Ferno's Building Notes
A collection of the practical building notes behind Ferno — the hearth, mortar, insulation, and cladding. These pages were shaped by real hands-on building experience, alongside guidance originally found on a few outside resources that are credited at the end of each section.
The oven shelter design that inspired Ferno's own — see the original design here.
The Hearth
The hearth is the true cooking surface of the oven — the floor your pizza actually sits on — so it deserves real care. It should be a level bed of clean, flat fire bricks, laid on top of a level bed of sand and fire clay mixed roughly 1:1 with water. Use high-quality, flat, clean bricks here specifically; any warping or roughness on the cooking surface will show up in every pizza you slide across it.
Sources & further reading: archived pinkbird.org guide, one of the resources that guided this build.
Mortar
When you're building a brick oven, you'll actually need two different kinds of mortar — and mixing them up (or using one where the other belongs) can cost you down the road.
Two mortars, two jobs
The two types are refractory (fire clay) mortar and general-purpose building mortar. They are not interchangeable:
- Refractory mortar goes between the fire bricks that make up the dome and hearth — anywhere the mortar itself will face direct heat.
- General-purpose mortar is for the parts of the build that aren't exposed to oven temperatures — the outer structural brickwork, for example.
Refractory mortar mix
For the fire-facing joints, mix sand, fire clay, and a cement binder. Two ratios work well depending on what you have access to:
- Sand : fire clay : Portland cement : lime — 10 : 6 : 2 : 3
- Or, if you can source calcium aluminate cement: sand : calcium aluminate cement : fire clay — 10 : 3 : 1.5
The lime is doing real work here — it's what lets the mortar tolerate the heat once the Portland cement itself would otherwise degrade. Use grey Portland cement, not white.
General building mortar mix
For the non-fire-facing brickwork, it's simpler: sand : Portland cement : hydrated lime at 6 : 1 : 1. You can skip the lime entirely and go sand-to-cement at 4 : 1, but the lime makes the mix more workable and gives you more time before it sets.
Mixing and applying it
Combine the dry ingredients thoroughly before adding any water — a wheelbarrow or a flat board works fine. Add water gradually until you reach a consistency like thick peanut butter: it should hold its shape in a ball without slumping, but still spread with a trowel.
Keep the mortar layer thin — about 1/4 inch (6mm) between bricks. Any gap wider than that should be filled with a shaped piece of fire brick instead of a thick blob of mortar, since thick mortar is what tends to crack and shrink over time. Inside the hottest part of the dome, aim for bricks touching directly with only the thinnest skim of mortar for strength, not to fill space.
Curing
Don't rush the drying. If the mortar dries too fast — direct sun, hot dry climate — cover it with plastic sheeting or a damp cloth and mist it periodically over the first day or so. And before you light the oven for real, make sure everything is fully cured and dry; trapped moisture turning to steam under heat is exactly how you get cracks. A series of small, gradually hotter fires before committing to a full burn is time well spent.
Sources & further reading: archived pinkbird.org guide, one of the resources that guided this build.
Insulation
This is the step that determines whether your oven just gets hot, or actually holds onto that heat long enough to be useful. Skimp here and you'll be feeding it wood constantly; get it right and the residual heat alone will bake bread hours after the fire's out.
The foil layer, first
Before any insulation goes on, wrap the dome in a layer of heavy aluminum foil. This does two things: it keeps the insulation from drawing moisture back out of your dome and mortar before everything's fully cured, and it adds a small amount of reinforcement on its own.
Vermiculite-cement insulation
The most common approach for a home build is a mix of vermiculite (or perlite) and Portland cement, generally somewhere between a 4:1 and 6:1 ratio of vermiculite to cement, mixed with just enough water to hold together without going soupy. It's inexpensive, forgiving to work with, and handles the oven's expansion and contraction well without cracking.
The tradeoff is thickness: vermiculite/perlite mixes need considerably more depth than more modern options like ceramic fiber blanket to do the same job. For the dome, a commonly recommended range is 4 to 6 inches (10–15cm). Thinner ceramic fiber insulation (1–2 inches) can achieve similar performance but costs more and isn't something most of us have lying around the garage.
Applying it
Mix the dry vermiculite and cement together first, then add water gradually — don't dump it all in at once, or you'll end up with a mix too wet to hold its shape. You're aiming for a consistency that packs and stays put on the dome without sliding off. Build it up in layers if you're going for real thickness, letting each layer set before adding the next.
Why this step matters more than it seems
A well-insulated oven isn't just about saving wood — it's the difference between a wood-fired oven and a "wood-fired hot surface." Good insulation gives you the long, steady heat that makes it possible to bake bread and pizza in the same session, hours apart, off a single firing.
Sources & further reading: archived pinkbird.org guide, one of the resources that guided this build, along with Forno Bravo and Crust Kingdom for insulation thickness and mix-ratio guidance.
Cladding
Cladding is the final outer layer of the oven, sitting over the insulation. It's easy to think of it as purely decorative, but it's doing real functional work too — and living in the Pacific Northwest, where rain is basically a lifestyle, it's not optional.
Why bother with cladding
- It adds another layer of insulation and extra thermal mass.
- Because it holds heat longer, your oven cooks more efficiently and stays usable longer after the fire dies down.
- It slows the rate the oven cools between uses.
- And yes — it's also what makes the finished oven look like a finished oven.
The foil barrier, first
Before any cladding goes on, lay a layer of aluminum foil over the insulation. This stops the cladding's moisture from being drawn out prematurely by the insulation or fire bricks underneath — if that happens too fast, you can end up with a weaker outer shell. Go for the heavier-duty foil sold at hardware stores if you can; ordinary kitchen foil works in a pinch. A layer of chicken wire over the foil also helps the cladding grip properly.
The cladding mix
A simple concrete-like blend works well: sand (or another aggregate) : Portland cement : lime at a ratio of 4 : 1 : 1. Any concrete coloring you like can be mixed in at this stage too, if you want to skip a separate finishing coat.
Applying it
Trowel the mix on over the foil in a layer about 1 inch (2.5cm) thick — some builds go closer to 2 inches if water exposure or cracking is a real concern in your climate. Let it firm up slightly, then go back over it with the trowel to smooth the surface.
Avoiding cracks
The oven dome expands and contracts a fair amount with heat, and if that movement transfers straight through to the outer cladding, cracks are what you get. A proper insulation layer between the dome and the cladding absorbs most of that movement before it reaches the outer shell. Beyond that, cracking after the initial cure is usually just the mix drying too fast: use the least water you can get away with, and if the oven sits in direct sun, drape a damp cloth over it while it sets.
Sources & further reading: archived pinkbird.org guide, one of the resources that guided this build.
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