Laboratory Crucibles: Materials, Styles, Temperature Limits, and Selection

Estimated reading time: 10 minutes

Laboratory crucibles are heat-resistant vessels used for ashing, ignition, calcination, fusion, melting, and other high-temperature sample-preparation procedures. Although their construction is simple, crucibles are not interchangeable. A material that performs well during routine ashing may fail rapidly in a molten alkali flux, contaminate a trace-element analysis, or crack after repeated thermal cycling.

Crucible selection should therefore consider temperature, sample chemistry, furnace atmosphere, contamination tolerance, heating and cooling rate, and intended analytical procedure. Maximum temperature alone is rarely sufficient.

Common Crucible Styles

Crucible geometry affects heating, containment, sample access, and evaporation.

High-Form Crucibles

High-form crucibles are relatively tall and narrow. Their deeper geometry helps contain powders and reduces losses from spattering or vigorous reactions. They are commonly used for ignition, ashing, calcination, and general gravimetric procedures.

Low-Form Crucibles

Low-form crucibles are wider and shallower. Their larger exposed surface area can improve sample access and facilitate evaporation or heating. They are useful when rapid heating and handling are more important than maximum containment.

Crucibles with Lids

A fitted lid reduces contamination and sample loss while allowing some gas exchange through the gap between the crucible and lid. The lid position depends on the procedure. During oxidative ashing, for example, sufficient oxygen must reach the sample. A lid may therefore be tilted or removed during part of the heating cycle.

Filtering Crucibles

Filtering crucibles incorporate a porous or perforated base. A precipitate can be collected by vacuum filtration, washed, dried or ignited where appropriate, and weighed in the same vessel. Sintered-glass filtering crucibles are useful for filtration and drying but should not be treated like refractory ceramic crucibles. Many laboratory sintered-glass products have maximum-use temperatures around 500°C or below, depending on glass composition and construction. The manufacturer's temperature specification should be checked before ignition.

Porcelain filtering crucibles and other ceramic designs are available where higher-temperature treatment is required.

Combustion Boats

Combustion boats are elongated, shallow vessels designed for tube furnaces and combustion systems. Their geometry allows a sample to be positioned within a furnace tube while maintaining gas flow around it.

Alumina, porcelain, and fused-silica boats are commonly available.

Common crucible materials

Porcelain Crucibles

Porcelain is one of the most widely used materials for routine laboratory crucibles. Laboratory porcelain is a vitrified ceramic that combines reasonable chemical resistance, mechanical strength, and low cost.

Common applications include:

• Ash determination
• Gravimetric ignition
• Loss on ignition
• Calcination
• Heating inorganic precipitates
• Routine teaching and research

Depending on composition and manufacturer, laboratory porcelain crucibles commonly have useful temperature ratings in approximately the 1,000–1,200°C range, with some products rated higher for limited exposure.

A stated maximum temperature should not automatically be interpreted as a continuous-use temperature. Repeated cycling near the maximum rating can shorten crucible life.

Porcelain is also not chemically inert under all conditions. Strong alkalis and molten alkaline fluxes can attack the glaze and ceramic body. For trace-element work, potential contributions from the ceramic composition should be considered.

For routine furnace ashing at approximately 500–600°C, however, porcelain is often an economical and appropriate choice.

Alumina Crucibles

Alumina crucibles are manufactured predominantly from aluminum oxide (Al₂O₃). Laboratory products are available in a range of purities, commonly from approximately 95% to 99.8% Al₂O₃ or higher.

High-purity alumina provides substantially greater refractory capability than ordinary porcelain. Depending on purity, design, atmosphere, and manufacturer, alumina crucibles are commonly used at temperatures in the 1,500–1,700°C range.

Applications include:

• High-temperature calcination
• Ceramic synthesis
• Powder processing
• Materials research
• Thermal treatment
• Furnace experiments

Alumina nevertheless has important limitations. It is brittle and can crack under severe thermal shock. Molten alkalis, certain metal oxides, slags, glasses, and flux systems may react with alumina at elevated temperatures.

Contamination must also be considered. If aluminum is an analyte of interest at trace concentrations, an alumina crucible may be inappropriate even though it is mechanically and thermally suitable.

Porcelain vs. Alumina Crucibles

These two materials cover a large proportion of routine laboratory furnace work.

*Representative ranges only. Product-specific manufacturer ratings should govern use.

The higher temperature rating of alumina does not make it automatically superior. A porcelain crucible used well within its temperature and chemical limits can be more economical for routine analytical work.

Fused-Silica and Quartz Crucibles

Products commonly called quartz crucibles are generally manufactured from fused silica (SiO₂).

Fused silica combines high chemical purity with an exceptionally low coefficient of thermal expansion. It consequently has substantially better thermal-shock resistance than many conventional ceramic materials.

Applications include high-purity sample preparation, specialized thermal treatment, semiconductor-related processes, and procedures where metallic contamination must be minimized.

Its limitations, however, are important.

Devitrification

Fused silica is an amorphous material. During prolonged or repeated high-temperature exposure, it can progressively crystallize, particularly to forms of crystalline silica such as cristobalite. This process is known as devitrification.

Devitrification becomes increasingly important during prolonged or repeated exposure above roughly 1,000–1,100°C, although there is no single universal threshold.

Surface contamination—particularly alkali metals such as sodium and potassium—can accelerate the process substantially.

A devitrified crucible may become:

• Cloudy or opaque
• Roughened
• More brittle
• Increasingly susceptible to cracking during cooling

This means that the practical lifetime of a fused-silica crucible may be limited by devitrification well before the material reaches its nominal softening temperature.

Clean handling is therefore particularly important for quartz crucibles used repeatedly at high temperature.

Chemical Limitations

Hydrofluoric acid attacks silica rapidly. Strong alkalis and particularly molten alkaline compounds can also attack fused silica.

Quartz is also inappropriate where Si contamination would compromise the analysis.

What Should Be Used with Hydrofluoric Acid?

HF is unusual because it attacks silica-containing materials, including ordinary laboratory glass and fused silica.

For HF digestion at moderate temperatures, PTFE and PFA fluoropolymer digestion vessels are commonly used because of their excellent resistance to hydrofluoric acid. These are digestion vessels rather than conventional high-temperature furnace crucibles.

Platinum is also resistant to HF under many conditions and may be appropriate for certain analytical procedures. However, compatibility must be evaluated for the complete sample-reagent system, not HF alone.

A material that tolerates pure HF may still react with another acid, oxidant, flux, or sample constituent present in the procedure.

Specialized Crucible Materials

Some analytical procedures require crucible materials selected specifically for the reaction chemistry.

Platinum

Platinum crucibles are widely used for demanding analytical procedures, including certain silicate and borate fusions, geological analysis, gravimetric procedures, and sample preparation for X-ray fluorescence.

Their advantages include high purity, high temperature capability, and broad chemical resistance.

Platinum is not universally inert. Certain metals, sulfides, phosphates, carbon-containing materials, reducing conditions, and molten mixtures can alloy with, embrittle, or otherwise damage platinum.

Handling is particularly important because platinum crucibles are both expensive and susceptible to contamination by other metals.

Hot platinum should not be handled with ordinary steel tongs. Suitable platinum-tipped or appropriate non-contaminating tongs should be used according to the laboratory procedure. Contact with other metals at elevated temperature should be avoided because alloying can permanently damage the crucible.

Strong reducing conditions and reducing flames should also be avoided unless the specific procedure is known to be compatible with platinum.

Nickel

Nickel crucibles are commonly selected for certain alkaline fusion procedures, including methods involving sodium hydroxide or sodium peroxide.

Nickel provides good toughness and is more resistant than silica-based materials to some strongly alkaline melts.

Its limitations include oxidation at elevated temperature, reaction with certain fluxes and samples, and potential Ni contamination.

Nickel should therefore be selected for a specific validated fusion chemistry rather than used as a general-purpose furnace crucible.

Zirconium

Zirconium crucibles are used for selected aggressive alkaline and peroxide fusion procedures. They provide useful mechanical properties and chemical resistance under conditions where conventional ceramic materials may fail.

Their suitability remains method-dependent, and Zr contamination must be considered when zirconium is an analyte of interest.

Silver

Silver crucibles are used in selected alkaline fusion procedures.

Silver melts at approximately 962°C, so it is unsuitable for the very high temperatures tolerated by refractory ceramics or platinum. Analytical procedures generally operate substantially below the melting point, often around 700°C or lower, depending on chemistry and required mechanical stability.

Silver is also chemically incompatible with certain reagents and analytes and should only be used where specified by the analytical method.

Iron and Steel

Iron or steel crucibles are inexpensive and mechanically robust and may be suitable for certain specialized fusion procedures.

Their obvious limitation is Fe contamination. They are inappropriate where iron is being determined at low concentrations or where the sample chemistry attacks the metal.

Graphite

Graphite crucibles provide excellent thermal conductivity, good thermal-shock resistance, and exceptional high-temperature capability under suitable atmospheres.

They are commonly used for metal melting and specialized high-temperature processing under inert gas, vacuum, or appropriate reducing conditions.

Atmosphere is critical.

Graphite begins oxidizing in air at elevated temperature, with oxidation becoming increasingly significant from approximately 400–500°C upward. Consequently, a graphite temperature rating without an accompanying atmosphere specification is of limited value.

Graphite can also introduce carbon or establish reducing conditions that alter sample chemistry.

Comparison of Crucible Materials

The table is a starting point rather than a compatibility chart. Sample and flux chemistry can dominate crucible performance.

Crucible Size: Nominal Capacity Is Not Working Capacity

Crucibles are commonly sold according to nominal volume—for example, 15, 30, 50, or 100 mL.

This does not mean that a 30 mL crucible should routinely be filled with 30 mL of sample.

Ashing, decomposition, and fusion reactions can involve:

• Foaming
• Expansion
• Gas evolution
• Spattering
• Melting
• Changes in bulk density

Adequate headspace should therefore be maintained.

For gravimetric work, sample mass rather than nominal crucible capacity may be the more relevant specification. The appropriate loading should follow the analytical method and allow uniform heating without sample loss.

Crucibles for Ashing and Loss on Ignition

Ashing is among the most common crucible applications.

Food, biomass, wastewater solids, soils, polymers, and other materials may be heated in a muffle furnace to remove volatile or combustible components and determine the remaining residue.

Porcelain is commonly suitable for routine procedures around 500–600°C, provided the analytical method permits its use. Alumina may be selected for higher-temperature or chemically demanding procedures.

For quantitative gravimetry, the crucible itself must have a stable tare mass.

A typical procedure involves conditioning the empty crucible, cooling it in a desiccator, weighing it, adding the sample, carrying out the specified thermal treatment, cooling again in a desiccator, and repeating the heating/cooling/weighing cycle until the method's constant-mass criterion is met.

Hot crucibles should never be placed directly on an analytical balance.

Identify Crucibles Used for Gravimetry

When several crucibles are used simultaneously, each vessel should have a permanent or otherwise validated identification system so that its tare mass remains associated with the correct sample.

Depending on crucible material and laboratory procedure, identification may involve heat-resistant markings, engraved identification, manufacturer-applied numbering, or another method that survives the heating cycle without contaminating the sample.

Ordinary adhesive labels and many common marker inks are obviously unsuitable for furnace use.

Accurate crucible identification is particularly important when multiple samples undergo repeated heat-cool-weigh cycles.

Crucibles for Fusion

Fusion places much greater chemical demands on a crucible than ordinary ashing.

A powdered sample is mixed with a flux and heated until a molten phase forms. Fluxes may include borates, carbonates, hydroxides, peroxides, or other reagents.

At fusion temperatures, materials that appear inert under ordinary laboratory conditions may react rapidly.

There is therefore no universal fusion crucible.

Platinum or platinum alloys are widely used for many borate fusion procedures. Nickel, zirconium, silver, iron, or other materials may be required for particular alkaline or peroxide fusion methods.

The analytical method should determine crucible selection. Substituting another material solely because it has a higher temperature rating can contaminate the sample, destroy the crucible, or change the fusion chemistry.

Temperature Rating vs. Chemical Compatibility

Maximum temperature is one of the least useful specifications when considered by itself.

A crucible rated for 1,700°C may be chemically attacked at 900°C by an incompatible molten sample. An inexpensive porcelain crucible, meanwhile, may perform reliably for years in routine 550°C ashing.

Before selecting a crucible, consider:

1. Operating temperature
2. Time at maximum temperature
3. Heating and cooling rate
4. Furnace atmosphere
5. Sample composition
6. Flux or reagent composition
7. Elements being measured
8. Acceptable contamination
9. Required sample volume and headspace
10. Expected number of reuse cycles

For analytical work, chemical compatibility and contamination are often more important than maximum temperature.

Thermal Shock and Heating Practice

Porcelain and alumina are refractory but brittle materials. Rapid temperature gradients can generate internal stresses sufficient to cause cracking.

Avoid unnecessary thermal shock. Where appropriate, use controlled furnace heating and cooling programs and follow the manufacturer's recommendations.

Never quench a hot ceramic crucible in water simply to accelerate cooling.

Fused silica has substantially lower thermal expansion and better thermal-shock resistance, but repeated high-temperature cycling introduces the separate problem of devitrification.

Metal crucibles require their own handling procedures because oxidation, softening, alloying, and chemical reaction can depend strongly on atmosphere and temperature.

Handling Crucibles

Clean crucibles used for quantitative work should not be handled with bare fingers after conditioning and weighing. Fingerprints can introduce contamination and measurable mass.

Use clean tongs suitable for the crucible material and geometry.

Avoid excessive gripping force on ceramic crucibles, particularly while hot. Concentrated mechanical stress can contribute to cracking.

Platinum requires special care. Avoid ordinary steel tools on hot platinum and prevent contact with incompatible metals.

After furnace treatment, gravimetric crucibles are commonly cooled in a desiccator before weighing. This protects the balance from heat and limits uptake of atmospheric moisture during cooling.

Cleaning and Reuse

Cleaning procedures should be selected according to both crucible material and previous sample chemistry.

Porcelain and alumina crucibles can often be cleaned by removing loose residue, soaking or using a compatible acid treatment where appropriate, rinsing thoroughly with high-purity water, and reheating if required by the analytical procedure.

Aggressive mechanical scraping should be avoided when it damages the surface.

Quartz crucibles should be kept particularly clean when used at high temperatures because surface contamination—especially alkali contamination—can accelerate devitrification.

Metal crucibles require material-specific cleaning procedures. A treatment suitable for platinum may be unsuitable for nickel, zirconium, or silver.

For trace-element work, cleaning is part of analytical quality control rather than simply cosmetic maintenance.

When Should a Crucible Be Replaced?

Replacement should be considered when a crucible develops:

• Through-cracks or significant structural cracks
• Severe glaze deterioration
• Deep chemical corrosion
• Significant deformation or warping
• Persistent contamination
• Substantial wall thinning
• Chipping that compromises sample containment
• Loss of stable tare mass for gravimetric work

For fused silica, substantial clouding and surface roughening associated with devitrification may also indicate that replacement is appropriate.

Discoloration alone does not necessarily mean a crucible has failed. Many crucibles develop permanent staining during normal service.

Practical Selection Guide

For routine ashing and ignition, porcelain is usually the economical starting point.

For high-temperature calcination and ceramic/materials research, high-purity alumina is commonly preferred.

For high-purity heating where metallic contamination is undesirable, fused silica may be appropriate if temperature and chemistry are compatible.

For HF digestion, use an appropriate PTFE/PFA digestion vessel or another method-specified HF-resistant material rather than glass or fused silica.

For borate fusion and demanding analytical work, platinum or an appropriate platinum alloy may be required.

For alkaline or peroxide fusion, nickel, zirconium, silver, or another method-specified material may be appropriate.

For extreme-temperature processing under inert atmosphere or vacuum, graphite may be useful.

The appropriate crucible is not the one with the highest temperature rating. It is the one whose material, geometry, purity, chemical compatibility, atmosphere tolerance, and contamination profile match the procedure.

Porcelain and Ceramic Crucibles from Longreen Lab

Longreen Lab supplies porcelain and ceramic crucibles for routine laboratory ashing, ignition, calcination, gravimetric analysis, educational laboratories, and general high-temperature sample preparation.

This guide also discusses specialized materials such as platinum, nickel, zirconium, silver, graphite, and fused silica because selecting a crucible requires understanding when a general-purpose ceramic crucible is—and is not—the appropriate choice.

When selecting a crucible, consider the required temperature, sample and reagent chemistry, furnace atmosphere, analytical contamination limits, crucible geometry, and working sample quantity rather than nominal capacity or maximum temperature alone.