Drying Green Bricks

Part of Brick Making

Controlled drying of unfired bricks to prevent cracking and ensure firing success.

Why This Matters

A freshly molded brick contains 20-30% water by weight. That water must leave the brick slowly and evenly before firing. If drying happens too fast, the outside of the brick hardens while the inside remains wet, creating internal stresses that cause cracking, warping, and breakage. If bricks go into a kiln still containing moisture, the water turns to steam during firing and can cause explosive spalling.

Drying is where many first-time brick makers fail. The forming went well, the clay was good, but impatience or ignorance during the drying phase destroys 30-50% of the batch. Proper drying is not complicated, but it requires patience, planning, and attention to conditions.

A well-dried green brick is strong enough to stack and handle, dimensionally accurate, and moisture-free throughout its entire thickness. Achieving this consistently is the difference between a brick-making operation that works and one that wastes enormous labor.

The Physics of Drying

Understanding why bricks crack during drying prevents it from happening.

How Water Leaves a Brick

Water in a green brick exists in three forms:

1. Free water (pore water): Fills the spaces between clay particles. This evaporates first, from the surface inward. As it leaves, the brick shrinks — clay particles move closer together.

2. Adsorbed water: A thin film coating each clay particle. Leaves after free water is gone. Causes additional minor shrinkage.

3. Chemical water: Bound within the crystal structure of clay minerals. Only released during firing above 500 C. Not a drying concern.

Why Cracks Form

Cracks form when different parts of a brick shrink at different rates. The surface dries and shrinks first. The interior is still wet and has not shrunk. The surface is now too small to fit around the still-expanded interior, so it cracks — exactly like dried mud flats.

The solution is to slow surface evaporation so the interior has time to lose moisture at a rate close to the surface. Every drying technique is essentially about controlling the rate of surface evaporation.

Setting Up a Drying Area

Requirements

• Flat ground: Level to within 2 cm per meter. Uneven ground causes bricks to warp as they dry under their own weight.
• Protection from rain: Even a brief shower can dissolve unfired bricks. A simple roof or tarp is essential.
• Shade: Direct sun causes rapid, uneven surface drying — the top and sun-facing side dry faster than the bottom and shaded side.
• Air circulation: Gentle airflow removes moisture-laden air from around the bricks and replaces it with drier air. Avoid strong wind, which dries one side faster than the other.
• Clean surface: Lay bricks on boards, straw, sand, or dry grass — not bare dirt, which wicks moisture unevenly and causes sticking.

Drying Hack Layout

Arrange a drying hack (the traditional term for a brick drying area):

[Roof or tarp overhead]

  |  B  B  B  B  B  B  |   <- Row of bricks on edge
  |                      |
  |  B  B  B  B  B  B  |   <- 10-15 cm spacing
  |                      |
  |  B  B  B  B  B  B  |
  |                      |
  [Open sides for airflow]

• Space bricks 10-15 cm apart on all sides
• Lay bricks flat for the first 1-2 days, then turn on edge to expose the bottom
• Arrange rows so prevailing breeze passes between bricks, not along them

The Drying Process

Stage 1: Initial Setting (Day 1-2)

Freshly molded bricks are soft and fragile. Leave them in the mold or on the molding surface for 2-12 hours until they are firm enough to handle without deforming. In hot weather, this may be as little as 2 hours. In cool or humid weather, allow overnight.

Key actions:

• Keep in shade
• Do not move until firm enough to lift without fingerprints remaining
• Cover with damp cloth or straw if conditions are very hot and dry

Stage 2: Slow Drying (Days 2-5)

Turn bricks onto their edges to expose the bottom surface that was against the ground. This is now the wettest part.

Weather Condition	Drying Strategy
Hot and dry (>30 C, <40% RH)	Cover with straw or damp cloth, expose gradually
Warm and moderate (20-30 C, 40-60% RH)	Shade only, good air circulation
Cool and humid (<20 C, >60% RH)	Maximum air circulation, consider gentle heat
Rainy	Keep under cover, may need 2x normal drying time

During this stage, rotate bricks every 1-2 days so each face gets equal exposure to air. This prevents one side from drying faster than the other.

The Fingernail Test

Press your fingernail into the brick surface. If it leaves a deep mark, the brick is still too wet to move to the next stage. If it barely dents, the surface has set properly.

Stage 3: Final Drying (Days 5-10)

As bricks approach their final moisture content, they can tolerate more aggressive drying:

• Move to a sunnier location
• Stack in open lattice patterns with air gaps
• In dry climates, bricks may be fully dry in 7 days total
• In humid climates, allow 14-21 days

Stage 4: Verification

A fully dried brick is ready for firing when:

1. Color change: Dried bricks are noticeably lighter than wet ones — typically shifting from dark brown/gray to lighter tan/brown
2. Temperature test: Hold a brick against your cheek. A moist brick feels cool (evaporative cooling). A dry brick feels neutral or warm.
3. Weight test: Weigh a brick. Weigh it again 2 days later. If the weight has not changed, it is dry.
4. Sound test: Tap two dry bricks together. They should make a clear clinking sound. Wet bricks make a dull thud.

Troubleshooting Drying Problems

Cracking

Crack Pattern	Cause	Fix
Surface cracks radiating from edges	Drying too fast	Slow drying, cover bricks
Long straight cracks parallel to length	Uneven drying, one side in wind	Rotate bricks, improve shelter
S-shaped or spiral cracks	Clay too plastic, insufficient temper	Add more sand to future batches
Corner cracks	Sharp corners dry fastest	Slightly round mold corners, slow initial drying
Base cracks	Bottom stayed wet while top dried	Turn on edge sooner, improve base drainage

Warping

Bricks warp when one face dries and shrinks more than the opposite face. Prevention:

• Dry on a flat, non-absorbent surface
• Turn bricks regularly
• Ensure air reaches all faces equally
• Stack only after bricks are firm enough to support weight without deforming

Efflorescence (White Powder)

White deposits on the surface indicate soluble salts migrating to the surface with evaporating water. This is cosmetic during drying but may indicate a clay quality issue. Solutions:

• Add barium carbonate (if available) to the clay mix at 0.5-1% — it converts soluble salts to insoluble ones
• Without chemicals, fire to a higher temperature to fuse salts into the brick body
• Accept minor efflorescence on common bricks — it does not significantly affect structural performance

Climate-Specific Strategies

Arid/Desert Climates

The main challenge is drying too fast. Slow everything down:

• Dry entirely in shade
• Cover with damp burlap or straw for the first 3 days
• Sprinkle water on covers daily to maintain humidity around bricks
• Total drying time: 5-7 days

Tropical/Humid Climates

The challenge is drying too slowly, risking mold growth and extended timelines:

• Maximize air circulation — open-sided drying shelters
• Use fans of woven fronds to move air if natural breeze is insufficient
• Build drying racks to stack bricks vertically, increasing air contact
• Consider a low-temperature pre-drying fire (small smoky fire upwind of drying area)
• Total drying time: 14-21 days

Temperate Climates

The Goldilocks zone for brick drying. Standard procedures work well:

• Shade for first 2-3 days, then gradual sun exposure
• Turn every 1-2 days
• Total drying time: 7-14 days

Never Fire Wet Bricks

If you are unsure whether bricks are fully dry, wait another 2-3 days. The cost of patience is nothing compared to the cost of a batch ruined by steam explosions in the kiln. When in doubt, dry longer.