A very common question I get from people is about the difference between slurry and swarf. What is Slurry? What is Swarf? Here's a quick crash course.
Slurry = Loose particles of sharpening stone abrasive. This is the "paste" that forms when you flatten your sharpening stones and is the color of the stone you are using. Slurry can increase the cutting speed of a sharpening stone.
Swarf = Loose metal particles that are formed when sharpening a knife. This mixes with the slurry while you are sharpening and is blackish or grayish in color. Swarf can increase the polishing effects of a stone, BUT it does not increase the cutting speed of a sharpening stone.
Forming a slurry at lower grits is encouraged because it increases the speed of bevel setting and scratch cleanup. On higher grit stones it is a different story.
On higher grit natural stones, a slurry is encouraged because it will break down and form an even higher grit in the process. This is why some Japanese natural stones span a grit of 6,000 to 10,000 grit, or 10,000 to 15,000 grit, and beyond. On a synthetic stone, however, or when using the "Murray Carter" method of sharpening, a slurry is not encouraged due to the particles running into the fine edge you've created. This can sometimes slightly dull the edge. Synthetic stone particles have a much harder time breaking down, thus, they tend to finish at the grit they were designed for.
Feel free to shoot questions or add to this, please!