First Fire: The Bisque Firing Process

Introduction

After days or weeks of careful drying, our ceramic pieces face their first trial by fire. The bisque firing (or "Schrühbrand" in German) transforms fragile, dusty clay into permanent ceramic material – a dramatic chemical and physical change that sets the stage for everything that follows.

Quick Summary:

• Bisque firing is the first kiln firing that transforms clay into ceramic
• This firing typically reaches temperatures of 900-1000°C (1652-1832°F)
• The process burns out organic material and chemically changes the clay
• After bisque firing, pieces are stronger but still porous enough to accept glaze
• This step requires careful loading, programming, and monitoring

What Happens During Bisque Firing

The bisque firing is far more than simply heating clay – it's a carefully orchestrated series of chemical and physical transformations:

The Chemical Transformation

As temperature rises in the kiln, the clay undergoes permanent changes:

• At 100°C (212°F): Physical water evaporates (any remaining moisture turns to steam)
• Between 350-500°C (662-932°F): Chemically bound water is driven off
• Around 573°C (1063°F): Quartz inversion occurs – a critical point where silica minerals in the clay change their crystal structure
• Between 700-900°C (1292-1652°F): Organic materials burn out completely
• By 950°C (1742°F): Clay particles begin to fuse, creating ceramic material

Once these changes occur, there's no going back – the malleable clay is gone forever, replaced by rigid ceramic. This irreversible transformation is what makes ceramics so durable throughout human history.

Preparing for Bisque Firing

Final Inspection

Before loading the kiln, each bone-dry piece undergoes a final inspection:

• We check for hairline cracks that might expand during firing
• We ensure all pieces are completely dry (even slight moisture can cause explosions)
• We gently clean any dust or debris from surfaces
• We check that signatures or maker's marks are clear

Loading the Kiln

Proper loading is crucial for successful firing:

• Pieces must not touch each other as they'll shrink during firing
• Air circulation needs to flow evenly throughout the kiln
• Heavier items go on lower shelves, lighter pieces above
• Space is used efficiently while maintaining safe distances

We typically stack pieces on specialized kiln shelves separated by kiln posts, creating layers that maximize the number of pieces we can fire while ensuring nothing is crowded.

The Firing Schedule

A bisque firing isn't simply "turn on and walk away" – it follows a carefully controlled schedule:

The Slow Heat-Up

We begin with a very slow temperature increase:

• 0-150°C (32-302°F): Extremely slow heating (often 50°C/hour) to allow any remaining moisture to escape gently
• 150-600°C (302-1112°F): Moderate heating rate as water of crystallization leaves the clay
• 573°C (1063°F): Hold temperature briefly during quartz inversion
• 600-950°C (1112-1742°F): Faster heating as the clay becomes more stable

This careful ramp-up prevents thermal shock that could crack pieces.

Hold and Cool Down

• We typically hold at peak temperature (around 950°C/1742°F) for 15-30 minutes to ensure all pieces reach full temperature
• Cooling happens naturally as the kiln shuts off, usually taking 12-24 hours before we can open it

A complete bisque firing cycle in our studio typically takes about 18-24 hours from start to cool-down.

Special Considerations for Different Forms

Different ceramic forms require special attention during bisque firing:

Thin-Walled Pieces

Delicate forms like fine cups or thin vases are particularly vulnerable during the water smoking phase (the initial heating where physical water turns to steam). For these pieces:

• We use an even slower initial heating rate
• We sometimes place them in the cooler areas of the kiln
• We ensure they're completely bone dry before firing

Large or Thick Forms

Substantial pieces present the opposite challenge – they need extra time for heat to penetrate evenly:

• We place them where temperature is most consistent
• We might extend hold times to ensure they're fully processed
• We give them extra space to accommodate more dramatic movement

Complex Shapes

Pieces with attachments or unusual forms receive special placement:

• We support vulnerable areas with kiln furniture
• We position them to minimize stress on joints or thin sections
• We sometimes fire them separately from standard production pieces

The Kiln Opening: A Moment of Truth

After cooling, opening the kiln reveals the first major transformation in our ceramic journey. It's always a moment of anticipation – will all pieces survive? How has the firing changed their appearance?

What We Look For

When examining bisque-fired pieces, we check:

• Overall shrinkage (typically 4-6% from dry size)
• Color changes (often from grey to warm terracotta tones)
• Ring test (properly fired pieces make a clear ring when gently tapped)
• Surface texture (now harder but still somewhat porous)
• Any firing flaws that need addressing before glazing

Post-Bisque Processing

Before moving to glazing, bisque-fired pieces often need:

• Light sanding to remove sharp edges
• Dusting to remove any kiln debris
• Waxing on bottoms where glaze shouldn't adhere
• Organization by form for consistent glaze application

The Technical Side: Our Kiln Setup

In our Frankfurt workshop, we fire our bisque in an electric kiln with:

• Digital controller for precise temperature regulation
• Multiple zones for even heating
• Programmable schedules we've refined over years
• Pyrometric cones as backup verification of temperature
• Ventilation system for safe operation

This equipment allows us to create consistent, reliable results while monitoring energy efficiency.

Why Bisque Fire at All?

Some potters practice single-firing (going straight from bone-dry to glaze firing), so why do we take this intermediate step?

Bisque firing offers critical advantages:

• Creates stronger pieces that are easier to handle during glazing
• Allows for cleaning, sanding, and precise glaze application
• Removes all organic materials before the glaze seals the surface
• Provides an opportunity to catch and address any issues before applying expensive glazes
• Creates the perfect surface porosity for glaze absorption

The extra time and energy are investments in quality and reliability.

The Bisque Aesthetic

There's a particular beauty to bisque-fired ceramics – a warm, matte surface that shows the pure clay body before glaze transforms it. This stage reveals:

• The true color of the clay body
• Fine throwing marks and intentional textures
• The pure form without the distraction of glaze variation

Some ceramic traditions celebrate this aesthetic, leaving portions of pieces unglazed to showcase the bisque surface.

What Comes Next?

With bisque firing complete, our pieces are ready for their colorful transformation. In our next post, we'll explore the art and science of glazing – how we add color, texture, and functionality to our stoneware.