Acryloid B-72 — marketed interchangeably as Paraloid B-72 and commonly abbreviated to B-72 — is a thermoplastic acrylic copolymer resin (a copolymer of ethyl methacrylate and methyl acrylate) that has become one of the most trusted reversible adhesives in both lapidary practice and professional museum conservation. Dissolved in acetone or, where lower volatility is preferred, toluene or xylene, it forms a clear, colourless solution that bonds a gemstone to a dop stick without the application of heat, making it indispensable for materials that cannot safely tolerate thermal stress.

Why Reversibility Matters in Dopping

Conventional dopping waxes require a heat source — typically an alcohol lamp or a dedicated dop-wax heater — to achieve adhesion. For the majority of hard, stable gem materials this presents no difficulty. However, a significant range of commercially important stones is vulnerable to thermal shock, dehydration, or structural damage at even modest temperatures. Opal, with its water-bearing amorphous silica structure, can craze or lose play-of-colour if heated unevenly. Amber and other organic gem materials soften or distort. Doublets and triplets — assembled stones bonded with optical cement — risk delamination the moment warmth reaches the adhesive layer between components. For all of these, a cold-setting adhesive that releases cleanly in solvent is not merely convenient but essential.

B-72 addresses this requirement directly. The lapidary applies the dissolved resin to the dop stick, seats the stone, and allows the solvent to evaporate at room temperature. The resulting bond is firm enough to withstand the mechanical forces of grinding and polishing on a standard flat lap or cabochon machine, yet the adhesive remains fully reversible: immersion in acetone for a short period dissolves the resin completely, releasing the stone without mechanical stress and leaving no residue on the girdle or pavilion.

Physical and Chemical Properties

B-72 is chemically inert once cured, non-yellowing over long periods, and resistant to moisture — properties that distinguish it from older natural-resin adhesives such as shellac, which yellow, embrittle, and respond unpredictably to humidity. Its glass-transition temperature (approximately 40 °C) is low enough that the cured film remains slightly flexible at room temperature, reducing the risk of stress fractures in fragile specimens, yet high enough to maintain a stable bond under normal lapidary working conditions. The resin is soluble in a range of common organic solvents, giving conservators and lapidaries flexibility in choosing a working medium appropriate to their ventilation and safety constraints.

Applications in Lapidary Work

• Heat-sensitive cabochon materials: Opal, turquoise, malachite, amber, jet, coral, and shell all benefit from cold dopping with B-72.
• Assembled stones: Doublets (opal doublets, garnet-topped doublets) and triplets are safely dopped cold, avoiding delamination of the cement layer.
• Faceting delicate crystals: Some collectors facet fragile or low-hardness materials — fluorite, calcite, selenite — where heat could cause cleavage; B-72 provides adequate hold without risk.
• Re-dopping mid-project: Because removal is clean and solvent-based, the lapidary can re-orient and re-dop a partially worked stone without surface contamination affecting subsequent adhesion.

Use in Museum and Specimen Conservation

Beyond the lapidary bench, B-72 has been adopted as a standard consolidant and adhesive in natural history and geological museum collections worldwide. It is used to stabilise friable mineral specimens, consolidate crumbling matrix, and re-adhere detached crystal fragments. Its acceptance in conservation practice rests on the same properties that recommend it to lapidaries: chemical stability, optical clarity, and — critically — the principle of reversibility that underpins modern conservation ethics. A treatment applied with B-72 can be undone by a future conservator without damage to the original specimen, a requirement codified in the standards of organisations such as the American Institute for Conservation.

Practical Notes for the Lapidary

B-72 is typically used at concentrations of 20–40% by weight in acetone for lapidary dopping; lower concentrations (5–10%) are common in conservation consolidation work, where deep penetration into porous material is desired. Working time is governed by solvent evaporation rate and ambient temperature; in warm, well-ventilated conditions the bond may be ready for light work within thirty minutes, though allowing a full cure overnight is advisable before applying significant lateral pressure. Standard wooden or aluminium dop sticks are compatible. Because acetone is highly flammable, B-72 solutions should be handled away from open flames — a particular consideration when the lapidary's bench is otherwise equipped with an alcohol lamp for conventional wax dopping.