carelesslove, here!
Many years ago, I was lucky enough to meet up with an old printer who poured & machined linotype and he "tried" to teach me about linotype & casting it.
He had harsh words for those printers that used soft lead as spacers and swore he only used linotype - and in his words - "if you let the tin burn off, or put ANY other metal in your pot, your type metal will go soft".
My seemingly intelligent question was "how will you know if the tin has burned off?". His reply was quick & terse - "Hell boy, just look at it!".
Knowing then, that I was technically dead in the water, I asked him what to if it went soft. He replied - I'm going to give you a bucket of "Parting Metal". If you need to bring up the hardness, just knock off a chunk of this in your pot and it will "sweeten it right up!"
After that, the only question I had was "What do I owe you, Sir". I took all the metal he would sell me + the "Parting Metal" and ran home like I had stolen something.
To this day, I do not know the constituency of the mysterious parting metal, but a chunk of it did make my bullets instantly harder and made the sharp edges on my bullets even sharper.
Success without knowledge is usually rare, but I enjoyed it.
Later in my casting life, I figured out, that for my revolver shooting, a "push thru" fit in the cylinder throats was more important than bullet hardness, so I use my linotype metal for alloying - not casting bullets from it.
I am absolutely certain the old printer knew what he was talking about. I just wasn't smart enough to figure it out.
Now, the old printers are pretty much gone - and along with them, their stories.