Fiorelli’s method, introduced in 1863, was to pour liquid plaster of Paris into each cavity and allow it to set. Once hardened, the surrounding volcanic material could be carefully chipped away to reveal the cast of whatever had occupied the void. The technique worked on bodies, on wooden furniture, on plant roots — any organic material that had decayed within the volcanic ash and left a cavity matching its form.
The plaster fills the precise shape of the void, which means it captures not the body itself but the space the body occupied at the moment the ash set around it. The skin surface, the drape of clothing, the position of limbs — these are preserved in the cast. Inside the cast, the skeletal remains of the actual person are often still present, in varying states of fragmentation, mixed with the plaster and the ash. Fiorelli’s technique used high-quality plaster reinforced with wood, and his casts have survived in better condition than some of those made later with inferior materials.
By the time the 2024 DNA study was conducted, 104 casts had been made at Pompeii. The techniques for studying them had expanded far beyond visual inspection. CT scanning of casts has allowed researchers to examine the skeletal remains inside without damaging the outer surfaces — revealing ages, evidence of healed injuries, possible health conditions. Strontium isotope analysis, conducted on skeletal material, can suggest where an individual spent their childhood by comparing the isotopic signatures in their teeth against local geological values. Ancient DNA extraction, where bone or tooth material is preserved well enough to yield genetic information, can add biological sex, genetic ancestry, and kinship data.