Rockwool is made from molten rock that is spun into long, glass-like fibers. These fibers are then compressed into bricks and cubes, or sold loose as "flock." Rockwool has long been used to insulate buildings as an alternative to fiberglass, and has been a mainstay in commercial hydroponics for the last twenty years. It readily absorbs water and has decent drainage properties, which is why it is used widely as a starting medium for seeds and a rooting medium for cuttings. In my opinion, the premiere benefit of rockwool is its sterility from pathogens and just about anything else that could contaminate a hydroponic system. Some of the world's largest hydroponic greenhouses use rockwool slabs to raise numerous varieties of plants to full maturity, and they often reuse the rockwool slabs many times by steam sterilizing the slabs between crops.
Lately I have noticed a decline in the use of rockwool by the hydroponics hobbyist. From what I have observed, this may be due to a wider sense of environmental responsibility to avoid using products that cause pollution from their manufacturing process, as is the case with rockwool. A word of caution should you decide to work with rockwool. Many people find its dust is irritating to the skin, which leads me to believe there could be a similar effect in the lungs if its dust is inhaled.
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