Crucibles & Lids

Crucible capacity typically ranges from small classroom sizes to high-wall industrial sizes. Choose a crucible that leaves at least half of its volume empty once the sample is loaded, so ash residue does not spill over during heating or swelling reactions. For repeat gravimetric work across a class or lab, standardise on one size so every result is directly comparable.

Glazed crucibles have a smooth, non-porous interior that resists staining and cleans easily, making them the better choice for general laboratory and classroom work. Unglazed crucibles have a slightly rougher surface that copes marginally better with rapid temperature swings, making them a common choice in industrial ashing and fusion work with heavier daily use.

A matching lid reduces sample loss from spitting and airborne contamination, while still allowing gases produced during ignition to vent safely around its edges. Buy crucibles and lids as a matched set rather than mixing sizes, since a loose or oversized lid will not seat correctly and can slip off during handling with hot tongs.

Porcelain reaches very high temperatures quickly and holds heat long after the flame is removed, so always use crucible tongs, never bare hands. Let a heated crucible cool on a heatproof mat before moving it to a balance, and store cooling crucibles in a desiccator to prevent them reabsorbing atmospheric moisture and skewing your mass reading.

Standard laboratory porcelain crucibles are rated for continuous use up to around 1,200°C, well above the temperature of a standard Bunsen burner flame and suitable for most muffle furnace ashing procedures. Always check the manufacturer's rated maximum before using a crucible in a high-temperature furnace programme, and avoid exceeding it, since repeated overheating shortens the working life of the porcelain and increases the risk of hairline cracking.

Yes, but always support it in a pipe-clay triangle resting on a retort stand ring, never hold it directly in the flame by hand. The triangle distributes heat evenly around the base and lower walls, reducing the thermal stress that causes cracking. For very high or sustained temperatures, a muffle furnace gives more even, controllable heating than an open flame and is the preferred method for gravimetric and ashing procedures.

Cracking is almost always caused by thermal shock: heating or cooling the porcelain too quickly, or resting a hot crucible on a cold metal surface. Always bring a crucible up to temperature gradually, avoid contact between a hot crucible and any cold surface, and let it cool naturally on a heatproof mat rather than quenching it in water. Inspect crucibles for hairline cracks before each use, since a compromised crucible can fail suddenly under heat.

Glazed crucibles have a smooth vitreous coating that resists staining, cleans easily with water, and looks new for longer, making them ideal for general lab and classroom use. Unglazed crucibles have a matte, slightly porous surface that can absorb some residue over repeated use but tolerates rapid heating and cooling marginally better, which is why they see more use in industrial ashing and fusion applications with frequent furnace cycling.

Once fully cooled, most residues can be removed with warm water and a soft brush; avoid abrasive scouring on glazed crucibles as it can scratch the glaze and create sites for future staining. Stubborn mineral deposits or fused residue can often be loosened by gently reheating the crucible with a small amount of dilute acid or by soaking overnight. Never clean a crucible while it is still hot, as sudden contact with water can crack it.