September 29, 2023

Chemical Changes in Clay Firing

The high temperatures within firing kilns drive chemical reactions and elemental changes within clay that permanently alter its composition.

Ceramics pottery

A plain grayish lump of clay seems an unremarkable thing, but contained within is astounding potential for transformation. By applying fire, human ingenuity reshapes earth itself into exquisite and functional artforms. Yet the true alchemy unfolds at a molecular scale as heat drives profound chemical changes that permanently alter the clay's fundamental identity.

From Humble Ingredients

In its raw natural form, clay consists of a hodgepodge of quartz, feldspar, organic detritus, water and hydrated aluminum silicate minerals which lend plasticity when wet. This diverse makeup provides the palette of chemical constituents to be transformed by kiln heat.

Organic matter - whether naturally present or added by potters - can provide decoration, but also poses risks. As firing begins below 212°F, mechanically bonded water starts evaporating. Then between 500-900°F, any residual organic material decomposes rapidly in the heat through oxidation reactions and escapes the clay body as gases. Removing these volatile organics prevents defects like bubbling that could emerge later if organics were trapped.

Clay Crystals Under Fire

Intense heat doesn't just expel water and burn away organics - it fundamentally alters the clay's crystal lattice structure as molecules gain energy. Hydroxyl groups attached to aluminum ions break free around 600-900°F, allowing atoms to rearrange into new configurations as water vapor. This dehydroxylation permanently modifies the clay mineralogy in a process called decalcification or dehydration.

Certain clays also contain quartz crystals which can wreak havoc during firing. Around 1050°F, quartz undergoes an abrupt crystalline inversion from alpha-quartz to beta-quartz, expanding its lattice - potentially shattering the clay from the inside out. But as temperatures continue rising, other transformations prevent this damage.

Molten Clay and Crystal Formation

Around 1200°F, energy input allows clay particles to break free from their prior structure and enter a viscous melt phase. Silica and alumina ions liberated by the breakdown of kaolinite and feldspar begin fusing into silicate compounds. This liquefaction signals the onset of vitrification as the material starts to sinter into solid ceramic.

In some high-fire clays exceeding 2200°F, the newly mobile ions reorient into highly ordered sheet structures called mullite crystals reinforced by a glassy phase between sheets. However, excess crystallization can impart undesirable viscosity. Precise control prevents over-formation.

Cooling and Setting

While peak temperature drives critical changes, the cooling phase also matters. As the hot ceramic begins hardening, the viscosity of the glass phase changes, allowing the migration of ions that seed crystal growth. These interlocking formations provide hardness and strength.

The next time an ancient artifact or handcrafted mug catches your eye, consider the intricate hidden chemistry within the clay that grants it permanence. By manipulating heat and minerals, human will shape earth and stone into cultural treasures that endure through the centuries.

Get our email when new blog posts go live

Recent posts

Browse all posts