The Rebirth of Luna

Luna’s about 7 years old. For an oven built as a prototype, that’s getting on. She’s had a robust, quite eventful life so far. She’s lived in four locations, as well as a short stint living ‘on the road’, as the centrepiece to my first travelling bakery and classroom. She was designed to be a mobile, high volume wood fired oven. She was meant to be light, heat up quickly, and be able to bake 300 or more perfect loaves of bread in the space of a market - which would be around eight hours.

For this task, she was a complete failure. The entire mobile bakery enterprise had a number of flaws, as it turned out. I may well have covered these, and the mobile bakery, in a previous blog post; I can’t remember. Anyway, that’s not what this post is about.

That’s Luna’s bush hideaway. She’s in the box…

Luna found her place as a stationary oven. She lived on a fixed site, still on the mobile bakery trailer, at a bush hideaway in Ellalong, where she performed the weekly baking duties for local Saturday markets with incredible finesse. I knew the difference Luna made - a kind of crust that only a brick oven can give you.

She was always a bit tricky to work with - she liked to be pre heated for a good 5 hours before she would really begin to sing, for example. She had some hot spots (which became completely ‘worked around’, as one does with any old bakery oven), and she needed a major clean out and overhaul every year, or she would block up (and actually melt) in parts. I had to rebuild the firebox a couple of times, and used a crowbar to open up a pathway for flue gases when it fatigued after about 5 years use.

I learned the hard way with Luna, every time, but after each rebuild she returned to work, better than ever. She was, for many years, a ‘work in progress’. She eventually became an excellent oven, capable of baking an average of 30 average sized full sourdogh loaves an hour - provided I was on my game - and more if someone was helping me. She did her job as a test bed and we improved our Aromatic Embers ovens as a result.

When the first Bush Bakery at Ellalong came to an end, I packed Luna up in the trailer, took out her bricks, and parked her in a nearby paddock, where she lived for a few months. I towed her here to the farm, and she was parked again for a few more months. We removed her from the trailer after the Tour Down South, and a boilermaker began the task of refurbishing her, with design modifications we had now applied to some of our other ovens.

Luna was my third prototype. I was the test pilot and outside design consultant. Actually, I became the crash test dummy more often than not. The first two prototypes, both named Bertha, turned out to be absolute pigs of ovens, but pigs which were made to sing for their supper nonetheless, thanks to my need to bake decent bread.

Bertha 1 in Cafe mode. Note bricked plate warmer on top!

Luna was different. We really thought about Luna - all our mistakes taught us what NOT to do. So Luna was a decent oven from the getgo - but she developed some long term issues. That’s why I was always working on her - she took a fair bit of tweaking to make her really sing, I can tell you! So when I left her in the boilermaker’s capable hands, I gave him my wish list - or at least half of it. I knew I’d be doing the other half myself.

This time I wanted to make the flame generated from the fire really stretch, so that it could do the job of heating more cleanly, quickly and efficiently. We needed to get the bottom decks more even too. Back in the day, the area above the firebox was always the hottest part of the deck. It was so hot that I had to set loaves to one side so they didn’t burn.

The boilermaker takes Luna apart with the tynes of a tractor.

Stretching the flame in Luna’s firebox.

The top decks have always relatively slow, so we set out to improve the heat here at same time.

The boilermaker built a more sophisticated and heavy duty baffle system, based on my ideas. He made the baffle itself more angled so that flame was siphoned off better when it runs against it from the firebox. I later bricked the inside of the firebox to further enhance the ‘flamethrower effect’. He rebuilt the firebox to include more brick than before. He also incorporated a whole series of cleaning access tubes to the roof of the oven, so that the area nearest the flue could be cleaned - an issue which had reared its ugly head a couple of times in Luna’s life already.

I’m hoping the changes to the flue system will eliminate the problem of soot build up altogether. I’m fully aware that I may simply be experiencing a case of wishful thinking here. Every designer wants their latest and greatest innovation to work - we always wear rose coloured glasses, to a certain extent. Sometimes, though, it pays to take out insurance. If there was still a build up of soot, despite our new modifications, at least I can now clean it out more easily than in the past.

Luna’s cleaning tubes before they get bricked and mortared.

All this work took many months. Luna was positioned in the middle of a paddock full of farm equipment. When the boilermaker was on the farm, he’d carry Luna’s bits over to the shed on the tines of the old tractor. He’d weld and angle grind and rivet for hours on end. From time to time I’d ‘lackey’ for him - just as I did when Luna was fabricated here on the farm years ago. Only then, as I remember it, I was on crutches. That’s a whole other story. Not this time - I walk pretty well these days.

More heavy duty thermal mass is added before putting Luna back together.

As the work on Luna slowly got done, a bit here and a bit there, the dairy shed also got finished - in much the same manner. A few weeks ago we carried Luna’s 2 finished pieces over to the dairy shed on the tractor, and we put her back together for the first time in a year. Then we positioned her outside the new school classroom and bakery, where she will live for quite a while, I hope.

Luna 2 showing inner layer of brickwork. This will be wrapped with besser brick.

I’ve been bricking her up, inside and out, for the past two weeks or so. It’s slow, heavy work, as a great deal of the brick is in really hard to get at places - inside baking chambers, for example. These baking chambers are only 16 cm high and a metre deep. One slides the bricks in on the end of a metal peel, and manipulates them as best as one can from a metre away. Around the baking chambers there are two layers of brick, and another layer on the top of them. Getting the bricks in place involves climbing up a ladder with brick or bricks in hand, keeping a fresh mortar on the go all the while, for maybe a couple of hundred climbs. Each brick weighs between 2 and 5 kg, and so far I’ve put roughly 500 bricks in and through and around the oven, as well as another couple of hundred inside the baffles, which we did before she was put back together. I’ve only done enough, at the time of writing this, to fire the oven up and make it work.

First layer of brickwork done. The oven is functional here, though nowhere near thermally efficient.

All this extra thermal mass and insulation will become necessary when Luna goes into production mode for the ‘Steady State Bakery’. It will operate as a heat sink, as well as a kind of heat mirror; the thick walls of brick should hold heat for days. Luna will become a super efficient oven.

i was inspired in my design for Luna by spending time at Harcourt Historic Bakery with Jodi and Dave when I did the Tour Down South last year. Their oven is capable of holding high temperatures for days after firing, as it has some 72 tonnes of brick around it. It’s an incredible piece of kit for a 100 year old oven. Dave manages to keep it hot with very little timber each day.

My version only will have maybe four or five tonnes of brick when it’s finished. The principle is similar to the oven in Harcourt though - get the brick hot, and then once it’s hot, keep it there for as long as possible. I’ll be looking for new bakery customers very soon, that’s for sure!

Next layer done. Still have to complete the outer wrapping, add more mass to the roof, and fill the besser bricks with rubble and sand.

Upon firing her up after putting her in place here, I saw that Luna was really a serious piece of flame art now. The firebox works a treat, blasting flame a good few feet each side, in sheets, spread out right under the two baking decks. It takes the oven from cold to baking temperature in just 3 hours, but will do it substantially quicker when it is fired each day or two, as it will retain a lot of heat.

Just a teensie fire here. When I fully blaze the fire, it’s too hot for my camera!

At the time of writing I’m about two thirds of the way through the brickwork. There is an ‘inner outer’ layer of brick around the baking chamber, and another around the firebox. There are two layers on top, with two more layers of grog based on mortar and recycled crushed concrete. There will be another layer of brick and mortar on top as well. There is a layer of besser brick surrounding the three sides, and I’m currently filling these with rubble, glass and sand to add thermal mass, as well as to use up everything I can from a demolished brick wall I was given. I’m going to fill a void between the inner and outer layers with insulation board.

My experience so far has been a bit different to what the world of oven builders has been telling me. One commonly held belief is that insulation like ceramic blanket or rockwool or ceramic board will ‘reflect’ heat back into the structure. While this is possible, there needs to be an outer layer of brick or thick mortar wrapping up the blanket in addition to the blanket itself for any reflection effect to occur. The blanket will eventually dissipate its stored heat in both directions - back in and out. If you wrap your blanket in brick on both sides, the blanket still fills with heat, but it slowly dissipates the stored heat back into thermal mass surrounding it. If you don’t wrap your insular material in thermal mass, you will ultimately allow 50% of the stored heat run out into the atmosphere. In addition, your insular material will actually be absorbing heat from the bricks next to it, contributing to a slower heat up of the oven itself. I learned this with Bertha 2, which took over 18 hours to heat from cold, and really only started to get useful after the second bake for the week. Needless to say, once I figured out our insulation mistake, I got to pull her apart and replace the insulation with brick, and this sped her heating time up by many hours.

The advantage of brick is that while it absorbs heat, it also reflects heat. If you sit beside a brick wall in the sun, you will experience how brick reflects heat. When it becomes ‘soaked’ with heat, it then becomes a heat source - it actually ‘radiates’. So you get reflection, absorption and dissipation (radiation) of heat, in that order. Brick, as a material to work with storing heat, becomes more efficient over time.

One uses ones loaf to make a decent loaf. Or so they say…

So far I’ve used the oven for my standard bakes, and have kept the oven warm over multiple days doing various tasks - slow roasting on one day and baking pizza on another. Luna can hold baking temperature without fire for a couple of hours at the moment. In fact, the top decks increase in temperature for the first six hours of firing, and continue to increase without fire for the next three hours. On some nights I have finished the bake and checked the baking chambers about 10 hours later, and they still held over 120 C. I think I can improve on this by a significant amount by just beefing up the thermal mass and adding strategic insulation in some places.

I’ve done the big stuff, now I will do the little things. Watch this space.

If you would like to experience Luna first hand, I run workshops for the general public each month. Professional baking workshops are held four times a year. Check out what’s on offer.

Post script:

Just a little update regarding Luna’s thermal performance. Since writing this article, I’ve filled in all the besser bricks with rubble, bits of brick and crusher dust. I’ve also enclosed a sleeve of air surrounding the baking chambers with brick. I’ve bagged the outer shell with mortar, and I’m half way through adding a layer of bottles covered in mortar over the top. Once this is done, I’ll paint the top section in black bituminous paint. At this point, the oven holds an average 100 degrees C some 12 hours after a full bake of about 80 or so loaves. It takes just a bit less than 3 hours to reach baking temperature from cold, though if I really want the oven to be fully ‘soaked’, I’ll pre heat for about 5 hours. I’m yet to gather data on how long the oven takes to heat from 100 degrees, but I think it should re heat in just a couple of hours. All the little bits I’ve done to make it hold heat longer are making a difference; and I can see I’ll be doing more as the need to use the oven more often grows with demand for bread. I’m also noticing that to heat the oven takes less fuel now. This is a bit unscientific, because I’m using different wood from around the farm, but the effect is still noticeable. I still don’t have enough demand to fill 2 days baking, but this will gradually build as I get out and gather more subscribers.