Foundry grade bentonite binds moulding sand, while cat litter bentonite must absorb liquid and form a firm clump. Both come from the same family of clays, but they are bought on different properties, so a grade that suits a foundry may be wrong for litter and the reverse.
Start from the use, then specify the measurable properties. The general picture of grades is in bentonite grades exported from India.
What does a foundry need from bentonite?
In green sand moulding, bentonite is mixed with sand and water to hold the mould shape while metal is poured. Foundries test the sand system, so they care about how the clay behaves in it. Specify and ask for results on:
- Montmorillonite content, often shown by a methylene blue value, because it is the active mineral.
- Green compression strength and wet tensile strength, measured in a standard test sand mix.
- Fineness, usually percentage passing a fine sieve, since coarse material mixes poorly.
- Moisture, because it affects both the weight you pay for and how the powder handles.
- Whether the clay is sodium type, calcium type or soda-activated, since this changes bonding and thermal stability.
How do foundries approve a source?
A foundry will usually run its own trial before approving a source. Give the supplier your sand system details so the sample is representative.
Ask for results on both the raw clay and the activated product, if any, because processing changes the numbers. Retest a sample from the first container when it arrives and compare it with the pre-shipment sample, so any drift in grade shows up while you can still raise it with the supplier. Foundries that run several moulding lines often keep two grades approved, one for general use and one for demanding castings, so ask which your castings need.
What does cat litter bentonite need?
Clumping litter relies on high swelling, which usually means sodium bentonite or a sodium-rich blend. When wet, the particles must swell and bind into a clump that stays intact when scooped. Buyers normally specify:
- Swell index or free swell, as covered in the bentonite swelling index.
- Granule size distribution, because the screen range affects clumping, tracking and appearance.
- Low dust, which matters to consumers and to retailers.
- Moisture, which should be stable so the product does not cake in the bag.
- Clump strength, tested by wetting a measured amount and checking whether the clump holds.
Does crystalline silica matter?
Bentonite can contain crystalline silica, which some markets regulate for dust exposure and labelling. Ask your supplier for the silica analysis and check the labelling rules in your market with your regulator or customs broker.
Bulk or bags, and how to avoid the wrong clay
Foundries often buy bulk or large bags, while litter packers may prefer bulk for their own filling line. See bentonite in bulk or bags for the practical differences.
The commonest mistake is accepting a name instead of a number. Two lots both sold as bentonite can differ widely in swelling and montmorillonite content. Ask for a certificate of analysis for each lot and keep a sealed retention sample to compare if there is a dispute.
Next step
We shortlist bentonite producers in Kutch and elsewhere in Gujarat, have samples tested against your use, and supervise loading. To start with your foundry or litter specification, book a free sourcing review.