Perlite is actually an expanded stone (just like pop-corn). 
The raw material comes from Greece and this stone is volcanic. This material is sieved in several gradations and is shipped that way to Holland or Germany.
The Perlite producer will take this sand and heat it up to 1.000 oC and so it will expand and after cooling down you have perlite.
This sounds easy but you can imagine that the temperature is very important, also the speed of heating will have its influence on the quality of the perlite. For example if you heat it up too much or too fast, the perlite will be too weak and can be destroyed easily. If you heat it up too little, the perlite will probably be sturdier but also less airily.

The reasons for using perlite are:

  1. Perlite will bring air in the potting soil. If you take a seedling substrate, for example you know that the peat should be fine (otherwise machines will refuse or trays will not be filled properly) but that means that there is less air in the potting soil. By adding Perlite, you bring air in the potting soil and still it is very well possible to handle the potting soil in/on the machine.
  2. However, if you have a coarse mix, it does not bring air (the Perlite will “take its place” in the “air-holes” made by the coarse nuggets). In fact, if I take 10 m³ of course peat and 1 m³ of Perlite, totally it will have 10 + 1 = 10 m³. But there is another advance of Perlite: it does not absorb much water (only 2,5 times its own weight) but it does absorb the water very easily and fast. So if you want to add water in a dry potting soil, it will go a lot faster with Perlite.
  3. This only holds true if water is added from the top. The other way around will have the opposite effect, the capillarity will be less if perlite is mixed in the substrate.

If you have any questions feel free to send an e-mail.