N.S.P  techniques--Tonal performance
 
Let us interduce more about  the N.S.P , we can use the force of the draw to create different color levels. However, most children can't control the force, so we can use different soft and hardness pencils to make the effect of depth  of the picture. The principle is very simple: "The harder the pen, the darker the color; the softer the pen, the lighter the color" is the opposite concept of drawing with a pencil, but pay special attention to the fact that the printing effect will be too light and not obvious if the pen is too soft for engraving.  It is recommended not to use soft pencils above 3B; although the soft pencils will  be printed with a gray tone, but N.S.P will not be printed if drawing force are too light with soft pencil above 3B.
 

In addition, with density of sandpaper, different effects will be produced.

Usually I would suggest students: Use #600 sandpaper with 6H pencil to draw the outline of the theme ( black and darker) . Use #200 sandpaper with 2B( or HB) pencils for color blocks or background processing to enrich the tone and emphasize the theme.
 

The following is N.S.P  tone for reference.