Mobile Oven #1

[MarvinG]

Dear all,

Finally, after quite a while again, my Oven-Journey continues. Thanks to david I learnt a lot about my next project and would like to document it here. The first take-aways can be found towards the end of the page here, which convinced me to make it happen.

So, I decided against using a Kei Truck, for obvious reasons. It's too cute and not strong enough. Instead I now have this trailor:






The plan is to have a thick oven floor of about 8cm (Thick bricks, then 2.5cm big Floor tiles on top), all insulated with ceramic fibreboard, later Perlcrete and then stell cage - laying on something like a tire to balance shock. The trailor has a suspension, but still. more is more.

- The dome I'll do with 4:1:1:1 (Sand, Cement, Lime, Shamot-powder),
- Reinforcement with AK resistant glass fibres, AK resistent mesh cut into strips and put round, as well as PP Fibres and to help humidity escape, as well as steel needles.
- I think I'll build this around XPS and sand - will try to go quite low - neapolitan style low-dome. Let's see how long it last's. My attitude this time is. Make and learn - if necessary a new oven is done rather fast - home-brew is not that expensive. If it will bake for a few festivals, perfect.


Question: how much
PP fibres, AR Glass fibres and Steel needles should I put into my mix? Anyone knows a good ratio?

Stay tuned for the progress,

Greetings from Tbilisi
Marvin

[david s]

pp fibres 1% by volume
AR glass fibres 3% by volume
SS needles 2% by volume

Something has gone wrong with the pics you posted because they won't display, but you do need to match the estimated weight with the trailer spring capacity.

8cm cooking floor is extremely thick, most ovens go for around 2cm. An 8cm brick floor will add a lot of extra weight to lug around, take a very long time to reach pizza temperature, if at all and use a lot of fuel. Really thick floors are usually reserved for bread ovens which are fired daily and never really cool down completely.

"4:1:1:1 (Sand, Cement, Lime, Shamot-powder) " sounds to me too lean. The chamotte, which is crushed fired clay, is probably a better addition than powdered unfired clay because it will impart less shrinkage, but it will not have the binding effect like powdered clay, which makes applying the mix easier. It is not cemetious so should just be considered an aggregate and that further weakens the mix making it leaner. I believe a more suitable brew is 3:1:1:0.5 sand,OPC,hydrated lime, powdered clay (bricklayers clay). If you are really keen to use chamotte, just replace a proportion of the sand with it. Because of the temperatures to which we fire, I believe sand is a suitable aggregate, although the chamotte imparts more refractory qualities, it's just way more expensive than sand.

what is XPS?

[MarvinG]

Hi David,

Thanks for your reply.
When you say these additives are added by "volume", how do I do that? cause the fibres are fluffy...so weight would be much easier and precise no? I'm a bit confused.

Yes, 8cm is very thick. So far I built two ovens, both have around 5.5 thick floor. On the 2nd oven we recently baked around 50pizzas in an hour. The fact is, that each pizza cools out the oven floor around 5°Celsius. And when it is constantly covered with pizzas, after 30min you struggle to have a crunchy pizza (from the bottom).
This oven will be for festivals, so is designed to bake rater 100 pizzas an hour - therefore the thickness.


Yes I plan to use shamot-powder, because that is, what is available here, and its not expensive. They Call it shamot - but what it really is, if its fired or unfired clay, only god knows.
So if I hear you right, you would say 3:1:1:1 (sand, OPC, HL, Shamot)?! My experience from the last two brick ovens, where I used a rough 4:1:1:1 ratio, as the binder was that its very sticky. very nice to work with.

XPS is something like a styropor, used as insulation material. Basically air in solid form. It would reduce the amount of sand needed to build the form.

[david s]

Hi David,

Thanks for your reply.
When you say these additives are added by "volume", how do I do that? cause the fibres are fluffy...so weight would be much easier and precise no? I'm a bit confused.

Yes, 8cm is very thick. So far I built two ovens, both have around 5.5 thick floor. On the 2nd oven we recently baked around 50pizzas in an hour. The fact is, that each pizza cools out the oven floor around 5°Celsius. And when it is constantly covered with pizzas, after 30min you struggle to have a crunchy pizza (from the bottom).
This oven will be for festivals, so is designed to bake rater 100 pizzas an hour - therefore the thickness.


Yes I plan to use shamot-powder, because that is, what is available here, and its not expensive. They Call it shamot - but what it really is, if its fired or unfired clay, only god knows.
So if I hear you right, you would say 3:1:1:1 (sand, OPC, HL, Shamot)?! My experience from the last two brick ovens, where I used a rough 4:1:1:1 ratio, as the binder was that its very sticky. very nice to work with.

XPS is something like a styropor, used as insulation material. Basically air in solid form. It would reduce the amount of sand needed to build the form.

Hi Marvin,

Whilst recipes for various mixes should be made by weight, including water, for accuracy, their conversion to volume is the standard method.
Addition by volume is far easier than weighing all the ingredients when working on a job site. Often materials like sand and aggregate are damp or wet and this can create inaccuracies working by weight.The fibre additions I gave are not critical, so use the volumes I gave, but increase or decrease them somewhat if you want. The melt extract fibres (stainless needles) make working the mix difficult. Yo will certainly get a few needle stick injuries, usually not enough to draw blood, but enough to make them a nuisance. Because they are expensive and the AR fibre addition is also for strength you could leave out the needles and increase the AR fibres, which are great to work with. The pp fibres are difficult to disperse. They ball up like a cat fur ball if you try to mix them with the dry ingredients before adding water. I've found it better to pull them apart and add them in very small quantities as the water is added, then continue to mix them for about double the time that you'd think they're mixed in. Inspect the dispersal as you mix.
Chamotte (Shamot) is fired clay that has been ground up, also commonly called grog. Depending on the type of fired clay used, creates varying degrees of refractoriness (resitance to temperature).
I'm surprised it would be cheaper than sand. Un fired clay imparts a lot of shrinkage because the particles are so small. Once fired and ground the particles become vastly larger, reducing shrinkage of the mix on drying. But, as I said previously it will not add the stickiness in the way powdered clay does. It behaves like sand does in the same way when mixed with water.

A far cheaper (free) way of filling out the volume of the sand mould, is to use some polystyrene fruit boxes and pile the damp sand over them. They are really easy to remove, break them apart if necessary.
Buying XPS sounds like an unecessary expense. Forming the damp sand is pretty easy and if you want to get technical use dry sand and add 10% water by weight.

Dave

[MarvinG]

Hi Dave,

Thanks for the info. Finally I ordered my fibres online, they are not available here.
I now bought 1kg of 18mm pp fibres, and 3kg of 25mm ak glass fibres. I hope I chose the right lenght. there are options from 5-30mm.. so I thought longer is better?!
Going with your sugguestion, I will not use needles.

Good tipp regarding the fruit boxes, i'll def. do that. they are widely available here.

the next step is planning the details of the oven. my table size is 150x170cm of the trailor - so I hope to come up with a good design.
I'll keep you posted!

best,
Marvin

[MarvinG]

Meanwhile a question has arised:


- My trailor dimensions are 150cm wide, 170cm long (as a maximum)
- I'm planning to have maximum insulation, that means for sure 10cm. With perlite towards 15cm (I dont know how much Ceramic Planket I have left)
- For the floor I will have 6cm - the same what I have used in the past: Fire-Bricks. (not 8cm)
-> how thick should I mortar the oven? also around 6cm?
I mean I want a solid stability, but of course not too much mass which makes the oven heat up too slowly. Also if the trailor is not ridiculously heavy I would also be happy. But it should of course be suitable for festival production. What's a good balance?

Thanks for your opinions.

best,
Marvin

[david s]

Are you planning on building the dome with bricks or casting a dome? There have been reports of brick domes rattling to bits from road bumps and vibration. For this reason most mobile ovens seem to go for cast domes, although I have seen a brick mobile oven that does a lot of travel and is surviving ok. To avoid this problem you will need to fit shocks to the trailer springs as well as work out the trailer springs load capacity. Calculation of the oven weight will need to be done for this or you can do a guesstimate and maybe either replace the springs after you’ve completed it or fit some after market booster springs.
With blanket thickness of 100mm that’s plenty of insulation so the vermicrete layer over it does not need to be as insulating. A 5:1 mix layer 30mm thick would be enough to restore a good dome form and provide a firm substrate for the outer render.

[david s]

My trailer is of a similar size (1.46 X 1.75 m internal dimensions) with a single axle. So, I presume yours is also a single axle. Apart from the weight that has to be supported by the axle, the load on the drawbar is also more important for a single axle trailer than on a dual one. Also for safety, there is a danger of the load, which has its centre of gravity well above the axle, creating an accident causing the drawbar to elevate violently. To overcome this problem and to be able to adjust the balance point over the axle, I made a trolley that the oven sits in, held in place in the trailer with chains and turnbuckles allowing fine adjustment to the balance over the axle. This also allows me to roll the whole oven off the trailer, onto a steel stand and use the trailer for other purposes.
If you plan on doing festivals you will need to cook multiple pizzas and box them. My oven really only works best cooking one pizza at a time, although I can cook 2, if pressed. I can cater comfortably for a crowd of 40, prepping, cooking, cutting 2 min. pizzas on my own, provided everyone shares whatever comes out of the oven. No other catering method could come close. It’s a different story doing a festival where people buy a whole pizza. This also requires an extra hand which increases operating costs. To get around this problem I sell pizza by the piece. After doing 20 pizzas I’m a bit over it, so now only hire out my oven for others to operate. I don’t want to get sick of cooking pizza.