Putting LIne to Work
Before computers, before photography, before magic markers even, images were reproduced in woodcut, engraving or etching. From the 15th sixteenth to the 17th century visual information was encoded into lines, shapes or dots. This was the vocabulary of reproduced images.
In your sketchbooks you often have a felt tip marker or a sharpie to work with. How do you make shades of grey with a tool like this? You can achieve a broad range of grey tones by overlapping layers of line with different angles and spacings. It's called crosshatching.
Using just a felt tip pen or sharpie, draw for an hour from a subject that presents you with a broad range of value. Drapery forms or arrangements of planes will have a variety of continuously changing value. Shadows cast on walls and floors, cloud forms, and the classic rumpled paper bag will give you good material to work with. The aim is to avoid depending too much on contour to identify the forms. Emphasize the value ON the form and inside the forms, work from the interior toward the edges.
Draw for on hour and try to extract at least five levels of value from the tools you choose.
Work inside some kind of framing device as well. Draw rectangles or squares in your sketchbook and draw inside them. Get used to working with the edges this way.
Check out these links for more information on this technique.
http://www.dannygregory.com/2005/09/cross_hatching.php
http://drawsketch.about.com/library/blinktexture.htm