Tutankhamun, the legendary woodworker, would have no doubt used the finest hide glue in the making of his furniture. Other craftsmen and furniture makers from centuries past such as the great Stradivari would also have utilised the glue. Georgian joiners too, that had the privilege of fitting the finest sash windows would no doubt have covered their tools in the hide glue, used to join the fine joints together. Hide glue made from animal bones and hides boiled in water, was for centuries the default material used for the joinery of the vast majority of wooden items. No one thought any worse of it. It was a fine and versatile material.
However, in the mid-twentieth century, along came PVA which gripped harder and was cheaper to purchase than hide. So PVA became the universal glue and the hide was forgotten from most workshops. The problem with PVA however, is that it cannot be ‘un-done’.
Hide glue can be ‘dissolved’ (as it were) with the application of heat and moisture, and then the joint will come apart cleanly with no sign of having been previously mended. This property appears to be a weakness, but it is precisely this that makes it so suitable for use in conservation work. In contrast to more modern materials, which are designed to last ‘forever’, the materials used in conservation work should all be reversible, i.e. should all be such that any intervention made using them can be ‘un-dosed’ by later conservators. This is the underlying principle behind Historic England’s guidance on repairing historic timber structures
Hide glue also has a property known as creep, meaning that it slowly yields to constant pressure. This is just as well for a joiner, as wood frames subject to humidity changes will expand and contract. Hide glue allows for a tiny amount of movement within a joint, whereas other adhesives will resist this and can even cause the wood to split under long-term stress as the timber wants to expand and the adhesive is resisting.
A sash window joiner today would deliberately use a hide glue or a consolidant with similar properties, within a sash window repair, as he knows that he is doing work that will be capable of being opened up two hundred years later by someone who has every intention of undertaking further restoration work.
Sash Window Repair is covered in more detail at sashwindowpreservation.co.uk/services/sash-window-repair/.
The glue that comes undone is the very glue that saves the two hundred year old sash window for future repair!
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