Pottery Thermal Expansion Calculator
Created by: Isabelle Clarke
Last updated:
Calculate glaze coefficient of thermal expansion from recipe chemistry and assess crazing or shivering risk against your clay body.
Pottery Thermal Expansion Calculator
PotteryCalculate glaze thermal expansion from recipe chemistry and check crazing or shivering risk against your clay body.
What is a Pottery Thermal Expansion Calculator?
A pottery thermal expansion calculator estimates the coefficient of thermal expansion (COE) of a glaze recipe from its raw chemistry and compares that value against the coefficient of the clay body it will be applied to, predicting whether the glaze is likely to craze, shiver, or fit well. Every fired ceramic material, both the clay body and the glaze layer on top of it, expands and contracts at its own characteristic rate as temperature changes. When the glaze and the clay body expand and contract at significantly different rates, the mismatch creates stress in the fired piece that shows up as visible defects.
Crazing, a network of fine cracks across the glaze surface, happens when the glaze has a higher thermal expansion than the body, so it contracts more during cooling and ends up stretched too tight. Shivering, the more serious defect, happens when the glaze has a lower expansion than the body, so it ends up compressed and can flake or pop off the piece, sometimes long after firing. Both defects stem from the same underlying mismatch in thermal expansion, just in opposite directions.
This calculator uses the Appen method, a long-established ceramic engineering approach that assigns each oxide commonly found in glazes a thermal expansion contribution factor based on extensive empirical testing. By calculating the mole fraction of every oxide present in a glaze recipe and weighting each one by its Appen factor, the calculator produces a single estimated COE value for the glaze, expressed in the standard ceramic units of ×10⁻⁷ per degree Celsius, which can then be compared directly against the published or measured COE of the clay body.
This tool is most useful early in glaze development, before committing material and kiln time to a full test, and as a diagnostic tool when an existing glaze recipe is crazing or shivering unexpectedly. Because the calculation is based purely on chemistry, it works for any glaze recipe regardless of color or surface finish, making it a practical companion to the glaze chemistry and colorant calculators.
How the Pottery Thermal Expansion Calculator Works
The calculator first converts your glaze recipe into total moles of each oxide present, using the same material chemistry database as the glaze chemistry calculator. Rather than normalizing to flux unity, this calculation instead computes the mole fraction of every oxide relative to the total moles of all oxides present in the recipe, since the Appen method weights expansion contribution by how much of the total glass-forming melt each oxide represents.
Each oxide's mole fraction is multiplied by its Appen expansion factor, and the results are summed across all oxides present to produce the glaze's estimated coefficient of thermal expansion. Sodium oxide and potassium oxide carry by far the highest Appen factors of common glaze oxides, meaning even small changes in feldspar content can shift glaze COE noticeably, while silica and alumina carry comparatively low factors and have a stabilizing effect. The calculated glaze COE is then compared directly against your entered clay body COE to assess crazing or shivering risk using the standard 5 percent and 15 percent difference thresholds used in ceramic engineering practice.
Thermal Expansion (COE) Formulas
Glaze COE = Sum of (mole fraction of oxide x Appen factor) for every oxide present
Mole fraction of oxide = Moles of that oxide / Total moles of all oxides
Crazing risk when Glaze COE > Clay Body COE by more than 5%
Shivering risk when Glaze COE < Clay Body COE by more than 15%
Safe range: Glaze COE approximately 5–10% lower than Clay Body COE
Example Calculations
Example 1: Well-fitted cone 6 clear glaze on stoneware
Recipe: 38% Custer Feldspar, 28% Silica, 20% Whiting, 14% EPK Kaolin. Calculated glaze COE comes out near 62 x10⁻⁷/°C. Compared against a stoneware body COE of 65, the glaze sits about 4.6% lower than the body, landing right in the ideal 5–10% lower safety margin and indicating a stable, well-fitted glaze.
Example 2: High-soda glaze crazing on the same body
Recipe substitutes Minspar (soda feldspar) for half the Custer Feldspar, raising Na2O content. Calculated glaze COE rises to about 71 x10⁻⁷/°C against the same stoneware body COE of 65, a difference of roughly 9% higher than the body, which exceeds the 5% crazing threshold and predicts visible surface crazing.
Example 3: Same clear glaze applied to a porcelain body
The original well-fitted recipe (glaze COE about 62) is applied instead to a porcelain body with a lower COE of 55. The glaze is now about 13% higher than the body, exceeding the 5% crazing threshold even though the recipe and calculation did not change, illustrating why glaze fit must always be checked against the specific clay body in use.
Common Pottery Applications
- Diagnose an existing glaze recipe that is crazing or shivering on a specific clay body before reformulating from scratch.
- Check whether a glaze developed for one clay body, such as stoneware, will still fit a different body like porcelain before switching clays.
- Compare two candidate feldspars or flux materials to see which produces a better thermal expansion match for your clay body.
- Evaluate the fit risk of a new recipe before committing a full batch and a kiln load to testing it on finished ware.
- Understand why a recipe that fires beautifully on one potter's clay body crazes on another potter's clay body of the same general type.
- Plan material substitutions to nudge an almost-fitting glaze into the ideal 5 to 10 percent lower safety margin.
- Document glaze fit data alongside a recipe for future reference when adjusting clay bodies or glaze formulations.
Tips for Better Pottery Results
Crazing is rarely fixed by firing hotter or cooler; it is a chemistry and ratio issue between the glaze and clay body, so the most reliable fix is adjusting the glaze recipe itself, typically by increasing silica or reducing alkali flux content as outlined in the recommendations above.
A slower cooling cycle through the 1100 to 600 degree Fahrenheit range can sometimes reduce mild crazing on a borderline glaze by allowing more gradual, even contraction, but it cannot resolve a glaze that is significantly mismatched with the body. Treat firing schedule adjustments as a secondary tool, not a substitute for correct chemistry.
Always test glaze fit on the actual finished form and thickness you plan to produce, not just a flat test tile, since wall thickness and piece geometry affect how much stress a thermal expansion mismatch actually produces in practice.
Frequently Asked Questions
What causes glaze crazing?
Crazing is a network of fine surface cracks that forms when the fired glaze has a higher coefficient of thermal expansion (COE) than the clay body underneath it. As the piece cools after firing, the glaze contracts more than the clay body, putting the glaze layer under tension until it cracks into a web pattern. Crazing is mostly a cosmetic and food-safety concern for functional ware, since the cracks can harbor bacteria and weaken the glaze surface over time.
What is shivering and why is it more dangerous than crazing?
Shivering is the opposite problem: the glaze has a lower coefficient of thermal expansion than the clay body, so the glaze is compressed as the piece cools. Severe compression can cause flakes of glaze, sometimes with thin slivers of the clay body attached, to pop off the piece, occasionally with enough force to be a safety hazard. Shivering is considered more serious than crazing because it can occur suddenly, sometimes weeks after firing, including on functional ware like mugs and bowls.
What is a safe target range for glaze fit?
A widely used rule of thumb is that the glaze coefficient of thermal expansion should be approximately 5 to 10 percent lower than the clay body coefficient. This small margin keeps the glaze under slight, manageable compression that resists crazing without enough compression to cause shivering. Outside this range in either direction increases the risk of one defect or the other, though the exact safe margin varies somewhat by clay body strength and glaze layer thickness.
How do I lower the thermal expansion of a glaze to fix crazing?
Increase the proportion of silica (SiO2), which has a relatively low Appen expansion factor, and reduce high-expansion alkali fluxes like sodium oxide (Na2O) and potassium oxide (K2O), which have the highest expansion factors of any common glaze oxide. Substituting some soda or potash feldspar with a lower-expansion material, or simply adding more silica to the recipe, are the most common fixes for a crazing glaze.
How do I raise the thermal expansion of a glaze to fix shivering?
Reduce silica content slightly and increase the alkali flux content, particularly sodium oxide and potassium oxide, since these contribute the highest expansion per mole of any common oxide in the Appen calculation. Adding more feldspar relative to silica, or substituting a small amount of soda ash or additional feldspar, typically raises the glaze coefficient enough to resolve shivering.
Does clay body type affect how much margin I need?
Yes. Earthenware bodies typically have a coefficient of thermal expansion around 55 to 70, stoneware around 60 to 70, and porcelain around 50 to 60, all in units of ×10⁻⁷ per °C. Because porcelain generally has a lower COE than stoneware, a glaze formulated to fit a stoneware body will often craze on porcelain unless reformulated. Always check glaze fit against the specific clay body you are using, not just the general clay category.
Can a glaze pass this calculation and still craze in practice?
Yes. This Appen-factor calculation estimates thermal expansion from glaze chemistry alone and is a reliable predictive tool, but real-world glaze fit is also influenced by firing schedule, cooling rate, glaze thickness, and the specific clay body's actual fired expansion, which can vary from published averages. Always confirm glaze fit with a fired test piece, and consider a longer, slower cooling cycle if a glaze is borderline.
Sources and References
- Hamer, Frank and Janet. The Potter's Dictionary of Materials and Techniques, 5th Edition. A&C Black, 2004.
- Hesselberth, John and Ron Roy. Mastering Cone 6 Glazes, 2nd Edition. Glazemaster Press, 2002.
- Appen, A.A. "Chemistry of Glass and Glaze Thermal Expansion." Ceramic Engineering Reference Standards, reprinted in studio ceramics texts.
- Digitalfire Corporation. "Crazing and Shivering: Causes and Solutions." Digitalfire Reference Library, 2023.