When you think of ceramics-plates, tiles, vases, or even bathroom fixtures-you probably don’t picture a white powder. But that powder, calcium carbonate, is one of the most important ingredients behind every durable, high-quality ceramic piece you’ve ever used. It’s not just filler. It’s the silent engine that makes ceramics strong, smooth, and stable through firing. Without it, many ceramics would crack, warp, or fall apart before they even reach the shelf.
What calcium carbonate actually does in ceramics
Calcium carbonate, often called chalk or limestone in its raw form, is a mineral compound made of calcium, carbon, and oxygen. In ceramics, it’s added to clay bodies and glazes in precise amounts-usually between 5% and 15% by weight. Its main job? To act as a flux.
During firing, ceramics are heated to temperatures above 1,000°C. At those extremes, materials melt and fuse. Calcium carbonate breaks down around 825°C, releasing carbon dioxide and leaving behind calcium oxide. That calcium oxide then melts into the glassy phase of the ceramic, lowering the melting point of silica and other oxides. This helps the body vitrify properly, making it less porous and more durable.
Think of it like baking bread. Yeast doesn’t give you flavor-it helps the dough rise. Calcium carbonate doesn’t add color or texture directly. It enables the other ingredients to transform.
Why it beats other fluxes
There are other fluxes-feldspar, borax, dolomite. But calcium carbonate has unique advantages. First, it’s cheap. It’s mined in massive quantities across Europe, the U.S., and China. Second, it’s clean-burning. Unlike some fluxes that release sulfur or heavy metals, calcium carbonate breaks down cleanly into lime and CO₂, leaving no toxic residue.
Third, it improves thermal expansion matching. In ceramics, mismatched expansion rates between the body and glaze cause crazing or shivering. Calcium carbonate helps balance this. A study from the British Ceramic Research Association in 2023 showed that adding 8% calcium carbonate to a porcelain body reduced glaze cracking by 62% compared to formulations without it.
It also helps control shrinkage. Too much shrinkage during drying and firing leads to warping. Calcium carbonate stabilizes the clay matrix, allowing more consistent drying and reducing defects.
How it shapes glazes
In glazes, calcium carbonate doesn’t just melt-it modifies the surface. It creates a matte or satin finish when used in higher concentrations (above 10%), which is why many artisanal pottery glazes use it to avoid glassy, slippery surfaces. In lower doses (3-6%), it adds gloss and durability.
It also reacts with other oxides. Add copper to a glaze with calcium carbonate, and you get rich greens. Add iron, and you get earthy browns. It’s a key player in traditional celadon and shino glazes from Asia, and in modern studio pottery worldwide.
One potter in Stoke-on-Trent told me he switched from feldspar to calcium carbonate in his white glaze after years of cracking. His new recipe-6% calcium carbonate, 12% kaolin, 2% silica-produced a flawless, bright white finish that survived 300+ firings without a single crack. He still uses it today.
Industrial use: Tiles, sanitaryware, and beyond
The ceramics industry doesn’t stop at handmade pots. Over 70% of the world’s ceramic tiles-used in kitchens, bathrooms, and public buildings-rely on calcium carbonate. In tile manufacturing, it’s added to the body to improve green strength (the unfired hardness), making tiles easier to handle on fast-moving production lines.
In sanitaryware, like sinks and toilets, calcium carbonate is critical for achieving the smooth, non-porous surface that resists stains and bacteria. A 2024 report from the European Ceramic Society found that tiles with 10% calcium carbonate had 40% lower water absorption than those without, making them better suited for wet areas.
Even advanced ceramics-like those used in medical implants or electronics-use high-purity calcium carbonate as a sintering aid. In zirconia-based dental crowns, it helps control grain growth during firing, ensuring the final product is both strong and translucent.
Purity matters
Not all calcium carbonate is the same. Industrial-grade material must be over 98% pure. Impurities like iron oxide turn white glazes gray. Magnesium can cause bloating. Clay suppliers test each batch with X-ray fluorescence to ensure no trace metals sneak in.
High-purity calcium carbonate is often ground to less than 2 microns-finer than flour. That’s because particle size affects how evenly it melts. Coarse particles leave white specks in glazes. Fine particles dissolve completely, giving uniform results.
Some manufacturers use precipitated calcium carbonate (PCC), made chemically from limestone and CO₂. It’s purer and more consistent than ground limestone, but more expensive. Most large factories use ground limestone. Small studios use PCC when they need flawless surfaces.
Common mistakes and how to avoid them
Many beginners add too much calcium carbonate, thinking more is better. But over 15%, it becomes a problem. Excess calcium oxide can cause over-firing, leading to bloating, blistering, or even melting the piece. It can also make glazes too fluid, causing them to run off the surface.
Another mistake? Skipping the calcination step. Raw limestone contains water and carbon dioxide. If you add it directly to wet clay without pre-firing, the gases escape during drying or early firing, creating pinholes or cracks. Always calcine it first-fire it to 900°C, then grind it before mixing.
And never assume all chalk is the same. Classroom chalk is often gypsum, not calcium carbonate. Using the wrong material can ruin a whole kiln load.
Environmental and sourcing trends
With rising concerns about carbon emissions, the ceramics industry is looking at ways to reduce the CO₂ released when calcium carbonate decomposes. Some manufacturers are experimenting with recycled ceramic waste as a partial substitute. Others are blending calcium carbonate with fly ash from coal plants-a waste product that also acts as a flux.
In the UK, most calcium carbonate comes from the Peak District and the Jurassic Coast. The UK has over 20 active limestone quarries, and many ceramic factories are located within 50 miles of them to cut transport emissions. That’s why British ceramics often have a reputation for consistency-they’re made with local, well-tested materials.
What’s next for calcium carbonate in ceramics
Researchers are now testing nano-sized calcium carbonate particles. Early results show they can reduce firing temperatures by up to 100°C, saving energy and cutting emissions. Others are combining it with bio-based binders to make ceramics more sustainable from start to finish.
Meanwhile, craft potters are rediscovering traditional recipes. In Japan, artisans still use local limestone from Shikoku for their raku ware. In Mexico, potters in Oaxaca mix crushed seashells-natural calcium carbonate-with clay to create their signature black-on-red pottery.
So while technology advances, calcium carbonate remains unchanged. It’s still the same mineral that helped ancient Romans make durable amphorae. It’s still the same powder that makes your coffee mug strong enough to survive the dishwasher.
Is calcium carbonate the same as chalk?
Yes, chalk is a naturally occurring form of calcium carbonate. But not all chalk is suitable for ceramics. Classroom chalk is often made from gypsum (calcium sulfate), which melts at lower temperatures and can cause defects. Only use limestone-derived calcium carbonate that’s labeled for industrial or ceramic use.
Can I use baking soda instead of calcium carbonate?
No. Baking soda is sodium bicarbonate. When fired, it breaks down into sodium oxide, carbon dioxide, and water. Sodium oxide melts too early and makes glazes overly fluid, causing them to run. It can also make ceramics brittle. Calcium carbonate behaves completely differently under heat.
Does calcium carbonate make ceramics white?
It helps. Pure calcium carbonate is white and doesn’t introduce color. When used in porcelain or white glazes, it enhances brightness by reducing iron impurities and promoting even vitrification. But it won’t make a brown clay body white-you still need a white-burning clay like kaolin.
How much calcium carbonate should I add to my clay body?
For most stoneware and porcelain, use 5-10%. For earthenware, 3-6% is typical. Higher amounts (10-15%) are used in glazes for matte finishes. Always test small batches first. Too much can cause bloating or warping.
Why do some ceramic glazes craze even with calcium carbonate?
Crazing happens when the glaze contracts more than the clay body during cooling. Calcium carbonate helps reduce this by adjusting thermal expansion, but it’s not a fix-all. If your clay body is too porous or your glaze is too silica-rich, you’ll still get crazing. Adjusting the silica or adding feldspar may be needed.
Christine Mae Raquid
November 2, 2025 AT 02:19Okay but why does everyone act like calcium carbonate is some magic potion? I used to work in a tile factory and they just dumped the powder in like it was sugar. Half the time it clumped and we had to scrap whole batches. Not rocket science.
Torrlow Lebleu
November 3, 2025 AT 00:21Wow. Just wow. You didn’t even mention the fact that calcium carbonate’s decomposition releases CO₂ at a rate of 0.44 tons per ton of material. This isn’t ‘silent engine’-it’s a climate liability disguised as tradition. And you call that sustainable? The ceramics industry is clinging to medieval chemistry while the world burns. Pathetic.
Zach Harrison
November 3, 2025 AT 19:37Actually, I think Torrlow’s being a bit dramatic-but he’s not wrong. The CO₂ thing is real. But here’s the thing: the industry’s already adapting. I saw a plant in Ohio that’s blending 30% fly ash with limestone. The glazes are actually better. Less bloating, more consistency. Maybe we don’t need to ditch calcium carbonate… just refine how we use it.
charmaine bull
November 4, 2025 AT 20:42As someone who’s fired over 2000 pieces, I’ll say this: calcium carbonate is the unsung hero. I used to use feldspar, but my glazes kept crawling. Switched to 7% PCC and suddenly my celadons looked like they came out of a 12th-century kiln. No more pinholes. No more cracking. Just pure, quiet magic. Also-please don’t use baking soda. I’ve seen it happen. It’s a disaster.
Terri-Anne Whitehouse
November 5, 2025 AT 23:17How quaint. You all treat this like some artisanal secret. In Europe, we’ve been using high-purity PCC since the 80s. The UK’s Peak District limestone? Overmined. The real innovation is in China-they’ve got nano-calcium carbonate with 99.97% purity, fired at 100°C lower. You’re all still arguing about chalk while the future’s already glazed.
LeAnn Raschke
November 6, 2025 AT 14:45That’s actually really interesting. I’m a teacher and I’ve been showing my students how ceramics work. We did a little demo with chalk, clay, and a propane torch. They were amazed that something so simple could make things so strong. I think it’s cool how science is hiding in plain sight.