Pure water-soluble graphite in a stick!
There are so many things you can do with Derwent Watersoluble Graphitone; use it like a graphite pencil, peel off the wrapper to use the stick whole (no wood casting here!), or break it into chunks to create broad sweeps of tone really quickly.
Just add water and watch the graphite dissolve to create line and wash sketches.
You can choose from 4 wash strengths from a light to a very dark wash.