Choosing the Right Clay
Beyond clay type, labels include firing range, grog, shrinkage, and intended use. Reading these correctly helps prevent cracks and warping.
Reading Clay Specifications
When you buy clay, you'll see numbers and terms. Here's what they mean:
Cone Number Cone indicates heat work (temperature plus time), not temperature alone.
- Low fire is usually around cones 022-01 (common bisque/glaze ranges include 06-04)
- Mid fire is commonly cone 5-6
- High fire is commonly cone 9-10
Grog Content Grog is pre-fired clay that's been ground up and added back. It:
- Adds texture and "tooth"
- Reduces shrinkage
- Helps prevent cracking
- Makes clay easier to handbuild with
Shrinkage Rate Clay shrinks during drying and firing. Typical ranges:
- Earthenware: 4-8%
- Stoneware: 10-13%
- Porcelain: 12-15%
Color Matters
Natural Clay Colors
- Red/brown: Iron content (earthenware)
- Gray: Stoneware
- White/cream: Porcelain or white stoneware
- Buff: Stoneware with some iron
Why It Matters The clay color affects your glaze results! Dark clays make colors appear different than white clays.
Texture & Plasticity
Smooth vs. Grogged
- Smooth: Better for wheel throwing, finer detail
- Grogged: Better for handbuilding, large pieces
Plasticity How easily the clay bends without cracking. More plastic = easier to throw. Less plastic = better for handbuilding.
Matching Clay to Project
Wheel Throwing Choose a smooth, plastic body labeled "throwing."
Handbuilding Use medium grog for structure and reduced warping.
Large Pieces Higher grog content prevents cracking and warping.
Delicate Work Use fine stoneware or porcelain only when you can control drying well.
Storage Tips
Keep It Moist
- Store in airtight plastic bags
- Add a damp sponge if it's drying out
- Wrap tightly to prevent air exposure
Aging Clay Clay actually gets better with age! The longer it sits, the more plastic it becomes. Some potters age clay for months.
Reclaiming Save your scraps! We'll learn how to reclaim clay in Unit 14.
Common Beginner Mistakes
- Choosing porcelain too early
- Ignoring shrinkage when making lids
- Mixing clay scraps from different firing ranges
Pro tip: Start with one clay body for several weeks so your hands can learn its behavior.
Test Batch Exercise
Make three small test forms from your chosen body:
- One thin cup
- One thicker bowl
- One small lid fit test
This reveals plasticity, shrinkage behavior, and fit issues quickly.
Keep Exploring
Pyrometric cones measure heat work (time plus temperature), not just temperature alone, which is why potters trust them over thermometers. The concept of vitrification explains why stoneware becomes waterproof while earthenware stays porous. If you want to understand what makes clay plastic at a molecular level, the kaolinite article covers the plate-like mineral structure that gives clay its workability.
