When we talk about shading, we refer to a sandcarving technique that resembles airbrushing, except we do not use paint but our blaster to accomplish a similar effect. This requires a low pressure setting (about 1 to 5 pounds of pressure) and also a minimal abrasive flow. The objective is to create various areas within a design that are blasted to different shades of gray.
With an airbrush or a spray bottle, you control the amount of color delivered by the trigger or depressing the button more or less. In sandcarving, it is a bit trickier: you keep a much greater distance to the glass and move much faster with the nozzle.
There are two different styles of shading: one, uniform area shading, where any particular area is blasted to the same saturation all over; and two, variable area shading, where there is a gradation within any given shape.
-Ruth Dobbins, EtchMaster