cassette

A cassette is a sealed or semi-sealed ceramic container used in kiln firing to protect ware from direct flame, kiln atmosphere, and dust. Also called a saggar, it functions as a miniature kiln chamber within the larger kiln, creating a controlled microclimate around the pieces inside. The cassette walls are typically 12 to 25 mm thick, made from a refractory clay body formulated to withstand thermal shock and repeated heating cycles without warping or bloating.

Cassettes come in several configurations. A closed cassette is sealed on all sides with only a small spy hole for observation; this is used when absolute isolation from kiln gases is required, such as when firing high-temperature glazes prone to vapour interaction. A muffle cassette is open at the bottom to allow kiln atmosphere circulation while protecting the top and sides. Lidded cassettes have removable covers, making them faster to load and unload than fully enclosed types. Industrial production facilities commonly use stacked cassette arrays to maximize kiln volume: a single kiln firing run might contain twenty or more cassettes stacked vertically.

The cassette's primary job is to prevent glaze blistering, crawling, and discoloration caused by kiln volatiles, reducing scrap rates significantly in production ceramics. Cassettes also allow the same kiln to fire different ware types simultaneously: one cassette atmosphere for bone china, another for stoneware. However, cassettes add thermal mass and slow the kiln's heat rise; a kiln packed with cassettes typically fires more slowly and consumes more fuel than an open kiln.

Refractory wear and replacement

Cassettes degrade with use. Repeated thermal cycling causes spalling at corners and edges. Salt glazes and high-alkali melts attack the ceramic body directly, creating pitting and erosion. A well-made cassette lasts 50 to 200 firing cycles depending on temperature (higher temperatures shorten life), kiln atmosphere (reduction is harsher than oxidation), and the chemistry of the glaze or clay being fired. Once the walls thin below 8 mm or develop through-cracks, the cassette is retired to avoid contamination and thermal failure during firing.

The term cassette is less common in British ceramic traditions, where saggar remains standard; in North America and parts of Europe, cassette is preferred. The etymology reflects French influence in industrial ceramics terminology. Cassettes differ from thimbles (small protective tubes for individual pieces) and stilts (refractory props that support ware directly), and they are distinct from kiln wash or bat wash, which are liquid coatings applied to kiln shelves rather than removable containers.