Lamb calls the casting process "lost foam", as it is similar to the lost wax technique but substitutes a wax original for one of expanded polystyrene.
Here's the text from Lamb:
Sacrifice and Investment - Bronze Poly Chair
A series of unique chairs cast in Bronze using the 'lost foam' casting process.
The Bronze Poly Chair combines two processes I have explored in previous projects - my Pewter Stool that was cast directly into a hand-carved mould on a sandy beach in Cornwall, and my Polystyrene Chair that is carved from a solid block of low density expanded polystyrene (98% air) and then coated in rubber.
Each Bronze Poly Chair is hand-sculpted in polystyrene foam and then buried in sand ready to be sacrificed.
Ingots of bronze are heated to over 1100 degrees C and the 'red hot' molten metal is poured through the sprue into the sand, consuming the delicate foam and investing the hidden cavity in bronze.
Over two hours of waiting and the solid bronze replica of the foam object is broken from the sand.
With only one attempt to invest the foam in bronze, every sacrifice is a risk.
Successful or not, each casting requires a new foam pattern ensuring no two chairs are the same.