Quote Originally Posted by gainer View Post
Aside from all that, I no longer recommend heating above the flashpoint and have not for quite some time.
I do understand that and I'm glad.

But the description of the behaviour of flammable liquids (and yes, glycols are flammable liquids) that PE and I have given here are accurate.

The only requirement you need to achieve to get a flash fire is to heat the flammable liquid enough to get a suffient quantity into the air AND an ignition source. The best way to avoid one of these fires is to not have either condition met.

At a lab I worked in, we used to do fat extractions using petroleum ether which has a 30-60C boiling range. It is a very volatile solvent. As a safety precaution, we used light bulbs to heat the refux flasks. Safer right? No Bunsen burners.

Well, the flash point of Pet Ether is less than room temp, and the autoignition temp is 473F. The Pet Ether the guy spilled evaporated and worked its way over to one of the hot light bulb in the apparatus and ignited. No open flame, and it was not spilled directly on to the hot bulb.

So I'm just trying to point out that it's pretty easy to meet the conditions needed to start a flash fire. And if the conditions are just right, an explosion can occur as well. It really is best to work with flammable materials well below the flash point whenever possible.

As to your question,
Quote Originally Posted by gainer View Post
does the vapor actually behave as you suppose?
Yes, it does. It's not a supposition.

Quote Originally Posted by gainer View Post
There are temperature gradients to consider. The container will be at a higher temperature than its contents during heating.
No, you do not need to worry about temperature gradients of the container. You need to worry about the concentration of the vapour in the air.

I'm not an expert in microwave oven design. But I have personally experieinced a fire in a microwave due to failed electronics. Also, we don't know if the person doing this will be smoking which would give ample source of ignition if enough vapour is in the air when the door to the microwave is opened. Lots of things we can't control that may happen.

Again, glad you don't recommend this practice anymore. But why are you so stubborn about general safety recommendations? We should all be promoting practises that will make everyone's photo lab experience safe and enjoyable.