weeks, or months - require the use of mediums. I will discuss this further 
in the section on <a href="#marks">Making Marks</a>.</p>
<p>As a more or less separate issue, there are <i>solvents</i>. You can use a 
solvent as a painting medium, and it has the effect of making the paint more 
fluid, transparent, and faster drying. But solvents are also used for cleaning 
brushes, both between strokes and at the end of a session, and for other general 
cleanup of oil paint. And a rag dipped in solvent can 