Chloroform is the preferred solvent used with a Hedstrom file to effectively dissolve and soften gutta-percha, facilitating its removal during root canal retreatment. Its strong solvent action helps break down the thermoplastic gutta-percha without damaging the dentinal walls. This characteristic allows for efficient mechanical instrumentation using the Hedstrom file to clean the canal thoroughly. Other solvents like ethanol or xylene are less effective in dissolving gutta-percha, and formalin is not used due to its toxicity and lack of dissolving properties. Thus, chloroform optimizes both chemical and mechanical removal of gutta-percha in endodontic retreatment procedures.

Reference: Pathways of the Pulp, Stephen Cohen, 12th Edition.