Violin Forum/Message Board

[bacon]

I have done many small partial casts as well as a few full violin plate casts with a fair amount of success. I am very nervous of a full table cast for a cello.javascript:emoticon('')
I usually use plaster but I am concerned it will set up too fast. Could anyone with experience in this offer me any helpful advice?
Stephen

[Jack H.]

should be no problem to do a full cast.
First thing is to make a frame for the top.
Next is to glue newspaper to the frame and then when that is dry to glue the top to the frame ( on the newspaper side, the newspaper makes it easy to remove the top from the frame. The top then needs to be sealed with either saran wrap or tin foil, the saran wrap can be stuck to the top and smoothed so there and no wrinles or air bubbles. the tin foil needs some kind of oli or vasaline so it will stick and smooth down with no wrinkles or air bubbles. i preffer the saran wrap, but the foil offers better protection.
next thing is to get some kind of skirting for the frame.
I used to use the rubber skirting that is used in kitchens for floor molding, it can be tacked into place with push pins or small nails. now you also need a backing for the plaster cast. should be very close to the full size of the frame, ply wood is good for the frame and the backing.
next thing is to mix the plaster and pour. if you think the plaster is setting too fast then you should mix it and pour it quickly. When mixing plaster start with the correct measuremnt of water and then slowly add the plaster to the mix while constantly stirring. I recommend using a mixer with an electric drill. when you have added the correct amount of plaster for the water adn it is thouroughly mixed, pour.
A mistake often made is to over mix the plaster, that makes it set too fast.
once there are no bits and it is homogenous, pour. then set the backing into the plaster. after a few minutes the plaster should set enough to remove the skirting and then the plaster cast.
Do not let it sit on the top too long, the plaster warms when it cures adn could damage the varnish!!!!!

ust make sure all the tools for mixing plaster are very clean!

Good luck
J

[bacon]

Jack thank you. I have constructed my walls of poster-board , but rubber skirting sounds worth trying out. It seems that when I have done full violin table forms that it took only five minuets for the plaster to set. I will conceder the mixing time and might do another violin that needs a form for doubling, that is not a priority , first for the feel.
Maybe I'll even try your power drill idea, done it with my fingers before. I just hate the idea of it all setting up before I finish the pour.
Thanks again for the reply.
Stephen

[Geemac]

I am new and would like to know why a casting is needed?
_________________
GeeMac

[Jack H.]

there are quite a few reasons why castings are used.
I use them mainly for doublings.
When you double the edge of an instrument you need a flat plane of refference. You make a cast of the area to be worked on and then you can remove the wood to flat specifications and then glue the flat doubling material in place with out risk of deformation!
Sometimes I make small casts in flexible areas when I have to fit studs over repaired cracks. once again, it prevents deformation of the top during gluing and allows a better fit while fitting.
Another reason for a plaster cast is for arching correction. Very hard to do but great results!

[Jack H.]

[jonathan vacanti]

Important points:
-*the colder the water is the slower the plaster sets*
-if you gently shake the table the air bubbles will rise to the surface
-the first layer should be thin so it follows the plate closely, thick plaster will leave large air pockets
-too thick of a cast will deform the plate from it's weight
-don't leave on too long or heat will damage varnish
-plaster should dry for a few weeks if possible

[bacon]

"-the first layer should be thin so it follows the plate closely, thick plaster will leave large air pockets "

Jonathan are you suggesting multiple cured layers or just making sure that the surface is finely covered before adding the bulk of the plaster? Thank you for your list.
My method to date has been as recommended by Weisshaar/Shipman. The plate is glued to paper glued to plywood. Hopefully this will retain the arching as is. My plan is to keep it with in around 2-cm thick of plaster in the center reinforced with wire mesh and a holed plywood clamping board. I do check to see if I need further wedging for the f hole wings.
Baking soda as well has been suggested to slow down the process. For a cello form would it be advisable to add glue to the mix?
Stephen

[Jeffrey Holmes]

Sorry to come in so late on this... but had a couple of alternate materials to mention.

Dental foil is unbeatable for detail, but I very rarely use it for an entire plate.

I've used saran wrap... but find it really difficult to work out the wrinkles. Maybe a vacuum box would help...

What I find works well for me is very thin latex sheeting. It stretches a bit and is very easy to work with as a barrier... reusable too. It can be found through suppliers like McMaster-Carr.

Especially for a 'cello cast, I'd avoid the cardboard (poster-board) method (Weisshaar). Chances are you'll have yourself a mess.

I had always eyed the dense foam insulation available at the home improvement stores... thinking it might make a good cast frame (it's only a few bucks, comes in a variety of thicknesses and can be glued with contact cement if needed). Finally, I had the need to make several at once... Figured it was worth a try. Glad I tried it. Takes very little time to cut out the outline on a bandsaw (use a vacuum attachment). Better than a wood frame for me, as each frame is custom (fits the violin rather well).

I simply trace the plate on the insulation, cut it out, tack glue the plate (w/ folded paper squares) to a rectangular board, make any minor adjustment required (support), cover with latex, clamp the insulation to the board (might want to use something like a thin piece of plywood over the top side of the frame... gives you something to clamp on), install wire if needed can be held up w/ toothpicks stuck into the foam), make and pour the plaster (slowly), agitate (shake; to get the bubbles out), place the clamping surface in the top of the wet plaster, let it set a while (watch the heat!), and disassemble. The insulation frame can be trimmed away once things dry out a bit.

I use Hydrocal White Gypsum for casting. Set up time is very tolerable.

Cheers!
_________________
Jeffrey

http://holmesviolins.com

[brucesown]

Hi there,

I do plaster casting all the time, just on teeth. The setting time is a function of temperature as previously mentioned, but also a property of the plaster. It comes in different formulations that will all have a different inherent setting time. If you want more working time, get a slower setting plaster.

It also comes in different colours and hardnesses. There are also epoxy materials that can be used.

Cheers,

Bruce Burgess
_________________
Sometimes you're the windshield, sometimes you're the bug.

[jonathan vacanti]

I don't mean multiple layers, I just meant that the plaster should be thin as it hits the plate, you can't put it on when it's like pudding or you'll have air pockets. If you want it thin, use cold water.
_________________
I am a professional luthier with 13 years experience building and restoring violin family instruments and bows.

[bacon]

I would like to thank everyone for their input. I have finished my cast and it came out wonderful. The power stirrer was great. I did end up using the rubber skirting. I used a new material, to me, for sealing the top. It was the plastic wrap developed for windows. You tape it around the form and use a hair drier to shrink wrap it. It worked incredibly well. I heard about at David Kerr's shop.
Because I was so worried about time I made a bad mistake and ended up doing the job twice. I read in Mayer that one percent hide glue will slow down the set. So I thought perhaps a little more would work as well as make the form stronger. Six hours after pouring there was no sign of any harding reaction so I undid it all. The stuff did eventually harden but I had a commitment and I wanted to be around to flip and trim the mold before it set. My second round was with just plaster and cold water and I had plenty of time. I must have done something odd for my last violin form.
Stephen

[Jack H.]

Glad it worked out well for ya, that heat shrink plastic insulation for the windows sounds like a good idea!

Good thing you remembered to put the wire in, I forgot to mention it. Muct be from too many smaller casts!


J

[Oded Kishony]

Thanks all, some interesting new things to try.

Oded Kishony