Hey folks! We are back with another week of firing must-knows and know-hows. This week we are sharing everything arch and arch form adjacent.
Any arch form depends on something called a "keystone brick", or key brick. This is the final brick that is placed in the very center of an arch — that keeps the arch from falling. This keystone brick can either be cut to size, mortared into place very carefully, or cast with a soft material like refractory castable.
Illustrations from The Kiln Book, Fred Olsen
That being said, let's take a deep dive into arch forms...
One of the most recognizable arches is a Catenary Arch, one of the most stable arches you can build — due to the vertical force of gravity being redirected into compression forces that are evenly distributed throughout the arch's curve. Fun fact, catenary arches follow the curve of the hyperbolic cosine function (for all you math nerds out there!)
The easiest way to create a catenary arch is to hang a chain from either end of where the arch needs to start, and trace the line that the curve makes. The ideal rise-to-span ratio (height of the arch : width of the bottom of the arch) is 1:1, but it can be variable.
The catenary style arch is unique because it doesn't actually require any skewback bricks at each end (or bricks cut to a certain angle) in order to keep the arch stable.
Illustrations from The Kiln Book, Fred Olsen
Bill Bracker using the chain method to measure a caternary arch (Left), and Anne Bracker removing the arch form after the bricks have been placed around it (Right)
Another very common arch we see throughout kiln design is a sprung arch, or an arch that springs outwards from certain angled bricks — called skewback bricks. These integral skewback bricks, along with reinforcements (such as steel frames) are what support this shorter and more gentle arch form. Commonly used for doorways, or the "ceiling" of a kiln, and considerably easier to load than a steeper catenary arch.
We might find this kind of arch in many types of kilns — but especially in gas, wood, and hybrid kilns!
Great example of sprung arch built by Justin Lambert, for a kiln in Cleveland, OH.
Barrel arches are another interesting arch used within kiln design. This type of arch curve is basically a segment (or half) of a circle, often sitting on an elevated part of the kiln wall. This is a fairly common arch design, often used at the front of traditional Anagama or long tube kilns. Over time, this kind of arch has the potential to flatten out at the top (and potentially fall in) without proper reinforcements on the sides.
Barrel arch kiln, with plywood arch form inside.
Classic barrel arch doorway on John Reinking's Anagama Tube kiln.
Compound arches are also a little less common, but this is basically when the arch changes throughout the kiln — in size, dimension, or curve. We might find this kind of arch in experimental kilns! This takes some extra precaution when building the complex plywood arch forms, but it can create some unique firing conditions and flame paths.
A hybrid arch built by Chad Brown in Moore County, NC.
Zeynep Uner is the current kiln and wheel repair apprentice at Bracker’s, working directly with David Sturm (the head tech) on repairs large and small. She has been repairing kilns for approximately 3 years now, and has been absorbing everything she possibly can from David’s 30+ years in the field.
Zeynep enjoys atmospheric firing, taking film photos, and throwing on her old Brent kick wheel.
