Shivering Risk Calculator

Enter your glaze recipe components and firing conditions to estimate shivering risk, COE differential, and silica-alumina balance.
Luis GonzalezCreated by Luis GonzalezLast updated:

How to Use This Calculator

  1. 1

    Enter Silica (SiO₂) content

    Input the amount of silica in your glaze recipe, typically measured in parts or percentages.

  2. 2

    Specify Alumina (Al₂O₃) content

    Provide the alumina content, crucial for calculating the silica-to-alumina ratio.

  3. 3

    Input Flux Oxides content

    Enter the total amount of fluxing oxides (e.g., CaO, MgO, K₂O, Na₂O) in your glaze.

  4. 4

    Define Firing Temperature

    State the peak firing temperature in degrees Fahrenheit for your glaze.

  5. 5

    Select Cooling Rate

    Choose your kiln's cooling rate: 'Slow', 'Normal', or 'Fast' (crash cool).

  6. 6

    Review your shivering risk assessment

    The calculator will display a shivering risk score, COE index, and ratio assessments.

Example Calculation

A potter is testing a new glaze recipe with 70 parts silica, 15 parts alumina, and 10 parts flux, fired at 2300°F with a fast cool. They want to assess the shivering risk.

Silica (SiO₂) (parts)

70

Alumina (Al₂O₃) (parts)

15

Flux Oxides (parts)

10

Firing Temperature (°F)

2300

Cooling Rate

Fast (crash cool)

Results

94/100

Tips

Aim for Glaze COE Slightly Higher Than Clay

To prevent shivering, the Coefficient of Thermal Expansion (COE) of your glaze should ideally be slightly higher than that of your clay body, ensuring the glaze is under slight compression as it cools.

Adjust Silica-to-Alumina Ratio

A silica-to-alumina ratio between 6 and 10 is often considered balanced for glazes. If your ratio is too low, increasing silica can help raise the COE and reduce shivering risk.

Consider Cooling Schedule Impact

A fast cooling rate can exacerbate shivering, as it stresses the glaze-body interface. Implementing a slower cooling schedule or a hold (soak) at key temperatures can sometimes mitigate fit issues.

The Shivering Risk Calculator is an indispensable tool for potters and ceramic artists, offering a scientific approach to predicting and preventing a common glaze defect.

By analyzing key compositional elements like silica, alumina, and flux content, alongside firing conditions, it helps users achieve optimal glaze-body fit.

This precision ensures durable, aesthetically pleasing pottery, minimizing costly failures and enhancing the quality of ceramic work in 2025.

Why Glaze-Body Fit is Paramount in Pottery

Achieving proper glaze-body fit is paramount in pottery because it directly impacts the durability, functionality, and aesthetic integrity of a ceramic piece.

When a glaze and clay body do not contract at compatible rates during cooling, stress develops, leading to defects such as shivering or crazing.

Shivering, where the glaze flakes off, can render a piece unusable and even dangerous if shards are ingested.

Crazing, characterized by fine cracks, compromises hygienic surfaces and structural strength.

A perfectly fitted glaze creates a strong, stable bond that protects the clay, resists staining, and ensures the longevity of the artwork.

Deconstructing Glaze Composition for Shivering Risk

This calculator assesses shivering risk by analyzing the interplay of key glaze components and firing conditions, focusing on the Coefficient of Thermal Expansion (COE) balance between glaze and clay.

Silica / Alumina Ratio = Silica (parts) / Alumina (parts)
Total Oxides = Silica + Alumina + Fluxes
Silica Pct = Silica / Total Oxides
Flux Pct = Fluxes / Total Oxides

Glaze COE Index = 3.5 + (Flux Pct × 8) - (Silica Pct × 2)
COE Differential = Glaze COE Index - Clay Body COE (typically 6.0 for stoneware)

Shivering Risk Score (initial) = absolute(COE Differential) × 25 (if COE Diff < 0)
Shivering Risk Score (final) = Shivering Risk Score (initial) × Temperature Factor × Cooling Modifier

The Silica / Alumina Ratio and Flux Content are key indicators.

The Glaze COE Index estimates the glaze's thermal expansion, which is then compared to a typical Clay Body COE to determine the COE Differential.

A negative differential increases the Shivering Risk Score, which is further adjusted by Firing Temperature and Cooling Rate.

💡 Just as precise ratios are key in glazes, they are also essential in other art forms. Our Oxide Pigment Calculator can help you mix accurate colors for your ceramic work.

Analyzing Glaze for Shivering Potential

Let's assess the shivering risk for a new glaze recipe.

  1. Glaze Composition: 70 parts Silica, 15 parts Alumina, 10 parts Flux Oxides.
  2. Firing Conditions: Fired at 2300°F with a Fast (crash cool) rate.
  3. Calculate Silica/Alumina Ratio: Ratio = 70 / 15 = 4.67
  4. Calculate Total Oxides: Total Oxides = 70 + 15 + 10 = 95
  5. Calculate Silica and Flux Percentages: Silica Pct = 70 / 95 = 0.737 Flux Pct = 10 / 95 = 0.105
  6. Estimate Glaze COE Index: Glaze COE Index = 3.5 + (0.105 × 8) - (0.737 × 2) = 3.5 + 0.84 - 1.474 = 2.866
  7. Calculate COE Differential (assuming Clay COE of 6.0): COE Differential = 2.866 - 6.0 = -3.134
  8. Calculate Initial Shivering Score: Initial Score = |-3.134| × 25 = 78.35
  9. Apply Temperature Factor (2300°F is 1.0) and Cooling Modifier (Fast is 1.2): Final Score = 78.35 × 1.0 × 1.2 = 94.02

The calculated Shivering Risk Score is 94/100, indicating a very high risk of shivering due to the glaze's COE being significantly lower than the clay body, exacerbated by a fast cooling rate.

💡 Understanding how different components affect your glaze is similar to how various pigments create specific colors. Our Paint Color Mixing Ratio Calculator helps achieve desired hues accurately.

Achieving Glaze-Body Fit in Ceramic Arts

Achieving optimal glaze-body fit is a cornerstone of successful ceramic art, preventing frustrating defects like shivering and crazing.

This fit hinges on the Coefficient of Thermal Expansion (COE), which measures how much a material expands and contracts with temperature changes.

For a robust bond, the glaze's COE should be slightly higher than the clay body's, ensuring the glaze is under slight compression as it cools.

For common stoneware, a typical COE range is 5.5-7.0 x 10^-6 /°C.

Different glaze materials significantly affect COE; for example, adding more silica generally increases it, while excessive fluxes can lower it relative to the clay.

Potters in 2025 meticulously adjust recipes and firing schedules, often incorporating controlled cooling ramps and soaks to mitigate thermal stress and secure a perfect, lasting fit.

Typical Glaze Material Ratios and COE Values

In the world of ceramics, specific material ratios and Coefficient of Thermal Expansion (COE) values serve as crucial benchmarks for achieving desired glaze properties and preventing defects like shivering or crazing.

For mid-fire glazes (cone 5-6, ~2200°F), a common silica-to-alumina ratio often falls between 6:1 and 8:1, with total flux content around 10-15% of the dry weight, aiming for a COE in the range of 6.5-7.5 x 10^-6 /°C.

High-fire glazes (cone 9-10, ~2350°F) might have a slightly higher silica-to-alumina ratio, perhaps 8:1 to 10:1, and often rely on more refractory fluxes, with a COE target of 6.0-7.0 x 10^-6 /°C.

These ranges ensure compatibility with common clay bodies like stoneware (COE 5.5-6.5 x 10^-6 /°C) or porcelain (COE 5.0-6.0 x 10^-6 /°C), guiding potters in formulating stable and durable glazes.

Frequently Asked Questions

What is shivering in ceramic glazes?

Shivering in ceramic glazes is a defect where the glaze separates from the clay body, typically at edges or rims, due to the glaze having a significantly lower Coefficient of Thermal Expansion (COE) than the clay. As the piece cools, the clay body contracts more than the glaze, causing the glaze to be compressed and eventually flake or chip off. It is the opposite of crazing, where the glaze contracts more than the clay.

How does the Silica-to-Alumina ratio affect glaze fit?

The silica-to-alumina ratio is a critical indicator of glaze durability and fit. A higher silica content generally increases the glaze's Coefficient of Thermal Expansion (COE), making it contract less upon cooling. Conversely, a higher alumina content, especially relative to silica, can lower the COE. A balanced ratio, typically between 6:1 and 10:1, helps ensure good glaze melt and a proper fit with the clay body, reducing defects like shivering or crazing.

What role do flux oxides play in glaze shivering?

Flux oxides (such as sodium, potassium, calcium, and magnesium) are crucial for lowering the melting temperature of glazes. While they help the glaze melt smoothly, they also tend to increase the glaze's Coefficient of Thermal Expansion (COE). Therefore, a higher proportion of fluxes relative to silica can push the glaze's COE above that of the clay body, making it more prone to crazing rather than shivering. Balancing flux content is key to achieving optimal glaze fit in 2025.