If you’ve been following along with my “How to Build a Better Cob Oven” series — great! If you haven’t, you can catch up by reading part 1 and part 2. So far I’ve described how to site your new oven and build a shelter, how to prepare and build the foundation, install the hearth, door opening, and build the cob dome itself. So let’s see where we are now… I think it’s time to talk about insulation, plaster, and wrapping things up.

Here’s the final installment of How to Build a Better Outdoor Oven.

Straw-Clay Insulation

Now that the sand is removed from the dome, you can think about insulating the oven. If you’re in a very wet and shady location, and your cob dome is slow to dry, I recommend building a small (very small) fire just to speed the drying process up. If your oven is exposed to a good breeze and some sunshine, things tend to dry out fairly quickly once the sand is out of there.

I suggest insulating the oven in a thick layer of “light clay straw” or “straw-clay” insulation. Yes, I know, I dislike the name, too. It’s terribly general. However, light clay straw is nothing more than loose straw tossed in a very thin clay slip.

To prepare your light clay straw, grab a wheelbarrow (or tarp), and shake a few flakes of straw out. To make your clay slip, use a high-powered electric drill with a large paddle (like a Hole Hawg), and spin up a 5 gallon bucket partially filled with slaked clay. The clay slip should be very thin, like the consistency of chocolate milk. It really shouldn’t be any thicker than that. So a little bit of clay and a good amount of water will go a long way. If you don’t have a Hole Hawg, take some nicely soaked clay and use your hands and arm as a paddle to stir up the clay, dissolving it more completely in the water. It’s much more labor-intense this way, but it’s doable.

Now it’s time to pour your prepared clay slip slowly over your straw. The point here is that you really don’t need much of it. Just a little bit, I promise. People are always surprised at how little is required. The best way I can describe how much slip you should use is this: think of the straw as lettuce, and the clay as a vinaigrette dressing — you only want to coat each straw strand, not saturate the whole business. The clay is merely a binder, and you don’t need much at all the achieve the effect. So go light!

Mixing up the straw and clay should only take a minute or so. Once your straw is fully (lightly) dressed, it’s ready to use. You’re going to need a very healthy dose of this stuff, so if you’ve got a helper friend, keep the production line going.

To apply the straw to the dome, take armfuls and pat it directly against the cob dome. I’ll admit, it’s pretty tricky because straw is so springy. It may help to try to orient the straw in the same direction initially. You want to pack it as tightly as you can, since the insulation will be supporting a fairly thick (and heavy) layer of plaster. Do your best, and as you climb the dome, keep punching it down and match the curve of the dome, and maintain the same thickness of insulation as you go. I’ve found that using somewhat shorter straw (especially on the bottom-most layer) is easier to compress than very long straw fibers.

Halfway up, the oven will look like some kind of funky bird’s nest, and when you’re done the oven will have grown about double in size. It’s kind of a humorous sight. If you’ve done well, your insulation will be consistently packed tight, and match the dome shape of the oven.

Preparing the Oven for Plaster

We’re starting to hone in on the final product here… now that your cob oven is insulated, it’s time to think about plaster work. I always do two coats, not including an initial clay slip application. The first thing you’ll want to do is apply a sticky clay slip to the entirety of the straw insulation. This clay slip will give the actual clay plaster layer something to adhere to. Compared to the clay slip you just made for the light clay straw insulation, this mix will be thicker. It’s still just clay and water, but you want a nice smear-able consistency — something you can pick up and hold on to, and smear smoothly on the dome.

Like before, grab your Hole Hawg and stir up some slaked clay, but add less water. You’re aiming for a sludgey consistency, like a thick pudding. Once it’s ready to go, simply grab it by the handful and start applying it to the straw. It should go on in a smooth, consistent layer. We’re not trying to build up any kind of thickness here, but merely provide a sticky surface for the plaster to come. A note about timing here, too: it’s good to do this step immediately after you get the straw insulation on the oven. The sooner you contain things, the better. I particularly enjoy this step. Smearing raw clay feels really nice…

Base Coat of Plaster

Now that your oven is all slipped up, you can move immediately into plaster work. Again, it’s not a bad idea to do this step right away. The base coat of plaster will help refine the shape of the dome, and provide a smooth, even surface for your finish coat. You can apply it by hand or with a trowel, but in either case you want to create a consistent surface. You may have to build up in some spots, depending on how lumpy or irregular the surface may be.

To mix a batch of base coat plaster, I actually use the same exact recipe for my cob mixes. To refresh your memory, see my cob recipe here. The basic recipe is: 2 parts sand, 1 part soaked clay, and straw (not too much). Actually, you’ll want this plaster to be more wet than a typical cob mix, but you can decide just how wet to make it for yourself. It’s always possible to dip handfuls of the material in water as you go.

This base coat of plaster should be at least 1/2″ in thickness, and more than likely it will be thicker in some spots. Again, you want to plaster the entirety of the dome and really refine your form. If you’re clay slip undercoat is fresh, you shouldn’t have to pre-wet the surface at all. Start at the bottom of the dome and work your way up. If you want to do any decorative surface work like sculpting, this would be the time to start incorporating that stuff. I’m personally more of a fan of the plain and clean look, so I never fuss with anything too fancy. Either way, this is fun stuff, so savor it!

At this point, it’s highly advisable to let things dry out completely before you proceed with your finish plaster. Again, if your location is very shady, it’s a good idea to build a small fire to speed that process along. You definitely don’t want your straw insulation layer to start decaying under all of that mud.

Applying Finish Plaster

Can you smell those pizzas? It’s almost time to invite your friends over. Actually, if you’re impatient, there’s no reason you can’t begin using your oven now that the base coat of plaster is applied, and everything is dried out. The finish is merely decorative. But the oven is far more beautiful with a smooth finish coat.

This is the last major step of your new outdoor oven construction. There’s about a million ways to finish things out here, including different plaster recipes and clay paints, or a custom mosaic installation. (Actually, if you want to use colored glass or other material to make a mosaic on the surface of the oven, you need to have all of that stuff ready to go so you can push it into the freshly applied plaster.) Again, I tent to opt for simple and clean. Let’s start with how to make your finish clay plaster.

Please refer to my recipe for preparing and mixing finish clay plaster, so I don’t have to regurgitate all of that business here. You’ll need sifted sand, screened clay, fresh cow manure, cattails, and wheat paste.

With your materials fully prepared and mixed, you’re ready to go. Thoroughly pre-wet the surface of the oven before you apply your plaster. Try to put it on as thin as you reasonably can, and consistent all over the surface. 1/4″ or less is best. A good quality plaster trowel is essential for a shiny smooth coat. Please read here for tips on selecting a trowel for your needs. I can’t get into all of the minute details for proper plaster application here, but again… aim for a consistent, smooth surface. If you’re going to embed mosaic materials, do it as soon as humanly possible. Always make sure the undercoat is wet enough so the finish coat can properly adhere. Burnish your work with a flexible steel trowel or a yogurt lid once it has set up.

Stand back and admire your finished oven! Thanks to the very thick layer of insulation you recently put in, you should not experience any cracking from heat in your finish plaster layer. Any cracks that do appear will be from your mix or your technique. No worries, though. Everyone will be much more interested in your pizza than any imperfections in the finish.

Party Time!

Congratulations! You now have a finished outdoor pizza oven. It’s time to throw a party and celebrate. Eventually, I’d like to write about best practices when firing your oven and how to make delicious pizza, but that will have to wait a little while. This should be plenty of information to get you going for now.

Thanks for following along on this 3 part “Build a Better Pizza Oven” series! Your feedback is appreciated.