Back crack plus hole repair

[catnip]

This violin suffered from a fall and I am interested in how to repair it.  Weisshaar goes into detail about a through patch for the top (table) and the process of matching grain lines.  The two small missing pieces were found inside the violin so hopefully they will fit without having to match the flame.  I am thinking about removing the back to do a plaster cast and a reinforcing patch from the inside.    Can this repair be done by just removing the top and making a plaster cast with the back in place?

I have done a brief search to see if similar repairs were done but did not find any.   Suggestions and and advice would be appreciated,

   

[Mark Norfleet]

It can be done by just removing the top, but that will make access and clamping somewhat more difficult and wouldn't be worth the small advantage for me.  I've done a similar repair.
It's good that the missing pieces weren't!  Good Luck.

[jacobsaunders]

If you are going to repair the back, you need to remove the back. If you want to make a cast, you will need to puzzle the bits in first, because a cast of a hole is of little use. I hope you have the missing bits, otherwise its probably not worth doing.

[David Burgess]

12 minutes ago, jacobsaunders said:

 If you want to make a cast, you will need to puzzle the bits in first, because a cast of a hole is of little use. I hope you have the missing bits, otherwise its probably not worth doing.

Cling film, rubber dam, or foil (whatever will be used to cover the back anyway) can be used to bridge the gap when making the plaster cast. It will leave a bump in the cast where the weight of the plaster deflected the film into the hole, but that can be carved away.

[jacobsaunders]

23 minutes ago, David Burgess said:

Cling film, rubber dam, or foil (whatever will be used to cover the back anyway) can be used to bridge the gap when making the plaster cast. It will leave a bump in the cast where the weight of the plaster deflected the film into the hole, but that can be carved away.

Probably easier to make a new violin than do it arse over tit

[David Burgess]

6 minutes ago, jacobsaunders said:

Probably easier to make a new violin than do it arse over tit

Yes, if the violin isn't very expensive, and is insured, I'd probably "total" it.

Mark, how would you do this? Make a cast, correct the cast, glue the pieces in (as well as the other cracks), re-soften the glue used on the broken area, and vacuum bag it into the cast?

[jacobsaunders]

1 hour ago, David Burgess said:

 how would you do this? Make a cast, correct the cast, glue the pieces in (as well as the other cracks), re-soften the glue used on the broken area, and vacuum bag it into the cast?

The longer one works with plaster casts, the more important one finds getting the respective plate as near perfect as possible first, and then making a cast. Glueing such jagged breaks in maple all the more so, since it isn’t simply a case of pressing it downwards into a cast, but one will have to shove the jagged bits in all sorts of directions to get the breaks adiquately back together. Would be fascinated if Mark has any brilliant patent solution.

[catnip]

Thanks for all your suggestions.   I will remove the back in any case and take it from there.   Will update the progress later

[Mark Norfleet]

I've spent waaay too much time thinking about this repair for one that's not in my shop.

I notice in the farthest point to the left in the photo of the damage that there seems to have been a point of impact.  Hopefully there isn't too much wood missing and it's mostly compressed and can be swelled back out.

I cannot know for sure how I would do this repair without having it in my hands, and suspect that my approach would change during the repair depending on the sequence of break glueing I decided on.  I would probably make a plaster cast before doing any gluing, but agree with Jacob that in most cases it's best to do as much pre-casting correction as possible.  Sorting out the proper sequence for gluing things and thinking ahead is important and will reduce  difficulties later on.

For the similar repair I did I used a variety of techniques, including gluing a few things in the plaster mold, but no more than I had to.  There was a 30x30mm piece that was fractured and separated from the rest of the back.  I used pillars and clamps on that piece to get those cracks glued before putting it in place and dealing with all the smaller bits.  I used the pillars or lower blocks glued to the inside a lot since I couldn't clamp against broken edges without causing damage.  I don't recall if I used a vacuum bag for gluing anything, but think I might have used one to hold the back into the mold while doing the preliminary fitting of the patch that reinforced the worst of the broken area.

I'm not sure the idea that David wonders about of "Make a cast, correct the cast, glue the pieces in (as well as the other cracks), re-soften the glue used on the broken area, and vacuum bag it into the cast?"  would work in a situation like this.  I think there would be so much swelling of the wood in the process of getting the glue to soften that the area would grow significantly and buckle.  Vacuum bagging, at least alone..., would not provide enough pressure to overcome this.  I suspect that there's also a good chance that other damage could occur as a result.  I wish it could be that easy.

As an aside, I usually only use vacuum bagging with a plate on an internal plaster mold for very difficult biased cracks at an acute angle to the varnished surface.  I do this in large part to avoid compression and subsequent deformation of the wood at the crack due to glue saturation.  Experience and experimentation is critical.  This method is particularly fun with cellos... 

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Edited December 1, 2020 by Mark Norfleet
An attempt at clarity.

[Mark Norfleet]

This was my similar project.  I believe David Burgess has posted this picture in the past.

[jacobsaunders]

54 minutes ago, Mark Norfleet said:

This was my similar project.  I believe David Burgess has posted this picture in the past.

Have you done it yet?

[Mark Norfleet]

Yes, stuck back together years ago.  I just hope the young owner payed attention when I told her that the varnish in that area needed to be maintained.

[Pate Bliss]

It seems to me, from an aesthetic point of view, that unless the repair looks just perfect, a different kind of repair that does look perfectly done might be preferable?