When a glaze settles into two stubborn layers or powders off the pot, a very small helper, bentonite, can rescue it. This ultra-fine clay is added in low percentages to keep glazes suspended, improve handling, and strengthen the dry coat. In the following sections, we will look at the science behind it, the right addition rates, and the simple tricks that make everyday glazing far more predictable for studios and factories alike. Understanding these basics prevents waste, rework, and a lot of frustrated stirring.
Iran Bentonite Co. supplies ceramic producers with several grades of processed bentonite sourced from rich Iranian deposits and refined in modern plants. The company focuses on stable particle size, low impurities, and reliable swelling capacity, so glaze technicians can predict the behavior of every batch. Whether the user needs sodium-type material for strong suspension or Construction Bentonite, this supplier offers consistent quality, export-ready packaging, and technical guidance for glaze applications across tiles, tableware, sanitaryware, and studio work.
Bentonite in Glaze; How Does It Work?
Bentonite is a very fine-grained clay dominated by the mineral montmorillonite, famous for its ability to absorb water and expand. Because the particles are plate-like and extremely small, they create an enormous surface area in water. Even at low dosages, this surface area allows bentonite to interact with other materials in the glaze, forming a loose, supportive network around them. That network is what stops glaze ingredients from settling too quickly during storage and application, especially in low-clay or high-frit formulations.
What makes bentonite valuable in glaze is the combination of swelling, colloidal behavior, and a small contribution of alumina and silica. When hydrated, the clay thickens the water phase just enough to hold heavier minerals like feldspars, frits, zirconium opacifiers, or stains in suspension. At the same time, it improves the adhesion and strength of the unfired glaze layer, so the coating survives handling, waxing, and transport before the piece reaches the kiln, reducing losses and surface damage in production.
Main Functions of Bentonite in Glaze
Bentonite does several jobs at once in a glaze, but they all point to one outcome: a slurry that stays mixed and a coating that does not fail before firing. It suspends heavy ingredients, makes application smoother, and gives the dry layer a little toughness, especially in low-clay recipes or highly fritted glazes. In the following applications, we will look at each of these roles separately to see how they support reliable glazing in studios, craft workshops, and industrial lines that demand repeatability.
Suspension & Anti–Hard-Panning
Hard-panning happens when the heavy minerals slowly sink and form a compact mass at the bottom of the bucket. With only 1–2% bentonite, the water phase becomes slightly structured, so those minerals stay floating among the clay particles instead of settling. This prevents the cement-like layer that is difficult to re-disperse and keeps daily glazing fast, even after the mixture has been sitting for days in storage or transport, which is especially valuable for industrial users and teaching studios that mix in bulk.
Better Brushing / Application
Some glazes run off the bisque because they are low in clay or are formulated mainly with frit. Adding bentonite increases the glaze’s grip on the surface and slows the runoff, so a single dip or three good brush strokes produce an even coat with consistent color. This gives more control over thickness, reduces streaks, and makes glazing less dependent on the exact porosity or firing history of the ware, which is very useful in mixed batches of bisque from different kilns.
Dry Strength
A glaze layer that dries too weakly will chip when you wipe the foot or load the kiln. Because bentonite is plastic, it acts as a binder between the other particles and raises the dry strength of the coating. That means fewer touch-ups, less dusting, and fewer accidents when pieces are stacked close together on shelves before firing, especially in workshops where many people handle the same ware or where glazes contain a lot of glassy frit and almost no clay.
Improved Slurry Rheology
Rheology is the way a liquid moves, and glazes with poor rheology either settle fast or pour like water. Bentonite introduces a gentle thixotropy: the glaze stays thick when resting but loosens when you stir or dip. This makes it easier to keep an even thickness on vertical surfaces and reduces the amount of water you need to add during long production days, which keeps the recipe chemistry more stable over time and minimizes glaze defects.
How Much Bentonite Should Be Added?
Most glaze formulas need only a pinch of bentonite—typically 1–2% of the total dry weight—to stay suspended. If the recipe already contains 10–15% kaolin or ball clay, start at the lower end, because those clays also help suspension and dry strength. At this level, bentonite improves behavior without changing the color, melt, or surface finish of the fired glaze, so it is safe for glossy whites, celadons, and lightly opacified glazes used on tableware and sanitaryware.
Some glazes, especially highly fritted, low-clay, or special-effect recipes, may need 2–3% bentonite to stop rapid settling. Above that, you gain suspension but also add more shrinkage to the drying stage and slightly slow the drying time, which can delay loading on busy days. Too much bentonite can make the unfired glaze crack, peel, or crawl off the bisque, so increases should always be tested on a tile before full-scale use in production kilns or classrooms.
Sodium Bentonite vs. Calcium Bentonite for Glazes
Sodium bentonite is the more powerful suspender. It swells a lot in water, creates a thick, gel-like structure, and can keep even heavy, granular materials floating for long periods. This makes it excellent for glaze systems that travel, sit in storage, or are shared by many users in a teaching studio. However, its strong swelling sometimes makes the glaze too viscous, so small additions, good mixing, and occasional deflocculation with suitable chemicals are important to restore flow.
Calcium bentonite swells less and therefore gives a milder thickening effect. It is often chosen for light-colored or porcelain glazes where purity and flow are more important than extreme suspension power. When sourced from companies such as Iran Bentonite Co., calcium grades can deliver clean slurries, good adhesion, and predictable drying without turning the glaze into a jelly, which helps with brushing and smooth, flaw-free fired surfaces in decorative ware, sanitary pieces, and glossy white tiles.
Further Reading: Sodium Bentonite vs Calcium Bentonite
How to Add Bentonite?
Bentonite must be introduced carefully, because its fine particles love to form stubborn lumps when they meet water. The safest method is to hydrate it on its own, then blend it into the glaze while mixing with a drill or propeller mixer. Below are practical steps that work for studio batches and for larger industrial buckets where consistency, repeatability, and proper hydration are important, especially when using high-suspension sodium grades or refined calcium grades from Iran Bentonite Co. in export projects.
- Step 1 – Measure the dose:
Weigh bentonite at 1–2% of the dry glaze recipe, or slightly higher for low-clay mixes. Accurate weighing is important, because even small overdoses change viscosity, drying time, and sometimes the fired surface, especially in glossy or translucent glazes.
- Step 2 – Pre-slurry in water:
Put the bentonite in a separate container and add 5–10 times its weight of clean water. Stir or shake until no dry pockets remain. Pre-slurrying removes lumps and gives full hydration before the clay meets the glaze.
- Step 3 – Add to the glaze:
Pour the hydrated bentonite slowly into the main glaze bucket while mixing continuously. A drill mixer or impeller will spread the clay evenly and prevent local gel pockets that make the glaze feel rough or stringy.
- Step 4 – Sieve and rest:
Pass the glaze through a suitable mesh to break any remaining agglomerates. Let the bucket rest for several hours so the thixotropic structure can develop fully and the bubbles rise out of the slurry.
- Step 5 – Adjust thickness:
After resting, check the dipping or brushing feel. If the glaze is too thick, add small amounts of water and mix again. If it is too thin, a tiny extra bentonite slurry can be introduced and mixed thoroughly.
Glaze Problems That Bentonite Fixes
Because bentonite strengthens the water phase and the dry film at the same time, it can solve several common glaze issues without changing the main recipe. The following problems appear often in studios and small factories, especially when glazes are stored for days, shipped, or applied on less porous bisque. Addressing them with 1–3% bentonite is cheaper and faster than reformulating the glaze from zero, and it helps standardize behavior across different batches of water or seasonal temperatures in production rooms.
- Hard-panning and heavy settling:
Bentonite thickens the liquid phase so heavy minerals cannot sink and lock together. A well-hydrated 2% addition usually keeps the glaze fluid on stirring but stable during rest, saving time on remixing and sieving before every glazing session.
- Powdering or flaking dry coats:
If the dried glaze scratches off when you touch it, the slurry lacks plastic binder. Bentonite adds that binder and helps particles cling to the bisque, so you can wax, sponge, and move the piece without damaging the unfired surface.
- Uneven pickup on low-porosity bisque:
Some bisque or tile bodies do not absorb water quickly, so the glaze runs. Bentonite increases the slurry’s body and gives a slightly slower runoff, which produces a more even thickness on vertical or semi-vitrified pieces.
- Edge cracking in the dry stage:
Glazes that shrink too much when drying can crack around rims or carved areas. A small amount of bentonite balances the drying process and supports the coating so it remains intact until the heat of the kiln sinters it permanently.
Alternatives and Complements for Bentonite in Glaze
Although bentonite is the first choice for suspension in most ceramic glazes, it is not the only option. Some formulas or production lines prefer organic binders, mixed-layer clays, or simply a higher proportion of kaolin. These choices depend on the color sensitivity of the glaze, the type of application, and whether the glaze must travel or be stored for long periods without remixing or sieving, such as in export or distant workshops supplied from one plant.
The following materials can either replace bentonite partly or work alongside it to fine-tune glaze behavior. Using them in combination allows the technician to balance suspension, brushing quality, and drying speed without overloading the glaze with one clay. They are particularly useful for very white or translucent glazes where even small impurities or too much shrinkage would be noticeable to customers and quality inspectors in export markets with strict acceptance levels and documentation requirements for raw materials and additives used in production.
- Veegum or hectorite blends:
Give strong suspension and excellent brushing, often cleaner in color than ordinary bentonite. They cost more but are ideal for high-value glazes where appearance and stability must both be guaranteed in every production run, including sanitary or tableware lines.
- CMC or other organic binders:
Improve adhesion and brushing feel, especially for glazes sold in ready-to-brush form. They do not suspend as strongly, so a small amount of bentonite or extra clay is often kept in the recipe for long-term stability.
- Extra kaolin or ball clay:
Raising the built-in clay content of the glaze can remove the need for separate bentonite. This option is simple and cheap, but it may slightly shift the fired surface, so lab tests are necessary before changing production formulas.
