Arch Form Building

Part of Kiln Design

Constructing arched kiln roofs without modern forms — using temporary supports, wedge-shaped bricks, and ancient techniques.

Why This Matters

A kiln needs a roof that can withstand extreme heat, resist its own weight, and span an open chamber without steel reinforcement. The arch is the oldest and most reliable structural solution to this problem. Unlike flat lintels that crack under thermal stress, arches convert downward forces into outward thrust against supporting walls, distributing loads evenly through compression.

In a rebuilding scenario, you have no prefabricated arch forms, no refractory castable cement, and no steel framing. Every arched kiln roof must be built over a temporary support structure (formwork) that is removed or burned out after the arch is complete. Getting this right is essential — a collapsed kiln roof wastes weeks of brick-making and refractory preparation, and a failure during firing can be dangerous.

The techniques here apply to any arched structure: bread ovens, smelting furnaces, lime kilns, brick kilns, and even architectural doorways and bridges. An arch is an arch, whether it spans 18 inches or 18 feet.

Arch Geometry Fundamentals

Types of Arches for Kilns

Arch Type	Shape	Best Use	Difficulty
Semicircular	Half circle	Small kilns, ovens	Easy
Segmental	Shallow arc (less than semicircle)	Wide kilns with low headroom	Moderate
Pointed (Gothic)	Two arcs meeting at a peak	Tall kilns, reduced thrust	Moderate
Catenary	Hanging chain curve	Most efficient, minimal thrust	Hard (see separate article)
Barrel vault	Extended semicircular arch	Long tunnel kilns	Easy to moderate

For a first kiln, the semicircular arch or barrel vault is recommended. The geometry is simple, the formwork is straightforward, and the structural behavior is well understood.

Key Terminology

• Span: The horizontal distance between the two walls supporting the arch
• Rise: The vertical height from the spring line (where the arch begins) to the crown (highest point)
• Springer: The first brick on each side, resting on the wall
• Voussoir: Any wedge-shaped brick in the arch ring
• Keystone: The central brick at the crown that locks the arch
• Haunch: The lower third of the arch on each side — the region most prone to failure
• Thrust line: The path of compressive force through the arch

The Golden Rule of Arches

An arch works entirely in compression. Every brick pushes against its neighbors. If the thrust line stays within the middle third of the arch thickness, the arch is stable. If it exits the arch body at any point, cracks develop and collapse follows.

Building the Formwork

Formwork is a temporary structure that supports the arch bricks during construction. Once the keystone is placed and the mortar has set, the formwork is removed.

Wooden Formwork (Most Common)

For a semicircular barrel vault kiln:

1. Cut two identical semicircular end boards from planks. Mark a semicircle with a compass (string tied to a nail at the center point, pencil at the radius). The radius equals half the kiln span.

2. Cut the boards along the semicircle line. These are the “centering” pieces.

3. Space the two boards apart by the depth of the kiln (the barrel length) using cross-pieces nailed between them.

4. Cover the curved top surface with thin, flexible strips of wood (lath), bark, or split bamboo — these form the surface the bricks rest on.

5. Support the assembly on legs or stacked bricks at the correct height (the spring line of the arch).

Earthen Formwork (No-Lumber Method)

When sawn lumber is unavailable:

1. Build a mound of damp sand or earth inside the kiln walls, shaped to the desired arch profile
2. Pack firmly and smooth the surface
3. Cover with a thin layer of ash or dry sand to prevent bricks from bonding to the mound
4. Lay the arch bricks directly on the mound surface
5. After the arch is complete and mortar has dried, dig out the sand/earth through the kiln doorway

This method was used throughout the ancient world and remains practical. The disadvantage is that removing the fill is labor-intensive, especially if it compacts during construction.

Woven Formwork

In bamboo-rich or willow-rich environments:

1. Weave a basket in the shape of the desired arch interior
2. Coat the exterior with a thick layer of clay slip to create a smooth surface
3. Let dry, then set in position
4. Build the arch over the wicker form
5. Burn out the formwork during the first firing — the wicker burns away cleanly

Formwork Removal Timing

Never remove formwork until the mortar has fully dried — minimum 3-5 days for clay mortar, 7 days for lime mortar. Premature removal is the most common cause of arch collapse in amateur construction.

Making Wedge Bricks (Voussoirs)

In a true arch, every brick is wedge-shaped — wider on the outer face (extrados) than the inner face (intrados). This wedge shape allows bricks to follow the curve while maintaining tight joints.

Calculating the Wedge Angle

For a semicircular arch with N bricks in the ring:

• Total angle around the semicircle = 180 degrees
• Angle per brick = 180 / N degrees
• Each brick’s taper = this angle applied across its depth

For example, a semicircular arch made of 15 voussoirs: each brick tapers by 12 degrees across its depth (180/15 = 12).

Forming Wedge Bricks

Method 1: Cut from standard bricks

If you have rectangular bricks, cut the taper using a wire (for unfired clay) or a brick saw (a flat stone with abrasive sand):

1. Mark the taper on both end faces of the brick
2. Cut along the marks
3. Smooth the cut surfaces

Method 2: Mold tapered bricks

Build a brick mold with tapered sides:

1. Make a rectangular mold frame from wood
2. Tilt one side inward by the calculated angle
3. Fill with clay, strike level, unmold, and dry
4. Fire alongside your regular bricks

Method 3: Thick mortar joints (Quick and Dirty)

Use standard rectangular bricks with thicker mortar joints on the outer face. This avoids making special bricks but creates weaker joints. Acceptable for small kilns (span under 24 inches / 60 cm) but unreliable for larger ones.

Always Make Extra Voussoirs

Fire at least 20% more wedge bricks than calculated. Bricks crack, warp, or break during handling. Running short mid-arch means dismantling and starting over.

Laying the Arch

Preparation

1. Confirm wall alignment — both supporting walls must be plumb, level, and at the same height. An uneven spring line creates an asymmetric arch that concentrates stress.
2. Set formwork precisely centered between walls
3. Dry-fit — lay all bricks on the formwork without mortar to verify fit, adjust wedge angles, and identify the keystone position
4. Prepare mortar — refractory mortar for high-temperature kilns (see parent article), lime mortar for lower-temperature structures

Laying Sequence

1. Start from both springers simultaneously — place the first brick on each side of the arch, resting on the supporting wall
2. Work upward from both sides equally — never build one side higher than the other. Unbalanced weight shifts the formwork.
3. Apply mortar to the bed joints (between bricks) — keep joints as thin as possible (1/8 to 1/4 inch / 3-6 mm)
4. Press each brick firmly against its neighbor, squeezing out excess mortar
5. Check alignment frequently — sight along the barrel length to ensure bricks are not wandering sideways
6. The keystone goes in last. It should require firm tapping with a mallet to seat — this pre-loads the arch in compression

The Keystone

The keystone is the most critical brick. It must:

• Be the correct taper to fill the remaining gap exactly
• Be tapped in firmly — a loose keystone means the arch has gaps
• Not be forced so hard that it displaces the neighboring bricks

If the gap is slightly too large, use a slightly oversized keystone or add thin shims of fired clay. If too small, trim the keystone with a rubbing stone until it fits snugly.

Multi-Ring Arches

For larger kilns (span over 30 inches / 75 cm), a single ring of bricks may not provide enough structural depth. Build multiple concentric rings:

1. First (inner) ring: Laid on the formwork as described above. Use your best refractory bricks here — they face the fire.
2. Second ring: Laid directly on top of the first ring, with joints staggered (offset by half a brick length). Use a slightly wetter mortar that bonds the rings together.
3. Additional rings: As needed for structural depth.

Each ring beyond the first is self-supporting against the one below, so no additional formwork is needed. The combined rings behave as one thick arch with greatly increased strength.

Handling Thrust

An arch pushes outward at its base (the springers). If the supporting walls cannot resist this thrust, they spread apart and the arch collapses. Solutions:

Buttresses

Thicken the walls at the springer points. A buttress extending 12-18 inches (30-45 cm) beyond the wall face provides adequate resistance for most kiln arches.

Tie Rods

An iron rod or heavy wire running across the kiln interior at spring line height, connecting the two walls. This directly resists outward thrust. The rod must be heat-resistant — wrap in refractory clay if it passes through the firing chamber.

Earth Backfill

Pack earth or sand against the outside of the kiln walls to the full height of the haunches. The passive pressure of the backfill resists outward thrust. This is the simplest solution for small kilns.

Buried Construction

Build the kiln partially below ground level. The surrounding earth provides natural thrust resistance. Many traditional kilns worldwide are built into hillsides for this reason, with the added benefit of insulation.

Common Failures and Prevention

Crown collapse: Usually caused by premature formwork removal or insufficient keystone compression. Prevention: wait longer before removing formwork; tap the keystone firmly.

Haunch separation: Cracks at the lower third of the arch, where bricks pull apart. Caused by insufficient wall thickness or missing buttresses. Prevention: buttress or backfill.

Thermal spalling: Brick faces flake off during firing due to moisture trapped in incompletely dried bricks or mortar. Prevention: cure the arch for at least 2 weeks before first firing; conduct a slow warm-up fire (12+ hours of gentle heat) before any full-temperature firing.

Uneven settlement: One wall sinks, tilting the arch. Prevention: build on solid foundation; ensure both walls rest on undisturbed soil or stone footings below frost line.