Having emailed a reply to a plea from a desperate reader at the weekend, I thought I might as well publish it here for the potential benefit of others.
Animal glue is mildly hydrophilic which alone, enables it to maintain its adhesive property. Glue that has been utterly deprived of humidity will become brittle and subsequently fail. Luckily for those who restore glued articles, this same action can be replicated chemically.
Alcohols are hydrophilic in varying degrees (methanol has the highest affinity for water, though ethanol rates a very satisfactory second) and restorers and furniture-makers normally have a supply of ‘dry’ ethanol on hand for making spirit varnishes.
Ethanol dehydration can be employed to reduce animal glue to a crystalline state, breaking its bond and thereby permitting dismantling of a loose or damaged joint. Ethanol is injected into the joint with the aid of a syringe whereupon the glue progressively relinquishes its moisture – often accompanied by a crackling sound – as the alcohol wicks its way in.
Further pulling, wiggling and possibly tapping of the joint is usually required to persuade the now granulated glue to crumble away. Larger chunks of crystalline glue can either be chipped or scraped from the open joint; however it’s not critical, as any residual glue will be rejuvenated with the application of fresh hot glue when the joint is reassembled.