Crucible

Specifications

Typical capacity

20 – 250 kg

Material

Fused silica, clay-bonded mullite, or high-alumina refractory

Maximum temperature

1350 – 1500 °C depending on body

Wall thickness

20 – 45 mm at the base

Thermal shock

Fused silica is the most resistant; clay bodies the least

Working life

3 months – 2 years, strongly dependent on duty cycle

Form

Usually a tapered bowl with a pouring lip or a flat rim

Technical parameters

Fused silica

Best thermal shock, best for borosilicate, most expensive

Mullite / clay-bonded

Good all-rounder for soda-lime, cheaper

High-alumina

Most chemically inert, longest life, heaviest

Compatibility

A crucible contaminated with a different glass must be retired

Preheat rate

Typically under 100 °C/h when new or after a cold spell

Cost band

Medium to high, and recurring

Overview

The crucible is the vessel the glass is melted in. It is the only part of a furnace that touches the glass, and it is a consumable: a working crucible lasts anywhere from a few months to a couple of years, and when it fails it usually fails by cracking and leaking a hundred kilograms of molten glass onto the furnace floor. Choosing one is a compromise between thermal shock resistance, glass compatibility and cost.

Where it fits in the process

Crucible is used in gather. The stage decides how hot the glass has to be when the tool touches it, which in turn decides how much of the work it can do in one pass.

Safe use

• A cracked crucible can release the entire melt. Inspect for weeping at the base and shut down at the first sign.
• Never add cold cullet to a nearly empty crucible. The thermal shock can split the wall.
• Dust from a broken refractory pot contains respirable silica. Wear a P100 respirator when removing one.
• Hot crucible fragments stay hot for hours after removal. Quarantine them somewhere nothing can walk into them.

How it is used

1. Preheat a new crucible slowly — under 100 °C per hour — and hold it at temperature before the first charge.
2. Keep a dedicated crucible per glass colour family. Iron contamination from a previous melt will grey a clear glass for months.
3. Never let the glass level fall below a third full; the exposed wall above the melt overheats and spalls.
4. Record the date it went in. Life is predictable enough that a calendar beats an inspection.
5. Keep a spare on the shelf. A furnace down for two weeks waiting on a crucible is a studio that has stopped.

Compatible glass types

Matching the tool to the glass family matters less than matching the glass to itself — see the COE compatibility calculator before combining two glasses.