Cob - A Clay, Sand, and Straw Story (March 2024)

From a very young age, I’ve enjoyed McGyvering. In my terrible 8s, I built a fort in the woods near my house. It was wedged into a flat landing on an otherwise steep hill, a landing on which I would only arrive after pinballing off of trees to which I had nailed chunks of old carpet. I retrospectively recognize my penchant for hosting also arose during that time. Perhaps the most recent development relevant to this tale is my joy for fine-grained sediments. I take sediment samples; Russian-peated, Vibracored, syringed, scraped, and et al’ed for academic pursuits. Generally, I subsample mere grams of these samples to infer decades of change. So when privyness revealed that a mixture of clay, sand, and straw in some haphazard proportions might constitute a cob pizza oven around which I might convene friends, my particle senses were particularly tickled. Thus began the search for sediments, far and wide (but no wider than 64 microns).

First, like any good structure worth its weight in sand, I needed a foundation. Luckily, the backyard of the house I reside in is riddled with 3-inch thick islands of concrete. Grass is overrated, or something. It was atop one such island, perhaps the old entrance to the driveway, I planned to erect the structure. I next needed to find a way to elevate the to-be pizza oven to a chest-high stature. A more earthen build, in a more orogenous region, would use stones pried from underfoot. Alas, on the coastal plain, boulders of this size are either foreign or forced. I found the latter. Luckily, one of the byproducts of the evidently insatiable urge to gobble up quaint(er) waterfront homes and replace them with oversized and deflavorized vacation cubes are old cinderblocks. Before being carted off to landfills, I took the liberty to repurpose some of these well-shaped blocks for the noble cause of ovendom. After arranging them in a square with one side missing and a central support pillar, I spanned a reclaimed piece of 4’ x 4’ plywood across them and weighed it down with a layer of solid 8” x 16” x 4” cinderblocks. These blocks provided a sturdy, porous, and somewhat level surface to adhere a ring of bricks to.

The project hovered at this intermediate stage as I perused many a local marketplace for firebricks - those that withstand great magnitudes of heat and do not crack upon rapid cooling. The force of temperature on rock, manmade or otherwise, cannot be overlooked. Even the carbonaceous near-earth asteroid Bennu shows evidence of differential heating resulting in fractured rocks. In that case, solar radiation rather than wood-fired pizzas is the cited culprit. And so, with a level of frugally-driven patience that would drive some insane, I waited. And in due time, the bricks came along. A batch of close to 100 firebricks, dimensions 4” x 8” x 1.25” (cobbles, as far as grain size goes) from marketplace Max. I also sourced some pro-bono standard red bricks from discount Debbie which required exhumation from her fire-ant ridden yard. Tarps were a key player at most junctures, in this case containing the dirt (and wayward ants) to some discrete pile in the trunk of my station wagon.

With the base formed, it was time to stick some things together. I chalked a 4’ diameter circle onto the solid cinderblocks using a paracord pivot-irrigation technique. I then began placing the standard bricks upright, their narrowest face sitting on the cinderblocks below, their widest face hugging their nearest neighbor. Wedging extra mortar in the seams of the bricks on the outer edge of the circle provided an outward fanning design needed to turn rectangles into a continuous circle. The mortar mix slowly locked them in. Before closing the circle, my housemate Nic and I created an arch from the same bricks which would come to form the oven door. We aimed for a width of 16” and a similar height to allow air intake, as this would be the only apertured portion of the finished oven. A combination of extra bricks and the bark of nearby supported the arch as the mortar cured.

I had long wanted to use old bottles in the build, and had begun collecting glass bottles for some time leading up to this stage; each containing the final drops of some dinner party story. These bottles would provide dead space beneath the firebricks that form the oven floor, limiting heat transfer out of the bottom of the oven to the cinderblocks which rest below. I sprinkled and spread a layer of perlite. Expanded Perlite, often found in potting soil mixes and often confused with vermiculite, is produced by heating hydrated obsidian until it pops from internal steam production - a la Orville! And, depending on where you’re reading this, you might have Perlite overhead - it’s what ceiling tiles are made of. I nestled a first layer of bottles atop this vesicular volcanic veneer, then buried them in more of it before laying down the second layer. One more liberal spreading of Perlite and a leveled layer of sand and I was ready to set down the firebricks. image

I worked from the oven door inward, fitting together the firebricks and using a rubber mallet to hug them together. It’s important here to focus on keeping a smooth, catchless surface for the pizza and the pizza peel to slide over. I spent a decent amount of time tinkering with the bricks, cautiously moving sand around beneath until they sat as happily as they might - of course some settling would occur during cooking. Again, the build sat here for some time as I toiled over finding the right quality and quantity of clay. Not impossible, but not immediately underfoot.

Many Countless tides and crashing waves have winnowed away the clays from our beaches- often carrying it far offshore where it settles beneath the wave base. Perhaps I could find these fine grains nestled between the blades of Spartina alterniflora on the marsh, mixed with fragments of periwinkle shells and a healthy dose of salt. Or, perhaps I could have exhumed a Pleistocene deposit (2.58 million to 11,700 years ago) which formed during past marine transgressions. This one was particularly enticing; I knew it to sit beneath my feet. We often encounter this deposit as a stratum that refuses to accept our coring tubes - a point-of-refusal telling us we’ve successfully captured all the Holocene information above. But being so far down, and with no obvious outcrops, I decided to look upland. On a trip to Charlotte North Carolina, I used publicly available satellite imagery to reveal an accessible reddish patch of cleared forest 30 minutes out of the city, on a bluff draining into the South Fork of the Catawba River. There, I met a dense air-dried ledge of red-clay, tough to the shovel, but warming to the heart. With many a jump-pry-scrape and conveyor buckets of the “spice melange” loaded into a tarped trunk, the cobbing could begin.

Now, cobbing is an art form better passed through familiar hands than through internet fora. It’s easy to get bogged down in the minutiae of others’ builds; step-by-step instructions, measurements of openings, and cob-tests to name a few. Retrospectively, these were barriers that hung mental caution tape around the build, and if you are so inclined to try and build one yourself, I suggest trying substitutions and eyeballing if the technicalities feel overwhelming.

That being said… we mixed an 8:4:1 volumetric ratio of Sand:Clay:Straw on a holy old tarp. Interleaving shovelfuls of sand, clay, straw, and water onto a mound that others danced on, we reconstituted a sticky strong fibrous mass of misplaced earth. Fellow cobber Mark even performed a ceremonial backflip to summon a successful mix. Folding the tarp across itself again and again, and dancing yet more, the first shapes of “cob” took form. Before beginning to mix, we mounded wet sand over the firebrick floor of the oven to form a mold which the cob could cure over and which could later be scoured out and replaced with wood (and pizza). image

Using our hands, we packed cantaloupe-sized warbled spheres of cob and began throwing them at the base of the mounded sand mold. Malleable brick by modgeglobbed brick, uprose the walls of the oven, sealing in darkness as they melded on top. We placed thicker globs higher up, as overburden would compress and widen the ones below. Cobbing music playing, we stomped together more cob and layered it on the dome. Once massaged together, the walls were roughly 15-centimeters thick. And now, a healthy helping of patience. image

One month later: Moisture evacuated. Platelets of clay, angular grains of sand, and strands of straw all commingled into an oven dome as hard as, well, rock! After vigorously scooping out the sand with a shovel and a noodling arm, the structure held.

PhD Student in the Rodriguez Coastal Geology Lab

My research interests include saltmarsh and barrier island geomorphology, change detection via remote sensing, and drone imagery collection techniques.

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