green strength

Pick up a dried but unfired ceramic part — a pressed tablet, a cast cup, an extruded rod — and it holds together in your hand, but only just; squeeze or drop it and it crumbles. That modest, just-enough toughness of the raw part is its green strength: the mechanical strength of a green body, the force per area it can bear before breaking while it is still unfired. It is what lets a fragile shaped-and-dried body be lifted, trimmed, stacked, and moved to the kiln without falling apart.

A green body has no fired bonds, so its strength comes entirely from weak, temporary forces holding the powder together: bridges of organic binder (usually the biggest contributor, which is much of why binder is added at all), friction and mechanical interlocking between packed particles, van der Waals attraction, and — if the body is still a little damp — capillary suction pulling the particles together. The numbers are humble: green strength is typically only a fraction of one up to a few MPa, perhaps a hundredth or less of the strength the same body will have after firing. It rises with more binder, denser packing, and finer particles, and (up to a point) with a trace of residual moisture.

Green strength is a practical, not a glamorous, property — but it decides whether parts survive the journey from forming to firing intact, and it is a major reason binders exist. Too little green strength and parts chip, slump, or break during handling, demoulding, machining, and loading; too much (from heavy binder) can make burnout harder and can crack the part as the binder is removed. Where parts are shaped by green machining — cutting the soft green body to final shape before firing — green strength must be high enough to machine cleanly yet low enough to cut easily. It is the strength that gets the body to the kiln; firing then replaces it with the real thing.