Glazed porcelain is the standard acid-resistant material for crucibles, evaporating dishes, mortars and storage jars because its vitrified body and hard glaze are chemically inert to most acids and fire at high temperature. Hydrofluoric acid dissolves it, hot strong alkali slowly etches it, and a lead glaze can poison what it holds. Heat it slowly, keep walls even and round the inside corners, because thermal shock and a crazed glaze are the usual causes of failure.
Part of Porcelain
Chemical vessels are porcelain and stoneware pots, dishes, crucibles and jars used to hold, heat and react corrosive chemicals where metal would corrode and glass might crack.
Why Porcelain Resists Acids
The silica-alumina glass and mullite in a fired porcelain body, covered in Porcelain, are not attacked by hydrochloric, sulfuric or nitric acid, and the vitrified body has no open pores for liquid to enter. The corpus advises matching container to contents in Container Selection, noting glass is the best overall for acids and alkalis, and that the wrong material can dissolve, crack or react. Porcelain is the companion material where heat or a thick wall is needed: heating dishes, ignition crucibles and mortars.
What Porcelain Does Not Resist
- Hydrofluoric acid. It dissolves the silica of porcelain and glass alike. The hydrofluoric acid warning in Aluminum Smelting applies: it penetrates skin and is extremely dangerous.
- Hot strong alkali. Boiling lye slowly etches the glaze and then the body. Prefer iron or nickel for hot caustic work.
- Sudden temperature change. See Thermal Properties.
- Lead glaze. Glaze Materials warns against lead because it poisons food and water, and an acid will leach it faster. Never use leaded or unknown glazes for acids or food.
Design Features That Prevent Cracking
- Even wall thickness. A thick section lags a thin one in temperature, causing tension.
- Round interior corners. A sharp corner concentrates stress.
- Glaze fit. The glaze should shrink slightly less than the body on cooling, which leaves it in light compression. If it shrinks more it goes into tension and crazes into a fine crack network that traps liquid and lets it creep into the body.
- Slow heating. Warm a dish gently and keep the flame spread, not on one spot.
Check glaze fit by soaking a fired piece in coloured water or ink and wiping it: dye lines show crazing. A crazed vessel is unsuitable for strong acids or for food.
Practical Uses
- Crucible and ignition dish. Fired to a high temperature, porcelain can be heated red hot, within the limits above. Crucibles covers the choice for metals.
- Mortar and pestle. A hard, non-porous surface grinds without contaminating.
- Storage jar. The body is impermeable and tight lids hold vapour. The acid-and-alkali choices are covered in Container Selection, and glass lab ware in Laboratory Ware.
Corrosive contents, heat and glassy shards
A cracked vessel can release hot acid without warning, so inspect before every use, work over a tray that will contain a spill, and wear eye and hand protection. Add acid to water, never water to concentrated acid. Never heat a closed vessel. Broken glazed ware has sharp edges.
Common Mistakes
- Putting a cold vessel on a hot flame — it shatters.
- Using ware with unknown glaze for acid — lead poisoning.
- Ignoring crazing — the crack network leaks and weakens.
What’s Next
- Learn next: Glass-Ceramics — a more shock-resistant vessel material
- Combine with: Container Selection — matching vessel to chemical