 drying, others 
to slow it down. Some are designed to make paint more fluid, some to make it 
more like paste. Other mediums are designed to make paints more transparent.</p>
<p>Strictly speaking, mediums are not necessary in order to paint with oil, and 
many artists - especially plein air painters, who are always looking to cut down 
on unnecessary supplies - do not use them at all. But some paintings techniques 
- especially ones that involve painting in multiple layers over the course of 
days, 