Restoration of Egyptian Oud - Part 4, the Bowl

The bowl is in a fairly rough shape with cracks, badly executed repair work and consequent damage to the bowl to sound board joint face.

To first assess the possibilities for corrective repair work to the joint face, the alignment of the joint face to surface of the fingerboard was evaluated using 'winding sticks' - a simple tool familiar to wood workers.
Winding sticks are straight, parallel sided strips of wood used to determine twist in timber by sighting along the tops of the strips.
The attached images show the placement of three winding strips on the oud - one at the nut, one at the neck joint and one across the bowl at 30 cm from the neck joint. The strips are placed at 90° to the centre line of the oud.

Note that there is no twist in the fingerboard which is otherwise perfectly flat along its length.

The amount of 'twist' in the bowl (relative to the fingerboard surface) is determined by packing up the winding strip laid across the bowl (at the bass side) until it aligns with the other two strips. In this case the packing measures 9 mm thick - quite a discrepancy between treble and bass side !

When the bowl is inverted on a flat surface the overall discrepancy is less and so it should be possible to level the bowl (perhaps with some 'scoop') but this will require that the neck be removed and reset so that the fingerboard surface properly aligns with the sound board joint face.
Due to damage to the bowl joint face it will likely be necessary to cut away the damage and compensate for the loss with a thin strip of new material glued around the edge of the bowl.

More on this to follow

Hi Samir - that is a good question worth further investigation.

Due to the damage over the years to the joint face of the bowl it is not possible to be sure what the original condition may have been.
It should be noted, however, that both of the side ribs are more or less equal in dimensions measuring 20 mm maximum width. (these two ribs are narrower than the other ribs of the bowl that all measure 25 mm maximum width). Note also that the plane of the fingerboard surface is parallel to that of the upper joint surface of the neck block.

As is evident from the previous tests using winding sticks the bass side of the bowl is lower than the treble side - using the upper surface of the neck block as a reference plane.
Placing the bowl inverted on a flat surface - marked with a grid - the deviation of the bowl joint face from the reference plane was measured (using a taper gauge) at intervals of an inch around the perimeter of the bowl.

The attached image is a plot of the profile of the bowl joint face relative to neck block upper joint face as a reference plane.
From this it can be seen the the treble side of the bowl is - for the most part level but about 2 mm higher than the reference plane.
On the other hand the bass side is 'scooped' - from zero at the neck joint and bottom of the bowl to a maximum depth of
7 mm. This correlates with the 9 mm total difference between treble and bass sides measured using the winding sticks.

So - returning to Samir's question - could this cut out on the bass side rib be intentional - an example of 'scooping' to provide additional string clearance in the area below the sound hole? Or is this just action on the part of the luthier to correct asymmetry in a less than perfectly constructed bowl?

Bowl 'scooping' to provide extra string clearance is said to be found in some old ouds (Nahat ouds for example) - and is also said to be found in some old surviving lutes and guitars of the 16th and 17th C (although this may be disputed on instruments that are 400 years old and so subject to damage and deterioration over time).
As far as I am aware, 'scooping' is symmetrical - equal amounts being removed from the side ribs. However, there would seem to be no logical reason why 'scooping' should not be asymmetrical.

From the measured data, plotting the geometry in both longitudinal and transverse dimensions, it would appear that this asymmetric geometry might work. The string clearance below the sound hole is greater on the bass than treble side - which is the way to go. However, at the neck joint the string clearance over the finger board is less on the bass side than the treble - not the way to go. Nevertheless - if my rough calculations are correct - this clearance may be maintained to an acceptable 3 mm.

So a question. Is there any evidence to confirm that 'scooping' in old ouds - if it can be confirmed to exist - might be either symmetrical or asymmetrical?

The attached images are plots of the string clearances under the first and sixth courses relative to the sound board and fingerboard derived from the measured bowl profiles previously reported.

A nut height of 1mm is assumed and string height at the bridge is 8 mm. The fingerboard at the nut end is about 1 mm higher than at the neck joint.

It can be seen that string clearance at the neck joint is 4 mm for the first course and 3.5 mm for the sixth course. This is the measured geometry with no string tension.
String clearance over the soundhole is about 7 mm for the first course and about 7.5 mm for the sixth course.

Now with a better understanding of the geometry of the oud and based upon these figures it should not be necessary to reset the neck or modify the current asymmetry of the bowl as previously thought. It will, however, be necessary to patch the upper edges of the side ribs of the bowl with veneer to restore a smooth contact surface for the sound board. This is consistent with the objective of this project which is to make minimum repairs to bring the instrument to playing condition - i.e. to conserve as much as possible of the original oud.

Here are some detailed images of the edges of the side ribs showing some of the damage and repairs that have occurred during the life of the oud.

The edges have been cleaned of glue and show that the knife marks are areas where damage to the edges has been roughly repaired with wood veneer. The small nail holes are located at the brace positions so no doubt nails were used to temporarily clamp the ends of the braces when they were being re-glued for repair.
This oud has seen better days!

My guess is that the nails were used to clamp poorly fitting brace ends when the sound board was being re-glued after maintenance repairs. There are nail holes in brace positions # 2, #3 and #4 bass side and brace #3 treble side.

The attached image shows matching nail holes in the ends of braces #3 and #4.

There are now no nails in evidence but most likely thin headless veneer nails were used.
The veneer nails in the attached image measure 15 mm long by 0.7 mm diameter. Veneer nails are used to pin veneer to work so that the veneer stays in position while being glued in place (traditionally ironed in place using hot hide glue). The nails - being so thin - do not split the veneer and leave no visible mark after either being removed or just driven through the veneer.

Not the way to repair an oud!
But it gets worse .........

The neck block is split on the treble side - the crack extending from the bottom corner of the dovetail joint (an inherent weakness of this type of neck joint due to the wedging action of the dovetail). This had been 'repaired' in the past by gluing a scrap piece of wood across the split. Note the small nail hole in the piece of wood at the top - the nail being used to hold the wood in place until the glue had cured.

Clean up of the old surplus glue on the neck block and removal of the scrap of wood revealed a multitude of sins otherwise hidden by the attempted repair work. The split is gaping wide near the dovetail and extends probably to the full depth of the neck block. The dovetail joint has been packed with wood - and lots of glue - on one side.
Also the neck joint has been shimmed with veneer packing and is showing signs of deterioration of the glue.

No attempt will be made to remove the neck. The neck block will be repaired in three stages.
1 - Thin hot hide glue will first be run into the split to completely fill the open crevices.
2 - A series of holes will be drilled along the length of the split that will then be plugged with wooden dowel glued in place. This is essentially to create a new joint face for the repair.
3 - Finally the front face of the neck block will be levelled and a thin softwood reinforcing plate will be glued to cover the entire face.

The deteriorated shim at the neck joint will be cut back and replaced with new material.

Of interest to luthiers - note that the grain direction of the neck block is vertical, perpendicular to the longitudinal axis of the oud.

Apart from all of this ........

There are four small cracks in the ribs that will be repaired by reinforcing from the back with glued paper strips. The most severe of the cracks has been caused by impact as seen in the attached image. The damage has been filled with wax the residues of which can still be seen.
For some reason, a repair of the crack was attempted some time in the past by heating the crack with either an open flame or red hot iron. This action almost set things alight as the wood around the crack has been turned to charcoal! Scraping away the burnt wood reveals that the wood remaining is so thin that it can be flexed with light finger pressure.
Due to the fragility of the wood the crack will be left 'as is' and reinforced with glued paper.

The rib joints have been reinforced in the usual way with glued paper strips - however in several places these were not originally glued in contact with the joints so are useless as a reinforcement. These offending areas will be cut away and replaced with fresh glued paper strips.

I shall use some antique hand made 'laid' paper for the repair - made by renowned English paper maker J. Whatman - salvaged from an old diary - watermark dated 1803. This paper is made from 'rags' - cotton and linen fibres - so should be strong enough and acid free.

The paper recommended for this application by 14th C. Ibn al-Tahhan al-Musiqi is 'the best mansuri'. Not sure what
14th C 'mansuri' paper is but likely it does not differ much from Whatman's paper - made in the same tradition for 5 centuries.

First step is to remove the sections of paper reinforcing strips that are not (and never were!) glued to the rib joints. This was done with a dental pick (to locate the voids under the paper) and the offending areas were then scraped down to the wood with a curved scraper.
The original paper strips are a 'heavy' grade of paper - cartridge paper or the like - that is too stiff to fully conform to the irregularities of the joints. The replacement antique laid paper is only half the thickness (about 0.08 mm) but stronger than the original paper and flexible enough to be easily worked into the irregularities with a glue brush. Most of the original reinforcing strips were left intact and no attempt has been made to clean up the great 'globs' of glue left inside the bowl as a consequence of repair work in ages past.
The cracked ribs were also repaired by patching with paper and glue. In the case of the larger crack previously reported this was reinforced with a thin cotton patch (rather than paper) for additional strength.

The hide glue remaining in the pot was diluted to a slightly thinner consistency so that it ran in a steady thin continuous stream from the brush. With the oud held in a vertical position glue was then run into the open crack in the neck joint so that it was filled to overflowing with glue. Hopefully all of the open spaces of the crack will have been completely filled with glue. This is the first step in the repair of the neck block.

The second step to repair the crack in the neck block is to 'chain drill' the crack and insert wooden plugs to bridge the gap. This type of repair is similar in principle to that used in the engineering world to 'stitch' repair cracks in large castings.

To ensure that the drilled holes do not accidentally go right through the neck block, a full size layout drawing of the neck block is first prepared so that the correct drill depth for each position may be determined. Three or four plugs will be fitted for the repair.

The holes will be drilled with a 6 mm diameter spur drill to provide a clean accurate hole - using a hand operated drill for maximum control. The correct depth for each hole is marked with a piece of masking tape.
The glue used to fill the crack - as previously reported - is only to provide a solid matrix to allow the plug holes to be drilled accurately.

The correct diameter for the plugs is 5.94 mm (15/64 inch) allowing a smooth sliding fit for rapid assembly - necessary when using hot hide glue. The plugs are made from standard 1/4 inch diameter dowel shaved to the correct diameter by hammering the dowel though a 15/64 hole drilled in a metal plate. Each dowel is then 'pointed' with a pencil sharpener - again to facilitate rapid assembly.

Plugs are to be assembled one at a time allowing 24 hours in each case for the hide glue to fully cure before drilling the next hole. To instal, each plug is dipped into hot hide glue and immediately inserted into the plug hole - with a slight twisting action - to the full depth of the hole. Speed is of the essence here - two to three seconds before the glue gels making it then impossible to move the plug.

So plug #1 successfully fitted and glued - so on to the next one ...

The damaged edge of the bowl is to be cut back about 1.5 mm in depth to remove the chips, scars and file marks.
As it would be impossible to find wood to exactly match the side ribs, edging strips of black African Ebony will be inlaid and glued in place to create a new edge.

Two ebony strips have been prepared - cut from a 1.5 mm thick sheet on a bandsaw with a fine tooth metal cutting blade.
The sawn edges have then been smoothed and each strip reduced to a uniform width of 2.2 mm using a thicknessing tool. This is simply a block of hardwood with slots of different depths cut across its width. The strips are thicknessed by placing each strip in a slot of the required depth, holding a block plane over the strip at an angle and then pulling the strip through the slot. The plane does not move during this operation.

Each strip will be hot bent to exactly fit the profile of the bowl.

Three repair plugs have now been glued in place to bridge the large open crack in the neck block. The plugs have been cut flush with the face of the neck block (Lee valley Flush Cut saw) and the face filed flat with a small rasp.
The third and final step of the neck block repair is to fit and glue to the face a reinforcing plate of quarter sawn Sitka Spruce 4.5 mm thick. The profile of the plate was determined with a paper 'rubbing' and the shape transferred onto a spruce blank and cut to size with a fine bladed fret saw.
The plate has been glued with hot hide glue and held in place with - yes - veneer nails! The nails will be removed after the glue has fully cured.
After trimming the plate it will be marked with my maker's brand to confirm that this addition is not an original feature - to those who may be repairing the oud in future.

The repair plate has been trimmed to size, veneer nails removed, reinforced with paper strips and marked with my maker's brand to complete the job. The top edge of the plate has been chamfered so that it does not contact the soundboard. The reduced volume of the bowl due to the plate is insignificant.
Note that the bowl cross section is not perfectly semicircular but is deeper.

Now that the cracks in the bowl and the neck block has been repaired the bowl can be more easily handled without risk of further damage.

The final step will be to cut back by the top edge of the bowl by about 1.5 mm and fit the ebony repair strips.

The two ebony strips for the repair of the bowl edges have been hot bent to the bowl profile (traced onto a piece of card to avoid handling the bowl unnecessarily).

My bending iron is made from a length of heavy walled brass tubing 'squashed' to an oval cross section in a vice. An adjustable slot fitted at the top of the bending iron is designed for bending lines or purfling. The bending iron is heated with a propane torch.

The ebony strips were taped together at each end and the slot in the bending iron adjusted so that the strips were a loose sliding fit. The strips were moistened with water before being bent.

Next to plane down the damaged edges in preparation for fitting the strips.

To cut back the edge of the bowl to remove the damaged areas tape reference markers were first placed around the edge as a guide.

The edges were then cut back to a smooth surface using a mini plane and spokeshave (available from Lee Valley Tools). These are used with one hand the other hand supporting the bowl against flexing.

At the neck block a rebate was marked to the required depth (1.5 mm) with a purfling cutter and then cut out with a chisel. To make a cutter like this check out 'Make a Purfling Cutter' on this forum - posted back in 2007..

After planing the edge of the bowl it is finally levelled using a flat board with sandpaper glued to one side.

The hot bent ebony strips have been glued to the levelled edge of the bowl with hot hide glue. Glue is first applied to the bowl edge and each edge is then ironed in place - the hot iron remelting the gelled glue. This is done bit by bit - taping each strip down with masking tape as the ironing proceeds. The pieces of tape are spaced about 20 mm apart so that it can be verified that the strips have been properly glued in position with some glue 'squeeze out'.

When the glue has cured the tape will be removed and the inside edge of each strip will then be reinforced with glued paper strips.
The edging appears to be perfectly fitted but one advantage of using ebony as a line or purfling strip is that should there be any imperfections in the joint these would not be visible.

Finally, the ebony edging will be made perfectly level using the sanding board - ready to receive the sound board.