A drilling contractor who accepts a bentonite lot on the word "drilling grade" alone can have it rejected at the rig when it yields under 16 m³ of acceptable mud per tonne — the API 13A Section 9 floor — and the cost of trucking off-spec clay back from a remote wellsite dwarfs the discount that made it look cheap. Bentonite is sold under one generic name into at least five end-uses with non-interchangeable specifications, and the single most expensive procurement error is treating "bentonite" as a commodity rather than a spec class. This guide walks the spec sheet a buyer should hand to a mud engineer, a foundry metallurgist, or a pellet-plant operator before signing the next lot.
The 60-Second Read
Bentonite is montmorillonite-rich clay in two commercial families: sodium bentonite (high swelling, 24+ mL per 2 g) for drilling and sealing, and calcium bentonite (lower swelling but stronger bonding) for foundry and bleaching. Bare Syndicate supplies API 13A drilling grade from Khushab (Punjab), AFS foundry green-sand grade from Kohat (KP), montmorillonite-rich pelletizing grade, acid-activated bleaching earth, and soda-activated drilling grade. Global production is ~17–19 Mt/yr per USGS Mineral Commodity Summaries 2026; Pakistan contributes an estimated 100–200 kt/yr. The buyer's question is never "is it bentonite" — it is which test method the lot was certified against, and whether your application needs the sodium or the calcium family.
API 13A Drilling Grade: The Spec That Gates the Mud
API Specification 13A Section 9 is the gate for drilling-grade bentonite, and it is a performance spec, not a composition one. The three numbers that decide acceptance: swelling of 24+ mL per 2 g of clay, fluid loss below 15 mL in the 30-minute filter-press test at 100 psi, and a yield of at least 16 m³ of 15-centipoise mud per metric tonne. Material is milled to roughly 200 mesh so it hydrates fast in the mud pit. Only natural sodium bentonite — in Pakistan, the Khushab District deposits — reliably meets API 13A without chemical treatment; this is the same drilling-fluids discipline covered for weighting agents in our API 13A barite spec decoder. A lot certified only for "viscosity" without the fluid-loss and yield figures is not an API 13A lot.
Foundry Green-Sand Grade: Bonding Clay by AFS Method
Foundry bentonite is the bonding clay in green-sand moulding, and it is graded against American Foundry Society (AFS) methods rather than API. The two figures a foundry buyer checks: methylene blue absorption of 35–40 g per 100 g (the proxy for active montmorillonite content) and green compression strength of 8–12 psi on the bonded sand. Calcium bentonite — from the Kohat and Nowshera horizons in Khyber Pakhtunkhwa — is preferred here because its thermal stability and dry-strength behaviour suit repeated mould cycles better than high-swelling sodium clay. Confusing the foundry methylene-blue spec with the API swelling spec is a category error: a clay that excels at one routinely fails the other.
Pelletizing Grade: The Iron-Ore Binder
Iron-ore pelletizing uses bentonite as the binder that holds green balls together before induration, dosed at roughly 0.5–1.0% by mass of concentrate. The spec is montmorillonite content of 70% or higher at or finer than 200 mesh. Because every kilogram of binder is silica and alumina that dilutes the iron unit, pellet plants pay for montmorillonite activity, not bulk clay — so the methylene-blue number matters as much as it does in foundry use. This grade pairs with the magnetite and hematite feeds on our iron ore page.
Activated Bleaching Earth and Civil Grades
Two further grades sit outside the drilling-and-casting axis. Acid-activated bleaching earth is calcium bentonite treated with mineral acid to raise its surface area to roughly 200–300 m²/g, used to decolorize palm, soybean, and sunflower oils — a processed product and the highest per-tonne tier in the family. Civil-engineering grade supplies the swelling clay in geosynthetic clay liners (GCLs) and slurry-wall trenches, where the same 24+ mL sodium-bentonite swelling that makes good drilling mud also makes a self-sealing hydraulic barrier.
Sodium Activation: When Calcium Clay Becomes Drilling Grade
Where natural sodium bentonite is unavailable, calcium bentonite can be soda-activated — treated with sodium carbonate (Na₂CO₃, soda ash) so sodium ions exchange for calcium on the montmorillonite lattice, lifting the swelling index toward the API 13A 24+ mL threshold. It is a legitimate, widely-used route, but the buyer should know what they are buying: soda-activated clay is contractually drilling grade only after activation, and lot-to-lot consistency depends on the activation process, not just the parent clay. Per IMA-Europe's bentonite working group, activated and natural grades are distinct product classes and should be specified as such.
Where Bentonite Procurement Trips Up
- Treating "bentonite" as one product. Drilling (API 13A), foundry (AFS), pelletizing, bleaching, and civil grades have different test methods and price tiers; a single "bentonite" line on a PO specifies nothing.
- Assuming all Pakistani bentonite meets API 13A. Khushab sodium bentonite qualifies natively; much Kohat calcium bentonite does not without soda activation.
- Accepting "viscosity" in place of the full API 13A panel. Without the fluid-loss (<15 mL) and yield (≥16 m³/MT) figures, the lot is not certified to Section 9.
- Confusing sodium and calcium families. High-swelling sodium clay underperforms as a foundry bond; strong-bonding calcium clay fails the drilling swelling spec.
- Quoting one "bentonite price." Per USGS, drilling, foundry, pelletizing, cat-litter, and bleaching-earth markets are separate price tiers — bleaching earth runs several times raw clay.
- Citing a Pakistani deposit's reserves without a Geological Survey of Pakistan or PMDC source. Occurrence is not a certified reserve.
What to Verify Before Signing
- Grade and test method — API 13A Section 9 (drilling), AFS methylene-blue and green-strength (foundry), or montmorillonite-percent and mesh (pelletizing), named explicitly.
- Family — natural sodium, natural calcium, or soda-activated, with the activation status stated.
- Certified figures — swelling (mL/2g), fluid loss (mL), yield (m³/MT), methylene blue (g/100g), or surface area (m²/g) as the application requires.
- Independent assay — third-party laboratory results (SGS, Bureau Veritas) on the shipped lot, not a generic datasheet.
- Origin documentation — Certificate of Origin and mining-licence reference; HS code 250810 for customs classification.
- Moisture and packaging — paid on dry basis; 50 kg bags, jumbo FIBC, or bulk per logistics route.
Next step: Review grade specifications and origin documentation for bentonite and the companion drilling-grade barite, or request a delivered-cost indication with the API 13A, AFS, or pelletizing figures spelled out for your application. The full portfolio sits under the Minerals & Mining division, and our Pakistani galena and sphalerite spec guide follows the same procurement-first format for base-metal concentrates.
Additional Market Context
The foundational data sources for bentonite procurement are the USGS Mineral Commodity Summaries (annual clays chapter), API Specification 13A for drilling-fluid materials, and American Foundry Society standards for green-sand bonding clays. For Pakistani-origin material, the Geological Survey of Pakistan and PMDC publish deposit-level disclosures; independent inspection by SGS or Bureau Veritas at origin is the contractual default for any lot of meaningful tonnage.
Last reviewed: 2026-06-30. Spec ranges and test-method references are sourced to USGS MCS 2026, API 13A, and AFS standards; verify current certified figures against the named methods and an independent assay before contract execution.
