Glaze Fit Calculator (COE Mismatch, Crazing & Shivering Risk)

How it works

strain = (α_glaze − α_body) × (T_set − 20 °C) · biaxial stress σ = E × strain ÷ (1 − ν), E ≈ 70 GPa, ν ≈ 0.22 (+ = tension) · crazing likely above ≈ 30 MPa tension; shivering risk above ≈ 120–200 MPa compression (thinner glaze tolerates more) · safe target: glaze COE 2–8 × 10⁻⁷ below the body

A glaze becomes a rigid glass around 500–600 °C on cooling; from there to room temperature it and the clay body shrink at their own rates. If the glaze shrinks more (higher COE) it ends up stretched over the pot and cracks — crazing. If it shrinks much less it is squeezed, and glass is strong in compression, so a little squeeze is exactly what you want; too much and slivers pop off edges — shivering. This calculator converts the COE difference into strain and stress with a simple thin-film model to tell you which side of the line you are on and roughly how far to move the glaze chemistry.

Worked example

A glaze with COE 68 on a stoneware body of 58 (× 10⁻⁷/°C), setting at 550 °C, wants to be 0.053% smaller than the body at room temperature — about 48 MPa of tension, well past the ~30 MPa a glaze surface tolerates, so crazing is likely. For functional ware the glaze should sit around COE 49–56; lowering it by about 15 units — trading part of the feldspar for a boron frit and talc and adding a few percent of silica — gives a safe gentle compression.

Frequently asked questions