Making a partial plaster cast

Some repairs require a plaster cast. However, it is not always necessary to make a cast of the entire plate. Someone recently brought to my shop a violin that had been damaged by a large falling object. Luckily it had only hit the corner, but there were several parallel cracks quite close together, as well as some cross-grain cracks (right).

After removing the top plate I pushed everything in place as well as I could, but the result was not satisfactory. The cracks were too close together to set clamps successfully, and the cross-grain crack would not settle back smoothly where the piece had torn away from the purfling. I therefore decided to make a partial plaster cast of the top plate. This allows for corrections, and forms the perfect counterpart when it comes to clamping.

Drawing the outline on the polystyrene

1 I use a 5cm-thick polystyrene sheet to contain the plaster when casting. I place the plate on the polystyrene and draw the relevant section of the outline, ensuring there is at least 3cm spare all the way round. I am always careful to include either a corner, an f-hole, the button or the neck recess, whichever part of the plate is being cast. This ensures that the plate will fit snugly without shifting every time it is removed from and returned to the cast.

Cutting out the mould A piece of material protects the top Tidying up the mould

2 Having drawn the outline, I cut out the waste using a fretsaw. The resulting hole is the size of the future partial cast (2a). To contain the plaster, I take the shape of the arching with a contour gauge. Then I place a strip of material on the top plate to protect it from dents or scratches (2b). I draw the arches on to the sides of the polystyrene, cut them out with the fretsaw, and tidy it all up with a rasp (2c).

The mould is secured on the plate

3 Next I place the top plate on to a flat board and glue it lightly to ensure it does not distort. Then I cover the plate with a latex sheet (or some other thin foil) which acts as a barrier between the plaster and the varnished top. (I never use oil or vaseline as a barrier, as the process is not safe and the fluid can easily get on to the areas to be glued.)

I place the polystyrene shape over the plate and locate it exactly, clamping it down lightly at the corners. Any gaps between the polystyrene and the plate can be sealed with small pieces of modelling clay, to stop the plaster running out. To support the finished cast, I make a plywood backing board which fits into the recess. Two screws with only a couple of turns serve as handles.

The granules represent the amount of water to be used

4 Now I need to work out how much plaster w ill be needed for a partial cast. I find that a ratio of one part water to three parts plaster results in a creamy consistency that pours well.

I place a thin bag into the casting area, and then fill the recess with something granular. Rice, sand or (in this case) glue granules can be used, depending on the thickness of the backing board.

The amount of water needed will be 200ml

5 The height of the granules in a glass m easuring jug represents my one part of water, in this case 200ml.

Weighing out the plaster

6 I weigh 600 grams of Moldano Blue plaster and pour it into the water while stirring continuously. It is important to sprinkle the plaster powder slowly enough for it not to form lumps, but fast enough to prevent the mixture from starting to set. Mixing takes about a minute, and then I have four minutes to pour the plaster into the mould before it begins to harden.

The mould filled with plaster

7 Before pouring, I knock the mixing jug on the w ork surface a few times, to encourage any air bubbles to rise to the surface and pop. Then I pour the plaster gently into the mould, making sure it runs into all the corners and fills up the recess, allowing a bit of room for the backing board.

The restored corner prior to gluing

8 I give the assembly a gentle sideways shake so that the surface levels out, before placing the backing board on to the plaster and pushing it down softly. Separately, I place a dollop of leftover plaster on to a scrap of wood, just so I can observe the setting process.

Moldano Blue sets in about twelve minutes and at this point it becomes quite warm.

The backing piece in place

9 Now is the time to take away the clamps from the polystyrene, r emove the screws from the backing board, place another smallish board on top of the assembly and flip the whole thing over. The flat board with the plate can now be lifted out, the latex peeled away and the plaster allowed to set further. Once the plaster has cooled down, the polystyrene can be removed by gently bending it away from the plaster until it is freed. Save the mould; it can be reused in the future for other, similar projects.

After about six hours, the plaster is firm enough to be shaped. Any correction that needs to take place can be carried out within the next 48 hours. After that, the plaster sets so hard that any attempt to correct it further will ruin your tools. But if necessary a second cast could easily be produced from the same mould.

When using the cast it is important to protect the varnished plate from moisture still held in the plaster by using cling film.