Clay Shrinkage Calculator

What is Clay Shrinkage?

Clay shrinkage is a fundamental and unavoidable property that all potters and ceramic artists must manage. It refers to the reduction in the size of a clay body as it loses water during the drying and firing processes. As water evaporates from the clay particles, they move closer together, causing the entire piece to contract. This process occurs in two main stages:

• Drying Shrinkage: This happens as the clay goes from its wet, plastic state to a completely dry, "greenware" state. The rate of drying shrinkage depends on the type of clay and its particle size.
• Firing Shrinkage: This occurs inside the kiln. At high temperatures, the clay particles fuse together in a process called vitrification. This chemical and physical transformation causes further, often significant, contraction.

The total shrinkage is the sum of both drying and firing shrinkage and is usually expressed as a percentage. For example, a clay with a 12% shrinkage rate means that a piece will be 12% smaller in its final fired state than it was in its initial wet state. Understanding and accounting for this shrinkage is crucial for creating pieces that are the correct final size, especially for functional pottery like lids that need to fit, or sets of items that need to match.

How to Use the Clay Shrinkage Calculator

This calculator is designed to help you with two key tasks in pottery: predicting the final fired size of a piece based on its wet dimensions, and determining the necessary wet dimensions to achieve a desired final size.

Mode 1: Calculate Fired Size

Use this mode when you know the size of your wet clay piece and want to know how big it will be after firing.

1. Select Mode: Choose "Calculate Fired Size" from the dropdown menu.
2. Enter Wet Size: Input the measurement of your wet clay piece.
3. Enter Shrinkage Rate: Input the total shrinkage rate of your clay, which is usually provided by the clay manufacturer. Enter this as a percentage (e.g., for 12%, enter "12").
4. View Result: The calculator will show you the estimated final size of your piece after firing.

Mode 2: Calculate Wet Size

Use this mode when you have a target fired size and need to know how large to make your piece in its wet state.

1. Select Mode: Choose "Calculate Wet Size" from the dropdown menu.
2. Enter Desired Fired Size: Input the final measurement you want your piece to be.
3. Enter Shrinkage Rate: Input your clay's total shrinkage rate.
4. View Result: The calculator will show you the required wet size to achieve your target fired size.

The Formulas Behind the Calculation

The math for clay shrinkage requires careful application of percentages.

Calculating Fired Size:

To find the final size, you subtract the shrinkage percentage from the original wet size.

Fired Size = Wet Size × (1 - (Shrinkage Rate / 100))

Calculating Wet Size:

To find the necessary wet size, you must divide the desired fired size by the percentage of the clay that remains after shrinkage. This is a common point of confusion; you cannot simply add the shrinkage percentage to the fired size.

Wet Size = Fired Size / (1 - (Shrinkage Rate / 100))

Practical Example

Let's say a potter wants to make a set of dinner plates that are exactly 10 inches in diameter when they come out of the kiln. The potter is using a clay that has a total shrinkage rate of 14%.

The potter needs to know how large to make the plates when they are wet.

1. Select Mode: They choose "Calculate Wet Size".
2. Enter Desired Fired Size: They input "10" inches.
3. Enter Shrinkage Rate: They input "14" percent.

The calculator uses the formula: Wet Size = 10 / (1 - (14 / 100)) = 10 / 0.86

The result is 11.63 inches.

This tells the potter that they must make each wet plate 11.63 inches in diameter to ensure that after 14% shrinkage, they will end up with a perfect set of 10-inch plates. If they had incorrectly added 14% to 10 inches (getting 11.4 inches), their final plates would have been too small. This demonstrates the importance of using the correct formula, which the calculator handles automatically.

Further Reading and Resources

• The Spruce Crafts: Understanding Clay Shrinkage
• Ceramic Arts Network: A Potter’s Guide to Clay Shrinkage

Internal Links

• Length Converter
• Percentage Calculator
• Oven Temperature Conversion