Cannabis sativa extracts have become increasingly popular with consumers for their purity as well as their potency. Demand for lighter colored extracts has led some extractors to develop a process known as ‘polishing.’ This process effectively removes unwanted compounds from a cannabis extract, including pigments.
We have previously discussed how activated charcoal and silica are used in the extraction polishing process. Another effective polishing material is naturally occurring fine-grained bentonite clay, which has been used commercially since 1888 when it was called “Taylorite.” 
Bentonite consists of sheets of alumina (Al2O3) and silica (SiO2) since it is primarily composed of the smectite mineral montmorillonite.  As a clay, it has a moderate-high cation exchange capacity (montmorillonite estimated at 70-110 milliequivalents per 100 grams) , which refers to its ability to attract and hold positive ions such as potassium (K+) and calcium (Ca2+), two important ions in soil nutrients.
Bentonite is usually strip-mined and appears in formations known as “lenses” which are thick in the middle while tapered on the ends.  It readily adsorbs water and organic materials.  The chemical composition—SiO2 and Al2O3 as well as magnesium, calcium, and potassium oxides— plays a role in its adsorbent capabilities as one study noted when testing bentonite as a possible defluoridation method for drinking water.  But what about extract polishing?
A recently published patent application describes bentonite as a useful tool in removing unwanted pesticides and heavy metals from cannabis extracts, acting as a medium for chemisorption where pesticide analytes can be adsorbed chemically. However, bentonite is also described as a solution for “colorless” or “very light yellow” cannabis oil.
Activated bentonite is heated with sulfuric acid to enhance adsorbent/bleaching capacity. The degree of activation or clay enhancement depends on desired outcomes. For example, Carbon Chemistry produces two grades of activated bentonite: T-5™ with a neutral pH for fast filtering, and T-41™ with a low pH and activated carbon for more thorough refinement.
One study found that activated bentonite significantly reduced chlorophyll and carotene content in cold-pressed hemp seed oil to “produce a lighter and pale colored oil.”  Furthermore, ultrasonic treatment used in conjunction with activated bentonite also increased the oxidative stability of the oil, which should improve shelf-life. 
Whether bentonite will be adopted as a standard polishing material across the cannabis industry remains to be seen. Regardless, clay minerals undoubtedly illustrate efficacy as adsorbents in multiple industries.
- Clem AG, Doehler RW. Industrial applications of bentonite. Clays Clay Miner. 1961;10:272–283. https://doi.org/10.1346/CCMN.1961.0100122. Times cited = n/a, Journal impact factor = 1.507
- Thakre D, et al. Magnesium incorporated bentonite clay for defluoridation of drinking water. Journal of Hazardous Materials. 2010;180(1-3):122-130. https://doi.org/10.1016/j.jhazmat.2010.04.001. Times cited = 96, Journal impact factor = 9.038
- Liang J, et al. Reduction of chlorophyll in cold-pressed hemp (Cannabis sativa) seed oil by ultrasonic bleaching and enhancement of oxidative stability. European Journal of Lipid Science and Technology. 2018;120(4):1700349. https://doi.org/10.1002/ejlt.201700349. Times cited = 4, Journal impact factor = 2.056
Photo courtesy of Josh Boaz at Unsplash