Perlite is an aluminum potassium sodium silicate mineral of volcanic origin. The natural mineral contains water in its chemical structure. When heated to a suitable point in its softening range (around 1600F) the expanding water creates tiny bubbles (in a manner similar to popping corn). These greatly expand the volume and create a very light-weight granular material of excellent insulating properties. The nature of the rock and manner of production can create a wide range of densities and sizes of granules. Material of larger granule size is of most interest in ceramics.
Perlite is not refractory in a ceramic sense, being a volcanic glaze it melts fairly early. Notwithstanding that, it can service sell to about 1200F, making it suitable in many applications. A very novel use of the perlite is making insulating boards and shapes by cementing the granules using sodium silicate (there are many videos on youtube about this). Sodium silicate acts as a sticky, inorganic glue to create a surprisingly strong and super-light material. This is not refractory enough for firing typical ceramic ware, but is suitable for forging steel. If an alumina oxide lining is employed, slightly higher temperatures are feasible. Perlite certainly could be used with refractory ceramic fibers, the latter on the interior of the kiln and the former out the outside.
Granular perlite can also be used as an aggregate material to improve drying properties in sculpture clay bodies, reduce density, or add texture.
Perlite has a wide range of chemistries, the one shown here is an example. However the chemistry of this material is not normally a factor in the reason for its use.
Since particles can be extremely fine and are microscopic glassy shards, breathing them is hazardous.
This body is a plastic fireclay base having 13% 20/48 grog and 10% 65 mesh silica sand. But the texture is far coarser than one would expect. That is because it has 4 cubic feet of perlite per thousand pound batch. If desired the surface can be trowel smooth. This works well partly because the perlite particles are soft and easy to crush.
Perlite at Wikipedia
How perlite is used to create insulation materials
YouTube video: How to make a coffee can forge furnace and stainless steel burner
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.