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Author
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Topic: Emergency plaster/hydrocal question!!!!!
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thx1138
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posted 09-20-2000 08:02 PM
I just whipped up a large batch of it and i need to know what consistency it should be??? Its like a soup now but im wondering if it should be thicker like clam chowder (sans lumps)? (im also hungry btw) man, I hope someone is on. ------------------
Fan Film Sound Effects For your star wars fan film. |
thx1138
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posted 09-20-2000 08:07 PM
also, should i put more plaster in to make it thicker? |
FXMan
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posted 09-20-2000 08:24 PM
Well truthfully the plaster should be added to the water until it looks like a dried river-bed. That's enough water at that point and only adding a few drops at a time to loosen it a little bit at a time should be done. It should be about the consistency of mayonnaise or thick cream. The only problem with too much water in the plaster means it will take longer to dry out or it might weaken the finished product. Ideally the plaster ratio to water should be about 35 parts water to 100 parts gypsum (Ultracal).If you want to get real exact, use a 300 ml. mixture of water to 1000 grams of plaster. Of course you have to have a way to measure these accurately. If you already have it mixed and need to know if you can add more the answer it yes but only little bits at a time. It just has to flow smoothly and not be too thick like cement and not too thin like milk. But a thick cream or stiff meringue is about right. Good luck FXMan
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thx1138
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posted 09-20-2000 08:28 PM
allright! thats what it ended up being the consistency of. I just finished pouring it before you posted and its getting stiff already. I'll share the results tonight. Thanks fx man. ------------------
Fan Film Sound Effects For your star wars fan film. |
thx1138
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posted 09-20-2000 08:31 PM
Oh and also how long, generally, should i let it harden? ------------------
Fan Film Sound Effects For your star wars fan film. |
FXMan
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posted 09-20-2000 11:25 PM
Well THX it often depends upon the type of plaster used. If it's Ultracal 30 it takes about an hour to set hard but can easily take days to dry out completely. Other plasters vary in setting and curing times. Dental stone for instance, sets really hard fairly quickly and can be used quite soon after it's set. But for general use, try to leave it at least 24-48 hours if you can. You can also often bake it VERY slowly in a warm oven, barely on to 200degrees for a few hours to speed the drying process but I don't recommend doing it if you have the time to let it dry naturally. I have left Ultracal molds dry for at least 3-4 days and as much as a week before doing anything with it, just to be safe and sure.Good luck. FXMan
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thx1138
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posted 09-20-2000 11:33 PM
thanks again, fx man. Looks like i'll be sharing the results in 24-48 hours than.------------------
Fan Film Sound Effects For your star wars fan film. |
FXMan
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posted 09-21-2000 07:32 AM
Can't wait to hear of your success. Good luck.FXMan |
Jeff F
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posted 09-21-2000 11:25 AM
Since I have always bought gypsum products in 100 pound bags, it takes me a while to use up the last bit. I've noticed that setting times increase with older plasters, presumably due to small amounts of moisture getting into it. I minimize this by placing the bags into a plastic trash bag or two, closing the tops of the bags, and dropping that into a trash can with a lid. I do this before cutting open the original bag and it minimzes the mess involved as well.------------------
Jeff F - Moderator
Magic and FX
Amazing the Masses |
thx1138
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posted 09-22-2000 03:11 AM
well it went ok. Not great but it could have been worse. I ended up with lots of air pockets that left many a hole in my mold. Fortunately most of them can be patched up successfully. Most of the really bad spots are on the outside area of the actual cast, on the keys/teeth (whatever you want to call them). So those need to be fixed before i can move on.------------------
Fan Film Sound Effects For your star wars fan film. |
FXMan
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posted 09-22-2000 07:41 AM
Well the one thing you need to be careful of when you are mixing plaster and before you pour it, is that you shake it down or tap it down to get as many of the airbubbles out as you can. Then when you pour it do it slowly so as not to create air pockets. If you can, you should try to do a "splash" coat first to eliminate any chance of bubbles, missed spots that would create empty spaces or air pockets, and let it set before pouring in the rest of the plaster. But glad to hear it worked for you. Be careful the keys too. They must be clean and well taken care of for proper fit.Good luck. FXMan
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Jeff F
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posted 09-22-2000 12:23 PM
Dental labs use vibrating platforms to drive the bubbles out of their plaster mixes, but these are pricey and suited for amounts of plaster generally too small for a lot of FX mold work.I simply try to avoid any more mixing than needed when stirring my splash coat, vibrate the bowl by striking the side with a pair of sticks (like a rock drummer on a solo), bouncing the bowl a bit, etc to drive the bubbles to the top where they can be popped or scooped off and discarded. You can blow on the splash coat as you apply it, popping bubbles that way. I was once doing a demonstration of life casting as part of a lecture at a convention, and mentioned that despite all the precautions, you never got all the bubbles, and would usually have to watch for at least a few when pouring on the splash coat. Of course this was the one time in my entire life that I got rid of every last single bubble!  I was so surprised that I started laughing and told the audience not to expect that result themselves. ------------------
Jeff F - Moderator
Magic and FX
Amazing the Masses [This message has been edited by Jeff F (edited 09-22-2000).] |
Cinetech
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posted 12-04-2000 01:37 PM
A good trick to get rid of bubbles in tyour bowl of mixed Plaster, Is to fill a spray bottle with alcohol (Regular Rubbing Alcohol will do.)and after tapping the bowl to make the bubbles rise to the surface, Spray a shot of Alcohol onto the surface and they will all magically pop. Keep repeating the tapping and spraying and you will lessen the chance of bubbles in the splash coat. But toget detail on the surface of your negative mold, You must do the splash coat first. Or else you will always have pockets of trapped air. Scultures can take a long time to do, so It's worth the extra step to insure that all of the detail will be captured. |
Thea
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posted 12-11-2000 07:52 AM
Ultra-Cal 30 ususally takes 30 minutes to set up, that's why they call it Ultra-Cal '30'. There is an Ultra-Cal 45 but it's even harder to find.
FX Man has the mix right. Dont' mix it until you put all the plaster in you need. And don't pull your piece too soon or it will weaken the plaster and you'll find it powdering over.
I always put in a spash coat (first thin layer of plaster). I have a friend who likes to hook a chip brush to a stick and use it to slosh around in deep molds, scrapping against the sides to loosen bubbles.?
Smacking the table works for me.------------------
Thea
www.stagedoorstudios.com |
morpherguy
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posted 12-11-2000 04:06 PM
I use to use thin splash coats when I made molds but I realized that the splash coat is actually the weakest part of the mold. Thats bad because that surface coat needs to be the strongest. Now I use the thickest batch that I can that will still flow into detail with the aid of compressed air. This makes the mold much stronger. Too much water makes for a weak batch of Ultracal. |
