This 4000 grit (3.68 micron) GlassStone (50103) is a medium grit, so cuts quickly but leaves a fairly fine finish.
The Shapton GlassStone HR series of ceramic sharpening stones are the most advanced series of stones that Shapton makes - they are designed to cut both modern tool steels and high carbon steel very quickly, but wear very slowly, so don't need to be flattened as often as most waterstones. The stones have a 5 mm thick layer of very consistent high quality ceramic abrasive which is backed with glass (which is where the name comes from) to provide a very flat base. The glass supports the ceramic abrasive, so that the entire thickness can be used without cracking or breaking. The glass stones measure 210 mm x 70 mm x 10 mm (8-1/4" x 2-3/4" x 3/8") and are made in Japan.
The abrasive particles in the Shapton stones are very consistent in size and this uniform size allows the stone to cut quickly while leaving a very consistent finish. Another great advantage of the glass backing is that the grit and micron rating is marked on the underside of the glass and is visible through the glass side of the stone which makes for very easy identification.
Unlike traditional waterstones, Shapton glass stones do not require soaking and we recommend that they should never be soaked in water because this can lead to softening of the stone. All that is required is wetting the surface of the stone with water, using a spray bottle.
Although these sharpening stones don't need to be flattened as often as waterstones, they still do need to be flattened occasionally - we recommend using either diamond plates such as the Shapton DGLP, DMT or Atoma, or for those on a budget, wet and dry sandpaper on a piece of float glass - in a pinch we have even used a piece of wet and dry sandpaper on the glass back of one GlassStone to flatten another. A useful trick when checking for flatness is to scribble pencil across the stone and check to see when the pencil marks have disappeared. When flattening a GlassStone, it's surface should be lightly sprayed with water, just as when sharpening. The stones should also be rinsed off and wiped dry after use.