Lye is caustic, true, but that high temp is needed to saponify the oils into soap! Then, yes, it basically burns out. It has done its job. You can also use ph strips to find the ph level in soap you make. I used them a lot with batches, so using different amounts of lye, or milks, or fats etc all came out with different ph levels. It was pretty fun! I felt like a mad scientist!