Sound Post Crack

If a violin is has the misfortune to develop a crack in the
sound post area this will effect the instruments performance
dramatically .


As the sound post tension is critically important to the
acoustic response of the violin a crack in this area will result
in the instrument becoming unplayable.


In order to repair a sound post crack the  violin Top must be
removed, and a “Support Cast “ made to assist in the fitting
of the repair patch.


The following article is a brief compilation to illustrate the
stages required in carrying out this repair.
The method described here is only one of many and may
differ in a number of details to alternative methods.










Prepare and shape patch.
  
Firstly we must Design the Sound post template  to suit the
individual Crack conditions  , The repair patch must cover a
sufficient area of the "Violin Top" to distribute the
sound-post force and protect the damaged area from
reopening.



The Patch wood must be selected to match the wood in the
violin Top grain . Using Seasoned wood is essential to
eliminate distortion of the patch with time.





Prepare area to patch
    
The Sound post crack in the Violin Top is cleaned and
re-glued before the Patch is fitted.


The position of the patch is very carefully calculated and
marked, both on the Patch and the Violin Top to facilitate
accurate location of the Patch during the fitting process.


Locate the Violin Belly plate into the Support Casting , using
 the small wooden Clamps .


Gouge initially inside the Patch outline mark, ensuring the
patch hole develops a smooth gradual curve leading to the
centre of the patch resulting in a central area which is
relatively flat.








The Patch is shaped by using chalk to highlight the contact
points , these high points are removed with a knife, and once the contact area fits 100% with the patch the patch is ready.


Leave  approx.1mm belly plate thickness at the center of the
patch.

Scraping the patch smooth and curving gently to the outer
outline.
DO-NOT   scrape past the Outline as this will result in a poor
outline for the patch


Fit the patch to prepared area
    
Glue Size the Patch once it has been prepared, then re-scrape
smooth once fully dry. This will ensure that the pores in the
wood are saturated by the hide glue and will help to make a
better glue joint when fitting the Patch. 

The patch must have a 100% surface area contact with the
hole to ensure an effective glue joint.     
             Patience is a virtue!!!!



Glue patch , shape and Finish
    
Once the Patch is fitted , Test Clamp the Patch in position
and prepare the Clamping method before gluing.
A  wooden insert in the Bottom of the plaster cast will
support the Casting , preventing Cracking During the
clamping process.
Great care must be taken not to use too much clamping force
when gluing the patch into position as it may distort the
instrument top .
 Use Strong thin Hide glue and  allign  the Patch to the
Location marks on the plate.Leave the Patch Clamped for at
least 8 hours before removing the clamp.






Shaping the Patch -
      
Cut back the Patch using a gouge or Thumb Plane initially.
Use a Scraper to Finish off the patch to the Desired Thickness
 approx. 3.2mm under the sound post position.










         Finished Sound Post Patch