the solidus
/ SOL-ih-dus /
The solidus is the liquidus's partner and its mirror. If the liquidus is the temperature where the first crystal appears on cooling, the solidus is the temperature where the last drop of liquid disappears — or, going the other way, the temperature on heating where the very first bit of liquid is born. Below the solidus everything is solid; above it, at least a trace of liquid is present. It is the floor of the melting range, the line where solidity finally ends.
Between the solidus and the liquidus lies the two-phase mush where solid and liquid coexist, and the two lines are the boundaries a tie line runs between. On the solidus you read the composition of the solid that is in equilibrium with the melt; on the liquidus you read the composition of that melt. In a simple eutectic system the solidus often runs flat along the eutectic temperature — a horizontal line — because below that temperature no liquid can survive at any composition. In solid-solution systems the solidus is a sloping curve giving the changing composition of the growing crystals.
For a ceramist the solidus is the honest warning line: the temperature at which the first liquid appears on heating is the temperature at which a body can begin to sinter with liquid help, deform, or lose load-bearing strength. That is why a refractory's safe upper service temperature is set not by its bulk melting point but by its solidus — the moment any grain-boundary liquid forms, creep accelerates and the part starts to sag. Impurities that create a low-melting eutectic pull the solidus down and quietly steal service temperature.
A magnesia refractory brick contains a little silica impurity. On heating, its solidus — the temperature where the first grain-boundary liquid forms — can be hundreds of degrees below pure MgO's 2800 degrees C melting point. That solidus, not the melting point, is the real ceiling: above it the brick begins to creep under load.
The solidus is the melting-begins line on heating; below it all solid, and a refractory's service ceiling sits here.
The solidus, not the pure-component melting point, sets a refractory's safe upper temperature — a tiny impurity that drops the solidus can cost hundreds of degrees of service headroom even when the bulk melting point looks reassuringly high.