Also, I let the lye temperature drop too low; I assumed I couldn't heat it up, so I mixed up another lye/milk mixture and put the original in glass jar with a tight-fitting lid. Can I save that? If so, how do you reheat a lye mixture?
As the others have said- whether your lye is cool or hot, it will still make soap.
For future reference, you can heat up a lye solution that's too cold for the particular temp at which you like to soap if you wish, although I don't know that I would try it with a milk/lye solution. I do it all the time with my 50/50 lye master-batch which I keep stored at room temp (about 78F), but it's a water/lye solution.
I just place it in a hot-water bath on my counter to warm up while my hard fats are melting. Basically, I heat up some tap water in a pot just to the point where I see tiny air bubbles starting to form on the bottom of the pot. Then I pour it off into a container that's larger than my plastic lye-solution cup. Then I place my water/lye solution-filled cup into the hot water-filled container to warm up.
I wouldn't try it with a milk/lye solution because of the fats and sugars in the milk, which could more than likely prove to be problematic.
For the same reason, I would not store a lye/milk solution for future use, either, especially not in glass.
It's perfectly fine to store a
water/lye solution, though..... as long as you use a proper storage container made out of either HDPE #2 plastic or Nalgene.
IrishLass