Digory is processed using additive manufacturing, the working time is significantly reduced,” points out Rath. “As surface finishing remains a craftwork process, [our approach] represents a perfect combination of innovative digital production and traditional techniques.”
The team plan to commercialize Digory and develop a scan-and-print service for artefacts.