Ceramic Bentonite and Loss on Ignition: What Does LOI Really Tell Us?

Bentonite is often added to ceramic bodies to improve plasticity, forming behaviour and green strength. When evaluating a new bentonite, however, ceramic manufacturers may notice that its loss on ignition (LOI) is higher than that of some other raw materials. Does a higher LOI make the bentonite unsuitable?

The answer depends on what is being lost, when it is released during heating, and how much bentonite is used in the complete body. LOI is an important screening value, but it cannot predict firing performance on its own.

What is loss on ignition?
LOI is the percentage of mass a sample loses when heated under a defined laboratory procedure:

LOI (%) = (mass before ignition − mass after ignition) ÷ mass before ignition × 100

The result depends on sample preparation, starting moisture condition, final temperature, holding time and atmosphere. For a meaningful comparison, the supplier and ceramic manufacturer should use the same method and report whether the starting mass is measured on an as-received or pre-dried basis.

LOI also needs to be distinguished from ordinary moisture content. Drying a sample at approximately 105°C removes much of its free and adsorbed water, but the amount of interlayer water remaining in bentonite can depend on the clay and the drying procedure. Subsequent heating can release additional water and other volatile components.

Why does bentonite lose mass during firing?

Bentonite commonly contains montmorillonite, a smectite clay mineral. Its mass loss may come from several sources:
Surface and interlayer water: Released mainly during the earlier stages of heating.
The amount can vary with humidity, exchangeable cations and sample preparation.
Structural hydroxyl groups: At higher temperatures, the clay structure undergoes dehydroxylation and releases water. This is part of the mineral’s thermal transformation, rather than evidence by itself of organic contamination.
Accessory minerals and other constituents: Carbonates, if present, can release carbon dioxide. Organic matter can also contribute to mass loss during firing.
These processes can overlap. A single LOI number reports how much mass was lost, but does not identify the source or temperature of each loss. Thermogravimetric analysis (TGA), ideally considered alongside mineral identification such as X-ray diffraction (XRD), gives a clearer picture.

Does higher bentonite LOI mean more firing defects?
Not necessarily. Defect formation depends on the whole ceramic formulation and firing schedule. Gas release becomes more concerning if a body is becoming dense while gases are still trying to escape. Depending on the process, this may contribute to pores, bloating or surface defects. Drying conditions, heating rate, body thickness, permeability, glaze application and peak firing temperature all matter.

The bentonite addition rate matters too. As a simplified illustration, replacing 3% of a near-zero-LOI raw material with a bentonite measured at 10% LOI would add about 0.3 percentage points to the calculated body LOI, assuming both figures use the same dry basis. This calculation is a formulation estimate, not a prediction of the fired product’s defect rate.

Conversely, selecting the lowest-LOI material without testing its plasticity, green strength or fired behaviour may solve the wrong problem. In a published porcelain study, increasing bentonite addition increased measured ignition loss while also changing shrinkage, density, porosity and strength. Those results belonged to that specific formulation and firing temperature; they are not universal targets for other ceramic bodies.

How should a ceramic manufacturer evaluate bentonite?
A useful comparison starts with the customer’s actual process:

Define the application. Is the bentonite being used in a tile body, porcelain body, sanitaryware, glaze or another formulation? What forming method and firing schedule are used?

Compare LOI on the same basis. Agree on pre-drying, ignition temperature, holding time and reporting method before comparing suppliers’ figures.

Identify the cause of mass loss. If LOI is a concern, use thermal analysis and mineralogical testing to distinguish water release, dehydroxylation and possible accessory-mineral effects.

Test the complete body. Compare the current and proposed formulations for workability, green strength, drying behaviour, firing shrinkage, water absorption, fired strength and visible defects.

Review consistency across lots. A stable raw material and a repeatable production process are often more useful than an isolated “good” LOI result from one sample.

The practical conclusion
Ceramic-grade bentonite should not be judged by LOI alone. The important questions are where the mass loss comes from, when it occurs relative to densification, and what the bentonite contributes at its intended dosage.
At HERMANO NEW MATERIALS, we recommend evaluating a candidate bentonite against the customer’s forming and firing conditions. Share your ceramic application, current body formulation, bentonite dosage, firing schedule and target properties with us so that we can discuss an appropriate material for laboratory trials.

Technical references
Experimental study on the heat treatment reaction process of bentonite, Scientific Reports.
Effect of bentonite addition on some properties of porcelain, Journal of Engineering.