By Jonathon Kelly
The previous post opening the bellows explains how the bellows work, assesses their condition and shows historical evidence found inside them.
This post outlines the replacement of the outer skins and remedial treatment of the bellows assembly.
The approach taken was, as far as possible that any work done should: maintain the integrity of the original or existing object, be reversible and take into consideration the use and long term preservation and interpretation of the object as a whole.
The justification for replacing the entire outer bellows skin (rather than further patching of previously repaired holes) came from one of the key project aims negotiated by the clock owner Temple Newsam House; the organ should once again play music. To play music the bellows have to work efficiently and reliably.
Upon inspection it quickly became clear that there were just too many leaks coming from holes and tears in positions awkward to patch. Some of the holes had already been patched or were very near to old patches (fig. 2). So for the organ to play music the skins had to be replaced.
Removal of the outer skins
For this project it was important to retain the existing skins complete. To do this the skin was carefully removed along the joints with the wood (fig. 3).
Once the skins were removed the design and dimensions of the reservoir and pump sections were recorded. (Fig. 4) The internal surfaces of the bellows were also revealed.
The existing skins had been bonded to the wooden surfaces with traditional animal (or hot melt) glue throughout the bellows (see fig. 5).
The strong bond of the glue was softened using Laponite. This gel like fluid was used as a poultice to re-introduce moisture to the glue, thereby enabling removal of the glue without damaging or having to refinish the edges of the bellows wood.
Selection of replacement materials.
The key properties required for the bellows membrane or skin are: flexibility, durability and of course being air tight.
For bellows of this size the options were to either replace with alum tawed skin or "pneumatic" skin. Pneumatic skin or leather is a trade term for a modern tanned skin or leather widely used for air applications such as bellows. It was selected primarily because of the Lead Conservator Malcolm Archer's previous experience of using it on similar bellows.
Rather than using organic hot melt glue, archival quality PVA (neutral pH) was selected because it is reversible and more easily removable than the hot melt glue which sets very hard. It is also tried and tested, based on Malcolm's previous experience of re-skinning bellows.
Card produced from 100% cotton fibre (neutral pH) was used to make the stiffening panels for the skins.
Treating the bellows hinge screws.
The steel hinge screws had been corroding. The loose surface rust was removed and after degreasing they were soaked in a tannic acid solution. This reacts with the surface of the iron to form ferric tannate, a black layer on the surface of the iron which inhibits further corrosion.
Microcrystalline wax was applied to the screws before reassembling the screws and hinges into the bellows.
Replacing the skins.
The bellows design was copied exactly. The card was cut to shape and glued to the new skin in the same arrangement which promotes efficient folds (Fig. 8).
The card on the skins was left under pressure for the glue to dry and form a strong bond (Fig. 9).
The skins were then clamped into their folded position to form a crease under controlled conditions (Fig. 10).
Attaching skins to the wooden bellows boards
The skins were carefully glued along the edges of the wood to ensure an airtight seal and positioned to avoid creasing, hence avoiding more holes.
Once assembled checks were made for correct folding action of each moving section and for any air leaks particularly where skin joints had to be overlapped.
Finally the bellows shown being fitted into the bellows frame (Fig. 16)