How bentonite is tested, and what the numbers mean
Written for the engineer who specifies the purchase, not only the buyer who places it.
What each test actually measures
Swell index
ASTM D5890Two grams of dry clay is sprinkled into 100 mL of water and left to settle. The settled volume in millilitres is the swell index, the most quoted number in the bentonite trade.
API filtrate
API 13B-1A mud sample is held at 100 psi against filter paper for 30 minutes. Low filtrate means a thin, tight filter cake that stops fluid escaping into the formation.
Viscometer readings
API 13B-1A rotational viscometer reads the mud at 600 and 300 r/min. From these come apparent viscosity, plastic viscosity and yield point.
Marsh funnel viscosity
Marsh funnelThe time for 946 mL of slurry to drain from a standard funnel. The quickest site check for piling and HDD slurry.
Methylene blue
MB titrationMethylene blue dye is titrated into a slurry until the clay adsorbs no more. It measures how much active clay you are actually buying.
Enslin water absorption
Enslin-NeffHow much water the clay draws up over time. Pellet plants use it to compare binders at equal dosage.
Green compression strength
AFSA rammed sand specimen is crushed and the peak load recorded. It decides whether a mould holds its shape. Foundries buy this number.
Residue on 75 µm
Wet sieveGrit left on a 200-mesh sieve after washing. Above 2.5% a clay fails OCMA grade, and grit wears pumps and mixers.
For what the standards require, see API 13A and OCMA explained.
Where Kutch bentonite comes from
Bentonite in Kutch (Kachchh) is a weathering product of Deccan Trap basalt, formed in the Palaeocene, roughly 65 to 58 million years ago. Bentonite, white clay and laterite are levels of the same weathering profile.
Where drainage was poor, silica stayed in place and montmorillonite formed: that is bentonite. Where drainage was efficient, silica was flushed out and the clay became kaolinitic: the kind of deposit our plastic white clay comes from. Further leaching leaves laterite.
The ore bodies are lenses and pockets a few metres thick, not continuous seams. Clay changes from lens to lens, which is why we keep faces separate, stockpile by grade and test every lot rather than every season.
Test it against what you buy today.
A 2.5 kg sample with its certificate of analysis, sent by DHL or FedEx. Tell us your current supplier's figures and we will tell you honestly how ours compare.