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Spectral analysis devices are used for the rapid chemical analysis of a material by means of the spectrum of its components. The sample is exposed to an electric arc. The light emitted by the arc is refracted into its different spectrums, showing the spectral lines of all the elements contained in the material. The qualitative analysis of the individual elements contained in the sample is carried out by identifying these lines. As a rule 8-10 elements are identified.

Soot is produced in special furnaces by means of the incomplete combustion of gases, liquids or solids containing carbon. Soot is a carbon product with a particularly small particle size.

The solidification temperature is the temperature at which a material changes from the liquid into the solid state. Above the liquidus temperature, a material is completely liquid. The material is completely solid only once the solidus temperature is reached. The range in between is called the solidification interval.

Liquidus, solidus and the solidification interval

Once the melt cools below the liquidus temperature, solidification starts: the first solid crystals form while the rest of the melt is still liquid. For a pure metal such as iron or copper, liquidus and solidus temperature coincide: the material solidifies at a single temperature, the same way water freezes at 0 °C. For an alloy, the two temperatures are usually apart, because the composition of the remaining melt keeps shifting throughout solidification. Only at eutectic compositions, for example eutectic cast iron with about 4.3% carbon, do liquidus and solidus temperature converge to a single point again.^[1] The solidification interval, the gap between the two temperatures, largely determines the flow and feeding behaviour of a casting alloy: a narrow interval solidifies shell-like from the outside in, while a wide interval solidifies mushy across the whole cross-section, which makes feeding harder and increases the risk of shrinkage porosity.

Practical use: why foundry engineers need liquidus and solidus temperature

For the engineer, the liquidus temperature is first of all the minimum temperature an alloy must reach in the furnace before it is castable at all; in practice the melt is superheated somewhat above it so it does not solidify prematurely while filling the mould. The solidus temperature marks the point from which a casting is dimensionally stable enough to be shaken out or unmoulded without any liquid melt still escaping. Together, both values also determine whether an alloy is suitable for semi-solid casting (rheocasting, thixocasting), and they underpin riser design: the wider the solidification interval, the larger the riser must be sized to avoid shrinkage porosity in the last region to solidify.

Liquidus and solidus temperatures of common foundry materials

The following values are rounded reference figures and vary with the exact alloy composition.

Material	Liquidus temperature	Solidus temperature	Solidification interval
Pure iron	1536 °C[1]	1536 °C[1]	0 K (pure metal)
Eutectic cast iron (~4.3% C)	~1153 °C[1]	~1153 °C[1]	~0 K (eutectic)
Hypoeutectic cast iron (~3.2% C)	~1200 °C[1]	~1150 °C[1]	~50 K
Aluminium casting alloy AlSi12 (near-eutectic)	~577–590 °C[2]	~577 °C[2]	narrow
Brass CuZn37	920 °C[3]	902 °C[3]	18 K

Frequently asked questions about liquidus and solidus temperature

What is the solidus temperature?

The solidus temperature is the temperature at which an alloy’s solidification is complete. Below this temperature, the material is fully solid.

How are liquidus and solidus temperature measured?

The common method is thermal analysis: a sample of the melt is cooled under controlled conditions while its temperature is recorded over time. The start and end of solidification show up as kinks or plateaus in the cooling curve, because latent heat is released during the phase change.

Why is a narrow solidification interval an advantage for the foundry engineer?

A narrow interval leads to directional, shell-like solidification from the mould wall inward. This can be fed well with risers and produces a denser casting with fewer shrinkage defects than mushy solidification with a wide interval.

Standard reference

DIN 51007 describes the fundamentals of thermal analysis (including differential thermal analysis), the method used in operational and laboratory practice to determine liquidus and solidus temperatures.

References

1. maschinenbau-wissen.de: Eisen-Kohlenstoff-Diagramm (iron-carbon phase diagram), liquidus line and eutectic point. German-language source, no English edition available. Accessed 27 August 2026.
2. materialmagazin.com: Aluminium-Gusslegierungen (aluminium casting alloys), Al-Si eutectic at 577 °C. German-language source, no English edition available. Accessed 27 August 2026.
3. Deutsches Kupferinstitut (German Copper Institute): Werkstoff-Datenblätter CuZn37 (material data sheet), section 3.2, solidus and liquidus temperature. German-language source, no English edition available. Accessed 27 August 2026.

Solidification is the transition from the molten to the solid state. Contraction occurs during solidification, and the resulting volume reduction on the mould has to be compensated for by the addition of more molten metal.

Slag control agents are added to the melting furnace as slag additives in order to alter their composition, or to bind the slag being formed, so that this can then be removed with suitable tools, i.e. tapping the slag. Such agents are also used to remove heavy slag deposits from the furnace walls. Normal soda is also often used for this purpose. When using soda however, care must be taken to ensure that the crucible wall itself is not damaged.

Slag consists of oxidic materials which are largely insoluble in molten metals, and
instead separate from them to form a separate layer on top of the molten metal.
A distinction is made between basic and acidic slag.

Function and reuse

Slag has a lower density than the metal, which is why it forms a separate layer on top of it; the slag layer also insulates the melt thermally and shields it from atmospheric oxygen^[1]. Processed metallurgical slag is widely reused as a secondary raw material, for example as ground granulated blast-furnace slag in cement production or as aggregate in road construction^[1].

References

1. Emergency reference (secondary source): Wikipedia: Slag. Retrieved 27 Aug 2026.

A siphon outlet enables slag-free casting from foundry ladles. In channel furnaces, the inlet and outlet are arranged according to the siphon principle. In this way, the molten metal is always fed in and removed below the bath surface. When emptying below the minimum sump level, the function of the siphon must not be interrupted for filling, i.e. the furnace may only be tipped back slowly, in relation to the filling level. If the furnace is tipped back into the basic position while still under-filled, the ingress of air, and thus also oxygen, is unavoidable. The furnace atmosphere is therefore enriched with oxygen, leading to increased slag formation on the melt sump. The ceramics in the transitional area between the siphon and the furnace vessel can also be damaged.

Sintering is the compaction of crystalline, granular or powdered materials by means of accretion of the crystallite under corresponding heating, without any of the components being melted. With the use of sintering agents, the reaction temperature can sometimes be reduced by up to 200 °C. For example, boracic acid or boric anhydride are used in the case of acidic tamping compounds. With the addition of 0.8%, the sintering point of pure quartzite is lowered from 1,700 °C to 1,550 °C.

Simplex operation refers to the melting of solid materials in a furnace without the addition of any molten charge.

Silicosis is a lung disease. It is caused by breathing in fine dust that contains alpha-quartz or another crystalline form of silicon dioxide. A respirator mask with nose filter should be worn whenever working with SiO2 materials^[1].

References

1. Occupational Safety and Health Administration (OSHA), U.S. Department of Labor: Silica, Crystalline – Overview. Accessed 27 August 2026.