How big of a chunk can I get away with?
If it will fit in your crucible it will melt. It doesn't take any
more or less energy to melt 50 grams in one chunk than in several
hundred pieces of casting grain.
Some casters have the idea that every casting should be measured out
in casting grain only. I have never understood why they think they
will come out ahead if they melt old sprues and buttons an extra time
to make them into grain first. I have seen it explained that the
smaller bits of metal will be liquid before the bigger chunks and
that elements of the alloys might be lost from that portion of the
metal before the bigger chunks melt. I would counter by showing that
sprues and buttons have already been melted once. If you make them
into shot before you recycle them for a casting you have melted the
material three times. Why isn't it better to skip the shot and melt
it only twice? Less work and less time molten and exposed to the
risks of oxidation and evaporation.
I mix big chunks and casting grain all the time and have been doing
it for years. I cast ten flasks today and most of them had buttons
from previous casting in them. Some of the buttons were over 100
grams. I see not problem with that.