Clay Shrinkage Calculator
Calculate how much your pottery will shrink from wet clay to finished piece. Works for stoneware, porcelain, and earthenware.
Clay Shrinkage Calculator
Calculate pottery size changes from wet to fired
Fired Size
8¾"
10" thrown → 8¾" fired
12%
Standard Shrinkage
6%
wet to bone dry
6%
bisque to glaze fire
1.20"
total difference
Quick Reference Table (12% shrinkage)
| Thrown Size | Fired Size |
|---|---|
| 2" | 1.76" |
| 3" | 2.64" |
| 4" | 3.52" |
| 5" | 4.4" |
| 6" | 5.28" |
| 8" | 7.04" |
| 10" | 8.8" |
| 12" | 10.56" |
Cone Temperature Reference
Formulas
Understanding Clay Shrinkage
All clay bodies shrink as they dry and fire. This happens in two stages: drying (as water evaporates) and firing (as clay vitrifies). Knowing your clay's shrinkage rate is essential for making pieces to specific dimensions.
Shrinkage by Clay Type
- Earthenware: 9-11% total shrinkage (Cone 06-02)
- Stoneware: 11-13% total shrinkage (Cone 6-10)
- Porcelain: 13-15% total shrinkage (Cone 6-10)
- Raku/Sculpture: 8-10% (high grog content)
Practical Tips
For Precise Work
- Make shrinkage rulers for your clay
- Test each new batch of clay
- Throw lids and bodies same day
- Keep detailed records
Common Mistakes
- Using manufacturer specs blindly
- Mixing clay from different bags
- Inconsistent firing temperatures
- Not accounting for glaze thickness
Frequently Asked Questions
Why does clay shrink when fired?
Clay shrinks in two stages: drying shrinkage (as water evaporates) and firing shrinkage (as clay particles vitrify and fuse together). Total shrinkage is typically 10-15% depending on clay type and firing temperature.
How do I test my clay's shrinkage rate?
Make a test tile with a measured line (e.g., 10cm). Let it dry completely, then fire to your target cone. Measure the line again. Shrinkage % = (Original - Final) / Original × 100. Test several tiles for accuracy.
Does firing temperature affect shrinkage?
Yes! Higher firing temperatures cause more shrinkage as clay vitrifies more completely. The same clay body fired to Cone 10 will shrink more than when fired to Cone 6. Always test at your intended firing temperature.
Why do my lids never fit after firing?
Lids and their containers often shrink differently if thrown on different days or from different batches of clay. For best results, throw lids and containers from the same clay on the same day, and measure wet to ensure proper fit.
What causes different clay bodies to shrink differently?
Shrinkage varies based on clay particle size, grog content, and mineral composition. Porcelain (fine particles) shrinks most (13-15%). Sculpture clay with grog shrinks least (8-10%). Stoneware is typically 11-13%.
Can I reduce shrinkage?
You can reduce shrinkage by: adding grog (pre-fired clay particles), firing to a lower temperature, or using a clay body formulated for lower shrinkage. However, lower shrinkage often means less vitrification and durability.
How do I make a shrinkage ruler for my clay?
Roll a slab and cut a strip exactly 10 inches or 25 cm long. Mark even increments (1 inch or 1 cm). Let it dry and fire to your target cone. The resulting ruler automatically accounts for shrinkage when you measure new pieces.
Does glaze affect the final size of pottery?
Glaze adds a very thin layer (typically 0.5-1mm) but does not significantly change dimensions. However, glaze can cause slight warping during firing, especially on flat pieces. Account for glaze thickness only when extremely tight tolerances are needed.
Can I calculate shrinkage for custom clay blends?
Yes. Test tiles are essential for custom blends since published shrinkage rates only apply to standard formulations. Mix your custom blend, make several test tiles with measured lines, and fire to your target temperature to determine the actual shrinkage rate.
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