Violin Forum/Message Board

[Stefano]

I have made many casts of instrument backs and tops for various repair procedures. Right now I am for the first time trying to make a triple cast: negative (as usual), positive from the negative with carefully made arching corrections, and then, from this one, a new negative to use as a sand bagging jig. The problem I am having is finding a separating agent for the casts beyond the first one.

For the first cast I use plastic wrap and a vacuum suction system to get very fine detail. It always works very well. However, I am hoping to find a separating substance to apply to this plaster cast so that separation from the subsequent casting is no problem. The vacuum system is not practical here and the plaster does not weigh enough to push the plastic wrap snugly around the high archings. I have tried wax dissolved in turpentine, gloss spray paint, and olive oil. So far everything sticks. The next attempt will be Kiln Wash but I already find that this material is a bit thick in consistency, and I found out that it is also quite caustic; I burnt a couple of fingers accidentally by rubbing them on the cast to see if it had dried.

I'd welcome some suggestions!

Stefano

[FiddleDoug]

Sounds like you're really into some sophisticated restorations. I've never gotten into anything quite that complex. Some people use thin rubber dental sheeting to separate plaster castings. In my other vocation, I work with a glass artist. We often use plaster and similar materials for mold making. We've sometimes used liquid dish soap as a release agent to keep plaster from sticking to wood. It might work plaster to plaster. Are you using sandbags to press the plates into the plaster form?
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Doug Wall

www.wallindependent.com

[Stefano]

[Ed Shillitoe]

Hi Stefano:

The stuff that you need is used in dental labs quite a lot, and is called a 'Gypsum Separator'. Here is a link to some info:

http://www.plastermaster.com/usg/plaster201.htm

You can buy it from pearsondental.com. Their online catalog has about 4 varieties, but the printed catalog has more.

It's easy to use - you just brush it on and let it set. In England it's called "Cold Mould Seal" or at least it used to be.

Hope this helps.

Ed

[Stefano]

Thank you very much, Ed! I'm certain that this will do the trick. I've known about this forum for a long time and am amazed at the generosity and good will here; I should have joined a long time ago. I'll let you know about the results.

Stefano

[Stefano]

Yesterday I poured the plaster into the Kiln Wash coated mould which I had left drying for a number of days. Today the new plaster released from the mould with astonishing ease! There was some loss of detail, as I had suspected because of the Kiln Wash being somewhat uneven in texture, so I will be trying other methods suggested above. However, for this present repair, there is sufficient detail to continue. Thanks for the help, all!

Stefano

[Geemac]

Although more expensive, you will find that hydrocal is by far superior to plaster. The hydrocal dries harder and does not break as easily as plaster.
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GeeMac

[Stefano]

Thanks, Geemac. I actually do use hydrocal; I used the word "plaster" as a generic term. However, not being much of a chemist, I must confess that I know virtually nothing about plaster in general. How does the stuff work? I know there is a chemical reaction of some sort and that it heats up during the reaction (important topic for violin restorers since one must use caution not do damage plates while casting). What is the difference between plaster, plaster of Paris, hydrocal, and guaging plaster? Yesterday I used guaging plaster for a sound post patch mold. In one day I was able to pour the cast, let it harden, and cut and glue the patch. It seems to set and cure really fast.