Tempering

Part of Pottery and Ceramics

Adding grit, sand, or crusite material to clay to prevent cracking from thermal shock — a critical step for making cooking pots, kiln furniture, and any ceramic that will face rapid temperature changes.

Why This Matters

Pure clay shrinks 8-15% between its wet, workable state and its final fired form. That shrinkage is uneven — thicker sections dry and shrink slower than thin sections, surfaces dry faster than interiors, and rims dry faster than bases. The resulting internal stresses crack pots during drying, during firing, and during use when heated over a cooking fire.

Tempering — adding non-clay particles to the clay body — solves this problem. The temper particles do not shrink. They create a skeleton of rigid material throughout the clay body that resists the clay’s tendency to pull itself apart as it shrinks. Tempered clay dries faster and more evenly, fires with fewer losses, and survives thermal cycling (heating and cooling over a fire) far better than pure clay.

Every ancient pottery tradition discovered tempering independently. Without it, making reliable cooking vessels is nearly impossible. The question is not whether to temper, but what to use and how much.

How Tempering Works

When clay dries or is fired, the spaces between clay particles contract as water leaves. This contraction generates stress. In pure clay, the stress builds until it exceeds the tensile strength of the material, and a crack forms.

Temper particles interrupt this process in three ways:

1. They reduce total shrinkage — since temper particles do not shrink, replacing 20% of the clay with temper reduces overall shrinkage by roughly 20%.
2. They create micro-channels — the gaps between temper particles and the surrounding clay allow moisture and steam to escape more easily during drying and firing, reducing the pressure that causes cracks and blowouts.
3. They distribute stress — instead of stress concentrating in one spot and forming a large crack, temper particles deflect and distribute the stress across thousands of interfaces, preventing any single crack from propagating.

Common Tempering Materials

Material	Source	Particle Size	Best For	Notes
Sand (quartz)	Rivers, beaches, crushed sandstone	0.5-2 mm	General purpose	Most universally available; avoid very fine sand
Grog (crushed fired pottery)	Broken pots, kiln waste	0.5-3 mm	High-temp ware, kiln furniture	Best temper overall — same thermal expansion as clay
Crushed shell	Mussel, clam, snail shells	1-3 mm	Cooking pots	Excellent thermal shock resistance; burns out leaving voids
Crushed rock (granite, basite)	Any hard, non-calcium rock	1-3 mm	Structural ceramics	Crush and sieve; avoid limestone (causes lime-popping)
Plant fiber (straw, grass, rice husk)	Agricultural waste	5-20 mm lengths	Low-fire bricks, furnace linings	Burns out during firing, leaving porous, lightweight body
Ite (volcanic glass)	Near volcanic deposits	1-3 mm	Lightweight ware	Excellent for portable vessels; not available everywhere

The Best All-Purpose Temper: Grog

Grog is crushed, previously fired pottery. It is the ideal temper because:

• It has already undergone all shrinkage and chemical changes
• Its thermal expansion matches the clay body perfectly
• It is available anywhere pottery has been made (crush your failures)
• It produces a strong, thermally stable body

Making grog: Collect broken or failed fired pots. Crush them with a hammer or stone. Sieve the fragments through screens to sort by size. Discard powder finer than 0.5 mm (too fine to be useful). Keep fragments between 0.5 mm and 3 mm.

Save Your Failures

Never throw away a broken pot. Every cracked, warped, or otherwise failed piece of fired pottery becomes grog — the best possible temper for your next batch of clay. Pottery waste is a resource, not trash.

How Much Temper to Add

The correct proportion depends on the clay and the intended use:

Application	Temper Proportion	Why
Cooking vessels	25-35% by volume	Maximum thermal shock resistance
Water storage	15-25%	Moderate thermal resistance, some porosity reduction
General tableware	10-20%	Balance of strength and workability
Kiln furniture (shelves, stilts)	30-40%	Must withstand extreme thermal cycling
Decorative ware	5-15%	Minimal — smooth surface more important
Sculpture / large forms	20-30%	Prevents cracking in thick sections

Testing the Right Amount

Too little temper and the clay still cracks. Too much and the clay becomes difficult to work — it tears when bending, does not hold shapes well, and feels gritty and unpleasant to throw on a wheel.

Simple test procedure:

1. Make 5 small test tiles, each approximately 8 cm square and 1 cm thick
2. Mix each with a different temper proportion: 10%, 15%, 20%, 25%, 30%
3. Label each tile by scratching the percentage into the surface
4. Dry all tiles together under the same conditions
5. Fire all tiles together
6. Compare: Which ones cracked during drying? During firing? Which feel strongest? Which surface is smoothest?

Preparing Tempered Clay

Step-by-Step Process

1. Measure dry clay and dry temper by volume. For a 20% temper mix: 4 parts clay, 1 part temper.
2. Dry-mix first — Spread the dry clay on a clean surface. Sprinkle the temper evenly over it. Mix with hands or a tool until the temper is distributed uniformly through the dry clay.
3. Add water gradually — Sprinkle water over the dry mix and knead. Add water slowly — it is easy to over-wet tempered clay, and drying it back out is tedious.
4. Wedge thoroughly — 100+ compressions. The temper must be evenly distributed. Clumps of untempered clay surrounded by heavily tempered clay will crack at the boundary.
5. Rest overnight — Wrap the mixed clay in plastic or damp cloth and let it sit for 12-24 hours. This allows water to penetrate fully and makes the clay more workable.

Particle Size Matters

Temper that is too fine (powder-like) does not create the micro-channels needed for steam escape. Temper that is too coarse (larger than 3-4 mm) creates weak points and makes the surface rough and difficult to finish. The sweet spot for most applications is 0.5-2 mm — like coarse sand or fine gravel.

Special Tempering Techniques

Shell Tempering for Cooking Pots

Crushed shell (calcium carbonate) is traditionally the preferred temper for cooking vessels because it provides exceptional thermal shock resistance. The shell platelets create a network of flat voids in the fired clay that deflect cracks and allow the vessel wall to flex slightly under thermal stress.

1. Collect freshwater mussel shells, clam shells, or snail shells
2. Calcine the shells by heating them in a fire until they turn white and crumbly (this drives off CO2 and prevents lime-popping)
3. Crush the calcined shells to 1-3 mm fragments
4. Mix 25-30% by volume into the clay

Fiber Tempering for Furnace Linings

Adding organic fibers (chopped straw, grass, rice husks) creates a porous, insulating body after firing. The fibers burn out, leaving voids that trap air and reduce heat transfer. This is ideal for furnace and kiln linings where you want to keep heat inside.

1. Chop fibers to 10-20 mm lengths
2. Mix 20-30% by volume into the clay
3. Form the lining while the clay is stiff (fiber-tempered clay is difficult to throw)
4. Fire slowly — the fibers generate combustion gases during burnout

Sand Washing

If your only available sand contains limestone fragments (fizzes when you drip vinegar on it), you must remove them. Limestone in tempered clay causes lime-popping — small pieces of calcium oxide absorb moisture after firing and expand, blowing out craters in the pot surface weeks or months later.

1. Spread sand on a flat surface
2. Drip vinegar or weak acid on it
3. Pick out or sieve away any fragments that fizz
4. Alternatively, soak sand in vinegar for an hour, rinse, and dry — this dissolves small limestone particles

Troubleshooting Tempered Clay

Problem	Cause	Solution
Clay tears when thrown on wheel	Too much temper or too coarse	Reduce proportion; use finer particle size
Cracks during drying despite temper	Not enough temper, or pieces too thick	Increase proportion; make walls thinner
Rough, pitted surface	Temper too coarse	Sieve temper to smaller particle size; smooth surface with slip coating
Lime pops (white craters appearing after firing)	Limestone in sand temper	Remove limestone; use grog instead
Weak, crumbly fired body	Too much temper, not enough clay matrix	Reduce proportion to 15-20%
Uneven color after firing	Organic temper partially burned out	Ensure complete burnout with good airflow during firing

Tempering is one of those foundational skills that separates functional pottery from decorative experiments. Master it, and your kiln losses drop dramatically. Skip it, and expect to lose half your output to cracks — especially cooking pots, the vessels you need most.