Alternate Names: Bleaching clay
A natural clay akin to bentonite. It has a history of use as an absorbent for oil, grease, and animal waste, for bleaching and decolorising a wide range or organic oils and as a carrier for pesticides and fertilizers. When heat treated (calcined) the hardness and liquid-holding capacity of granules of this material can be greatly increased. RVM (regular volatile materials) are raw grades that break apart more easily with moisture, LVM (low volatile material) are heat treated grades.
Fuller's Earth is often called bleaching clay (although this also refers to smectite). While some use the term Fuller's Earth and Bentonite interchangeably they are quite different materials. However no common ceramic formulations call for Fuller's Earth.
Fuller's Earth is a very important mineral commodity. For example, in 2015 the USGS Mineral Commodity report claimed that 2 million tons were mined in the US alone the previous year at a price of $91/ton (the only common clay that exceeds this value is kaolin). While the value of common clays seems to climb with their purity and whiteness, Fuller's Earth can be quite high in iron and dark colored and yet still be an excellent for filtration and bleaching.
Fuller's Earth and bentonite are not formed by the same geological processes or from the same sources (the former is formed from volcanic ash, the latter from metamorphic rock). While bentonites are very high in plasticity, Fuller's earth is non-plastic or semi-plastic in character. Dry or dehydrated fuller's earth adheres strongly to the tongue. The absorption of water in sodium bentonite proceeds with a considerable increase in volume while Fuller's Earth does not.
To qualify as Fuller's Earth a clay must excel in certain filtration and decolorizing (bleaching) comparisons with known good materials. Clays having a wide range of physical appearances and chemistries can qualify, so chemistry information is not really relevant (do a google image search to see this). Still, we have supplied a bentonite-like chemistry but with lower Al2O3, higher MgO and higher LOI (from the chemistry info that we were able to find from various suppliers). Testing the filtration power of a clay can be as simple as passing an oil through a column of the clay (usually in granular form) and comparing the efficiency of decolorization against a known clay. Another variation of the test is to simply stir varying amounts of a clay into an oil, agitate the mix and filter the clay back out. The amount of clay needed to produce a given degree of decolorizing can be compared to other clays to establish its performance.
The MineralZone website has an excellent overview page to help you understand how these clays are tested and compared (link below).
Information from Mountain Rose Herbs
Fuller's Earth at Wikipedia
MineralZone Fullers Earth filtration, bleaching clay information page