11. Plastering – Are we there yet?

Two posts in as many days! Must be some kind of record.

Sorry if the last post seemed a bit dis-jointed. This WordPress blog site is flakey, with a capital shonky. It has a preview function; which you would imagine showed you what your post would look like before you hit the “publish” button – Wrong! The email bears as much resemblance to the preview as I do to George Clooney!

Today we embarked on the final stage of constructing the oven – plastering the dome. Apart from the cosmetic function, the plaster coat helps to waterproof the oven.

The first step in plastering is to cover the dome with chicken wire. This helps to keep the plaster from cracking. I am not sure why it is called chicken wire, when - I used what was left on the role from making my pig pen, so I guess this is pig. Grant reckons you could also use dead cabbage tree leaves. If you've ever had them wrap around your lawn mower blades you will know how fibrous they are.

The first step was to cover the dome with a frame of wire. This reinforces the thin coat of plaster and keeps it from cracking. People refer to this wire as “chicken wire” – I used what was left on the role from making my pig pen, so I guess it is actually “pig wire” in this case. Grant reckons you could also use dead cabbage tree leaves. If you’ve ever had them wrap around your lawn mower blades you will know how fibrous they are.

The more astute of you will have noticed, from the absence of the brick arch, that I actually placed the wire a few days ago. The next step was to lay on a rough skim coat of plaster. No photos of how I did that, unfortunately, as I was on my lonesome and it is hard enough balancing a mortar board and trowel without having to cope with a camera. The skim coat beds in the wire and provides a key for the thicker decorative top coat(s).
Fast-forward a couple of days to Sunday, with the brickwork happening in between. Mort and Keith came around to give me a hand with making the top coat look presentable. As none of us had ever done any solid plastering before, so we were all playing it by ear and relying on advice from YouTube.
Mort mixing

Mort decided that he would do the heavy lifting and leave the finer points of plastering to Keith and me, relegating himself to mixing duties. Quite altruistic of him we thought, until we discovered that after a brief flurry of placing all of the ingredients in the concrete mixer he was able to sit down and observe proceedings while the mixer did most of the work!
The plaster brew comprised four parts sand (we used the sand that formed the sandcastle for the dome – nothing like a bit of recycling to keep you feeling virtuous), 1 part ordinary Portland cement, and one part hydrated lime (this, I believe provides plasticity to the brew). On top of the basic plaster mix we added a waterproofing agent (not strictly necessary, but it does provide surety) and a colour to match the bullnose tiles in the mouth of the oven. Coincidentally, the colour turned out to be called “tandoori”. Now the oven can do double duty as a tandoor – how culturally appropriate. To provide further resistance to cracking we added a handful of polypropylene fibres to each mix. This spreads itself through the mix and, theoretically will hold everything together. The amusing thing is that the fibres stick out of the final coat and make it look as though a cat has been sleeping on it. Gaynor reckons it looks like a plucked chicken. I suppose they will disappear in time. If not, I might have to give it the old blowtorch to the Y-fronts treatment and singe them off.

Just prior to taking the above photo, Mort told Keith and I to “bugger off” and leave the mixing to him, as our advice was a case of too many cooks…

Keith adding more pig wire to prevent the plaster from cracking where it transitions from the dome to the bricks.

Keith adding more pig wire to prevent the plaster from cracking where it transitions from the dome to the bricks. Note the scoring in the skim coat to help the next coat to bond to it

Applying the plaster. Just as the rain came down. Note the mortar board in my hand - looks nothing like my graduation

Applying the plaster. Just as the rain came down. Note the mortar board in my hand – looks nothing like my graduation photo

The finer bit of edging the plaster around the bricks - fantastic job Keith

The finer bit of edging the plaster around the bricks –  Keith is the kind of guy that will figure out how to do the most difficult tasks.

After having applied a 40mm thick coat of plaster by float we left it to dry for about and hour and then 'polished" it  using, believe it or not, a block of polystyrene foam. This was like using sandpaper on wood, only better, as not only does it knock off the high spots, but it also fills in the low - amazing what you can learn on YouTube

After applying a thick coat of plaster by float we left it to set. This took longer than it should have due to regular rain showers.  Once the plaster became a little firmer we “polished” it using, believe it or not, a block of polystyrene foam. This was like using sandpaper on wood, only better. Not only does it smooth off the high spots, but it also fills in the low – amazing what you can learn on YouTube

The finished product. Now, we just have to wait until it dries before we can start cooking up a storm

Here’s the finished item. I’m really pleased with it. It even looks something like my original sketch, that I drew up nearly a year ago (see the blog header).

We’re not quite there yet. I will leave the oven to dry some more before firing it up. So, expect at least two more posts. The next one when I plaster the support columns (I am hooked on this plastering game now), and the final one will record the ceremonial unveiling and first cooked pizzas. I am aiming for New Year’s eve for that.

9. Dome insulation

The posts are coming to you thick and fast now. After dawdling along for nearly a year things are really taking shape.

During the week I removed the timber arch former and dug out the sand castle. The sand was packed very tightly and, surprisingly, was warm. I guess it was absorbing the heat generated by the exothermic chemical reaction of the cement in the fire clay. The newspaper, with which we covered the sand form, has stuck to the inside of the fire clay dome. So, that will have to remain in-situ until the first fire.

Sand excavated out and a fan heater circulating cold air into the dome. I was playing around with some bricks to get an idea of how I want the front to be finished

After having excavated out the sandcastle, I played around with some bricks to get an idea of how I want the front to be finished – you will have to imagine the arch bit. The good thing is that managed to salvage the timber formwork intact, so I can recycle that into the formwork for the brick arch. No, your eyes are not deceiving you. The arch has turned out a different colour from the dome. It is slightly yellower. I am not exactly sure why, given that Mort was scrupulous in measuring the ingredients.

Gaynor and I added the insulation layer this weekend. This was 125mm of pumice-crete. Five parts pumice to one part cement and 225 mls of the secret fire mix brew. The good thing about not having clay in this mix is that we could mix it in the concrete mixer, which certainly removed a lot of effort.

Building up the insulation layer. The pumice-crete is packed loosely onto the foil-covered dome. Note the insulation blanket over the hottest part of the stove. This is something from the space shuttle programme and, apparently, you can hold your hand on one side while there is a 1000 degree heat source on the other side.

Building up the insulation layer. The pumice-crete is packed loosely onto the foil-covered dome. Including beer bottles into the pumice-crete increases the insulation value. I used long-neck bottles toward the base and then VB and Bundaberg ginger beer bottles toward the top, where the radius becomes tighther.
The white blanket on the top of the dome and the arch is some sort of high-tech ceramic fibre insulation.  This, like the battery drill, derives from the U.S. space programme. It is so effective that you can, apparently, hold your hand on one side while there is a 1000 degree heat source on the other side. Given its cost I deployed it only on the hottest part of the oven and resorted to lower-tech glass for the rest.

To ensure a bit of authenticity, and Italian beer bottle

To ensure a bit of authenticity to what is, after all, an Italian invention. Mi piace Peroni Nastro Azzuro ma preferisco Moretti.

The finished dome

Eccolo! The finished dome. Very rustic looking, I know. However, the next layer will be smooth plaster. I think it looks less like a nuclear reactor now and more like the cupola of a church, or even one of those shrines to the Madonna you see on the side of the road in Italy (and I imagine every other strongly catholic country)

And now a dissertation on curing the oven prior to use.

If you try and fire up the oven before it is completely dry the water in the clay will boil and form steam within the fire clay. The steam expands and has to go somewhere, usually venting to the atmosphere in the form of a dramatic explosion (In a controlled fashion, this is what happens in a steam engine). This happened with Dan’s oven.

On the other hand, it pays to dry the clay very slowly, because the slower concrete dries (and let’s face it, the fire clay is a form of concrete) the stronger is will be (there are a whole host of other considerations relating to concrete strength, but that is another story). As you will recall, I have been keeping the dome cool and retarding drying by covering it with a wet blanket. The insulation layer now takes over that role. The wet blanket has switched duty to cooling the insulation layer.

I bought a fan heater from the local junk shop and am now using that to circulate cold air through the dome to assist with slow drying (see the first picture). I will increase the heat slowly as the days march on. Beats all of the other advice about burning progressively large fires. Anyway, the fire clay should dry nicely in this heat. It was 27 degrees today and yesterday and the forthcoming weeks looks as though there will be no respite from il sole. I have never seen the farm so dry in November. This is the first time ever that I have had to top up the swimming pool prior to the end of December. It will be just my luck if we get another total fire ban this summer. Pizza ovens fall into the category of outside fires, which were banned during the drought last Summer.

The next thing on the programme is to build the brick façade. I am a total neophyte when it comes to bricklaying, so I will leave this to the professionals. Unfortunately, I can’t get a brickie until mid December, so the project plan has to be juggled a little. The next stage for me will be to apply a skim coat of plaster to the insulation layer prior to the brickwork. When the brickie has done his thing, we will then apply the final plaster coat (note the “we” team).

Time to open a beer (purely to make more insulation, of course)

Keeping the Pot Boiling

Keeping the Pot Boiling

Since my last post I have come to the conclusion that to get Grant down from the Land of the Waving Cabbage Tree to help on this project may not happen in the very near future.

So, I took the bull by the horns and got the bit between the teeth; I then closed the book of metaphors and decided to get this show on the road.

Step one – making a test brick to ascertain the correct mix of the ingredients that I have.

This guy did not follow Grant's advice.  Too much silt and no cement

This guy did not follow Grant’s advice. Too much silt and no cement

Clay, as she is dug up from the ground, is a mixture of clay, sand, and silt. Clay and sand are good. Silt is bad as it is weak. Clay composition differs from location to location, so it pays to experiment up front with your clay, rather than end up in the position of the unfortunate that built the oven in the accompanying photo. This leaning oven of pizza was caused by, amongst other things, too much silt in the brew. Geoff, my local geology consultant, advises me that Auckland clays tend to be mainly silt. Therefore it would probably pay to steer clear of Queen City clay, although Dan’s oven is working fine – at last report.

So how do you tell what’s in your clay? Easy – just crumble up a sample and mix with water in a jar. Leave to settle overnight. The clay will rise to the top, the silt will form a layer in the middle and the sand will form on the bottom.

The hearth and internal dome of the oven are made from fire clay, which is a mixture of adobe, cement, and lime. To make adobe; mix one part clay with two-three parts sand (actually, I would call it gravel). Using the sand-clay ratio in your sample bottle decide how much sand to use to achieve one part clay and three parts sand in your finished product. The other test is to keep mixing sand with the clay until it just loses “stickiness”.

For sand I used crushed slag from the Glenbrook steel mill (It’s local, is a waste product and cost $12 for a trailer load – it cost more to hire the trailer than the slag itself). Grant recommends using PAP 7 grade sand (gravel). I used PAP 5 slag, which should be okay.

As I mentioned previously, I am using clay from Ngaruawahia. My clay sample revealed no discernible layering, so I deduce that it is all clay, no silt, and sod-all sand. Therefore my adobe mix should be around 3 sand to one clay. However, it pays to mix the sand gradually with the clay until you have achieved the right mix (loss of stickiness).

I am lucky that my clay is dry, having come from under-cover storage at the brickworks. It was, therefore, easy to mix with the sand. It’s a devil of a job mixing the two when the clay is in a plastic state. It started getting interesting when I added water. The mix becomes verrrrry sticky. By the time I finished I had clay on my clothes, in my hair, on the table, on the deck, and even on the cat! Add the water in small amounts and just enough to make a plasticine-like consistency. As it turned out, stickiness started to fade at 2.5 parts  gravel. Maybe that’s because PAP 5 is finer than the PAP7.

Now that I had the adobe mix it was time to turn it into fire clay. To eight parts adobe mix add one part cement (normal Portland or Grade O) and one part lime. The conoscenti will tell you not to use cement because it breaks down at x degrees and your oven will operate at x + n yadah, yadah, yahdah. However, Grant has made around 80 ovens using this brew, so I guess it is not such a big issue. Use hydrated lime, not garden lime. Make up a paste at least a week before, by slowly adding hydrated lime to water and mixing until it forms a stiff cream. Be careful, hydrated lime is nasty stuff and will burn skin, eyes, and lungs. I wore gloves, goggles, and a respirator when I mixed my lime brew.

Born on the 13th June; a bonny bouncing  brick weighing in at 11.6 Kilos. The damage at the bottom was caused by the brick "bouncing" when I dropped it

Born on the 13th June; a bonny bouncing brick weighing in at 11.6 Kilos.
The damage at the bottom was caused by the brick “bouncing” when I dropped it

Having now made my fire clay I shaped it into a brick of roughly 300 x 300 x 100, which I then set aside to dry. The next day the brick was quite firm. I weighed it so that I could tell when it was dry (on the basis that when it stopped losing weight it would be dry. Unfortunately, I dropped the brick in the process of weighing it. This damaged the edges, which you can see in the photo. I left the brick in the lounge to dry for a couple of weeks and then put it in front of the fireplace for another couple of weeks. So far so good. No explosions, no cracking, only some minor crumbling from the damaged areas; otherwise it seemed quite sound. By this time it had stabilised at 10.4 kilos; a drop of 1.2 kilos, which is about the amount of water I added while mixing the adobe.

It was now time to ratchet things up a cog and get cooking with gas. I turned to the gas grill to see how my mix would handle heat. My Broil King grill ostensibly reaches 400 degrees C (according to the dial). I baked the brick for three hours at maximum setting. At its base (the side to the flame) the brick reached 385 degrees, while the top got as high as 250 degrees.

Now for the heavy artillery. I wanted to really stress test the mix by firing it to around 600 degrees in a pottery kiln (your average pizza oven operates at around 400). However, a potter advised me that it would be better if I subjected it to direct flame by putting it in a brazier. Not having a brazier (or indeed any women’s underwear) I used the fireplace in my lounge.

Dante would have been proud of this inferno

Dante would have been proud of this inferno

I placed the brick in the bottom of the firebox and loaded it up with dry gum. After two hours of fierce burning I swept the coals aside and measured the temperature of the brick, using an infrared thermometer – 550 degrees and no discernible damage (to the brick or the lounge)! To quote Borat, “Great success”. It seems that I have achieved close-to the ideal recipe at the first attempt. I now have to resist the usual Romi urge to tinker with a winning formula.

So what next? Grant usually builds an oven over two days. Rather than go flat out, I think I will build in stages. First the hearth, then the internal dome, then the insulation layer, and then the final rendering. I will make each stage a half to one day exercise. I intend to do the hearth sometime in the next couple of weeks, with the internal dome probably on the second or third weekend in August, all depending on the weather. Anyone interested in helping out and learning? let me know by commenting on this site or sending an email to claypizzaoven@gmail.com.

5. Ready to start building the oven

I had really hoped to have the whole project finished by now and to be enjoying freshly baked wood-fired pizza on a regular basis – However, the road to hell is paved with good intentions (the weather has certainly been less than hell-like around here).

My advisor, Grant, has dropped off the radar, so getting into the meat of things was not possible, as I would rather he were here to take advantage of his experience. The weather has been crappy anyway, so progress probably couldn’t have been made. I have had to resort to the old pizza stone in the gas grill to get my pizza fix.

Since my last post I have been pottering around between rain showers and the demands of clients and have managed to finish the oven platform. I have got things to the stage where I am ready to start building the oven itself. I am not sure when that may happen, but as Rachael Hunter said, “…it will happen”.

First of all, there was a slight, well major really, design change. I decided to add a side table by extending the oven support slab out to the left. This gives me somewhere to line up the pizzas prior to them going into the oven. By backing my gas grill up to it I now have an almost-complete outdoor kitchen. I say “almost” because a wood-fired barbecue or a churrasceria would round things off nicely; that will probably be the next project – aye caramba! (yes, I know that is sort-of Spanish and that they speak Portuguese in Brazil, but you get the idea)

I guess you could say that the next thing I did falls into the realm of building the oven itself – forming the insulation slab for the base of the oven. This will slow down heat transfer from the cooking surface, enabling pizza after pizza to roll off the production line.

I used pumicecrete for this layer. Being full of gas bubbles, pumice is a good insulation medium. I got the idea of using it for insulation when I dismantled an old hot water cylinder and found it lined with pumice. You can use vermiculite or perlite, both of which have superior insulation qualities to pumice. However, at a cost of $35 for a trailer load from the greenhouse supply yard up the road I couldn’t argue with the economics and convenience of pumice. What I don’t use I can throw into the compost bins to help break up the compost. Grant makes his insulation mix from clay and wood shavings. It works very well – in Dan’s oven you can touch the outside of the oven without getting burned. If wood can do that then pumice can only be superior, given it’s volcanic origin (is pumice actually brimstone?)

Lastly in this episode I tiled the slab to cover up the less than perfect finish of the concrete slab. I have now become quite proficient in using an angle grinder to cut masonry – both quarry tiles and concrete.

While the tiling went reasonably well the grouting was an exercise in frustration. I grouted on a beautiful fine day, and stood back to admire my handiwork. Five minutes later it poured down with rain (where did that come from – thanks Yahoo weather!). While the rain didn’t seriously damage the grout it did cause the grout lines to leach, resulting in a cement haze drying on the face of the tiles – bugger! I thought that I might be faced with replacing them. As quarry tiles are not glazed and are quite porous I though that the cement had penetrated irretrievably into the tiles. Luckily, application of a proprietary haze remover (a phosphoric acid substitute) cleaned them up nicely – phew! Thanks for the advice, Drew.

I am making an 850mm diameter oven, therefore the insulation slab is 1050 mm dia (850mm oven internal diameter + 100mm walls). I marked out the position of the insulation slab using a home made compass, as shown

I am making an 850mm diameter oven, therefore the insulation slab is 1050 mm dia (850mm oven internal diameter + 100mm walls). I marked out the position of the insulation slab using a home made compass, as shown. The axle was a screw placed through the arm into a hole drilled in the centre of the oven support slab. A pencil, 525mm from the axle allowed me to scribe a circle on the slab

Boxing for the pumicecrete was formed by sheet of 7mm plywood, scored on the table saw to allow it to bend nicely into a circle. The boxing is 190mm high (100mm insulation and 90mm of fireclay for the oven floor)

Boxing for the pumicecrete was formed from a piece of 7mm plywood, scored on the table saw to allow it to bend nicely into a circle. The boxing is 190mm high (100mm insulation and 90mm of fireclay for the oven floor)

The plywood boxing was laid out on the scribed circle and held in place by wooden blocks, which were clamped to the support slab. The more mathematically inclined of you will have calculated that to form a circle of 1050mm diameter I would need a piece of plywood 3.3. metres long. I only had a 2.7 metre long piece, so the balance of the circumference was formed using a strip of alcubond - This stuff is having a lot of use on this project. I hope that using my woodworking clamps on such an industrial project will not ruin them for fine woodworking!

The plywood boxing was laid out on the scribed circle and held in place by wooden blocks, which were clamped to the support slab. The more mathematically inclined of you will have calculated that to form a circle of 1050mm diameter I would need a piece of plywood 3.3. metres long. I only had a 2.7 metre long piece, so the balance of the circumference was formed using a strip of alcubond – This stuff is getting a lot of use on this project.
I hope that using my woodworking clamps on such an industrial project will not ruin them for fine woodworking!

The pumicecrete is made using 7mm pumice in a 5:1 mix with cement. A larger grade of pumice would probably have been better, but then I used what is available locally. Beer bottles add to the insulation value of the slab by introducing further air voids. I made the mix quite dry and simply screeded it off in the boxing. I didn't compact it as I want as much air as possible in the mix. It doesn't need to be absolutely level as the fireclay oven floor will be laid on top of it. In any event after emptying all of those bottles one was incapable of seeing straight, anyway. The end product has the appearance of rice bubbles and is quite crumbly.

The pumicecrete is made using 7mm pumice in a 5:1 mix with cement. A larger grade of pumice would probably have been better, but then I used what is available locally. Beer bottles add to the insulation value of the slab by introducing further air voids.
I made the mix quite dry and simply screeded it off in the boxing. I didn’t compact it as I want as much air as possible in the mix. It doesn’t need to be absolutely level as the fireclay oven floor will be laid on top of it. In any event after emptying all of those bottles one was incapable of seeing straight, anyway.
The end product has the appearance of rice bubbles and is quite crumbly.

Boxing removed and the completed insulation layer ready for the clay oven floor

Boxing removed and the completed insulation layer ready for the clay oven floor

The support slab and bench tiled and completed. Note the extension table off to the left. I will have to extend the deck a little so that I can roll the gas grill up to the edge. Now, time to empty some more beer bottles in case I need them for further insulation augmentation

The support slab and bench tiled and completed. Note the extension table off to the left. I will have to extend the deck a little so that I can roll the gas grill up to the end.
Now, time to empty some more beer bottles in case I need them for further insulation augmentation

1. Welcome

1. Welcome

Hi Pizza lovers and those who, like me, just need an interesting summer project.

This blog records my journey towards creating a wood fired pizza oven (WFPO from now on) at my farmlet in the South of Auckland City, New Zealand. It will track what I have done, from breaking the ground through baking the first Pizza and beyond. You can use it as a guide to motivate you, provide you with guidance, and to interact with me – you can ask questions, which I will try to answer, or you can add to the learning by providing your own thoughts. Please feel free to invite others to share in the experience.

For those of you who don’t know me, here’s a little background information. My name is Romi and, every now and then, I get these insatiable urges to build something. Over the years this has included parts of my house, a dam, bridge, workshop, farm projects, numerous items of fine furniture, and lots more than I care to (and can’t) remember. I love pizza, am a student of the Italian language, and appasionisto of all things Italian. All of these interests have collided into a perfect storm to create this project to satisfy all passions – well, for now at least.

The interest in building a WFPO was kindled about five years ago, when Hagen suggested that we build one. Hagen is a foodie of the finest kind and I think he was itching to get into the sort of activity denied to an apartment dweller. My motto of “you can’t rush these things”, combined with Hagen moving to Australia, meant the project got filed in the ‘one day’ drawer.

Well, one day arrived in the form of an invitation to attend a WFPO building workshop at Dan and Kazuyo’s B & B on Auckland’s North Shore. Grant Steven, a down-to-earth Kiwi bloke from the North of New Zealand was our guide. Grant has built many WFPOs and focuses on using materials readily at hand, rather than going down the kitset road. He reckons you can build a basic oven for around $NZ 300, if you can source the major materials out of your back yard (or nearest river). After a couple of days, and with the efforts of about ten people, we proudly stood back to admire what we had created from local clay, sand, cement, lime and, of all things, wood shavings – I was hooked.

So, the project jumped from the one day file to I want one, and I want it right now stage. However, the hottest summer I can remember intervened and I preferred to live in the pool, rather than singing breaking rocks in the hot sun with the Bobby Fuller Four (or the Clash for those of you who are of a generation that believes that Heard it Through the Grapevine was originally sung by the California Raisins).

It hasn’t got any cooler, so I decided to start anyway; working in the evenings so as to avoid mad dogs and Englishmen.

Most of what I will impart to you is based on Grant’s experience, mixed with some of my own design modifications and tips and tricks gleaned from various on-line communities.

So, that covers the WHY, now for the WHAT.

This project will be for a freestanding WFPO, made from raw materials that I can source locally, with an emphasis on using waste materials or by-products. It will be built on the edge of my deck and is designed to complement my house.

Here is a rough idea of what it will look like.  By “rough”, I mean very rough.  The design will evolve and I am already thinking of making the doorway and chimney more of an artistic statement than is currently depicted. For those of you with a drawing bent this was developed in Google Sketchup

Here is a rough idea of what it will look like. By “rough”, I mean very rough. The design will evolve and I am already thinking of making the doorway and chimney more of an artistic statement than is currently depicted. For those of you with a drawing bent this was developed in Google Sketchup


So, come along for the ride and bring your friends. I promise the posts won’t be as long as this introduction. I will focus on what I have done, why, and how, and there will be lots of photos. I will also detail quantities and costs of materials used.

As I said before, feel free to add comments or to ask questions on this site.

My next post will be details design and construction considerations.