Onggi are traditional Korean earthenware crocks used for fermenting kimchi, doenjang (soybean paste), and ganjang (soy sauce). The porous, unglazed clay allows micro-ventilation — gases produced during fermentation can escape without allowing air in, maintaining anaerobic conditions.
The air-gap lid
Traditional onggi lids are heavy and domed, resting inside a lip around the crock mouth. The weight of the lid maintains a seal under positive pressure from fermentation gases; gas escapes around the rim when pressure builds. This passive valve function requires no maintenance.
Modern equivalents
Large modern kimchi is made in food-grade plastic crocks or ceramic jars with locking lids. Neither replicates the micro-ventilation of onggi. The traditional clay produces a subtly different fermentation profile over months; for short fermentation (under 3 months) the difference is marginal.