Lead in glass is disolved in the glass and is held within the crystal
matrix, only a very small amount of lead is ever released from the
glass, that which was at or very near the surface. The point of putting
lead into glass is that it makes the glass less brittle, more likely to
bounce if you drop it rather than shatter.
I once saw a description somewhere that typography was like a window. It went something like this as I recall.
One might think the purpose of a window is to let someone see through it, but some windows purpose is to be looked at.
A clear plate glass window is there to give an unobstructed view of the outside whereas a stained glass window does not give a good view of the outside world it is there to be looked at.
Similarly there are fonts designed to convey the message of the writer, their purpose is to disappear, whether it is a sans serif or serif doesn't matter they are designed to be read.
Then there are advertising fonts, their purpose is not to convey a message, their purpose is to attract attention to themselves. You would not write a long passage of text in one of these fonts because you know the reader would never get to the end of it.
They are there to attract attention to themselves, to advertise.
Part of the art of graphic design is to know which one to use where.