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Dave Parvin
Joined:
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16-Mar-2005
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Started On : 21-Mar-2010 at 01:24:45 PM, #Views : 5976
Topic Subject : Re: Re: Lines, airbubbles, alginate and more |
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Ali. here are some more thoughts on your questions. When you pour the plaster into the alginate mold, as i explained in my last reply, do not over tap or over vibrate it. What I find is helpful is to rapidly make horizontal circles with the mold while the plaster is still liquid. Here is what I mean. Imagine that you have drawn a circle about a foot in diameter on the table or workbench. Move the mold rapidly around the circle. What this does is cause the plaster to spin in the mold dislodging any bubbles from the inside surface and drive them towards the center.
The very best way to eliminate bubbles is to use a vacuum mixer. They are made for dental labs and are pricey new but you may be able to find a used one. I have acquired three of them, one I traded a casting for and two were given to me. Please read my four articles in the ALI Library about vacuums and pressure for a thorough understanding of how to eliminate bubbles.
I do not know how long it takes the plaster you are using to set up, but I suspect that the longer it takes, the more likely you are to have problems caused by water in the alginate. Using a faster setting plaster might give you better results. You can also accelerate the plaster you have. I included a thorough discussion on how to do this in my article "An Alternative To Using Plaster Bandages As The Supporting Mold In Alginate Life Casting" which is in the ALI Library.
I have never used the alginate that you mentioned. I use ArtMolds products. For baby's hands and feet, I use the fast setting "Hollywood Impressions." For older hands and feet, "Regular Set." And for faces, bodies, etc., I use "FiberGel." I get excellent results. The only thing I can say about yours is that it seems to take far too much alginate for the amount of water you are using. A cup of water is 228 grams. If you are using 100 grams alginate for every 227 grams (1/2 half pound or 1/2 pint) of water, the alginate water ratio is 1 to 2.27. For the alginates I use, the ratio for hand casting would be about 1 to 4 or 5. A ratio of 1 to 2.27 would be so thick that you wouldn't even be able to mix it much less push a hand or foot into it. I do not doubt you numbers and have no idea why your alginate takes so much. I have to suspect that not only might you save money but you would get better results with a different alginate.
I would only attempt to mix very small amounts of alginate by hand, perhaps 1 cup of water size batch. For anything more, I would always use a "Jiffy" or paint mixer. I would also encourage you to not just blend in the alginate until it seems right, it takes to long. Once you know the proper ratio for your alginate, weigh out the alginate and the water and mix. You can measure water by volume but it is more accurate to weigh the alginate.
The proper consistency of the alginate is something I can show you easily but is difficult to describe. However, it needs to be runny enough to be poured into the mold but not too runny. Too runny will weaken the alginate. Also, if too runny, any bubbles in the alginate will rise and cling to horizontal surfaces on the hands or feet.
In order to eliminate bubbles on the surface of the hands, I smear alginate over the hands prior to submerging the hands into the alginate. By the way, I have never wet hands or any other body parts before applying alginate. I probably should try that to see if there is any advantage.
Do not use more water in plaster than is recommended by the manufacturer. Too much water weakens the plaster. The recommended plaster to water ratio should result in a runniness which should be just fine. I would suggest that you mix at least a few batches with careful weighing until you have a good feel for what is the correct consistency. After that you can probably just add plaster until the mixture seems right, something I would not do with alginate, however.
I realize that what I have written here may sound like arrogant pontificating. But I have cast a lot of hands. On just one day in July myself and twenty assistants cast between 650 and 750 hands. And I have been doing that for the last fifteen years.
Ali. please get back to me if you have any more questions. I would suggest that you contact ArtMolds and get my DVD on casting hands in which I go over all the above. Or better yet, jump on a plane and come to my next workshop April 12-16 during which we will cover everything you want to know about hands and feet and faces and bodies.
Dave
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