A key part of the ethics of restoration and conservation is to ensure that the treatments performed are reversible. In the case of structural joints this means using an adhesive that can be softened and the components separated without damage. ‘Animal’ glue, a reversible glue made from the rendered down parts of animals such as hooves, horn, hide, etc. has been used for centuries in England. This has generally been replaced in furniture manufacture in the twentieth century by modern substitutes which are easier to use but not necessarily better and generally not reversible.
Animal glue, also known as ‘Hide’ or ‘Scotch’ glue, is strong, flexible and compatible with that existing on antique furniture and is therefore the glue of choice of professional conservator-restorers such as myself. It allows the joints on a piece of antique furniture to be separated by repeatedly injecting warm water into the joint in order to soften the old animal glue. This is not always as easy as it sounds and great care must be taken. Once separated the old glue can then be cleaned from the joint components using more warm water and a scraper before repair and re-assembly using fresh animal glue.
Unfortunately, I frequently receive items of furniture that have been subjected to repair attempts involving a multitude of inapproriate and inexpertly applied adhesives, not to mention nails and sticky tape! The layman can be forgiven but there are some furniture repair practitioners whose idea of ‘restoration’ is to merely ‘squirt’ a modern adhesive into a joint on top of any old glue that is present. Beware! Regardless of the advertised strength and other properties of modern adhesives their efficacy is only assured when bonding correctly prepared wood-to-wood surfaces and not those covered in old glue, animal or otherwise. Such a repair is unlikely to form a good adhesive bond and its lifetime must be questionable. This is especially true if simply separating the components of a joint and injecting a modern adhesive is all that has been performed without addressing and correcting the causes of the joint being loose in the first place, such as shrinkage or fracture.
However, other than in joints there are roles for modern conservation-approved adhesives in restoration. These include consolidation of soft or woodworm damaged timber and the repair of fractures. If a component has been badly fractured, e.g. a broken chair leg, then a strong Epoxy Resin adhesive may be sometimes used to put the pieces back together again rather than the alternative approach of making of a complete replacement leg. By retaining the original leg we are satisfying another of the important ethics of restoration – that of maximum retention of original material.
In fractures such as these there are often occurences of small pieces of timber being lost from the mating surfaces and adhesives such as epoxy–resins are able to perform a ‘gap-filling’ function and provide a strong bond. However, there is a limit to the degree of gap-filling that some adhesives are able to perform. I have come across cases where PVA (or ‘white glue’ as it is sometimes called) has been used in an attempt to fill a gap of more than a couple of millimetres with the result that it has never fully hardened and many years later it still remains soft and ‘rubber- like’.
So, careful, ethical choice of adhesive for the task in hand is fundamental to good quality restoration as is using a qualified professional furniture restorer!