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Join the Plaster Party!

For many years we, especially Usa von Stietencron, have gained experience in working with plaster casts in repairs as well as during extensive restorations. By now there is literally no crack on ribs as well as on plates which we don’t glue without the support of a cast. This might sound time consuming and stressful at first, but as a result, getting cracks level has become so much easier, thus in the end saving a lot of time and nerves!  

We often faced problems reassembling instruments in the past, even though we had plaster casts of front and back. It was difficult to get an even overhang without adding a lot of stress on the instrument or altering the height of the ribs. Now, we always make the first casts of front and back on the closed instrument. With this method we can be sure that when closing the instrument, the freshly glued cracks and patches are under no stress or tension. Making a cast is much easier than you might think – it's fun and the advantages outnumber any initial training time. - Trust us! Once plaster casts are part of every repair and restoration job in your workshop, you will realise the initial extrawork will pay off as you encounter less surprises and achieve more stable results by limiting the amount of tension on the closed instrument while actually saving time and hassle.

Casting closed instruments: How to get started

 Start by preparing a mould and a frame – the frame to fit around the ribs and the mould around the plaster. We use XPS polystyrene which is used for insulating houses, you can get it at your nearest DIY shop (XPS = extruded polystyrene). We ’cut’ the material with a metal wire that we mount into a electric scroll saw. It doesn’t actually cut but melt its way through the foam due to the frictional heat - the Acusticus cello tailpiece wire works very well! 
pic. 1: scroll saw with the acoustics wire mounted

Preparations

Make sure the rib frame fits snugly around the instrument without distorting it. The idea here is for the frame to suspend the plate from the under edge. As for the mould (4-5 cm would be a good height for violin, 6-7 cm for a cello) an ideal outline is about 7mm larger than the plate. Make sure you don’t undercut the outline near the corners in the C’s, so the frame can be removed from the plaster without it breaking. 
As polystyrene is not exactly environmentally friendly, we keep old frames and especially for violins and violas it is possible to reuse old frames for several instruments. With cellos this is a bit more tricky. The advantage of the plaster being only 7mm larger than the outline, especially on cellos, is that your clamps reach in further and you save weight and plaster. Protecting the instrument from the plaster material is also a very important topic. We highly recommend using latex, which can be easily acquired from online shops (fourdrubber.com or latexwork.com or your local latex dress manufacturer). We use the thinnest latex available, 0.15 mm. After casting, clean the latex sheet with a damp sponge, let it dry and store it away from heat and UV light so you can use it for your next casts.

Plaster Time

We use type 3 dental plaster (DIN EN ISO 6873 - we use the brands Modelit and Moldano Blue) which has 6-8 minutes working time and 8 -12 minutes until it settles. The expansion of this plaster once hard is about 0.17%. There are plasters with less expansion but since we developed a new way of arching corrections, this is not an issue for us and the less expanding plasters come with their own set of disadvantages.

Mixing: We use the plaster in a thinner consistency than the manufacturers standard which does not have any effect on the final hardness of the cast. It should be like thin yogurt. Our ratio is 1 : 0.37 so for one kilogram of plaster we plan 0.37 litres of water. For a typical violin cast you need 3 kilograms of plaster which mix with 1.11 litres of water. [picture 8]
Pour the water into a clean bowl or bucket (no residues of soap or the like!), then add the plaster by pouring it steadily into the water.  
[picture 9]
In the end you will build up a small heap of plaster raising from the water. If you are doing a Cello cast you will need to prepare two buckets and you can actually leave one bucket by itself while you are mixing the other one. The water will slowly work its way through the dry plaster. Before mixing wait for 2-3 minutes to let the oxygen escape from the plaster, this will make the mixing much quicker. [picture 10] 
Then, get your hands in there, don’t use a whisk or a machine, just feel the lumps and mix them into the mass. Keep track of the time though, within 6-8 minutes you should finish the mixing and get onto pouring. Once you have achieved a homogenous mass, you can gently bang the bucket on the floor to get some airbubbles out, but actually this will be more effective once the plaster is on the instrument. 
Pouring the Plaster: [picture 11] Pour the plaster on the latex covered front, starting on one side so that no air gets trapped in the middle. Once all the plaster is inside the frame, gently shake the whole construction or knock on the wooden panels under the clamps with a hammer, the air bubbles don’t need to come out, we just want them to raise from the surface of the plate. [picture 12] Now you have to wait, but keep checking the cast to be able to react before it gets too warm for the varnish. [picture 13] 
Flipping: For a cello cast you will need a second pair of hands and a board and clamps to hold everything together. Violins are easily flipped by one person. Once the cast starts getting warm, remove the clamps. Now flip the whole construction and rest the cast on foam blocks. You can then lift the instrument with the rib frame. [picture 14+15] Leave the latex on another 10 minutes while the cast gets hotter. In our experience the surface of the cast gets nicer if you have that extra patience. Then remove the latex and the frames from the plastercast. [picture 16 + 17 + 18]

Cast Preparations

With a ruler scrape the underside of the cast flat so the clamps have a good surface to clamp on. This is important since during repair and restoration work we only put a little piece of cork in between the clamps and the plaster. Cut down the plaster outside the outline and leave about half of the edge thickness. [picture 19] 
Other Must Do’s: As the cast expands 0.17% in every direction, it’s necessary to remove all outside edges where plates could get stuck (f-holes on front, C-bouts in hook of corners on front and back). [picture 20] 
Corrections: You can do some corrections on this cast like getting rid of steps in crack areas or lifting up some sunken in regions of the arching, but keep in mind that anywhere you want to bring the arching up, it will want to go down in another place, so with only doing one cast you cannot do much corrections. If you want to delve (or dig) into arching corrections, watch this space as we are preparing another article on our unconventional method for this.

Glueing cling film into the cast : Start with spraying on the paper underlying your cast, then once the glue comes out evenly without bigger bubbles, pass over the cast vertically and horizontally. [picture 21] Less is more! Then place the cling film on top with some tension and gently rub it down with a piece of fine cotton cloth [picture 22]. We use ’3M Spray Mount Repositionable Glue’. In case the glue layer is too thick, it can also be removed with a special ’3M Cleaner Spray’ or acetone. Depending on your schedule you can glue in the cling film straight away or wait until the cast is fully dry. At any time of this process you can in fact decide to glue in the cling film which protects the varnish from the plaster and potential moisture. Trim off the excess cling film with a piece of rough sandpaper moving around the outside edge of your cast. 

IMPORTANT! Pour some talcum powder onto the cling film and disperse it evenly (a light dusting will be enough). This prevents sensitive varnish from sticking onto the cling film after glueing a crack. This should always be repreated before glueing the next crack.


This was a short explanation in which we tried to cover all the crucial parts, but do not hesitate to contact us if any questions arise. We have also done loads of variations of this technique, e.g.: If an instrument comes in which needs crack repair but the neck and fingerboard are fine, you can actually cast the instrument with neck and fingerboard still on. [picture 23 +24] If you need a full cast to work with, you can do an inside cast of the front and then another final cast from that inside cast. This is actually how we do arching corrections, but that is another story, which we will hopefully tell in the not too distant future!

Credits: Thank you to Martin Rainer for providing us with professional photographs and Brian de Boer for a thorough read through and helping with the english wording.

Text:  
Hanna Bozzetta (violin maker and restorer) hanna@bozzetta.org  
Usa von Stietencron (violin maker and restorer) usa@vonstietencron.com  
Pictures
Martin Rainer (violin and bow maker and restorer) info@martin-rainer.at