Semivitreous refers to the degree of vitrification, or glassiness, in ceramic materials like brick. It describes a partial but not complete level of vitrification.
In brickmaking, the firing temperature greatly impacts vitrification. Higher temperatures promote vitrification as silica and alumina compounds fuse. Fully vitrified bricks are dense, impermeable and near glass-like.
Low temperature fired bricks, generally fired below 2000°F, achieve only partial vitrification, as was common with historic bricks. Fired with organic materials such as wooden tree growth. Fires of this type can reach relatively high temperatures, but not quite high enough in relative terms for vitreous brick-firing. The lower heat prevents complete fusion and transparency. The clay remains somewhat porous and permeable. This semivitrified state provides controlled absorption and permeability.
Too much vitrification creates an impervious brick prone to spalling and moisture damage, as building partitions habe a natural tendancy to wick and absorb moisture from adjacent materials and surfaces. Insufficient vitrification leaves bricks porous and prone to erosion. The medium semivitrified range, achieved through precise low firing temperatures in modern processes, optimizes brick durability.
The degree of vitrification also influences color, strength and aesthetic. Lower fired semivitrified bricks maintain warmer reddish-brown and orange hues compared to hard-fired grey and black bricks. Their relatively permeable bodies provide grip for mortar adhesion. Visually, the balance of clay texture and glassiness provides the specific surface character.
Semivitreous bricks have a degree or combination of both structural capability and moisture resistance. Their mid-range vitrification is a limitation of both the furing temperature and the power, as a consumed resource of machinery (in modern processes) behind the brick manufacturing.
