Glaze LOI (Loss on Ignition) Calculator

Enter your glaze batch weight, fired weight, and material parts to calculate LOI by ingredient, total gas loss, and glaze fit risk.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Total Batch Weight
Input the total dry weight of your unfired glaze batch in grams. This is the sum of all raw materials before water is added.
2
Provide Fired Weight
After firing, weigh the remaining glaze material (e.g., from a test tile) and enter this 'fired weight' in grams. This allows the calculator to determine your actual, measured LOI.
3
Specify Silica Content (parts)
Enter the proportion of Silica (SiO2) in your glaze recipe in parts. Silica has 0% LOI and contributes no gas during firing.
4
Input Feldspar Content (parts)
Enter the proportion of Feldspar in parts. Feldspar typically has a very low LOI (around 0.5%) and acts as a flux.
5
Add Whiting / CaCO3 Content (parts)
Enter the proportion of Whiting (calcium carbonate) in parts. Whiting has a high LOI (~44%) and is a significant source of CO2 gas release.
6
Include Kaolin / EPK Content (parts)
Enter the proportion of Kaolin in parts. Kaolin has an LOI of about 14% due to chemically bound water released above 500°C.
7
Provide Talc Content (parts)
Enter the proportion of Talc in parts. Talc has an LOI of about 5% from structural water and CO2, acting as a mid-fire flux.
8
Specify Zinc Oxide Content (parts)
Enter the proportion of Zinc Oxide in parts. Zinc oxide has 0% LOI and functions as an opacifier and flux without gas evolution.
9
Review Your Results
Compare the Calculated LOI from your recipe against the Measured LOI from your fired sample. Analyze the Glaze Fit Risk and material contributions to gas loss.

Example Calculation

A ceramic artist is developing a new glaze and wants to understand its Loss on Ignition (LOI) to predict firing behavior and potential defects.

Total Batch Weight (g)

1000

Fired Weight (g)

920

Silica (parts)

Feldspar (parts)

Whiting / CaCO3 (parts)

Kaolin / EPK (parts)

Talc (parts)

Zinc Oxide (parts)

Results

11.3%

Tips

Moderate High LOI Materials

Materials like Whiting (CaCO3) and Dolomite have high LOI values, releasing significant CO2 during firing. Keep their proportion below 20% in the recipe to reduce pinholing risk in most glazes.

Compare Calculated vs. Measured LOI

Always compare your Calculated LOI (from the recipe) to your Measured LOI (from a test firing). A significant discrepancy might indicate an error in material composition, weighing, or firing conditions.

Consider Firing Schedule for High LOI

For glazes with a total LOI above 12%, a slower firing schedule with a prolonged soak between 800°C and 1000°C can help gases escape more gently, reducing common defects like blistering and pinholes.

Understanding Glaze Volatiles with the LOI Calculator

The Glaze LOI (Loss on Ignition) Calculator helps ceramic artists and manufacturers analyze the amount of volatile material that will burn off during firing. By comparing the calculated LOI from a glaze recipe with the measured LOI from a test firing, users can predict and mitigate potential firing defects like pinholes and blistering. This tool is essential for understanding how raw materials contribute to gas evolution, ensuring smoother, more consistent glaze surfaces, especially as artists experiment with new formulations in 2025.

Minimizing Glaze Defects and Firing Issues

Understanding a glaze's Loss on Ignition (LOI) is critical for preventing common firing defects that can compromise the quality and aesthetics of ceramic pieces. High LOI values, often from carbonates or hydrated clays, mean significant gas release during firing. If these gases cannot escape cleanly before the glaze surface vitrifies, they become trapped, creating pinholes, blisters, or even crawling. By predicting the LOI, potters can adjust recipes or firing schedules to allow for a more gentle gas evolution, leading to a smoother, defect-free surface. This proactive approach ensures consistent results, preventing costly rework or material waste.

Calculating Glaze Weight Loss During Firing

The Glaze LOI Calculator works by estimating the theoretical weight loss of a glaze batch based on the LOI percentages of its constituent raw materials, and then allows comparison with an actual measured LOI.

The formula for the Calculated LOI (percentage of dry weight) is:

Calculated LOI = (Σ (Material_parts × Material_LOI_%) / Total_parts)

The formula for Measured LOI (percentage of initial batch weight) is:

Measured LOI = ((Total Batch Weight - Fired Weight) / Total Batch Weight) × 100

By performing these calculations, the tool provides insight into how much material is lost as gas, helping to understand potential firing issues. High LOI materials like Whiting (calcium carbonate) or Kaolin contribute significantly to this loss.

💡 To accurately assess your overall production costs, including material expenses, use our Glaze Material Cost per Batch Calculator alongside your LOI analysis.

Analyzing Glaze LOI for a New Formulation

Consider a ceramic artist testing a new glaze formulation to ensure smooth, defect-free results.

Input Batch Weights: The artist prepared a 1000 g dry batch. After firing a test tile, the glaze residue weighed 920 g.
Enter Material Proportions: The recipe contains 30 parts Silica, 25 parts Feldspar, 20 parts Whiting (CaCO3), 15 parts Kaolin, 5 parts Talc, and 5 parts Zinc Oxide.
Perform Calculation: Based on standard LOI values for these materials, the calculator determines the theoretical weight loss. For example, Whiting contributes ~44% LOI, and Kaolin ~14%.
Compare Results: The calculator outputs a Calculated LOI of 11.3% from the material proportions. Simultaneously, the Measured LOI from the batch weights is ((1000 - 920) / 1000) * 100 = 8.0%.

The discrepancy between the calculated (11.3%) and measured (8.0%) LOI suggests that either the assumed LOI values for the raw materials are slightly off, or the firing process didn't fully burn off all volatiles, or there was some error in weighing. This prompts the artist to investigate further, perhaps adjusting the firing schedule or re-testing material purity to prevent potential glaze defects.

💡 Understanding LOI is just one aspect of glaze formulation. For a comprehensive view of how your glaze chemistry affects physical properties, explore our Glaze Fit Calculator.

Minimizing Glaze Defects and Firing Issues

Typical LOI Ranges for Common Glaze Materials

The Loss on Ignition (LOI) of glaze materials varies significantly, and understanding these typical ranges is crucial for successful glaze formulation. For instance, purified silica (SiO₂) and zinc oxide (ZnO) have an LOI of essentially 0%, meaning they contribute no volatile gases during firing. In contrast, calcium carbonate (whiting) boasts a high LOI of around 44%, releasing a substantial amount of carbon dioxide as it decomposes above 800°C. Hydrated clays like kaolin, often used as a suspending agent, typically have an LOI between 12-15% due to the loss of chemically bound water at temperatures above 500°C. Other materials such as feldspars and nepheline syenite usually have very low LOI values, often less than 1%, making them relatively stable during heating. Recognizing these inherent characteristics allows ceramists to formulate glazes with predictable firing behavior and minimize defects.

Frequently Asked Questions

What is Loss on Ignition (LOI) in glazes?

Loss on Ignition (LOI) refers to the percentage of weight a glaze material or mixture loses when heated to a high temperature, typically during firing. This weight loss occurs as volatile components like water, carbon dioxide, and organic matter burn off or decompose, transforming the raw materials into their fired state.

Why is LOI important for glazes?

LOI is crucial for glazes because it indicates the amount of gas released during firing, directly impacting glaze quality and appearance. High LOI can lead to defects such as pinholes, blisters, and crawling if gases cannot escape cleanly, affecting the smoothness and durability of the fired surface.

Which glaze materials have high LOI?

Several common glaze materials have high Loss on Ignition (LOI) due to their chemical composition. Notable examples include calcium carbonate (whiting) with approximately 44% LOI, dolomite, and certain clays like kaolin, which loses around 14% as chemically bound water. These materials are significant sources of gas during the firing process.

How does LOI relate to glaze defects?

High LOI in glazes is a primary contributor to common firing defects. As large volumes of gases like carbon dioxide and water vapor escape during vitrification, they can create pinholes, blisters, or craters on the glaze surface if the melt becomes too viscous too quickly, trapping the gas bubbles before they can burst and heal.