Repairing a bow thumb groove with epoxy and modelling clay

Makers reveal their special techniques

A non-invasive method for a fairly common task in bow restoration

Violin and bow maker based in Thessaloniki, Greece

One of the most common forms of wear on a bow comes at the point where our thumb touches the stick. Constant rubbing of the pernambuco with the thumb, in addition to excessive or acidic sweat, causes the wood to wear down, leaving a groove, in this case, a very big one. Restorers can fix this kind of wear using a two-component transparent epoxy resin, also called liquid glass.

One of the advantages of this technique is that all of the original wood will remain intact. It is not filed or levelled, just cleaned very well. The epoxy is durable and it will hold for many years. And if a restorer would ever like to bring the bow back to its original state before it was repaired, one can easily destroy the epoxy little by little, and when the thickness is very small, peel it right off the bow.

1b

1a A very big thumb groove on a cello bow

1 In this particular example I worked on a cello bow by the 20th-century bow maker Louis Henri Gillet. This is one of the biggest grooves I have ever seen on a bow.

Silver wiring unwrapped and secured higher round the bow

ALL PHOTOS IRIS ZHULLA

2 I gently remove and put away the old leather. Then I unwrap 1mm of the silver wiring and secure it higher around the bow with insulating tape. I clean the area multiple times, first with alcohol and then acetone.

3a Using pegs to stabilise the bow at a desired angle

3b The sticky tape leaves just the groove exposed

3 For this restoration I stabilise the bow with two pegs, to keep my hands free and to be able to turn the bow to whatever angle is needed to keep the level of the resin perpendicular to the groove’s depth.

The next step is important. I have to determine the surface that I want my epoxy to fill. One would think it is obvious, but sometimes it can trick you. You can easily see the groove in the centre, but you must make sure you have also found where the ends of the groove really are. The first few times I tried this technique, I found I was missing just a small amount of epoxy thickness round the ends of the groove, and I had to do it a second time just to fill those tiny areas.

I examine each of the facets of the octagonal section for wear; up to four of them can be affected. Then I mark the whole affected area with a soft white pencil, adding 1mm all around. Just like in any other repair, I try to not touch the original wood at any areas where it isn’t necessary. That’s why I also add doublesided sticky tape around the area, right on the white line. This will ensure that my epoxy won’t run out at other parts of the bow and that the modelling clay will not slip or slide if I have to use some heat later on.

Creating a nest for the epoxy with modelling clay

4 I start to build the nest for the epoxy with modelling clay, which can be found at any art store. First I use a small spatula or a rounded short-blade knife to set the base of the nest, right at the tape line. Then I start to build up the nest until it reaches the appropriate height (about 5–7mm). I am careful not to leave any paths where my epoxy can run on to the rest of the bow.

When I am happy with the result I take a pointed Q-tip and clean the wood inside the nest with acetone. Then I let it evaporate.

Where the crest of the facet will be formed

5 Now it’s time to prepare the epoxy resin. For this job I used Pébéo Gédéo two-component transparent epoxy. I have tried different resins and I find that this one sticks well to the wood, and is very resistant.

Be aware of the amount of epoxy you will pour into the nest. The first time I experimented with this technique I made the mistake of putting not quite enough epoxy into the nest. The epoxy tends to rise towards the walls of the nest and be hollower in the centre – but the centre needs to have enough epoxy, as that’s the point where the crest of the bow facet will be formed (as shown by the pink line). Sometimes you would think it’s enough but then checking with a straightedge you realise that there is material missing just at the highest point, and you have to repeat the process.

Cured epoxy ready to be formed

6 I am careful not to make bubbles when mixing and transferring the resin. If I transfer my epoxy from the mixing container into the nest little by little, there won’t be a problem. If I see many bubbles, there’s something I can do to make them come to the surface and pop, but I have to be very precise and quick: I take a small kitchen torch and place the flame on the surface of the epoxy pond for a split second. This bursts all the bubbles. I am also careful not to overheat the epoxy, as my modelling clay can change shape as a result. But it’s best just to have patience and transfer the epoxy drop by drop.

After filling my nest to the desired height, I let my epoxy dry for twelve hours. The epoxy is now cured and ready to be formed just like any wood. First I take off all the modelling clay and double-sided tape with a knife.

Working the epoxy with files

7 Then I use chisels, planes and files to form the facets on my epoxy. I am very careful when working on the ends of the epoxy, where the thickness is close to zero, because those are the weak points; the resin can start peeling off if I am not careful.

Using a straightedge to form the facets correctly

8 I work with a straightedge to make sure my facets are well formed and at an adequate height. I then use different grits of sandpaper (wrapped on a flat surface), just as if I were working with wood.

9a The finished repair

9b

9 I polish the repair with Tripoli powder and oil, and then French polish the whole frog area of the stick with shellac, and leave it to dry. Then I solder the silver wiring back in place. If the tape has left some glue residue on the wiring, I clean it with a little acetone. Lastly I make a brand new leather grip for the bow.