You may want to carve an image in multiple stages and create some depth within an area that was already blasted to a certain depth and no longer has any resist to define a shape within this shape. This is what we call freehand carving, where we use the blaster as a sculpting tool without the aid of resist. This requires not only great hand-eye coordination, but complete attention, since you must know exactly where you want to point your nozzle, how close you need to go, how fast or slow you need to move, and at what angle you must hold the nozzle to achieve the effect you are after. Last not but least, you must also adjust your pressure to fit the size and depth of the intended blast.
-Ruth Dobbins, Professional Glass Consultants