The alternative to Fiorelli’s method, developed in the 1980s, uses epoxy resin rather than plaster. Epoxy resin can be made transparent, which allows the skeletal material inside the cast to be studied visually without destroying the exterior surface. The transparent resin casts made since the 1980s are therefore more amenable to study than the earlier plaster casts, but they represent a small fraction of the total collection and a small fraction of the victims — most of the identifiable body voids were filled before the resin method was available.
The restoration project engaged with the trade-off created by Fiorelli’s method by using CT scanning to study the interiors of the plaster casts non-invasively. CT scanning sends X-rays through the cast at multiple angles and computes a three-dimensional image of the interior, allowing researchers to identify and study bone fragments, teeth, and objects without touching them. The DNA sampling required physical access to bone or tooth material, which was possible in cases where previous damage had already created access points in the cast.
This means that the DNA study was dependent on a prior history of damage to the casts — a somewhat uncomfortable irony. The casts that yielded usable ancient DNA were the ones whose interiors were accessible, which meant they were the casts that had sustained damage sufficient to expose the bone or dental material inside. Undamaged casts, whose skeletal remains are better protected, are also the casts from which ancient DNA sampling is most difficult.
The decision to sample bone from the casts was made carefully, as part of a research protocol that weighed the scientific value of the DNA data against the conservation cost of accessing the fragile interiors. The researchers did not drill into intact casts or create new access points where none existed. This constraint limited the sample to 14 of 86 casts — a significant limitation, but one that reflects appropriate conservation priorities.