12/06/2026
Number four and final post, for now, following the process of creating our unique marine art ceramics.
This one is a bit longer, describing:
What Happens When Glaze Goes Into the Kiln
When you put a glazed piece into the , it looks dull, muted, chalky, and completely different from the finished product. What happens next is a controlled chemical transformation. Essentially, we are melting a layer of powdered rocks, minerals, and glass directly onto a clay surface.
The process unfolds in a specific sequence as the temperature rises over several hours.
1. The Firing Sequence
Stage 1: Up to 600°C — Evaporation & Dehydration
Even dry-to-the-touch glaze contains microscopic water trapped in its powdered materials. In this initial stage, that chemical water evaporates and escapes out the kiln vent.
Stage 2: 600°C – 900°C — Off-Gassing & Calcination
Raw glaze ingredients like carbonates, sulphates, and organics begin to decompose. They release gases like carbon dioxide and sulphur oxide. This is called off-gassing. If the glaze melts too early before these gases escape, they can get trapped, causing small bubbles or pinholes.
Stage 3: 900°C – Peak Temp — The Melt & Interfacial Bond
The fluxes (melting agents) in the glaze begin to soften the silica (glass). The glaze turns into a boiling, bubbling cauldron of liquid glass. At the peak temperature, the glaze actively reacts with the clay body underneath it. They fuse together, creating an intermediate layer called the interfacial bond, which keeps the glaze permanently locked onto the clay.
Stage 4: Peak Temp down to Room Temp — Cooling & Solidification
The kiln is turned off, and the liquid glass smooths out. As the temperature drops, the molten glass thickens until it freezes into its final, rigid structure. If the glaze cools too quickly, it can crack or craze, if it cools slowly, beautiful crystals can grow inside the glass.
2. The Three Musketeers of Glaze
Every glaze is a fine-tuned chemical recipe balanced to ensure it melts properly without running completely off the piece and ruining the kiln shelves as well as the piece itself.
* Silica: The actual glass former (derived from quartz or flint). It has a very high melting point on its own—roughly 1710°C — which is hotter than most pottery kilns can even handle.
* Fluxes: The melting assistants (like sodium, potassium, calcium, or lead). Fluxes chemically lower the melting point of the silica so it can turn to liquid glass at standard kiln temperatures.
* Alumina: The stiffener (usually derived from clay). Liquid glass is incredibly runny. Alumina gives the molten glaze viscosity (stickiness) so it clings to vertical walls instead of pooling at the bottom of the kiln.
The Magic of Colourants: The bright greens, deep blues, and rich reds you see on finished ceramics don't exist in the raw bucket. Metals like copper, cobalt, and iron oxidize shift into their vivid final states only when subjected to the intense heat and atmosphere inside the kiln.
