Tempering

Part of Brick Making

Adding sand, straw, or grit to clay to control shrinkage and cracking during drying and firing.

Why This Matters

Raw clay shrinks as it dries — sometimes by 10-15% of its original volume. That shrinkage creates internal stresses that crack bricks apart before they ever reach a kiln. In a rebuilding scenario where every brick counts, losing half your production to cracking is a disaster you cannot afford.

Tempering is the practice of mixing non-clay materials into your clay body to reduce shrinkage, improve workability, and produce stronger finished bricks. Every brick-making tradition in history developed tempering methods suited to local materials, from the straw-tempered mud bricks of ancient Egypt to the sand-tempered fired bricks of Rome.

Understanding tempering transforms unreliable clay into a predictable building material. Get the ratio wrong and bricks crack, crumble, or warp. Get it right and you can produce hundreds of consistent bricks from a single clay deposit, building structures that last generations.

Identifying Your Clay Type

Before you can temper clay properly, you need to understand what you are working with. Different clays need different tempering strategies.

The Ribbon Test

Roll a handful of moist clay into a rope about pencil-thickness. Drape it over your finger:

Behavior	Clay Type	Tempering Need
Breaks immediately	Sandy/lean clay	Minimal — already has natural temper
Hangs 2-5 cm before breaking	Medium clay	Moderate tempering needed
Hangs 8+ cm without breaking	Fat/plastic clay	Heavy tempering required

The Shrinkage Test

Form a flat tile 10 cm long. Score a line exactly 10 cm with a straightedge. Let it dry completely in shade.

• Less than 5% shrinkage (9.5 cm or longer): Clay is already lean enough for basic bricks.
• 5-8% shrinkage: Needs moderate temper addition.
• 8-15% shrinkage: Needs significant temper — 25-40% by volume.
• Over 15% shrinkage: Extremely fat clay — may need 40-50% temper or blending with a leaner clay.

Always test before committing

Make 5-6 test bricks with different temper ratios before producing a full batch. Label each with a scratch mark showing the ratio used. The cost of 6 test bricks is trivial compared to losing 200.

Temper Materials and Their Properties

Sand

The most universal temper. Use sharp, angular sand (crushed rock) rather than smooth river sand — angular grains interlock better within the clay matrix.

Ideal grain size: 0.5-2 mm. Sift out anything larger than 3 mm and anything finer than dust.

Typical ratio: 20-35% sand by volume for medium clays.

Advantages: Reduces shrinkage dramatically, widely available, does not burn out during firing, produces strong bricks.

Disadvantages: Too much sand makes clay unworkable and bricks friable. Sand heavier than 40% usually causes problems.

Straw and Plant Fiber

Chopped straw (wheat, rice, grass) has been used for millennia in sun-dried (adobe) bricks. The fibers bridge cracks as they form, holding the brick together during drying.

Preparation: Chop straw into 3-5 cm lengths. Longer pieces create weak planes; shorter pieces provide less reinforcement.

Typical ratio: 5-15% by volume.

Advantages: Lightweight, excellent crack resistance during drying, adds insulation value to finished walls.

Disadvantages: Burns out during kiln firing, leaving voids that weaken the brick. Best for sun-dried bricks or low-fire applications. Straw must be dry — green plant material can ferment and create gas pockets.

Grog (Crusite Brick or Pottery)

Crushed fired clay — broken bricks, pot shards, or kiln waste ground to gravel and sand size. This is the professional brick-maker’s preferred temper.

Preparation: Smash reject bricks or pottery with a hammer, then grind between flat stones. Sift to 1-3 mm grain size.

Typical ratio: 15-30% by volume.

Advantages: Thermally compatible with the clay body (same expansion rate), zero shrinkage contribution, produces excellent fired bricks, recycles waste.

Disadvantages: Requires existing fired material to start — a bootstrap problem for the first batch.

Other Temper Materials

Material	Use Case	Notes
Wood ash	5-10% addition	Acts as flux, lowers firing temperature slightly
Sawdust	10-20% for lightweight bricks	Burns out during firing, creates insulating voids
Crushed shell	Coastal areas	Calcium content can cause lime popping if pieces too large
Animal dung	Traditional adobe	Fiber content + binding agents, must be well-dried
Rice husks	10-15%	High silica content, good for sun-dried bricks

Mixing and Preparation

The Soaking Method

1. Break dry clay into fist-sized lumps and spread in a shallow pit or trough.
2. Flood with water and let soak 24-48 hours. Fat clays may need 3-4 days.
3. Drain excess water until clay is workable — stiff enough to hold shape but soft enough to knead.
4. Spread clay on a clean, hard surface in a layer about 10 cm thick.
5. Sprinkle temper evenly across the surface at your target ratio.
6. Fold and knead — fold the clay over the temper, press flat, fold again. Repeat 20-30 times.

The Treading Method (Large Batches)

For production quantities, mix in a shallow pit:

1. Layer clay and temper alternately in a pit 30-40 cm deep.
2. Add water until the mix is thick mud consistency.
3. Walk through the pit barefoot, treading and turning the material with your feet for 15-20 minutes.
4. Let rest overnight, then tread again the next day.
5. The mix is ready when you can cut a cross-section and see temper distributed evenly throughout with no clay-only pockets.

The wedging test

Cut a ball of mixed clay in half with a wire or string. The cross-section should show temper particles evenly distributed. If you see streaks or clusters, keep mixing.

Aging (Souring)

After mixing, let the tempered clay rest in a covered, damp state for at least one week. Bacterial action breaks down remaining clay lumps and improves plasticity. Traditional brick-makers aged clay for months. Even 3-4 days of aging noticeably improves workability.

Getting the Ratio Right

The ideal temper ratio depends on your clay, your temper material, and your intended use. Here is a systematic approach:

Quick Ratio Guide

Clay Type	Sun-Dried Bricks	Kiln-Fired Bricks
Very fat (15%+ shrinkage)	30-40% sand + 10% straw	35-45% sand or grog
Medium (8-15% shrinkage)	20-30% sand + 5-10% straw	20-35% sand or grog
Lean (5-8% shrinkage)	10-15% sand	10-20% sand or grog
Very lean (under 5%)	None needed	0-10% grog

Adjusting by Observation

After making test bricks, look for these signs:

• Cracks during drying: Not enough temper, or drying too fast. Add 5-10% more temper.
• Bricks crumble when handled: Too much temper. Reduce by 5-10%.
• Surface flaking: Temper grains too large. Sift to finer size.
• Warping: Uneven temper distribution. Mix more thoroughly.
• Bricks strong but heavy: Normal. For lighter bricks, substitute some sand with sawdust or straw (sun-dried only).

Common Mistakes

Mixing wet: Adding temper to wet, sloppy clay makes even distribution nearly impossible. Work with clay that is firm but pliable — like stiff bread dough.

Inconsistent measurement: Eyeballing ratios leads to inconsistent bricks. Use a consistent container (a bucket, a basket) to measure both clay and temper for each batch.

Skipping the test phase: The single most common cause of wasted effort. Six test bricks take an hour. A failed batch of 200 bricks wastes days of labor.

Using fine dust as temper: Particles under 0.25 mm fill pore spaces but do not reduce shrinkage effectively. Your temper needs visible grain size — at minimum, coarse sand.

Firing straw-tempered bricks too hot: Organic temper burns out and leaves voids. This is acceptable for low-fire bricks (under 700°C) but weakens bricks fired at higher temperatures. For high-fire work, use sand or grog exclusively.