1. How to Use a Crucible
A crucible is a vessel or melting pot made from highly refractory materials such as clay, quartz, ceramic clay, or difficult-to-melt metals. Crucibles are primarily used for the evaporation, concentration, or crystallization of solutions, and for burning solid substances. Here is a detailed guide on how to properly use a crucible in the laboratory:

• Heating Solids: When a solid needs to be heated to high temperatures, a crucible is essential. Place the lid diagonally on the crucible to prevent heated objects from escaping and to allow air to enter and exit for potential oxidation reactions.
• Placement for Heating: Due to its small bottom, a crucible typically needs to be placed on a mud triangle for direct fire heating. It can be positioned directly or diagonally on an iron tripod based on the experimental requirements.
• Post-Heating Handling: After heating, avoid placing the crucible on a cold metal surface to prevent cracking from rapid cooling. Similarly, do not place it on a wooden surface to avoid burns or fire hazards. The proper method is to leave it on an iron tripod for natural cooling or place it on an asbestos mesh for slow cooling. Always use crucible tongs for handling.

Main Uses:

1. Evaporation, concentration, or crystallization of solutions.
2. Burning of solid substances.

Precautions:

1. The crucible can be directly heated but should not be suddenly cooled afterward. Use crucible tongs for removal.
2. Always place the crucible on an iron tripod during heating.
3. Stir during evaporation, and when nearly dry, use residual heat to complete the drying process.

2. Commonly Used Ceramic Crucibles in the Labware
Ceramic crucibles, often made from glazed pottery, have a high melting point (1410℃) and can withstand high-temperature operations. For instance, ceramic crucibles can be heated up to 1200℃ with minimal mass change, making them ideal for burning and weighing precipitation. High-profile ceramic crucibles are also suitable for handling samples under air isolation conditions.

Attention:

• The thermal expansion coefficient of laboratory ceramic vessels is (3-4) × 10⁻⁶. Thick-walled ceramic vessels should avoid sudden temperature changes and uneven heating during high-temperature operations to prevent rupture.
• Ceramic vessels have better stability against chemical reagents such as acids and alkalis compared to glassware, but they should not come into contact with hydrofluoric acid.
• Ceramic crucibles are not resistant to the corrosion of caustic soda and sodium carbonate, especially during their melting operations. Using inert substances like MgO and C powder as fillers, or wrapping alkaline flux in filter paper, can partially replace platinum products.
• Ceramic vessels are mechanically strong and cost-effective, making them widely used.

Types of Ceramic Crucibles:

Aluminum Oxide Crucibles:

• Made from nearly pure aluminum oxide, sintered at high temperatures.
• Features: High temperature resistance (melting point of 2045℃), high hardness, and good corrosion resistance to acids and alkalis.
• Usage: Suitable for melting and sintering certain alkaline fluxes, but the temperature should not be too high, and the time should be kept short. They can sometimes replace nickel and platinum crucibles but are unsuitable when aluminum interferes with measurements.

Quartz Crucibles:

• Made by melting natural colorless and transparent crystals at high temperatures. Translucent quartz is made from vein quartz or quartz sand.
• Features: Excellent physical and chemical properties, particularly acid resistance (except for hydrofluoric and phosphoric acids at high temperatures).
• Usage: Ideal for melting acidic fluxes and sodium thiosulfate. The operating temperature should not exceed 1100℃. Quartz crucibles are not resistant to hydrofluoric acid or strong alkaline solutions at elevated temperatures.

Use of Crucibles in Analytical Chemistry：

Ceramic crucibles with a capacity of 10-15 milliliters are commonly used in the quantitative analysis of analytical chemistry. They allow the analyte to react fully at high temperatures, with quantitative measurements based on the mass difference before and after the reaction.

Handling and Weighing:

• Due to their water absorption properties, ceramic crucibles should be dried and weighed on an analytical balance before use.
• Analytes filtered using ash-free filter paper can be burned in the crucible without affecting results.
• After high-temperature treatment, the crucible and its contents should be dried and cooled in a specially designed dryer before weighing, using clean crucible tongs throughout the process.

Ceramic crucibles are indispensable in laboratory settings due to their durability, thermal stability, and resistance to various chemical reactions. Their versatility and mechanical strength make them a preferred choice for many high-temperature applications.