Safe heating with the microwave
If the concern is that the glycol vapor may hit the hot light bulb in the oven and be ignited, let me describe a little experiment. Cut a 1/2 inch wide strip of paper towel or toilet tissue long enough to reach from top to bottom of the perforated grill that covers the bulb and tape an end of it at the top so you can see the direction of airflow through the grill by the angle of the dangle. Put a cup of water in the oven to prevent overloading the magnetron and turn the oven on for a few seconds. You will see that the flow is from the bulb enclosure INTO the oven.
Neither the direction of that flow nor the flashpoint of the liquid being heated will be an issue if no vapor can leave the container of glycol. There are several brands of plastic cling wrap that are microwave safe. It is a simple matter to cover the container of glycol so that vapor does not escape from it, nor is there a chance of a spark entering the container. Leave enough slack in the plastic wrap so that some part of it lies on the glycol surface after it is sealed. You will see that it does not leave the surface until the vapor pressure of the glycol exceeds the atmospheric pressure, which happens at about 370 degrees F where it begins to boil. You can test this with plain water in a pan on a stove top. The plastic wrap will lie on the water surface until the water begins to boil.
There is no need to reach the boiling point of water, much less the boiling point of glycol, and certainly not that of TEA. 150 degrees F (66 degrees C) are more than sufficient to speed up the dissolving of the chemicals in any of the developers that use glycol or TEA as solvent. That is well below the flashpoint of either propylene glycol or TEA. It is a simple matter to test an
individual microwave oven in steps to calibrate it for these solvents. My microwave heats 500 ml of propylene glycol from 70 to 150 degrees F in 1.5 minutes at highest heat.
Of course , common sense
So many like to theorise without actually doing any meaningful experimentation. Hats of to you Patrick for your willingness to take abuse from some for the willingness to sensibly experiment.
Not so much on this site, but with so many in the rest of my life. I know people that do not know how to experiment. They don't even know how to scratch cook. That can explain the difference between a yeast levened bread and a quick bread that was risen using baking powder, etc.
But all experiments need a bit of common sense. Usually that means talking incremental steps, measure, compare to expected behaviors if possible, and then proceed cautiously. Construct a non hazardous analogue to confirm a behaviour if necessary - as you have here.
I make soap at home, from scratch. I store my lye as a 40% W/V stock solution. Storing it 'pre-wet' allows me to make a batch up as soon as I have the oils just melted, rather than wait for hours for the fresh hydroxide solution to cool off.
To warm the hydroxiode solution I warm it in the microwave. Boiling hydroxide is no ones idea of fun. So I heat it, stir it, and measure its temperature at 20 second increments. I now know how long a given volume of hydroxide takes to bring up from about 64 to 90 degrees. There is a post- it on the side of the microwave recording the data, and I never heat it all the way there without a couple of stops to stir and measure temperature.
I am sure there will be a crowd of soap makers (the melt an pour crowd- kind of soap makers that real soapers treat as through they are point and shoot digital photogapaphers) who never would allow lye into the house, a second crowd that makes it fresh and recoils at the though of a microwave, and a much smaller fragment that have thought beyond a recipe and pondered if there is a better or more efficient way, and have strayed from the beaten path to experiment and find out if there suppositions are true.
Pat- please don't forget to warn people about the hazards of heating toxic chemicals in their home use microwaves and how they should get a second microwave that is dedicated to use in their lab/darkroom. Like you have.
It's not a good idea to be putting these materials into the household microwave where foodstuff are being prepared. Some things are just bad ideas for a multitude of reasons. Kind of like the person I knew that decided to "sterilize" their mercury fever thermometer by putting it into their diswasher and ended up breaking it... Just not a good idea for many reasons.
A simple stirring hotplate would work just as well without all the concern about poisoning your family.
By the way, where does any possible vapor go from your microwave? Are there any electrical componants that we should be concerned about in the pathway? And are all microwaves manufactured with the same airflow as you found in your microwave?
PS I like your suggestion about covering the container. That one is a good one.
How long do you have to stir Ascorbic Acid to fully disolve it at 60C ?
I find that it takes me more than 20 minutes of constant stirring at 80C to disolve 54g of AA into 500 ml TEA.
I also add 1.0g of bromide disolved in 15 ml of warm glycerol to control fog and any potential for streaking, as suggested Tom Hoskinson a few years ago. (Bromide is a bugger to disolve it TEA, but it disappears in a few seconds in in warm glycerin.)
As a matter of fact, I do have 2, one for darkroom and my own coffee cup, the other for the kitchen. However, both propylene glycol and TEA will be found in medicines and foodstuff. Some of the stuff I have heated for eating should have been pitched.
Originally Posted by Kirk Keyes
The air for cooling the lamp comes in from outside and exhausts on the other side of the oven. As you know, microwaves don't heat air hardly atall as they say here. I thought of taking my oven apart so as to trace the air paths. I think, though, that any exhaust path from the oven itself will not be used for cooling the magnetron or its associated circuitry, simply because that would not be any better for much of the cooking and reheating that is done in the ordinary household than for heating glycol or TEA. I could get something that would create a lot of smoke when heated and see where it goes. At least I know that if I were designing a microwave oven, hot gases from the oven would not be used to keep the magnetron cool. It would have a separate wind tunnel for that.
My darkroom oven has a Warm Hold setting that is handy.
TEA can be heated more than glycol, even in an open container, but it changes color around 250 F. Dow didn't say if that was good or bad for its uses, but I think it could be heated to 100 C without danger or change of color, and covering with cling wrap will keep any vapor from escaping anyway. Maybe I shouldn't be dogmatic about that. If there is water in the TEA, it will likely distill off long before the boiling point of the TEA and open the cling wrap. Still, the flashpoint of TEA and its boiling point are considerably higher than for glycol.
I don't know about all microwaves, but it seems the sensible way to do it. You guys can do the test on any that you have access to and let us know if you find the opposite path.
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