Less known facts about Excipients: Kaolin

This article is part of the “Less Known Facts About Excipients” series by Philippe Tschopp.

Kaolin: A natural adsorbent and glidant

Kaolin is one of the oldest mineral excipients used in medicine. For centuries, this naturally occurring white clay has been valued for its purity, adsorption capacity and chemical inertness. Even today, highly purified pharmaceutical-grade kaolin remains an important excipient in selected pharmaceutical and medical applications.

Unlike bentonite, which swells extensively in water, kaolin remains relatively non-swelling, making it particularly useful where adsorption and physical stability are required without significant viscosity development.

Kaolin is a naturally occurring hydrated aluminium silicate, composed primarily of the clay mineral kaolinite (Al₂Si₂O₅(OH)₄). It is formed over millions of years through the weathering of feldspar-rich rocks such as granite under warm, humid geological conditions.¹–³

How is pharmaceutical kaolin produced?

The production process consists of several carefully controlled steps:

Mining High-purity kaolin deposits are selectively mined from natural clay formations.
Blunging and Dispersion The crude clay is dispersed in water to separate fine kaolinite particles from sand, quartz and other mineral impurities.
Purification Hydrocyclones, centrifugation, magnetic separation and filtration are used to remove coarse particles, iron-containing minerals and other impurities.
Drying and Milling The purified kaolin is dried, milled and classified to achieve the particle size distribution required for pharmaceutical applications before complying with pharmacopeial specifications.¹–⁵

What makes kaolin unique?

Kaolin consists of plate-like kaolinite crystals arranged in a layered structure. Unlike swelling clays such as bentonite, the layers are strongly bonded by hydrogen bonds, preventing significant water penetration between them.

Its most valuable pharmaceutical characteristics include:

High chemical purity
Excellent adsorption capacity
Very low swelling
High whiteness
Chemical inertness
Good flow characteristics

These properties make kaolin an effective adsorbent while maintaining excellent physical stability in formulations.

Pharmaceutical applications

Pharmaceutical-grade kaolin is used in:

Oral suspensions
Topical powders
Dermatological formulations
Medical devices
Wound care products
Adsorbent formulations¹,²

Historically, kaolin was widely used in antidiarrheal products due to its adsorptive properties, although its use for this indication has declined in many countries with the availability of more effective therapies.

Its primary functions include:

Adsorbent
Suspending agent
Diluent
Protective agent
Opacifier

Considerations for formulators

As a naturally occurring mineral, pharmaceutical kaolin requires extensive purification to remove quartz, iron oxides and other naturally occurring contaminants. Modern pharmaceutical grades are manufactured under strict quality control to ensure compliance with pharmacopeial specifications.

Because of its high surface area and adsorption capacity, kaolin may bind certain active pharmaceutical ingredients, potentially reducing their bioavailability if formulation compatibility is not carefully evaluated.¹–⁴

Major pharmaceutical manufacturers

Examples of established manufacturers and suppliers include:

Imerys
Sibelco
KaMin LLC
Thiele Kaolin Company

Did you know?

The name “kaolin” originates from Gaoling (“High Ridge”), a village near Jingdezhen in Jiangxi Province, China, where the exceptionally pure white clay was first mined for the production of fine porcelain more than 1,000 years ago.

Its use in porcelain eventually led to its adoption in medicine, cosmetics and pharmaceuticals, making kaolin one of the few materials that has played a significant role in both art and science.

By Philippe Tschopp
Philippe Tschopp writes about pharmaceutical excipients, formulation and oral drug delivery, with a focus on excipient functionality, practical formulation considerations and lesser-known material properties.

Source:

Rowe RC, Sheskey PJ, Quinn ME (Eds.). Handbook of Pharmaceutical Excipients. Pharmaceutical Press / American Pharmacists Association.
European Pharmacopoeia (Ph. Eur.). Monograph: Kaolin, Heavy.
United States Pharmacopeia–National Formulary (USP–NF). Monograph: Kaolin.
Imerys. Pharmaceutical Mineral Solutions – Kaolin Technical Documentation.
KaMin LLC. Product Information and Pharmaceutical Grade Kaolin Technical Literature.
Murray HH. Applied Clay Mineralogy: Occurrences, Processing and Application of Kaolins, Bentonites, Palygorskite-Sepiolite, and Common Clays.
Guggenheim S, Martin RT. Definition of Clay and Clay Mineral: Joint Report of the AIPEA Nomenclature and CMS Nomenclature Committees.

Disclaimer

This article is based on publicly available information and industry sources at the time of writing and supported by AI. While every effort has been made to ensure accuracy, completeness, and fair representation, the author does not guarantee that all information is current, error-free, or reflective of the latest developments.

The content is provided for informational and educational purposes only and does not constitute technical, regulatory, or commercial advice. No liability is accepted for decisions made based on this material. Readers are encouraged to consult the original manufacturers and official sources to verify details before making professional or business decisions.