Rockwool is a synthetic material made of rock and sand that has been melted down, spun into thread and pressed together. Rockwool is primarily used for insulation purposes. But in the 1960's, rockwool's horticultural uses were discovered. Substrate hydroponics uses growing media to anchor the roots of plants. Because rockwool can hold large amounts of water and oxygen simultaneously, it makes an ideal substrate medium. However, be sure to buy high-quality horticultural grade rockwool. Insulation grade rockwool actually repels water.
Fill a container, large enough to easily accommodate the horticultural-grade rockwool with roughly 2 inches of water.
Place the rockwool flat in the container and allow it to absorb the water for 24 hours. Wet rockwool produces less loose fiber when it is cut.
Remove the rockwool from the water and place it on a few sheets of newspaper. Press down on it with your hands to remove some of the water. The rockwool should be saturated but not dripping wet.
Cut transplant holes in your rockwool. Oftentimes, seedlings are started out in small rockwool cubes that need to be transplanted into larger rockwool slabs as they grow. Place the cubes on the top of the rockwool where you would like to locate them. Trace the bottom of the cubes with a felt tip marker. Remove the cubes and draw two diagonal lines to connect opposite corners of the traced squares drawn on the rockwool. Use a sharp knife to cut along those diagonal lines. Cut roughly 1/2 inch deep and move your knife slowly and steadily to cut accurately and reduce debris. To insert the rockwool cube, peel back the four triangles created by the two diagonal cuts you made and push your rockwool cubes into the rockwool slab.
Cut large slabs of rockwool into smaller pieces. Use your marker to mark your cut lines. Use a sharp saw to saw through the rockwool. Apply moderate pressure and move from one end of the saw to another with each stroke so that you make a minimal amount of strokes to cut through the rockwool. This will give you even edges and limit the amount of debris created by the rockwool.