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Dave Parvin
Joined:
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16-Mar-2005
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Started On : 06-Jun-2009 at 05:46:07 PM, #Views : 5950
Topic Subject : Re: Re: Re: Re: Alginate volume |
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Ann, you are correct, too much water will weaken the alginate. Though one might think that a thin or runny mixture would produce a casting with fewer surface bubbles because any bubbles would rise to the top and escape the solution with a pour mold as you are describing. Also, a runny mixture will have a slower setting time allowing for more bubbles to escape. But one would be WRONG. What happens is that those rising bubbles come up to and attach to the bottom of of the casting producing "outies" that have to be repaired. You can mix the alginate to a consistency that is more runny that optimum for a skin mold but make it just runny enough to pour easily. By not delaying the setting time any more than necessary, you will also trap bubbles in the alginate away from the skin surface.
Another thing that I find helpful is that just before I pour alginate over the hands (or feet) or have the model(s) push his/her hand(s) into a container of alginate, I scoop some alginate into my hands and rub it over the skin to eliminate any bubbles on the skin surface and force it into ever nook and cranny.
I agree with Robert, always mix powder into liquid and not the other way around. Chefs are taught to do the same.
I also use a "jiffy" type mixer and a variable speed drill with a seed range of from 0 to at least 2,500 R.P.Ms. It is good to have some power so I want an amp rating of at least 7. I start mixing very slowly increasing the speed as the alginate becomes wet. I start a timer as soon as I combine the alginate to the water and mix until smooth which takes 40 to 60 seconds depending upon the amount. I generally do not try to mix more than 6 pounds (2724 grams) of water and the appropriate amount of alginate in one container. If mixing more, I divide the water and the alginate into two (or more) batches. Obviously, the larger the batch, the longer it takes to mix. Take too long and you may run out of time to get the alginate applied. I either mix half myself and have an assistant simultaneously mix the other half; or I mix half and as soon as I am done, have an assistant start the second batch. If my assistant is skilled at applying the alginate, we do it the first way. If she/he isn't so skilled and I pretty much have to apply the alginate, I do it the second way.
The tricky part is getting mixture just right which comes from testing and experience. I can't give you universal proportions because alginates really differ depending upon the brand. But when I get a new batch, I carefully test a small sample to know what I have. Even the same brand can vary somewhat. Not only do I weigh my test samples carefully but I use 80 degrees F. (27 C.) so I can verify setting time as well. I then mark the results on the container.
I usually weigh both the water and the alginate whenever I am mixing them to assure consistency. Robert said that he mixes by vol. as well. I do too in some applications. Remember, guys like us have been doing this for a long time and can tell at once from experience if we have to make an adjustment. I'll bet that Robert does what I do, has both water and alginate handy when mixing just in case.
While I sometimes use water warmer than 80 degrees to shorten the setting time, I rarely use colder to slow it down. To do that, I use AlgiSlo. Not only is 80 degrees comfortable for the model, but the published setting time for most alginates assumes water at 80 degrees.
Ann, this may be more than you wanted to know about alginate, but I hope it has helped. I am still planing on finding out how much the vol. of the water is increased by addition of the alginate and will post it here next week.
Dave
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