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FiddleDoug

Cast making

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OK, What's everyone using for making casts (full or partial) of violin plates for doing soundpost patches and other such work? Plaster, Hydrocal, dental compound, other?

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Hydrocal.... Though I really like Tecstone for some things, I'll probably pony up for a bag one of these days. With plaster I don't mess with a partial on a violin or viola (too much work). For small scale (doubling in the block area, or supporting a scroll graft ) friendly plastic or dental compound works for me.

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For years I have used nothing but plaster of Paris purchased at big building supply stores.  A few days ago, I made a full cast of a violin back for a button repair, and a few months ago I made a partial cast of a cello top for a soundpost patch.

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I use Hydrocal, mostly because it's worked well for me and I'm used to it.  Brad, you don't have a problem with shrinkage with the plaster of paris?  Doug, if your doing a cast, save yourself the time and don't do it Weisserhauer's way.  jeff

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Jeff (and others),

I do a lot of mold making in my part time job in the glass shop. We've been using foam for form making for years. Light, inexpensive, and easy to work with. For the galss work, we use a casting material called investment, which is plaster mixed with other materials to allow it to withstand the 1400 deg. F casting temperatures. This material has some of the same shrinkage problems as plaster.

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We (and a lot of people in europe) use dental plaster, hardness class 3, moldano blue if you want a name!… or equivalent. No problem whatsoever. So far…

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I use Hydrocal....  Brad, you don't have a problem with shrinkage with the plaster of paris?...

 

I have never noticed any shrinkage.

 

Where does one get Hydrocal?

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The sequence outlined in W & S is pretty complicated and the few attempts to make casts have been anything but easy; I am hoping someone who has a smooth technique would share  some tips on how to do this efficiently.  It seems to take too much time and adds to the expense of an already expensive repair.  I have done at least 10 soundpost patches, most of them without the benefit of a cast and never had a failure.  I do use a shaped work block and it keeps the top from rocking and rolling while I work it but doesn't fit any particular top.  My only attempt to use dental compound was a total failure. 

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 I have done at least 10 soundpost patches, most of them without the benefit of a cast and never had a failure.  I do use a shaped work block and it keeps the top from rocking and rolling while I work it but doesn't fit any particular top.

I'm thinkin' that wouldn't be good enough for a really fine repair. Depends on how you define "failure".

 

With experience, and materials on hand, a plaster cast can probably be made with less than an hour's labor. A dental compound or polyester resin partial cast in under 15 minutes.

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The sequence outlined in W & S is pretty complicated and the few attempts to make casts have been anything but easy; I am hoping someone who has a smooth technique would share some tips on how to do this efficiently. It seems to take too much time and adds to the expense of an already expensive repair. I have done at least 10 soundpost patches, most of them without the benefit of a cast and never had a failure. I do use a shaped work block and it keeps the top from rocking and rolling while I work it but doesn't fit any particular top. My only attempt to use dental compound was a total failure.

Check this out http://www.maestronet.com/forum/index.php?/topic/320511-casting/ That should get you on your way to something simpler than what is described in W & S. Jeff does a good job of giving an outline of a speedier process in the linked thread.

Closed cell insulation is available at most big box building supply stores - if you are desperate you can use open cell styrofoam and line it with either packing tape or duct tape. Passable latex (.006") can be had from McMaster Carr, thinner is better, but it is not regularly commercially available. When gluing the plate to the backer board you must take into consideration that plates rarely sit "flat" and will need to be shimmed with cardboard. Also, take care to provide support internally to the plate (small pieces of eraser etc so that plaster does not deflect the shape - you can cut holes in the back of the gluing board to insert objects).   Pull the latex taunt one side at a time after clamping.  You will need to release the clamps and then retighten on either end as your pour the latex in order to allow air to escape.   Never walk away from a cast after you've poured it, plaster can get mighty hot and you may need to pull the plate sooner than you anticipate.  I always start a timer to keep track of how long the plate has been under.  Remember, if your using hydrocal it needs at least a week to fully cure till it is safe to use (looses it's moisture content.)  If I recall correctly dental plasters like Moldano Blue and tecstone can be used much sooner.  

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I'm thinkin' that wouldn't be good enough for a really fine repair. Depends on how you define "failure".

 

With experience, and materials on hand, a plaster cast can probably be made with less than an hour's labor. A dental compound or polyester resin partial cast in under 15 minutes.

 

Agreed.  Casts make things a lot safer.   You're Quick David.

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   You're Quick David.

Well, that would have been with a re-usable oversized generic form made from plywood. Same thing as the foam, but more durable and less form-fitting. Didn't see much point in following the outline closely, or weight reduction on a fiddle cast. A cello cast, on the other hand, can get darned heavy.

 

An hour also wouldn't include arching tweaks prior to pouring the cast, which is the way I would always do it now, I think, if open cracks didn't preclude it. A little time spent jacking the arching around before pouring the cast can sure save a lot of time scraping or filling sections of the cast later.

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I meant quick posting. Not quick casting :-) No sooner had I sat down to respond, and you had already beat me to it.

I also agree that there are casts and then there are casts...

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UG Fiddlesmith, on 10 Jul 2013 - 9:09 PM, said:

The sequence outlined in W & S is pretty complicated and the few attempts to make casts have been anything but easy; I am hoping someone who has a smooth technique would share some tips on how to do this efficiently...

I entirely agree that the book's cast-making proceedure is unnecessarily complicated. When I get to it, I will start a new thread with some pictures showing how I do it based on the one I just did.

UG Fiddlesmith, on 10 Jul 2013 - 9:09 PM, said:

.... I have done at least 10 soundpost patches, most of them without the benefit of a cast and never had a failure...

I cannot imagine doing a SP patch without a cast made specifically for the instrument being worked on.

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Rick, In my experience, they both work fine.  What you are used to seems to be paramount for most people.  Both have very minimal shrinkage and are strong enough.  Interested to see what Jerry says.

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Rick Hyslop, on 13 Jul 2013 - 12:16 AM, said:

Jerry. What are you reasons for using Hydrocal, and what would be beneficial about using Moldano Blue as recommended in the workshop last year ?

r.

Rick,

Like Jeff said, I use hydrocal mostly because I'm used to it. I know roughly how long it takes before I need to pull a plate because the plaster is going to get too hot. I also like the fact that it is a dense hard plaster that holds up to use, but it is also a plaster that can still be worked with (corrected) weeks later.

Moldano blue is not available in the US as you may recall from Oberlin restoration 2012. Claire had to find a dental plaster that was available with similar densities and expansion rates. She found Tecstone (which is available from Clint Sales http://www.clintsales.com/dental.htm ). The Advantage that it gives, and why I would like to have some around, is that it cures incredibly fast. If I recall correctly from last year, it took roughly a day for it to loose it's moisture content and to be safe to use with a plate. ( Therefore, it can be used quickly if you need to have a faster turn around). The stuff is wickedly hard though. If you want to make any corrections they need to take place with in two days of casting. You could also argue, that since is has such a low expansion rate (as many dental plasters do) when curing that it should be capable of making a more accurate cast. However, it's really splitting hairs if fractions of a percent of expansion are going to make or break your casts. Thickness of the casting barrier will have a greater effect.

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Hey there guys, thanks for the information. Jerry, I will try to get some Hydrocal unless I am able to find Moldano Blue here in Canada. Thanks for the link to Jeff's picture essay.

 

Cheers.

 

r.

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Lots of really great tips on making casts-Thanks. Here's my question.  Almost everyone recommends a cast when fitting a soundpost patch but I've read nothing about making a cast when fitting a bass bar, neither new making nor replacing.  What is the difference?Thanks again.

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Lots of really great tips on making casts-Thanks. Here's my question.  Almost everyone recommends a cast when fitting a soundpost patch but I've read nothing about making a cast when fitting a bass bar, neither new making nor replacing.  What is the difference?Thanks again.

Are you actually fitting a patch (removing wood and inserting new wood) or just doing a patch overlay?  When wood is removed, there can be a possibility for the patch area to distort from the thinning out process and the cast will prevent that.  That being said, I do know of restorers that will use casts when fitting a new bassbar in an older instrument.

 

DGSR :)

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... Almost everyone recommends a cast when fitting a soundpost patch but I've read nothing about making a cast when fitting a bass bar, neither new making nor replacing.  What is the difference?...

 

The main difference is that the wood is carved very thin where the patch will be inlaid, and it needs to be supported because it is so thin.  Without the support, the wood is in danger of being distorted or broken by process of thinning the wood and fitting and gluing the patch.  Normally the wood is thick enough to withstand the stresses of fitting a bass bar without the support of a cast.

 

One situation when I think it advisable to fit and glue a bass bar with the top in a cast is after an arching correction.  Doing this in a cast will help ensure that the arching remains in its corrected shape.  The cast that has already been made for the arching correction can be used for installing the bar.

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