A crucible is a vessel or melting pot made of refractory materials such as clay, quartz, china‑clay or refractory metals. It is mainly used for evaporating, concentrating or crystallizing solutions, and igniting solid substances.

Crucibles and Their Applications

A crucible is required when solids are heated over a strong flame. During use, the crucible cover is usually placed aslant on the crucible to prevent heated materials from splashing out while allowing free air circulation for potential oxidation reactions. Given the small base of a crucible, it is generally set on a pipe‑stem triangle for direct flame heating. A crucible may be placed upright or tilted on a iron triangle support according to experimental requirements. A hot crucible must not be set immediately on a cool metal desktop, as rapid cooling will cause cracking. Neither should it be placed directly on a wooden desktop, which may be scorched or catch fire. The proper practice is to leave it on the iron triangle for natural cooling or set it on an asbestos wire gauze for slow cooling. Crucible tongs shall be used to handle the crucible.

1. Main Applications

(1) Evaporation, concentration or crystallization of solutions.

(2) Ignition of solid substances.

2. Precautions for Use

(1) It can be heated directly. Do not quench it after heating; handle it with crucible tongs.

(2) Set the crucible on an iron triangle support during heating.

(3) Stir the liquid during evaporation; rely on residual heat to evaporate to dryness.

3. Three Major Categories of Crucibles

Crucibles fall into three main types: graphite crucibles, clay crucibles and metal crucibles.

Common Laboratory Crucibles in Detail

Platinum Crucible

Platinum, also known as platina, is more expensive than gold. It is widely employed owing to its excellent properties. Platinum has a melting point as high as 1774 °C, features stable chemical properties, undergoes no chemical alteration upon ignition in air, does not absorb moisture, and resists attack by most chemical reagents.

1. Properties

Resistance to corrosion by hydrofluoric acid and molten alkali‑metal carbonates is a key property distinguishing platinum from glass and porcelain. Accordingly, platinum ware is commonly used for precipitation, ignition‑weighing, hydrofluoric‑acid fusion and carbonate fusion. Platinum exhibits slight volatility at high temperatures, so corrections are necessary after prolonged ignition. A platinum surface of one square centimeter loses approximately 1 mg upon ignition at 1200 °C for one hour, while platinum barely volatilizes at 900 °C.

2. Regulations for Using Platinum Ware

(1) Strict protocols shall be established for the collection, utilization, consumption and recovery of platinum.

(2) Platinum is soft, even alloys containing small amounts of rhodium and iridium. Do not apply excessive force when handling platinum vessels to avoid deformation. When removing fused melts, do not scrape the inner wall with sharp objects such as glass rods to prevent damage. Hot platinum vessels shall not be plunged abruptly into cold water to avert cracking. Deformed platinum crucibles or vessels may be reshaped with matched water‑filled moulds (yet fragile carburized platinum sections need uniform, moderate force during reshaping).

(3) Platinum vessels must avoid contact with any other metal during heating, because platinum readily forms alloys with other metals at high temperatures. Platinum crucibles should therefore rest on platinum triangles or supports made of ceramic, clay or quartz. They may also be heated on hot‑plates or electric furnaces fitted with asbestos boards, but must never make direct contact with iron plates or furnace heating wires. Crucible tongs used shall have platinum‑clad tips; nickel or stainless‑steel tongs are permissible only at low temperatures.

Gold Crucible

Gold costs less than platinum and resists attack by alkali‑metal hydroxides and hydrofluoric acid, so it often serves as a substitute for platinum ware. Nevertheless, gold has a low melting point (1063 °C) and cannot sustain intense high‑temperature ignition; its working temperature should stay below 700 °C. Ammonium nitrate corrodes gold noticeably, and aqua regia must never touch gold ware. Operational principles for gold ware are largely identical to those for platinum ware.

Porcelain Crucible

Laboratory porcelain is essentially glazed earthenware with a high melting point (1410 °C). It withstands intense ignition; for instance, porcelain crucibles can be heated up to 1200 °C with negligible mass change after ignition, hence frequent use for ignition‑weighing of precipitates. Tall‑form porcelain crucibles can be applied to sample treatment under air‑excluded conditions.

Precautions

The thermal expansion coefficient of laboratory porcelain ranges from (3‑4) × 10⁻⁶. In high‑temperature evaporation and ignition procedures, thick‑walled porcelain articles should be protected against sharp temperature swings and uneven heating to prevent fracture.

Porcelain outperforms glassware in chemical stability against acids, bases and other reagents, yet it must not come into contact with hydrofluoric acid. Porcelain crucibles are vulnerable to corrosion by caustic alkalis and sodium carbonate, especially during fusion operations.

By employing inert fillers such as magnesium oxide or carbon powder, silicate samples can be fused with alkaline fluxes wrapped in quantitative filter paper inside porcelain crucibles, partially replacing platinum apparatus. Porcelainware boasts good mechanical strength, low cost and extensive application.