the liquidus
/ LIK-wih-dus /
Imagine slowly cooling a hot melt and watching for the very first speck of crystal to appear. The temperature at which that first solid forms, for a given composition, is a point on the liquidus. Trace it across every composition and you get the liquidus line — the upper boundary on a phase diagram, above which the material is entirely molten and below which the first crystals begin to grow. It is the ceiling of solidity: cross it going down and freezing begins.
On a binary diagram the liquidus is the swooping curve (or pair of curves) that dips down to the eutectic valley. Its meaning is precise: at any temperature inside a solid-plus-liquid region, the liquidus tells you the composition of the liquid that is in equilibrium there. As you cool a melt below the liquidus, crystals of one phase come out, the remaining liquid shifts its composition by running along the liquidus line, and it keeps enriching until it reaches the eutectic, where the last of it freezes. Read together with the solidus, the liquidus brackets the mushy two-phase zone where solid and liquid coexist.
For firing and refractories the liquidus is the practical ceiling. The liquidus temperature of a body sets how hot you must go to fully melt it — critical for glass, where you must stay above the liquidus to avoid unwanted crystals (devitrification), and for glazes, which must melt and flow. For a refractory the liquidus warns how much liquid will be present at service temperature: the further below the liquidus you operate, the more solid skeleton survives to hold the shape. Impurities that drag the liquidus down are exactly why a nominally high-melting refractory can start to slump early.
For a soda-lime-silica glass batch the liquidus sits near 1000 degrees C. Melt and hold it above that, and it stays a clear liquid; let it linger just below the liquidus and cristobalite crystals nucleate and cloud the glass. Glassmakers deliberately work above the liquidus and cool fast through it precisely to dodge that crystallization.
The liquidus is the freezing-starts line: above it all liquid, below it the first crystals of the primary phase appear.
The liquidus gives the composition of the liquid, not of the whole sample. A common slip is to read your overall batch composition off the liquidus curve — the curve tells you what the coexisting liquid is, and the solid's composition sits on the other bounding line.