Repairs to Vellum- and Leather- hinged Damper Bodies

The Blüthner Damper Action — The picture on the right shows the damper action as found in Blüthner grand pianos that have the patent action. A kerf 0.5mm wide is sawn into the tapered end of the damper body and into this is glued a hinge of vellum about 0.4mm thick. These hinges are held firm in a fir or spruce clamp whose two halves are glued together and screwed down to the pine lift-rail. The damper drop is attached to the body in the normal way with a bushed centre using a pin of 1.28mm. The centre turns in the body, and not in the drop as in some actions

The purpose of this article is to describe the process of restoring such a damper action. Nobody who is not thoroughly familiar with the piano action and with the tools and materials of the Trade should attempt this task.

Cleaning and Measuring — It is not possible to clean the parts thoroughly before it is taken apart, but it should be thoroughly brushed and blown out before work commences. Take special care throughout the job not to knock the damper bodies, since even a gentle knock will stretch or tear a good vellum hinge and make it necessary to renew it. Number the damper bodies neatly in pencil on the under side.

Using a lath as a marking-stick (or a length of stiff card in normal conditions of humidity) record the position of the damper bodies and the ends of the clamp with pencil marks or a knife. The marks from this stick will be transferred to the clamp lower half when it comes to gluing on the hinges.

Carefully test each damper body for sideways movement, which indicates a damaged hinge, and mark the faulty ones. The lever should feel extremely firm without the slightest movement.

Once all necessary measurements and notes have been taken, the dismantling can proceed. First loosen the clamp from the lift-rail by removing the screws.

If only one or two hinges need renewing, it is possible to make fine saw-cuts across the top rail of the clamp down to the glue line, remove the piece and work on a single hinge, but in what follows we shall be removing all the damper bodies from the clamp, whether or not the hinges are good. This not only looks well but also greatly facilitates the re-centring of the damper drops, which to do without removing the bodies and without damaging the hinges makes a most tiresome job. The damper drops must be recentred since they will be corroded, and lubrication with magic potions will not effect a satisfactory long-term repair.

Parting the Clamp — The two halves of the clamp are glued together with what may well be a fairly strong and resistant glue. The utmost patience and care will be required to part them and one should be prepared to make a new lower half if the operation is unsuccessful. In the following sections and illustrations, the lower half of the clamp is replaced.

Secure the clamp upright in a vice with 6 - 9 inches standing above the jaws and cut a small notch in the top half as shown right down to the glue line. This will enable the blade of the knife to begin perfectly in line with the glue. Now wet the end of the clamp well with alcohol, insert the knife a very small distance along the glue line and keep feeding the joint with alcohol.

Secure the clamp upright in a vice with 6 - 9 inches standing above the jaws and cut a small notch in the top half as shown right down to the glue line. This will enable the blade of the knife to begin perfectly in line with the glue. Now wet the end of the clamp well with alcohol, insert the knife a very small distance along the glue line and keep feeding the joint with alcohol.

When the gap has opened a 2 or 3 millimetres insert a long fine chisel at the end to hold the two halves apart and apply a little pressure and more alcohol. Too much pressure at any stage of the operation will start a split along the grain of the wood rather than along the glue line. Allow time for the alcohol to work. As the crack opens a little apply a little more pressure and feed in more alcohol. As the crack approaches the jaws of the vice, move the assembly upwards. Patience and care, good knives and other implements will allow the clamp to be parted with little or no damage.

Removing the Damper Bodies — The top half of the clamp can now be set aside and the damper bodies torn from the lower half. Those with broken or defective hinges need no special care; those with good vellum can normally be pulled off the lower half without damage, provided that no sideways pressure or twisting is applied. Hold the body firm and straight at an angle of about 20° to the rail and pull slowly upwards. It is important that the vellum be perfectly dry when this is done, since wet vellum has little strength and will deform and stretch.

Cleaning up the Vellum — The exposed vellum is likely to be slightly curled after this operation and covered with the remnant of the glue and of the blue cloth strip from the top half of the clamp. Apply a little cold water to the vellum, wait for the glue to soften and carefully scrape off the residues with a chisel. Repeat this procedure until the vellum is quite clean, press it flat and allow it to dry. The picture shows the dampened vellum on the left and the cleaned on the right.

Removing the Hinge from the Body — This task is simple, fast and effective if the proper procedure is followed. The tools required are

  • A pan of water that is near boiling point but not boiling
  • A good quality tapering palette knife or a 0.4mm feeler strip
  • A pair of smooth-jawed pliers
  • Clean white cotton rags

Hold the hinged end of the damper body in the scalding water for 30 seconds. If the hinge is entire or only partly torn, it may be possible to pull out the vellum using the pliers. If not, proceed as follows : Immediately on removal from the water, line up the edge of the palette knife one side of the kerf and press against the vellum. This will then slide out of the kerf at the other side and can be gripped with the pliers and removed.

Pressing out the vellum with the palette knife

The Kerf

Once the vellum is removed, hold the damper body horizontally over the pan with the kerf vertical and draw the palette knife up a few times through the kerf and down into the water to wash out the remaining glue from the kerf. This done, blow sharply into the kerf, wipe the damper body dry and proceed to the next one. Keep the water at the right temperature with a spirit lamp or a very low gas flame.

Cleaning the Rails — If necessary, make new rails exactly to pattern. In this case, to imitate the original, choose good quality lengths of fir or spruce cut on the quarter. Provided the original rails are in good order, paint them with cold water and scrape off the softened glue. This will need to be done several times. Never waste time and risk damage by trying to remove hard glue. When the glue holding blue cloth strip is thoroughly soft, pull the strip away, further soak the remaining glue in the rabbet and clean this out with a hardwood stick.

If parts of the wood have been torn up during the parting of the clamp, make good with thin glue and clamps. There is no need to paper clean the inner surfaces of the clamp; some roughness is an advantage for when it is glued together again.

The old glue, softened by soaking
is scraped from the rail

The cloth pressure strip, well-
soaked, is pulled away

The Glue — The glue should be fairly fresh and neither thick nor too watery. It is assumed (perhaps wrongly!) that any technician undertaking this task will be thoroughly familiar with the behaviour and management of glue and the gluepot.

The Gluepot

Inserting New Hinges — The kerf in the damper body is 0.5mm thick and the vellum (or parchment) used originally for the hinge is roughly 0.4mm thick. Vellum of such a thickness can be obtained from piano supply houses or from various sources.

Cut the hinges a little wider and a little longer than the final size to allow for trimming.

Glue is applied to the kerf and to the damper body end of the vellum hinge using the palette knife. It is convenient to have a slot in the glue-well cover slightly larger than the blade of the palette knife so that this can be kept hot. With the damper body and vellum hinge in one hand, take the heated knife from the well, dip it in the glue and draw it through the kerf to coat the inside with glue; then wipe the hinge on the knife to coat it with glue and immediately insert the hinge into the kerf to its full depth. It is necessary to work quite fast in order to prevent the softening and swelling of the vellum, which will ensure quickly on contact with the hot, wet glue.

Provided the vellum has been properly measured and the kerf properly cleaned as described above, the hinge will slide smoothly into the kerf without any encouragement from pliers. Immediately clamp or tie the damper body and allow the glue to set for a while. When the glue is cold and gelatinous, but before it has begun to harden, take a well-squared hardwood or bamboo stick and pare off the excess glue both at the end and the sides. The clamp can be removed after 20 or 30 minutes.

Pare off the excess vellum each side of the kerf with a knife and cut the hinge to size with the shears.

Measuring the Vellum

Cutting the Vellum

Hot glue is drawn into the kerf
with the palette knife

The vellum is inserted and clamped

The extruded glue is removed with a wooden
stick after 20 or 30 minutes

Centring and cleaning — Since one good reason for taking apart the damper action will have been to facilitate a good re-centring job, it would be a shame to forget that at this point and remember only when the job is completed! Thoroughly clean the damper bodies and drops with plain water or oxalic acid and check that no glue remains where the hinge enters the kerf. Re-centre the damper drops using as fine a pin as possible. The original pins will be 1.28mm (N° 22¾), so a N° 23 (1.300mm) will be proper replacement.

Lubricating the set screws — The set screws were lubricated at the factory and are most unlikely to have seized, but it is a good idea to remove them and re-lubricate with the beeswax and tallow mixture so that they turn easily and will not tend to twist the vellum hinge during damper regulation.

The original centre pins

The damper drop re-centred

Gluing the Hinges to the Rail — It is most important to avoid getting any glue between the lower jaw of the damper body and the angle of the clamp. To this end, brush the glue on to the clamp towards the bevelled edge where the hinge bends, then quickly lower the damper body with the hinge on to the sharp angle. Next pull the hinge towards the back of the clamp while pressing down, so that the flat lower edge of the damper body is drawn tight against the angle of the clamp. A perfect right angle is bound to ensue if this work is properly done. Get the spacing as accurate as possible in the first place, but fine spacing and the flattening of the hinge will be done with the iron when the glue has cooled slightly.

When a dozen or so hinges are glued down prepare the iron for the process of final spacing and gluing flat. A heated chisel can be used as an iron but a more substantial block of steel will not only remain warm for longer and allow faster working but will also permit greater and more accurate sideways pressure to slide the hinge against the strong force of the glue.

The aims of this work are

  • To get the damper body so tight against the clamp that it is almost hanging horizontally
  • To have all the damper bodies at a right angle to the clamp
  • To have the damper bodies evenly spaced
  • To have all the damper drops vertical
  • To have the vellum thoroughly glued to the clamp and flat.

The glue is brushed towards the bevelled edge

The spacing iron is kept warm
under the gluepot

The damper bodies are spaced while
the glue is still fairly soft

The Completed Assembly