Jump to content
Maestronet Forums

Cello sound post patch tips


Guido

Recommended Posts

After many sound post patches in both violin tops and backs I’m now looking at a cello top.

for the violin I made a vacuum box that hooks up to the shop vac and my plaster casting is pretty honed in.

never really thought about how to do the same thing for a cello. I assume a local counter form of wood and chalk fitted to the relevant area of the top would be the way to go?

Can anyone share tips/ tricks/ experience installing a sound post patch to a cello top?

Link to comment
Share on other sites

I have done many violin patches, for which I make a plaster cast of the whole plate, using a latex rubber sheet as a protector.  I have only done several cello patches, for which I made partial casts.  I make a rectangular-shaped dam for the plaster out of four strips of thin plywood, 1/4 or 1/8 inch thick.  The edges of the strips that contact the plate need to be cut so that the rectangle, when it is assembled, will fit fairly closely to the arching.   Since I was doing top soundpost patches, I made sure that the casts included some of the F hole and the edge, so that I could use these features to locate the top on the cast during the patching procedure.  I joined the strips into the rectangle by wrapping plastic packing tape around the outside.  I held the rectangle in position on the plate with masking tape.  I draped the latex over the tops of the strips and down onto the plate inside the rectangle.

I have never used vacuum, but I think it might be useful.  Could you give more details about your vacuum box?

Link to comment
Share on other sites

When making a cello post patch, there are often pecuniary questions as well. A complete plaster cast for cello is unpractical, because it is so large and heavy. It takes up the whole table, and most cramps don’t reach far enough. In many cases one can successfully use a thermo-plastic dental counter form, although I would want to see your cello before expressing an opinion

Link to comment
Share on other sites

When I glued some ugly top cracks on a cello few years ago I used partial cast (including part of f-hole). I used thin foil (cling film?) stretched and applied to the surface it will copy incredible detail even without vacuum and created dam out of bricks of modelling clay (they were wrapped in foil as well to protect them from contamination and for future use). Tehn standard plaster pour and removal immediately as it hardens enough.

Link to comment
Share on other sites

6 hours ago, Brad Dorsey said:

I have done many violin patches, for which I make a plaster cast of the whole plate, using a latex rubber sheet as a protector.  I have only done several cello patches, for which I made partial casts.  I make a rectangular-shaped dam for the plaster out of four strips of thin plywood, 1/4 or 1/8 inch thick.  The edges of the strips that contact the plate need to be cut so that the rectangle, when it is assembled, will fit fairly closely to the arching.   Since I was doing top soundpost patches, I made sure that the casts included some of the F hole and the edge, so that I could use these features to locate the top on the cast during the patching procedure.  I joined the strips into the rectangle by wrapping plastic packing tape around the outside.  I held the rectangle in position on the plate with masking tape.  I draped the latex over the tops of the strips and down onto the plate inside the rectangle.

I have never used vacuum, but I think it might be useful.  Could you give more details about your vacuum box?

Thanks for spelling it out. Clicked for me. I had my head locked in thinking about wrapping the barrier film around the violin, rather than around the plaster. Makes no difference for a full cast, but is obviously the key ingredient for the partial cast.

I guess one of these countour gauges is very useful to get the shape of the top onto the lower edges of the dam boards.

IMG_4125.JPG

Link to comment
Share on other sites

7 hours ago, Brad Dorsey said:

I have never used vacuum, but I think it might be useful.  Could you give more details about your vacuum box?

I followed the design in the IPCI books, which was also discussed here in some detail a few years ago. See if you can find it before I type it up again :-)

With hindsight, the results aren't exactly revolutionary.

Link to comment
Share on other sites

4 hours ago, jacobsaunders said:

In many cases one can successfully use a thermo-plastic dental counter form, although I would want to see your cello before expressing an opinion

Thanks, I'll ask my dentist when I'm back home tonight. If she would only get the kids to bed a bit earlier...

Regarding the cello in question, it's not mine so I won't post it here. It's from the 1920s and apart from the sound post crack in very good condition. The crack is right on target, and extends about 2 cm in front and maybe 4 cm behind the bridge foot. It's old, black, but fairly closed. Apart form properly cleaning it I don't expect any complications.

What's the relevance of the cello or crack for the use of the dental thermo-plastic vs. the partial plaster cast?

 

Link to comment
Share on other sites

14 minutes ago, Guido said:

Thanks, I'll ask my dentist when I'm back home tonight. If she would only get the kids to bed a bit earlier...

Regarding the cello in question, it's not mine so I won't post it here. It's from the 1920s and apart from the sound post crack in very good condition. The crack is right on target, and extends about 2 cm in front and maybe 4 cm behind the bridge foot. It's old, black, but fairly closed. Apart form properly cleaning it I don't expect any complications.

What's the relevance of the cello or crack for the use of the dental thermo-plastic vs. the partial plaster cast?

 

A cello cast is a lot of work. If a cello literally only needs a soundpost patch, a cast seems a bit out of proportion, particularly if you are having a third person pay for it. On the other hand if there are several other things one needs it for, then one could think about it. One buys thermoplastic dental compound from a dental supply depot, not a dentist. 

Link to comment
Share on other sites

8 minutes ago, HoGo said:

Jacob, what is the temperature needed to make the stuff plastic?

I just warm it up till it melts. I don't have a thermometer

something like this Thermoplastische Abformmassen | Abformung | Praxisbedarf | denteris - ONLINE.DENTAL.DEPOT

there are different brands in different countries

Link to comment
Share on other sites

9 hours ago, Brad Dorsey said:

...I draped the latex over the tops of the strips and down onto the plate inside the rectangle...

 

2 hours ago, Guido said:

...I had my head locked in thinking about wrapping the barrier film around the violin, rather than around the plaster....

 Of course, when the latex is draped over the strips, it bunches up in the corners, leaving impressions in the cast.  As a result, you don't get as good an impression around the edges of the cast as you would if you were casting a whole plate.  So you might want to make the cast a little bigger to allow for the rough edges.

Link to comment
Share on other sites

On 10/24/2023 at 11:33 PM, Guido said:

After many sound post patches in both violin tops and backs I’m now looking at a cello top.

For a cello sound post patch, I make a cast across the center area of the top including the four corners, as I do end up using a clamp here and there when the top is in the cast to ensure the plate remains stable and true (especially around the treble f).  I use the thinest commercially available latex sheet and 2" insulation board (pretty common for casts these days) for the form with two strips of it cut to the contour of the arch. The stuff is easy to cut on the bandsaw, so the form and contoured strips go very quickly.  I've found a dental casting plaster/stone that is quite strong and has a very low expansion rate. This allows me to pour a slightly thinner cast than I could get away with using Hydrocal and other plasters... plus it cures and sheds moisture faster.

Below is a photo of a patch supporting a compound soundpost crack in a 'cello. My daughter (who I've been training) did most of the work, as she hadn't done one in a cello before.

 

Patch(1).jpg

Link to comment
Share on other sites

Thanks. Good looking patches seem to run in the family.

Talking a cast across the whole chest seems a good idea and is still only about the size of a full violin cast.

19 hours ago, Jeffrey Holmes said:

For a cello sound post patch, I make a cast across the center area of the top including the four corners, as I do end up using a clamp here and there when the top is in the cast to ensure the plate remains stable and true (especially around the treble f).  I use the thinest commercially available latex sheet and 2" insulation board (pretty common for casts these days) for the form with two strips of it cut to the contour of the arch. The stuff is easy to cut on the bandsaw, so the form and contoured strips go very quickly.  I've found a dental casting plaster/stone that is quite strong and has a very low expansion rate. This allows me to pour a slightly thinner cast than I could get away with using Hydrocal and other plasters... plus it cures and sheds moisture faster.

Thanks. Good looking patches seem to run in the family.

Talking a cast across the whole chest seems a good idea and is still only about the size of a full violin cast.

Regarding the plaster/ stone, could you share particular product and how thick your cast would be in the centre?

And if it's not too much rouble, what would be the weight of your cast? Obviously a particular cast may be a bit bigger or smaller and the arch will vary, but it'll still be useful to estimate the quantity of product to mix up.

I have this at hand:

IMG_4134.JPG

Link to comment
Share on other sites

21 minutes ago, Guido said:

Thanks. Good looking patches seem to run in the family.

Talking a cast across the whole chest seems a good idea and is still only about the size of a full violin cast.

Thanks. Good looking patches seem to run in the family.

Talking a cast across the whole chest seems a good idea and is still only about the size of a full violin cast.

Regarding the plaster/ stone, could you share particular product and how thick your cast would be in the centre?

And if it's not too much rouble, what would be the weight of your cast? Obviously a particular cast may be a bit bigger or smaller and the arch will vary, but it'll still be useful to estimate the quantity of product to mix up.

I have this at hand:

 

The product I use is called Tecstone,  From what I can tell from the label you illustrated, I believe the specs are similar.  How it handles may be different... no way to tell without actual casting comparison.  

I've found a 2" form will allow a cast that's plenty thick in the center, even with "moderately" high arch. On flatter arches, I stop the pour slightly early.  I've honestly never weighed the cast. It's manageable, however. A slightly thinner cast is easier to get the clamps over though.

Link to comment
Share on other sites

On 10/25/2023 at 1:59 PM, jacobsaunders said:

I just warm it up till it melts. I don't have a thermometer

something like this Thermoplastische Abformmassen | Abformung | Praxisbedarf | denteris - ONLINE.DENTAL.DEPOT

there are different brands in different countries

I abandoned thermoplastic cast material because it gets softer over time. But maybe I used a bad brand. How is your experience with that?

So now over time I got pretty quick in making a partial plaster cast. ( and don’t burn my fingers at the hot dental compound.)^_^

Link to comment
Share on other sites

I was never comfortable with larger partial casts, especially with cellos.  I've never had an accident, but having so much of the plate hanging unsupported off the relatively heavy plaster cast always made me nervous.  I also found making dams for each cast inefficient, so I made re-usable varnished plywood dams for violin, viola and cello decades ago.  Making a full cast is pretty quick and easy for me.  Yes the cello casts are big and take up a fair bit of bench space but it's worth it to me.  To get clamping access I simply cut away the areas which are in the way.  That said, if I didn’t anticipate making many cello casts, I would just make a partial cast for a soundpost patch.

It's been many years since I've used thermosetting dental compounds for small areas.   Putting heat on varnish sometimes yields unfortunate results.  These days for casting small areas for button patches or clamping cauls I use a polyester catalyzing wood filler.  It produces some heat as it cures, but If I'm paying attention and remove it at the point it gets stiff enough to hold its shape, all is well.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
  • Recently Browsing   0 members

    • No registered users viewing this page.


×
×
  • Create New...