Green sand is silica sand bonded with bentonite clay, activated by water, with a carbonaceous additive such as sea coal mixed in. Four materials, no chemical binder and no curing step.
The name refers to moisture: the mold is rammed and poured while it is still damp. It is also unrelated to the glauconite mineral sold as “greensand” for garden soil.
What Goes Into a Green Sand Mix
A green sand mix is four ingredient classes, and published percentages for them disagree from one source to the next because the alloy poured, the casting’s size and section thickness, and the amount of core sand the job drags into the system all move the numbers.
| Ingredient | Typical share | What it does in the mold |
|---|---|---|
| Silica sand | 85–95% | Refractory bulk; holds the cavity against the pouring temperature |
| Bentonite clay | 5–12% | The binder; more clay raises strength and lowers permeability |
| Water | 2–5% | Activates the clay and gives plasticity; past the temper point, more water weakens the mold |
| Carbonaceous additive | 2–10% | Forms a lustrous carbon film at the mold face against penetration and burn-on |
Base Sand
The base sand is bought for refractoriness and grain size. Silica at 98% SiO₂ fuses at about 3100 °F (1704 °C); sands with less silica, lake sand among them, fuse nearer 2800 °F (1538 °C).
Finer sand gives a better as-cast surface but demands more clay and water and vents gas less freely; coarser sand vents well and tends toward a rougher surface and metal penetration.
Chromite, zircon and olivine replace silica as base or facing sand where expansion defects or penetration will not go away otherwise.
Bentonite: Sodium or Calcium
Iron and steel foundries run sodium bentonite, or a blend weighted toward it, because it holds strength at mold-face temperature. Compared in the lab at 8% clay and equal compactability, sodium bentonite reached 575 psi in hot compression at 1800 °F against 110 psi for calcium, with wet tensile strength several times higher.
Calcium bentonite flows better and develops its green properties faster, and at the same 8% it gave the higher green compression strength, 14.3 psi against 11.8. Blending the two clays is common practice and lands the mold properties near the average of the two.
Carbon Additives and the Alloy Poured
The carbon additive is the ingredient that changes with the metal. Gray iron and ductile iron molds carry sea coal at around 5% of the mix. Some of that sea coal is now replaced with causticized lignite or gilsonite to cut emissions.
On the carbon steel side the same carbon is a liability. Steel practice uses iron oxide additives, and a great many steel molds are chemically bonded rather than clay bonded.
How a Foundry Controls the Mix
A foundry runs its sand to a set of measured properties, and it steers them with moisture. Compactability is the working proxy for water: the muller is set to hold a compactability number.
The methylene blue test reports active clay, the only clay still able to bond; the AFS clay test counts all the fine material, dead clay, ash and coke included. The carbon side splits the same way: loss on ignition counts every combustible in the sand, while the volatiles test at 900 °F (482 °C) isolates the coal still capable of working.
Green compression strength and methylene blue clay are daily tests, total clay and combustibles weekly. Ask a supplier for its control limits and how long its records run; a drifting system shows up on your castings as blows, penetration and sand inclusions.
Why System Sand Is Not the Recipe
The sand in the flask is recirculated sand, and its composition drifts on every trip through the molding line. Heat at the mold face destroys the bentonite’s bonding power for good, above roughly 600 °F (316 °C) for calcium bentonite and 1200 °F (648 °C) for sodium. The dead clay stays in the system as inert fines.
Depending on the atmosphere in the mold, the carbon additive returns to the muller as some mixture of coal, coke and ash, and only the unburned coal still does any work. So new clay, new carbon and new sand go in every cycle, sized to what was destroyed.
Mulling is what makes an addition count. Two plants working from the same recipe and mulling differently will turn out different molds.