Designing a kiln brick by brick

A kiln is a brick problem before it is a heat problem. You solve the heat later, with elements or a burner. First you have to stack the bricks so the walls stand, the roof holds itself up, and the corners do not open at 1,000°C (1,832°F). Getting the count right saves money at the yard. Getting the bond right saves grief a year in, when a wall that was laid with continuous seams starts to crack along them.

The kiln builder treats a kiln as exactly that: a stack of bricks. You enter interior dimensions and a brick size, and it places every brick, applies every saw cut, and counts what you need to buy. This post shows you how to drive the tool, then explains the masonry each shape is modelling and why the rules work the way they do.

Using the tool

Choose a shape

Four buttons across the top pick the kiln type: square (a flat top with a full-mouth front door), sprung arch, catenary, and round (a top-loading cylinder with a lift-off lid). Each shape has its own dimension inputs, and each remembers its own setup. Switch from square to catenary and back, and your square design is still there. The 3D model rebuilds as you type, so a change to the width or the rise shows up in the chamber immediately.

The dimension fields differ by shape because the geometry differs. Square takes width, depth, and height. The sprung arch takes a wall height and a rise per foot of span. The catenary takes a span and a rise as a percent of span. Round takes an inner radius and a height. Every dimension is an interior dimension, the space you actually load, so the walls grow outward from what you enter.

Read the brick count and cost

The right sidebar tallies bricks per component (walls, arch, door, lid) and a total. A price-per-brick field turns that total into a cost.

The count has two modes, switched by the toggle. Optimized applies cutting optimization: the tool groups the cut pieces, sorts them by their surviving dimension, and bin-packs them onto whole source bricks. It charges 1/8 in of kerf loss per cut, and it reuses an offcut only if it is at least 25% of a brick and long enough to serve. This is the accounting a builder does with a wet saw and a scrap pile. Max count treats every partial brick as a whole one. If you will not cut, or you want a conservative number to buy against, use Max.

Set the layout

A handful of toggles control how the bricks lay up.

Whole bricks only snaps every dimension to a brick multiple, so the model never cuts a partial brick to close a course. Account for mortar lays the bricks with a joint between them, defaulting to 1/16 in. The outer shell toggle wraps the chamber in a second leaf, counted as its own part. The door and lid toggles hide the removable closure so you can see the loading opening behind it. The explode slider lifts the courses apart to look inside the wall, and the view cube snaps the camera to a face when you click it or spins the model when you drag it.

Export

When the design is right, the export dock carries it out. SVG and PNG render the current drawing style (ink line, sketch, flat colour, or 3D shaded) as a file. The detailed PDF is a printable build sheet: three orthographic views (front, side, top), dimension lines, and a brick legend keyed to the count. Send to energy calculator carries the interior dimensions across to the next tool, so you do not re-enter them.

The masonry it models

The tool is faithful to the masonry in Frederick Olsen’s The Kiln Book. Every rule below is drawn from it, and every shape in the tool is one standard brick with saw cuts applied. You do not lay tapered voussoirs; you cut bricks. The model works the same way.

Running bond and the corners

Straight walls use running bond: each course is offset half a brick length from the course below, so no vertical seam runs continuously up the wall. That offset is structural. A continuous seam is a crack waiting to open; the offset forces any crack to step, and stepping costs it energy. Olsen shows this in Fig. 2-5, where “no joints run in a straight line above each other”.

At the corners the tool alternates the quoin bricks. On even courses the side wall runs through and owns the corner; on odd courses the back wall extends a full stretcher through it. The alternation ties the two walls into each other so the corner cannot peel apart. The cut piece that closes each row goes to whichever end you pick with the short-brick side control, or alternates by course.

Sprung arch: wedges and the skewback

A sprung arch springs from inside the wall, so the span equals the interior width. It rises at a rate you set in inches per foot of span. Olsen gives standard skewback rises from 1.5 in per foot (the minimum, a featheredge) up to about 3 in per foot, and notes that chamber arches commonly run steeper, 4 to 6 in per foot at a 4 to 8 ft span.

Each brick in the arch is a voussoir, a wedge cut with two radial planes that both pass through the arch centre. Cutting on the centre, rather than on the brick’s own edge, makes adjacent voussoirs share the same plane so they seat against each other along the full joint. The first voussoir lands on the skewback, a wall-top brick whose upper face is cut to the spring angle, and the arch seats flush on that slant. The force of a sprung arch runs down and out against the walls, which is why the skewback ties it to the wall and why the wall is built up past the spring line before the arch is turned. The crown key is cut and ground to fit, then set slightly below its neighbours, per Olsen’s Fig. 2-21.

Catenary: the shell that stands on its own

A catenary is the curve a chain makes when it hangs from two points. Invert it and the same curve stands in pure compression, with no bending anywhere along it. That is why a catenary kiln needs no skewback and no buttressing wall: the arch contains the walls and the roof in one curve, and the thrust runs along the curve to the ground. The tool computes the true catenary, y = a·cosh(x/a), then walks the outer arc and places a voussoir every brick width. At the feet the bricks stand on end and their tops are sheared to the foot angle, so the shell seats flush, the same idea as the sprung arch’s skewback. Potters commonly build the catenary at about a 1:1 rise-to-span ratio, which is the tool’s default.

Round: banded, not mortared

The round builder makes a top-loading cylinder. Each brick is wedge-cut to a true circle with two radial planes through the vertical axis, positioned so the inner face sits flush with the chamber and the taper opens at the outer face. A round wall holds itself together in compression, banded with a steel ring, so Olsen builds circle-brick walls dry and banded with “no need for mortar”. By default the tool stacks the bricks in aligned vertical columns, the look of a banded kiln. Untick aligned columns and it breaks the joints into running bond instead, following Olsen’s Fig. 2-13, which lays curved walls with the same alternating joints as straight ones. Either way the count is the same.

Why the mortar joint is thin

Insulating firebrick (IFB) is soft, porous, and light. It insulates because it is mostly air, so it takes a light joint, not a heavy bed. Olsen gives 1/32 in for IFB and up to 1/16 in for hard brick, troweled thin and tapped down. A thick joint would be a thermal bridge and a weak line at once. The tool defaults mortar to 1/16 in when you turn it on.

One rule the model follows quietly: a two-leaf wall is tied, not jointed. When you add an outer shell, the two leaves butt against each other and bond, with no mortar gap between them; Olsen ties the back-up leaf to the hot face every fourth to sixth course rather than floating it on mortar. And because the key of an arch carries the whole ring, it is cut to fit rather than left as a full brick, which is why the tool grinds it in and seats it low.

Next: how much power it needs

Once the brick problem is solved, the heat problem starts. You know the interior dimensions, the wall build-up, and the volume you have to bring to temperature. The energy calculator takes it from there and works out how much power the kiln draws to hold heat and how fast it climbs. If you sent your dimensions across from the export dock, they are already loaded. The post How the energy calculator works walks through the thermal model behind it.

Sources

• Olsen, Frederick. The Kiln Book (3rd ed.). Running bond and the rule that no joints run in a line (Fig. 2-5); alternating-quoin corners; voussoir wedges and sprung-arch geometry (Figs. 2-19, 2-21); skewback rises; the catenary as an inverted hanging chain (Fig. 2-27); round curved-wall bonding and steel banding (Figs. 2-13, 9-18); mortar joints (1/16 in hard, 1/32 in IFB) and two-leaf walls tied rather than jointed.
• Lou, Nils. The Art of Firing. Minnesota Flat Top (MFT) door construction, with the 4.5 in / 2.5 in lap on the side walls and ceiling.