Pottery Shrinkage Calculator
Use this calculator to determine the final size of your pottery piece after drying and firing, or to calculate the initial greenware size needed to achieve a specific fired dimension.
Calculate Fired Size
Enter the initial size of your greenware (wet clay) and the total shrinkage rate of your clay body to find out its final fired dimensions.
Calculate Initial Greenware Size Needed
If you have a target final fired size, use this section to determine how large your greenware piece needs to be to account for shrinkage.
Understanding Pottery Shrinkage
Pottery shrinkage is a fundamental concept for any ceramic artist. It refers to the reduction in size of a clay object as it dries and is fired in a kiln. This phenomenon occurs because water, which makes up a significant portion of wet clay, evaporates during drying, and organic materials burn out while clay particles fuse together during firing. Understanding and accurately accounting for shrinkage is crucial for creating pieces that fit together, maintain desired proportions, or meet specific dimensional requirements (e.g., lids fitting pots, tiles fitting a space).
Why Does Clay Shrink?
- Drying Shrinkage: As water evaporates from the clay body, the clay particles are drawn closer together. This process continues until the clay reaches the "leather-hard" stage, and then the "bone-dry" stage. The amount of shrinkage during drying depends heavily on the clay's water content and particle size. Clays with higher plasticity (more fine particles) and higher water content generally shrink more during drying.
- Firing Shrinkage: During the firing process in a kiln, further shrinkage occurs. This is due to several factors:
- Dehydration: Remaining chemically bound water is driven off.
- Organic Burnout: Any organic materials in the clay body burn away.
- Sintering/Vitrification: As temperatures rise, clay particles begin to fuse together, forming a denser, stronger material. This densification leads to significant shrinkage, especially as the clay approaches its vitrification temperature.
Factors Affecting Shrinkage Rate
The total shrinkage rate of a clay body is not universal; it varies based on several factors:
- Clay Type: Different clay bodies (e.g., stoneware, porcelain, earthenware) have distinct compositions and particle sizes, leading to varying shrinkage rates. Porcelain, for instance, often has a higher shrinkage rate than earthenware.
- Water Content: The more water initially present in the wet clay, the greater the drying shrinkage.
- Additives/Grogs: The inclusion of non-plastic materials like grog, sand, or kyanite can reduce shrinkage by providing a stable framework within the clay body.
- Firing Temperature: Higher firing temperatures generally lead to greater shrinkage as the clay particles vitrify more completely. A clay fired to cone 10 will typically shrink more than the same clay fired to cone 6.
- Thickness of the Piece: Thicker pieces may shrink unevenly if not dried slowly, but the overall percentage shrinkage is still determined by the clay body and firing.
How to Use the Pottery Shrinkage Calculator
This calculator provides two essential functions for potters:
- Calculate Fired Size: If you've just finished throwing or hand-building a piece and want to know its approximate final dimensions, enter its current "greenware" size and your clay body's known total shrinkage rate. The result will give you an estimate of the piece's size after drying and firing.
- Calculate Initial Greenware Size Needed: This is particularly useful when you need a piece to be a specific size after firing (e.g., a lid for a pre-existing pot, a tile to fit a specific space). Enter your desired final fired size and your clay's shrinkage rate, and the calculator will tell you how large to make your wet clay piece.
Example 1: Calculating Fired Size
You've thrown a bowl that measures 15 cm in diameter when wet. Your stoneware clay body has a total shrinkage rate of 12%. Using the "Calculate Fired Size" section:
- Initial Greenware Size: 15
- Total Shrinkage Rate (%): 12
- Result: Estimated Fired Size: 13.20 cm
This means your bowl will likely be around 13.20 cm in diameter after firing.
Example 2: Calculating Initial Greenware Size Needed
You need to make a lid that will fit a pot with an opening of 10 cm (fired size). Your clay body has a total shrinkage rate of 14%. Using the "Calculate Initial Greenware Size Needed" section:
- Desired Fired Size: 10
- Total Shrinkage Rate (%): 14
- Result: Initial Greenware Size Needed: 11.63 cm
You should aim to make your wet clay lid approximately 11.63 cm in diameter to achieve a 10 cm fired size.
Remember that shrinkage rates can vary slightly even within the same clay body due to variations in batch, firing schedule, and specific handling. It's always a good practice to test your clay and firing process to determine your own accurate shrinkage rates.