Reversibility and glass transition: B-72, B-76, cyanoacrylate

Every polymer you put on a fossil has a glass transition temperature (Tg): below it the film is glassy and rigid, above it the film is rubbery and tacky. A consolidant with a Tg near room temperature will creep in a hot car, pick up dust, and let a joint sag under its own weight. Conservators want a Tg comfortably above any temperature the specimen will meet.

Paraloid B-72 (ethyl methacrylate / methyl acrylate copolymer) has a Tg of about 40 °C, stays optically clear, does not cross-link with age, and redissolves in acetone or ethanol decades later. That combination is why it became the default consolidant and adhesive in preparation labs. Its weakness is a slightly soft film: a joint on a heavy bone can creep in a hot shed, and it needs a day to lose its solvent fully.

Butvar B-76 (polyvinyl butyral) has a higher Tg, around 60–65 °C, so it is stiffer and better in heat; it is more flexible in thin films and wets wet or damp bone better than B-72, which is why field crews in the western US favour it. It dissolves in ethanol, isopropanol or acetone and is reversible in the same solvents. It yellows very slightly over decades; B-72 does not.

Cyanoacrylates (PaleoBond, Starbond, ordinary super glue) are a different chemistry: a monomer that polymerises on contact with surface moisture. They set in seconds and bond hard, but the cured polymer is brittle, has a low Tg and degrades and yellows with age, and it cannot be redissolved — acetone and commercial debonders only swell it slowly and can carry stain into porous matrix. Reserve them for small, quick joins and note the use on the record.

Vinac B-15 and other polyvinyl acetates (PVA) have a Tg close to room temperature (about 30 °C) and are soft enough to collect grit; PVA emulsions (white glue) are water-based and can leave the film acidic. Neither is a good long-term choice, and either may be found on older specimens.

The practical rule: match the consolidant to the job (penetration versus gap-filling versus joining), pick the highest Tg that still wets the surface, prefer a polymer with a known removal solvent, and record product, concentration and solvent on the specimen's Preparation tab.