# Clay Properties & Selection

Unit: The Foundations of Clay
Topic: The Three Clay Types
URL: https://claybook.studio/learn/clay-properties-selection/

# 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](https://en.wikipedia.org/wiki/Pyrometric_cone) measure heat work (time plus temperature), not just temperature alone, which is why potters trust them over thermometers. The concept of [vitrification](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Kaolinite) article covers the plate-like mineral structure that gives clay its workability.

## Check your understanding

### Question 1: What does cone rating primarily indicate?

- [ ] A. Only kiln temperature at one moment
- [x] B. Heat work: the effect of temperature over time
- [ ] C. Clay color after bisque
- [ ] D. How plastic the clay feels when wedging

Tip: Cone rating indicates heat work (the combined effect of temperature and time) used for firing decisions.

### Question 2: What is grog?

- [ ] A. A type of glaze
- [x] B. Pre-fired clay that's been ground up
- [ ] C. A pottery tool
- [ ] D. A firing technique

Tip: Grog is pre-fired clay that's been ground up and added back to clay. It adds texture, reduces shrinkage, and helps prevent cracking!

### Question 3: How should you store clay to keep it moist?

- [ ] A. Leave it open to air
- [x] B. Store in airtight plastic bags
- [ ] C. Put it in the sun
- [ ] D. Freeze it

Tip: Store clay in airtight plastic bags to prevent it from drying out. Add a damp sponge if it's getting dry!