Kiln Glaze Transformation - From Clay to Ceramic
The Cast and Mould Process
In the creation of our unique marine art ceramics, the original fish or sea creature shape, sometimes called a 'cast, a 'model' or a 'positive', is made by the artist by hand from a hard silicone. It is detailed as close as possible to the real life creature in its form, its features and its texture. The colours and markings come later.
Using the silicone 'cast', a press mould, like a plaster cast, is made. Depending on the size of the piece, the press mould can contain multiple moulds of the same smaller piece or single moulds of bigger pieces. These press moulds look a bit like millstones, all of differing sizes. The press moulds are usually designed to be used in the machine that presses the wet clay into the mould. The wet clay starts out as powdered clay and is made up to a specific recipe depending upon the piece.
When the wet clay is pressed into the mould and then released, the newly born fish and sea creatures are stuck in the mould upside down. Each piece is peeled out by hand, any squidgy edges smoothed off by hand and then set aside ready for the next stage of its creation, the biscuit or 'bisque' process.
The Bisque Process
The biscuit, often called bisque, refers to clay that has been fired once in the kiln without any glaze applied to it. This is what happens to our marine creatures, following their pressing and any additional elements like extra fins or details being added, they are fired once to create the bisques.
Here is what you need to know about the biscuit stage:
The Transformation: During this first firing (the biscuit firing), chemical water is driven out of the clay, permanently changing it from fragile, raw clay into a durable, porous ceramic material. It will no longer dissolve or turn back into mud if it gets wet.
The Purpose: The biscuit state is deliberately left porous so that when the artist applies the colours, markings and liquid glaze, the piece acts like a sponge. It quickly absorbs the water from the glaze, leaving a perfectly even coating of raw glaze powder stuck to its surface, ready for the second firing.
The Etymology: The word comes from the French bis-cuit, meaning 'twice-baked'— referring to the traditional two-firing process used to create most finished ceramics.
The Painting and Decorating Process
After the cooling down from the first firing, the bisques lined up on their racks are ready for some colour and some glaze.
Using mini-spray guns to create solids and gradients and brushes for the stripes, spots, eyes, fins, gills, mouths and other details, the fish and sea creatures are painted in mute, pastel colours. These colours will come to life combined with glaze and firing and this is part of the magic and the skill of the artists.
Each piece is painted and glazed by hand, freehand.
When we say (and we say it a lot) that each piece is unique, we mean it. Just like in real life, each one is individual.
Once the glaze goes on, they are ready for the second firing in the kiln.
What Happens When Glaze Goes Into the Kiln
When you put a glazed piece into the kiln, it looks dull, chalky, and completely different from the finished product. What happens next is a controlled chemical transformation. Essentially, you 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 (1112°F) — Evaporation & Dehydration
Even dry-to-the-touch glaze contains microscopic water trapped in its powdered materials (like clay or kaolin). In this initial stage, that chemical water evaporates and escapes out the kiln vent.
Stage 2: 600°C – 900°C (1112°F – 1652°F) — 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 (up to 1300°C / 2372°F) — 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 (crazing); 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.
* Silica: The actual glass former (derived from quartz or flint). It has a massive melting point on its own—roughly 1710°C (3110°F)—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 pottery 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.