Building Bertha

Written by Warwick Quinton. Posted in Ovens

Bertha is our woodfired oven - or more correctly, she is the name given to our prototype woodfired baker's oven. There have been plenty of woodfired ovens built - but none like Bertha - she's a baker's oven for the 21st century.

She is an efficient, fast and 'easy to live with' oven, which is intended to be used in artisan bakeries - but she's 'off the grid' - meaning she happens to be powered by wood, or even food scraps, rather than electricity or gas. Yes, she can and does save money on energy costs too. And she burns very cleanly. That's the idea. Every baker's dream oven.

Her first incarnation, Bertha 1, lived in the SourdoughBaker Cafe, in West End, Newcastle. My mate Craig Miller and I designed her, over the phone, from scratch. There's more about that story on one of my blogs,  so I won't repeat it here. But if you are interested in SourdoughBaker Cafe, and it's oven, and other stuff, have a look at this blog too..


Bertha 1 - a comedy of errors

Bertha 1 was a bit of a rarity - she was much bigger than a domestic woodfired oven (typically an AGA or Wamsler or an Early Kooka), though she followed similar principles from a design perspective. She had her two fireboxes placed on one side, so that you can cook above one of them, and bake using side heat from both. She had two decks for baking bread, and a third below them which effectively became a 'heat bank', as its height and proximity to the firebox made it unuseable in a practical sense - so we filled it with brick to increase thermal mass.

There were two fireboxes at one side, one above the other. These fireboxes produced the heat for different parts of the oven. The top one heated the top deck and the cooktop, while the bottom one heated the lower deck. Each deck provided about half a metre of deck space. We could set up to twenty loaves or so at a time in the two decks.

It took about a year to properly tune Bertha 1. At first, her insulation was underdone, and a great deal of the massive heat she could generate with those two fireboxes simply went straight out into the cafe, or up the chimney. I made makeshift heat baffles so we could operate with her, then Craig came down from Bathurst to weld proper box baffles to her outer edges.

Once that was done, she behaved a lot more like an oven, rather than a massive room heater. When I say oven, I mean that in a very general sense - in my many years of baking, I had never experienced a more temperamental and uneven oven, with the exception of the first domestic oven I used to bake bread in Waverly. Bertha 1 was truly a bitch to work with - I had to turn the bread every few minutes to get anything resembling even crust colour.

To compound our problems, Paul West, our chef, was seriously struggling to make bacon and eggs on her cooktop - the sheet steel was way to hot, positioned as it was only a foot or so above the firebox. To compound his problems, there was no shielding between him and the raw heat of the firebox except for 5 millimeters of steel. No wonder he was getting angry every morning - I thought he was just a typical temperamental chef - but the hairs on his legs were burning off on an almost daily basis.

Little did I know that we were staring down the barrel of a years' worth of hard work to turn Bertha into something operable.

Woodfired ovens 101

Most bakery style woodfired ovens have the fire set inside them, or are 'scotch' ovens, which have a separate firepit below the baking deck - transferring heat via a tube directly into the decks - like a flamethrower.

Another word for these ovens is 'black' ovens - because they become black inside due to the smoke being inside the chamber.

Both of these types of ovens can't effectively be operated while the fire is flaming - instead, they heat the thermal mass (stones or thermal bricks) first, and then bake bread using the 'falling heat' contained in the stones. As such, they are notoriously inefficient, and so in the main have been surpassed by more modern ovens.


People lament their passing, and wax lyrical about the effect the wood smoke has on the bread - but this is largely mythology, as the ovens do not bake while the fire is burning. The crust, however, and the way they bake (using primarily thermal transfer rather than convection), makes them unique and very special - so I was keen to reinvent them along more efficient lines.

Craig, when I discovered his work, was using a separate firebox in his pizza ovens, so immediately we struck up a friendship. He was making what is known as 'white' ovens, or ovens where the smoke was flued away from the chamber directly by the use of a chimney.

I worked with Craig to create something like a 'setter' oven, based around a firebox, rather than gas burners. Ours was a different beasty altogether, with a completely contained firebox joined to the baking deck. Thus, the heat travelled via conducted means  through brick, rather than via convection. This was, and is, a very exciting challenge, as it combines the best of the old world technology with that of the new.

(I have to admit, I would be happy to be remembered as an oven designer equally much as a sourdough baker.)

Thermal Engineering 101

To those of us who have not completed degrees in thermal engineering, having the heat enter the baking chamber via thermal mass, or conduction, makes for a whole different type of baking.This is known as radiant heat, as the bricks get so hot that they become a heat source.

The main feature of heating a baking deck using large amounts of radiant heat is its steadyness - once the heat is in the deck, it stays there for a long time. But it takes quite a bit of energy to actually get it there.

We have been able to minimise the 'falling' effect of most woodfired ovens, making Bertha ideal for round the clock use.

This suits a cafe and bakery environment for a number of reasons - but one of them is that the oven can be used very flexibly, allowing for small and large bakes.

Any baker will tell you that everybody buys bread on Saturdays, so they gear up the bakery's capacity for that day.


But in a cafe, one needs a smaller, consistent capacity and heat to cater for weekdays as well. How do you do both efficiently?

The answer lies in utilising thermal mass to hold heat all the time, so that it can be warmed to baking temperature with minimal energy input, and kept there for long periods.

So far so good. Steady heat, long thermal storage.

As an artisan baker, I can tell you it's all about the crust.This presents another challenge, from a thermal engineering point of view.

The crust produced via an oven with a large thermal mass is far superior to the crust from any other oven. Bread from a brick oven is just very special. You can get good crust in other ovens too - oil filled ovens, and gas fired setter ovens for example - but brick is a unique material with unique qualities. Hence, unique crust.

Anyone who has eaten bread from a woodfired oven on a regular basis will be able to verify this. And if you've played with making one, you will appreciate it's all about the bricks.

So we've had to learn how to get thermal mass working efficiently and effectively. Bertha 1 gave us massive energy to deal with, so we learned some principles of thermal engineering on the job - and these are, in the main, counter intuitive - but brick plays a big part in getting the perfect crust.

Bertha 2, and Wal's part in her design

Since moving from our Hunter Street location to our larger premises at 15 Denison Street, Newcastle West, we've been making do without Bertha 1, and have instead been tuning Wal, an old caterer's oven I've modified to suit our sole style of baking.

Wal has about the same capacity as Bertha, but is an altogether different beast. Wal is gas fired, so he has a lot less grunt. This means I have had to approach the issue of running the thermal mass from an entirely different perspective - in effect, I've had to do more with less.


I've now made significant advances with Wal, and after extensive tuning, have been able to get something resembling speed out of him.

It has been a struggle - I've had to make the chambers in each deck smaller, and my limited thermal engineering prowess has been tested - but Wal can produce a very decent crust now, after a fair amount of brick has been added in and around his decks.

I've wrapped the whole thing up in a kind of insulation reflector, which hides all the bricks, but also keeps most of the heat reflecting back into them. Wal will be useful for a while as an oven specifically for breads 'in the tin', but Bertha 2 will still be for the sole breads, which are what it is all about from my perspective.

In the meantime, Wal has offered me a real learning opportunity in the art of thermal dynamics.

This will soon be put to good use.

Bertha 2 has been designed, and will soon be wheeled in. I will be putting lots of pictures up here about her, but she works along similar design principles. She will, however, be substantially bigger, and certainly more efficient.



Bertha 2 takes shape

When we designed Bertha 1, we were attempting to do many things at once - create a baker's oven, a smoker and a cooktop, all of which had to fit into a fairly small kitchen. Thus, we compromised on many things, not the least of which was the firebox.

No such issues with Bertha 2 - her sole function is to bake bread, and to do that, she has a truly massive firebox. The picture here shows the framework, which will be wrapped in steel sheet and brick. It is about nine or ten times bigger than Bertha 1. The holes at the bottom provide oxygen induction, and there will also be secondary heated induction added later.

This firebox will be positioned under the oven decks, and will power a heated flue system which will wrap around the decks.

It will be interesting to discover just how many decks the firebox will be able to power - initially we are putting on two decks, though both Craig and I believe we will have enough energy to heat three or more. Height may be the only issue.





Here's the deck framework showing the flueing and deck insulation space. These two decks will give us about 3 or four metres of baking real estate, which is pretty substantial. This type of oven actually cooks very fast - each deck can set upwards of twenty loaves and bake them in about 40 minutes - I'm talking sourdough language here, not yeast breads. Yeasted breads would cook much faster!

Between each deck, there will be 75 mm of clay bricks top and bottom - meaning that the flue gas will heat 150 mm of brick! This will provide the decks with massive radiant heat - and radiant heat is really what the baker is looking for when he wants to get a really good, colourful (but thin) crust. Having such a large volume of brick will also mean that the oven will achieve very steady heat within the chambers. Once Bertha is hot, she will stay hot!




Bertha's skeletonIn this photo, you can see some house bricks placed into where the bottom deck will be, to give you an idea of overall size of the oven, and how it will all fit together. You can see the firebox sitting underneath the decks - it is smaller, and will have flues running under the deck framework and up the sides. Effectively, each deck will have heated bricks on five out of six sides, with the door side being heavily insulated. As a result, each deck will create radiant heat for the crusts of the breads baked inside the chambers to soak up. Not only that, but there will be very little heat wasted - we anticipate you'll be able to rest your hand on any external surface of the oven, no matter how long it has been running! We believe Bertha 2 will be a far more efficient oven than anything commercially available, whether powered by wood or gas or electricity.





Firebricks above the fireboxHere is the top of the firebox, showing a layer of high temperature oven bricks, which will provide a heat buffer between the box and the base oven chamber - there will be another 75 mm of regular brick above this also. We are concerned that the heat on the bottom of the bottom deck will be pretty intense, as it is conductively connected directly to the firebox. We can, if need be, add another layer of brick - or insert heat blanketing, or just leave a few inches of air gap. However, a decision on this needs to be made before the oven is commissioned, as once the unit is assembled and filled with masonry, it will be virtually impossible to get to this part without a crane.

(We decided to leave about 25mm of gap which we can fill if need be. Here's hoping our design skills are reasonably sound!)





Bertha 2 nearing completionThis photo give you a better idea of Bertha's size. You can see how the doors sit level with the decks - though these had to be modified considerably, as they weighed over 60 kilos each and couldn't practically be used. In the end, Craig managed to trim their weight down to a more managable 25 kilos each, and coupled with a counterweight, the doors are workable. They are also well insulated - something we learned from Bertha 1!


Bertha 2 is installed!

Craig took just over a year to build Bertha. While a great deal of Bertha was planned and designed, there were lots of bits that were very much trial and error. For example, I was adamant that the doors needed to be properly insulated, as last time they weren't, and we spent half of the time turning loaves in Bertha 1 because the bit near the door would not crust.

So this time, Craig designed the doors with 3 millimetre steel and plenty of insulation to solve this problem with Bertha 2. However, when it came time to mount the doors on the oven, he realised that they could never be easily closed. Why? Because they ended up weighing over 60 kilos each!

So back to the laser cutter Craig went, and remade them out of much lighter metals. Finally, he managed to trim down the weight of each door to a mere 30 kilos - and redesigned a cantilever system so that they would balance properly as well. All this added about a month to Bertha's ETA.

But finally, in January 2013, we brought her in via a 3 tonne forklift. This took a day to do. Once she was finally in Large Woodfired ovenposition, we filled the upper parts of the baking chambers with sand rather than brick. This last modification was decided last minute, as the sheer number of bricks to fill Bertha was going to cost an arm and a leg. Having since baked with the sand on a number of occasions, we have decided to use sand in Bertha 3 as well. It works very well indeed!

The photo here shows her without her insulation baffling - this was added onsite to save space. She actually looks quite svelte without this added six inches of bulk right around her outside.

Craig is not a great quantum surveyor - before shipping her from Bathurst to Newcastle, I asked him what she weighed - just the metal before the brick and sand and extra insulation was put in. He did a few quick sums, and replied "mate, she weighs about one and a bit tonnes." Armed with this information, Craig rang the transport company, and booked the truck to move her from Bathurst to Sydney.

The transport company turned up a couple of days later with a small tabletop truck and a crane to lift her on. After almost destroying the crane, and then the truck, the transport company decided to run the loaded vehicle over the nearest weighbridge - just in case Craig underestimated just a smidgin. Two point two tonnes, Craig was told in a not too happy voice. Bertha ended up sitting on the back of that truck in a warehouse in Sydney right though Christmas because the transport company had to wait for a larger crane to get her off the truck!

(to be continued...)