Grog and Fireclay: The Skeleton of Your Clay Body

If you’ve ever tried to build a large slab or a tall vase out of fine throwing clay, you know the disappointment when it warps during drying or cracks in the kiln. Fine clay is excellent on the wheel, but it lacks an internal structure. That is where grog and fireclay come in. At Relyef, we view these materials as the skeleton of your work. Without them, large sculptures or architectural elements simply won't survive the firing process.

Watch our Reel and see the process.

Grog and fireclay are "temper" - pre-fired and ground-up ceramic materials added to raw clay. Their primary purpose is to reduce shrinkage during drying and firing, increase mechanical strength, and prevent warping or cracking in large objects. While fireclay is a naturally mined raw material, grog is typically created by crushing kiln-wasted pottery or bricks. A current trend is "localized tempering" and blending various particle sizes to achieve an ideal internal structure.

The Difference: Grog vs. Fireclay

Although both serve as temper, they have distinct origins and properties:

Why Temper Your Clay?

• Reduced Shrinkage: The more temper you add, the less the clay shrinks, as water evaporates only from the "binding" component between particles.

• Steam Escape: The gaps between particles act as micro-channels that allow residual moisture to escape, reducing the risk of explosions in the kiln.

• Texture: A coarse surface adds a rustic character to the work, which perfectly complements the textures or fine details created by Relyef stamps.

The Secret of Particle Size: Why Blend Them?

Tempering isn't about just throwing "something" into the mix. The key to perfect stability is combining different grain sizes (fractions). If you have access to coarse, medium, and very fine particles, always mix them in a specific ratio.

This creates a much denser and stronger internal network. Coarse grains provide support, while the finer ones fill the gaps between them. Using only coarse particles is generally not advisable unless it is a specific design choice, as the clay would lack the necessary cohesion.

Looking for a unique texture? See how mill scale can act as an experimental temper with dramatic firing effects in The Forgotten Secret of Blacksmiths in Ceramic Clay.

Technological Trick: Localized Tempering

An interesting variation is using grog or fireclay only on specific parts of the work that are under extreme stress during drying and prone to cracking. You don’t have to temper the entire clay body - just the critical points.

"I personally used this when making large platters. I threw the base with a coarser tempered clay and finished the rest with fine clay. It also worked brilliantly for large, conical pitchers. The cone has the largest base, and that’s where the stress is highest. By tempering only the bottom, I significantly eliminated the risk of cracking," explains Tomáš Macek, a member of the Relyef team.

Tip: Stamping on coarse grog

If you work with clay containing a high amount of coarse grog, achieving a crisp and readable stamp impression is difficult because the grains tear the lines. There is a simple trick to adding fine detail (like text) to a coarse, raw vessel:

1. When recycling or preparing your clay, keep a portion of the body completely free of temper.

2. Apply a thin layer of this fine clay (in the same color) to the spot you want to decorate.

3. You create a smooth "mirror" on the coarse base, into which Relyef stamps will impress with surgical precision and clarity.

Tomáš Macek's Experience:

"When working with large slabs at Relyef, I swear by fireclay. My advice: if you want a large vase base not to warp, mix a blend containing at least 25–40% temper with varying fractions of 0–2 mm (0–0.08 inch). Coarser grains can be unpleasant on your hands while modeling, but they create an incredibly strong wall and a rustic look."