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Author: Subject: Polymer Clay Curing (formerly 'Boyle's Law')

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[*] posted on 17-3-2005 at 11:08
Polymer Clay Curing (formerly 'Boyle's Law')

Hi, I need help and I found this forum and thought if anyone can help me they can.....

I am a clayer. I make jewelry with polymer clay, stone slabs, and lots of other found items.

I just recently had a group tell me I
couldn't do something, it wouldn't work and I was being unethical too!! So now I am a woman on a mission to find out for sure.

Polymer clay cures in an oven at 265 or 275 degrees on average and has to bake for 30 mins. per 1/4 inch of thickness.

When we bake, some of the colors will brown, or yellow or darken. Every one likes boiling them because the colors stay vibrant and true, but they say the clay is only cured halfway because it doesn't reach the proper temp. of 265/275 degrees.

I spoke with a man who is into mushroom culture and sells these pressure sterilizers. He said we should be able to pressure cure them at about 13 to 15 psi for a longer time and that the clay could be completely cured according to Boyles Law.

I would rather do this for many reasons, if the toaster oven spikes, the fumes are toxic, it can discolor clays, there is a strong plastics "burning" odor when baking, and toaster ovens will not let you do larger pieces. You cannot use a food use oven because they say the plasticisors that are burned off leave a toxic residue on the inside of the oven and have to be cleaned thoroughly before food use.

The labels say PVC and plasticisors. The translucents have bleach added. We have been told that if any plasticisors remain...even a molecule it will go to work softening the hardened piece and 6 months to 10 years from now it will crumble. People have sealed the clay with odd things like nail polish or hairspray only to have the piece go to mush days, weeks or months later.

Thank-you for any help. Sorry this post is so long.


Edit by chemoleo: Changed title.

[Edited on 19-3-2005 by chemoleo]
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[*] posted on 17-3-2005 at 14:19

Boyle’s law says that pressure * volume of a sample of gas is a constant. He was obviously playing you for a fool.

Could you give a little more information about the clay? What kind is it? What temperature units are you using to, C or F?

Some general thoughts I have with the information you have provided so far: I doubt very much that a single molecule would matter. Remember that an ounce of water has around 1,000,000,000,000,000,000,000,000 (10<sup>24</sup>;) water molecules, so a single one couldn’t do much harm, especially in a solid matrix where molecular motion is extremely limited. If you need a cheap oven, I suggest an air bath. Get an empty can and build a fire under it. Put your clay in the can (not directly on the bottom, of course) and monitor the temperature with a cheap thermometer.

edit: If you want more people to look at this thread, change the title to something more appropriate. ‘Boyle’s Law’ sounds like another middle school student asking for help with his homework.

[Edited on 17-3-2005 by neutrino]
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[*] posted on 17-3-2005 at 17:16

Originally posted by Featherzz2u
pressure cure them at about 13 to 15 psi

Just incase you were unaware, this is fairly close to atmospheric pressure.

\"It is dangerous to be right, when your government is wrong.\" - Voltaire
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[*] posted on 17-3-2005 at 17:28

I don't know where she is from, but in america we assume when someone is talking about pressure that the pressure noted is above atmospheric (1 bar, 14.7 psi I think) so 13 psi above atmospheric pressure was probably what the man was meaning to say.
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[*] posted on 18-3-2005 at 10:11

Well, I have read the replies and I have been called inappropiate, unawares and hailing from another planet.

You people are rude, and not user friendly. I know noething of science. That was why I chose this site because "real scientists" were too stuffy to even bother with my little queries. I would have rather you not have bothered than to be so insulting!! Feel free to make any more replies because I will not be checking back. None of you were born with your knowlege of science, you had to ask and read and learn. But apparently once you get to a point where you think you are learned is easy ( and apparently fun) to ridicule those who know little.

Just call me (OUTTA HERE!!!!)
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[*] posted on 18-3-2005 at 13:12

Well, I think I can attempt to answer a few of your questions should you decide to check this thread.

First of all, a pressure cooker should be able to do the job nicely for heating pieces to 275F, seeing as water boils at around 212F. You should be able to completely cure pieces by boiling them a bit longer, remember by heating them you are just increasing the reaction rate at which the monomers in the clay link into polymers (hardened clay). So temperature should be less of an issue as time that the pieces spend heated. At a lower temperature lets say twice the time may be needed, and some experimentation may be needed to find the additional time required.

However, if you insist that you heat them more, to cure them faster (or on time as the product suggests more than likely), you could stick them in a standard pressure cooker. More than likely this "mushroom culture sterilizer" is nothing more than a glorified pressure cooker that you can pick up for a few bucks from a supermarket. By increasing the pressure inside, the boiling temperature of the water rises, so that the liquid water will be heated to a higher temperature. Remember that when water boils at atmospheric pressure it only gets to ~212F and any additional heat simply makes it boil away faster without increasing the temperature.

Now the interesting part, if you maintain a high enough pressure in the pressure cooker you can acheive your desired cure temperatuer. According to my tables in my chemical engineering textbook, the vapor pressure of water at 275F is 45.41 PSI. This pressure is absolute, so subtract about 15PSI (atmospheric pressure) to get the gauge pressure that will be displayed on a pressure cooker with a gauge.

So, you will need a pressure cooker that will be able to hold about 30 PSI in order for the water to attain a temperature of 275F at its boiling point.
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[*] posted on 18-3-2005 at 23:48

Hey now, there's no need to become indignant. Perhaps you might imagine that these people can be human too, may be under stress, and therefore, may not treat inquiries with Madame Dulcier's finest manners.

Just to vouch for neutrino, I think (s)he was just trying to ascertain the specifics of your query. Without an accurate question... we can't really help anyone. Besides, so called scientists are well known for being aloof! Please don’t make the mistake of reading that as rudeness :)

Was it degrees Celsius, Fahrenheit or Kelvin? Are you sure of the pressure? What do you mean by “Every one likes boiling them because the colours stay vibrant and true, but they say the clay is only cured halfway because it doesn't reach the proper temp.” could you be a little clearer please?
I don’t know what you mean by “if the toaster oven spikes”, are you using a food grade oven to bake your jewellery with plastics in? If so, the discoloration of plastics is probably due to a temperature that is too high. I understand that you can cure polymer clays (especially translucent clays) at a lower temperature for a longer time to avoid ‘burning’ the plastics. This will also help you avoid flecks… much like tempering glass and some kinds of metal pieces.

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