Calculate the exact shrinkage rate or determine the target wet size for your pottery projects.
Calculate Percentage (%)
0%
Total Shrinkage Rate
Find Required Wet Size
0
Starting Wet Size Required
Understanding Clay Shrinkage in Pottery
Clay shrinkage is a critical physical transformation that occurs in ceramics as the material loses water. This process happens in two distinct phases: drying and firing. When water evaporates from the plastic clay, the particles move closer together, causing the object to contract. Later, during the firing process in the kiln, further contraction occurs as chemically bound water is released and the clay particles vitrify.
The Clay Shrinkage Formula
To calculate the percentage of shrinkage, potters use a simple ratio of the wet state compared to the final state. The standard mathematical formula is:
Shrinkage % = ((Original Wet Size – Final Fired Size) / Original Wet Size) × 100
Conversely, if you know your clay's shrinkage rate and want to reach a specific final dimension, you must calculate the "Wet Size" using this formula:
Clay Body Type: Porcelain typically shrinks more (12-15%) than stoneware (10-13%) or earthenware (5-8%).
Moisture Content: The wetter the clay is during the initial measurement, the higher the apparent shrinkage will be.
Firing Temperature: Higher temperatures lead to higher vitrification and more significant shrinkage.
Additives: The addition of grog (fired clay particles) or sand reduces overall shrinkage because these materials do not shrink themselves.
Practical Example
Imagine you are making a dinner plate that must be exactly 10 inches wide after firing. If your clay body has a known shrinkage rate of 12.5%, you cannot simply add 12.5% to 10 inches. You must divide the final size by the remaining percentage.
Calculation: 10 / (1 – 0.125) = 11.43 inches. You should throw your plate at 11.43 inches wide to account for the physical loss during drying and firing.
function calculateShrinkagePercent() {
var wet = parseFloat(document.getElementById('wetLength').value);
var fired = parseFloat(document.getElementById('firedLength').value);
var resultBox = document.getElementById('percentResultBox');
var resultText = document.getElementById('percentResultText');
if (isNaN(wet) || isNaN(fired) || wet wet) {
alert("Fired length cannot be larger than wet length in standard clay shrinkage.");
return;
}
var shrinkage = ((wet – fired) / wet) * 100;
resultText.innerText = shrinkage.toFixed(2) + "%";
resultBox.style.display = "block";
}
function calculateTargetSize() {
var firedTarget = parseFloat(document.getElementById('targetSize').value);
var rate = parseFloat(document.getElementById('knownRate').value);
var resultBox = document.getElementById('targetResultBox');
var resultText = document.getElementById('targetResultText');
if (isNaN(firedTarget) || isNaN(rate) || firedTarget = 100) {
alert("Shrinkage rate must be less than 100%.");
return;
}
// Formula: Wet = Fired / (1 – (Rate/100))
var requiredWet = firedTarget / (1 – (rate / 100));
resultText.innerText = requiredWet.toFixed(2);
resultBox.style.display = "block";
}