Sun Drying
Part of Brick Making
Air-drying bricks in hot climates without firing.
Why This Matters
Sun-dried bricks — adobe — are humanity’s oldest mass-produced building material. Settlements in Mesopotamia used them 10,000 years ago, and structures built from sun-dried brick in arid climates have survived for centuries. The method requires zero fuel, no kiln, and no specialized equipment beyond a simple mold. In a rebuilding scenario where fuel is desperately needed for cooking, metalworking, and warmth, sun-drying lets you produce building materials using nothing but earth, water, and sunlight.
The process is forgiving and scalable. A single person can mold and dry 50-100 bricks per day. A team of four can produce 300-500. Within two weeks, you can have enough bricks for a small shelter. The trade-off is durability — sun-dried bricks are weaker than fired bricks and vulnerable to prolonged water exposure. But with proper wall design (thick walls, generous overhangs, plastered surfaces), sun-dried brick buildings provide excellent shelter in arid and semi-arid climates.
Sun drying is also the essential first step for fired bricks. Every brick must be thoroughly air-dried before entering a kiln — wet bricks explode or crack violently when heated rapidly. Understanding sun drying is therefore fundamental to all brick production, not just adobe construction.
Climate Requirements
Sun drying works best in climates with:
- Daytime temperatures above 25°C (77°F) — higher temperatures speed drying
- Low relative humidity (<60%) — dry air pulls moisture from bricks faster
- Low rainfall — rain during the drying period damages unfired bricks
- Wind — even light breezes accelerate drying significantly
| Climate Zone | Drying Time (to handling strength) | Suitability |
|---|---|---|
| Hot arid (desert) | 3-5 days | Excellent |
| Hot semi-arid | 5-10 days | Good |
| Tropical wet-dry (dry season) | 7-14 days | Good during dry season only |
| Mediterranean | 7-14 days | Good in summer |
| Temperate summer | 14-28 days | Marginal; use covered drying |
| Humid tropical / cold | Not reliable | Use kiln drying or covered sheds |
Rain Destroys Unfired Bricks
A single heavy rain can dissolve the surface of green (uncured) bricks and ruin days of work. Always have tarps, mats, or a temporary roof ready to deploy. Even in arid climates, sudden storms occur. Losing a batch to rain is one of the most demoralizing setbacks in brick making — prepare for it.
Preparing the Drying Ground
The drying surface is as important as the brick itself. A poor drying surface causes uneven drying, warping, sticking, and contamination.
Level ground. The surface must be flat. Bricks dried on uneven ground warp as they shrink. Use a straight board to check flatness across the drying area.
Clean surface. Remove all vegetation, stones, and organic debris. Grass or roots under the brick trap moisture and cause bottom-face defects.
Non-stick surface. Spread a thin layer of dry sand (5-10 mm) over the drying area. This prevents bricks from bonding to the ground as they dry and allows easy removal. Straw or dry grass also works. Do not use loose soil — it becomes mud when the brick releases moisture and bonds to the brick’s bottom face.
Drainage. If possible, select a slightly elevated or sloped area so rainwater runs off rather than pooling around the bricks. A 2-3% slope is ideal — enough for drainage but not enough to cause bricks to slide.
Size the area. Each standard brick (290 × 140 mm) needs approximately 0.04 m² of ground space plus 30 mm gaps between bricks for air circulation. For 200 bricks, you need roughly 12 m² of prepared drying ground.
Molding Techniques
Open-Bottom Mold
The simplest mold is an open-top, open-bottom wooden frame — essentially a rectangular cookie cutter. It produces one or two bricks at a time.
Construction:
- Cut four pieces of hardwood plank (25 mm thick) to form a rectangle matching your desired brick size plus 10% for drying shrinkage
- For a brick that finishes at 290 × 140 × 90 mm, make the mold interior approximately 320 × 155 × 100 mm
- Join the corners with nails, screws, or lapped joints
- Smooth all interior surfaces — rough spots cause the clay to stick
Use:
- Soak the mold in water for 5 minutes before first use (wet wood releases clay more easily)
- Place the mold on the prepared, sanded drying ground
- Throw a lump of prepared clay firmly into the mold — the throwing action removes air bubbles
- Press the clay into corners with fingers or a flat stick
- Strike the top level with a straight edge, removing excess material
- Lift the mold straight up, leaving the brick on the ground
- Dip the mold in water, shake off excess, and repeat
Gang Mold
For higher production, build a gang mold that forms 4-6 bricks simultaneously. Same construction as a single mold but with internal dividers. Gang molds are heavier but dramatically increase output per cycle.
Adding Straw
Traditional adobe includes chopped straw at 1-3% by weight. The straw serves three purposes:
- Reduces cracking — straw fibers bridge forming cracks and hold the clay together during shrinkage
- Speeds drying — the hollow straw channels allow moisture to escape from inside the brick
- Improves tensile strength — dried straw acts as reinforcement against bending forces
Cut straw to 50-100 mm lengths. Mix into the clay thoroughly before molding. If straw is unavailable, dry grass, pine needles, coconut fiber, or even animal hair serve the same function.
The Drying Process
Drying proceeds in three phases:
Phase 1: Initial Set (0-24 hours)
The brick is soft and easily damaged. Do not touch, stack, or move it. The surface begins to lighten in color as surface moisture evaporates. In hot conditions, the surface may dry faster than the interior — this differential causes surface cracks if drying is too rapid.
Management: If cracking appears on the surface within the first few hours, the bricks are drying too fast. Cover them loosely with damp cloth or straw for the first day to slow surface drying. This is especially important for clay-rich mixes.
Phase 2: Leather-Hard (1-3 days)
The brick is firm enough to handle but still contains significant internal moisture. At this stage:
- Turn the bricks on edge. This exposes the bottom face (which has been against the ground and is still wet) to air. Stand them on their long edge, leaning slightly for stability.
- Check for defects. Now is the time to inspect. Bricks with significant cracks, warped faces, or soft spots should be recycled back into the clay mix immediately, before they dry further.
Phase 3: Full Dry (3-14 days, climate dependent)
The bricks gradually reach equilibrium with ambient humidity. They are ready when:
- Color has lightened uniformly across all faces
- The brick feels warm, not cool, to the touch (cool spots indicate trapped moisture)
- Tapping produces a clear sound, not a dull thud
- The brick has reached its final size — no further shrinkage is occurring (check by measuring a marked sample brick daily)
Stacking for Extended Drying
Once leather-hard, bricks can be stacked to free up ground space for new batches. Stack in open patterns that allow air to circulate around every brick:
Herringbone stack: Alternate layers at 90 degrees. Each layer is a row of bricks with 20-30 mm gaps between them. The next layer crosses perpendicular. This creates channels for air flow in both directions.
Crisscross stack: Similar to herringbone but with pairs of bricks rather than singles. More stable for tall stacks.
Maximum height: Do not stack more than 8-10 courses high for green bricks. The weight of the upper bricks can crush or deform those at the bottom. Once fully dry, stacks can go higher.
Shade vs. Sun
Counter-intuitively, drying bricks in partial shade often produces better results than full sun. Direct intense sunlight dries the surface too rapidly, creating a hard shell over a wet interior. This leads to surface cracking and internal weakness. Partial shade or a roofed but open-sided drying shed allows more even drying.
Shrinkage and Dimensional Control
All clay-based bricks shrink as they dry. Typical shrinkage is 5-12% depending on clay content and moisture level at molding.
Measuring shrinkage: Before your first production run, make 5 test bricks and measure their wet dimensions immediately after molding. Measure again when fully dry. The shrinkage percentage tells you how much to oversize your mold.
Controlling shrinkage:
- More sand in the mix = less shrinkage (but weaker brick)
- Optimal moisture at molding = less shrinkage (but too-dry clay does not mold well)
- Fiber reinforcement = less visible cracking (straw bridges micro-cracks)
- Slower drying = less cracking (moisture gradients cause differential shrinkage)
Target: Shrinkage under 8% is manageable. Above 10%, add more sand to the mix until shrinkage drops. Above 12% indicates far too much clay — the bricks will crack badly during drying.
Protecting the Finished Product
Fully dried sun bricks must be protected from rain until they are incorporated into a wall and plastered. Store them:
- Under a roof or covered with waterproof tarps
- Off the ground on wooden pallets or a layer of gravel
- Away from splash zones (at least 500 mm from drip lines)
Sun-dried bricks that get rained on can often be salvaged if they have not dissolved significantly. Let them re-dry fully and test with a drop test before use. Bricks that have lost more than 2-3 mm of surface material should be recycled into the clay mix.
Even in the finished wall, sun-dried bricks need protection. The three essential defenses are:
- Good hat: Generous roof overhang (minimum 300 mm, ideally 500 mm) to keep rain off walls
- Good boots: A stone, fired brick, or concrete foundation that lifts adobe walls at least 200 mm above ground, preventing rising damp
- Good coat: Lime plaster, clay plaster, or lime wash on all exposed surfaces to shed water and protect the brick face from direct rain impact