Prior to radiocarbon (14C) dating all samples are being selected and pre-treated according to the type, quantity and condition of material, and then graphitized. This is one of the most important phases in the carbon dating process, which requires highest expertise to achieve reliable and accurate result. Samples in Vilnius Radiocarbon laboratory are being pre-treated using exceptionally best-on-the-market chemicals and consumables and strictly following internally approved methodologies according to below given protocols:
Most organic materials, such as charcoal, plant fragments, fragile wood samples, skin, parchment, leather, keratin, textile also peat, soil, sediment samples are treated using the standard acid-base-acid (ABA) method (Molnár et al. 2013, Brock et al., 2010). The samples are treated with a sequence of 1M HCl, distilled water, 0.2M NaOH, distilled water (until the solution is colourless), and then 1M HCl. Woody plant material, skin, parchment, and leather samples may be subjected to a bleach treatment. The concentration of the solutions and the bleach (2.5–5.0% w/v), temperature and duration of storage of the samples in the solution depends on the size, fragility and preservation of the specimens.
Cellulose extraction of wood samples is performed using the base-acid–base-acid-bleaching method (Němec et al. 2010). This procedure is basically identical to ABA, but it is modified for modern wood samples with no or minimal degradation and is presented as current standard for wood cleaning procedure (Gaudinski et al. 2005; Anchukaitis et al. 2008). In brief: the samples are treated in 1M NaOH overnight, followed by sequential treatments in 1M HCl, 1M NaOH, and 1M HCl again for 1 hr each; then bleaching step of 30 min using 5% NaClO2 and 2 drops of 1M HCl is performed.
Bone, tooth dentine, antler, ivory collagen
Bone samples are ultrasonicated in ultrapure water, dried, grinded and sieved to get the appropriately sized sample fraction (0.5–1 mm). Bone collagen is extracted using an acid-alkali-acid procedure followed by gelatinization (Molnár et al. 2013, Brock et al., 2010). Samples are treated with 0.5M hydrochloric acid (~18 hr), 0.1M sodium hydroxide (30 min), and 0.5M hydrochloric acid (1 hr). Bone collagen gelatinization is performed in pH 3 solution at 70°C for 20 hr. Gelatin solution is filtered using a cleaned Ezee-filter and freeze-dried.
Teeth enamel samples were prepared for radiocarbon dating following Koch et al. (1997). Samples were cleaned of the matrix, and enamel was separated from dentine using a hand-held Dremel drill. At least 6 mg tooth enamel powder was collected and pretreated with 30% hydrogen peroxide and acetic acid (buffered 1:1 with calcium acetate).
Carbonized Organic Residues
A gentle pretreatment of carbonized residues on the exterior or interior of potsherds consists of demineralization with 1M HCl for 1 hr followed by 15 min ultrasonication in fresh 1M HCl. The samples are then rinsed in ultrapure water 4 times, then being ultrasonicated in fresh ultrapure water for 5 min approximately 6 times (until the water remains clear). The last step is to acidify the samples for 5 min in 1M HCl and rinse twice with ultrapure water (Brock et al., 2010).
Coral and shell samples are surface abraded using a rotary tool and coarsely cut or crushed before cleaning in MilliQ water with ultrasonication. The organic materials, if needed, are removed with H2O2. Samples are then etched using 0.2M HCl to remove the outer layer of potentially recrystallized carbonates. Then samples are treated by reacting with phosphoric acid.
Bone samples submitted for analysis as cremated bone must be gray and preferably almost white in color, indicating that they have been burned at >600°C. Cremated bone samples are treated using methodology applied by I. Major et al. (2018). After washing the samples with water and cleaning the external surface of the bone fragments, 4–5 g of the cremated fragments are treated with 2 × 50 mL of 0.25M sodium chlorite (24 hr, 20°C) in a centrifuge tube to remove organic residues. After that, the samples are washed in ultrapure water followed by treating the samples with 2 × 50 mL 1M acetic acid (24 hr, 20°C) to remove exogenous carbonates and less well-crystallized crystallites of apatite. Finally, all the samples were washed repetitively in ultrapure water again until a transparent liquid was observed and, after decantation, oven-dried at 60°C for an overnight.
In some cases when consolidants, preservatives, and/or other chemical contaminants (including oil paints on canvas) are known or suspected to be present in the samples, a solvent extraction is needed. We have adopted a Soxhlet extraction protocol similar to that applied by Molnár et al. (2013) and Hajdas et al. (2008). The contaminants are extracted in a sequence of hot solvents: hexane, ethanol, acetone and methanol, followed by a distilled water wash if a contaminant is suspected but its identity unknown. Where the exact contaminant is known, the choice of solvent is tailored to the specific contaminant (e.g., chloroform for latex and certain waxes; water, acetone, and methanol for PVA and Resistol; methanol for shellac).