One of the first times I wanted to mix up my lye water I had a fabulous brain fart. I reached into the cupboard and grabbed a plastic jar -- sometimes we wash out and keep jars from things like mayonnaise, peanut butter, coconut oil, etc. -- when I meant to grab a measuring cup. I don't know why, but it seemed perfectly reasonable at the time to add some distilled water to this *plastic* jar and then add the lye.
The lye heated the water to near boiling, and even though it was a thick, tough jar, it was obviously not designed to take that much heat. The results were... a little bit spectacular. Thankfully there really wasn't any chance of danger; I do all things soapy -- whether I'm mixing lye, stirring a bowl of soap batter, putting micas into oil, or whatever -- inside a plastic wash tub. The tub was on top of our stove, under the fan (for the fumes) and I had gloves and goggles on, of course. Even though I was pretty safe, I still scared myself. When the lye reacted with the water and heated up, the jar MELTED and collapsed onto itself! It was about 1/3 full initially, but by the time it finished shrinking and folding down it was pretty well full.
I grabbed the *proper* cup, poured the lye water into it, neutralized the weirdly shrunken/melted plastic with loads of water and threw it out, and then poured the lye water down the drain once it had cooled, with lots and lots of water. I didn't know if it would be contaminated with plastic, or just fine, and didn't want to take a chance.
From that day on, I had a dedicated lye cup made of steel, and it lives in my soaping cupboard. I don't do *anything* until everything is laid out and ready, and that includes the steel cup.
