Mobile Oven Dome in MIchigan

[szv9n5]

Let me first start by thanking the contributing members of this board....you guys are an unbelievable resource for so many. You just don’t see that on many boards these days. Thanks for all the help.

i am starting work on my second oven. The first was a brick dome build on cinder block frame on my patio. Now, I want to build an oven on a trailer so I can take it to events like tailgating. I have read many posts on this site and gathered tons of info but still have a couple I need help with....

Question 1: I assume because this is being built on pull behind trailer a cast dome would be better than a brick dome? I auromatically assumed the brinks would rattle loose .....do you guys agree?

[Karangi Dude]

szv9n5 welcome to the forum

To me it was a matter of weight and heat up time, cast ovens weigh a lot less and also heat up much faster than solid brick ovens

See my trailer build https://community.fornobravo.com/for...-s-mobile-oven

Cheers Doug

[david s]

I’ve heard reports of brick ovens rattling to bits and it doesn’t surprise me. Mine takes a pounding if I go off the bitumen. My first mobile oven was a one piece dome, but when I rebuilt it I made the dome castings in three pieces. It’s standing up pretty well. Some manufacturers offer cast ovens in one piece specifically for mobiles

[szv9n5]

Ok... cast it is.

after reading many posts on the topic of home brew vs proprietary.....I decided to buy this castable from Harrison Walker. Heck of a time to ask as I have already purchased it but any thoughts on this stuff? Anyone use it before. Data sheet attached....

[szv9n5]

Can I add the stainless needles and burnout fibers to this?

[david s]

That's good stuff. Most castable refractories already have the burn out fibres in the mix. To make sure they have take a sieve and sieve out the finer particles and you should see some very fine fibres left behind. It is important that these fibres are dispersed in the mix well, so mix it more than you would think necessary, but not for too long as the stuff goes off very quickly. If it's hot weather use chilled water it is very temperature dependent. so do not mix more than half a 25 kg bag in one batch and be sure to wash out your barrow and tools after each batch, otherwise it will accelerate the next batch. Stainless needles (not absolutely necessary) should be added min 2% by weight of dry material. They are not called needles for nothing so if you add them just handle the mix gently to avoid stick injuries.

[szv9n5]

The cure/dry out schedule for this product appears to be :
24 hours at 72 degrees
then increase temp 100 degrees per hour until 1,200 degrees is reached
hold 1,200 degrees for 1 hour for each inch of thickness (2.5 hours in my case).
then cool at a rate of -100 degrees per hour until room temp.

i have Understood a slow cure is better...keep it damp for 10 days. Then several fires starting very small and growing in temp over another 10 days. Is the slow cure only needed with the home brew because of the portland? Should I trust the product data sheet and cure and dry it in two days. It goes against most of what I have read....

[david s]

Here’s the recommended schedule for that product.
Typical dryout schedule for a single layer, 9” thick or less:

• Ambient to 250F at 75F per hour. Hold at 250F 1/2 hour per inch thickness.
• 250F to 500F at 75F per hour. Hold at 500F 1/2 hour per inch thickness.
• 500F to 1000F at 75F per hour. Hold at 1000F 1/2 hour per inch thickness
• 1000F to use temperature 75F per hour

It doesn’t matter if the castable is a CAC based one or homebrew, it is the slow elimination of mechanical water (steam) that is imperative. Suddenly expanding water will cause steam spalling splitting the walls of your casting neatly from the centre of the walls.Typically this will occur at around 250-300 C. The presence of water will also hold the temperature down in that area so it is important to try to dry the oven out evenly, otherwise uneven temperature will create uneven expansion which leads to stress.This is practically impossible with the crown drying off first so try not to allow flame impingement on the dome for the first few fires. CAC requires only 24 hrs to reach its full strength (give it two to be safe) so damp curing as would be done with anything contains Portland cement is unnecessary. Allowing the oven to cool off each day returns the whole structure back to an even temperature again.Further techemical and physical changes take place north of 600 C but as you won’t get the oven that hot it’s not necessary, although the ingredients in the castable allow the material to reach double that temperature. The above firing schedule refers to a more controllable heat source like gas. For a WFO the 7 fires in 7 days, getting progressively bigger is a better approach.

[szv9n5]

So, I made my sand form for my dome today. I started to cast the dome but I only got about half done. Meaning....the finished dome is planned to be 2 1/2” thick and I only got about an inch covering the whole dome. I’m assuming it is best to do the entire dome cast at one time. But how bad is it to do it over two days? I did scratch the surface so that the next layer will have something to bite into....

[szv9n5]

I built my sand form today for my 44” dome. I had trouble with it because of repeated collapses. I finally built it in layers of about 7” each and between each layer I laid a sheet of window screen. This worked perfectly. Just sharing In case anyone has this problem....

[szv9n5]

I would like to put a thermal break between my dome cast and the landing. I plan to put a piece of cardboard between the two when I pour my landing cast. How wide does this gap need to be? 1/16 or 1/8 inch?

[david s]

At 400 C the thermal expansion is in the region of 0.3%, so for a 1.0 m (36”) oven that’s 3mm (1/8”)

[szv9n5]

I plan to put a couple of inches of vermiculite over my blanket insulation. I understand the formula is 10 parts Vermiculie, 1 part Portland cement and water.

1). Will this proctuct be workable to apply and shape with a trowel?
2). I plan to get my Vermiculite from Uline....they have different sizes going from fine to course (see picture attached). I am guessing I should use the course as I would think it would have the best insulating properties?
3). Will this give me a good rough finish that I can apply the stucco to or do I need to rake it or apply lath before the stucco layers?
4) Any suggestions on how I estimate how much vermiculite I will need? Dome is 49” outside diameter plus 2” of blanket to total outside diameter is about 53”. It is sold in 4 cu ft bags.

Thanks everyone!

-Steve

[david s]

1. The leaner you make it the less workable it becomes. I find 10:1 about as lean as you can go. If you add a handful of powdered clay for every litre of cement it will assist in making it more workable. The correct water addition is essential. For every 10 parts vermiculite add 3 parts water. I find it best to apply by hand (wear gloves), but use the flat of the trowel to tap gently the surface flat once you’ve finished.
2. Course is less workable than fine, but fine requires more water. So a good compromise is medium grade.
3. You will be convinced that it will be inadequate, but it dries nice and firm and makes a good substrate to stucco on to.
4. Work out the volume of the layer using 4/3 Pi x r3. Subtract inner volume from outer volume and divide by 2 because it’s a hemisphere. You will get approx 20% reduction in volume of vermicrete when you add cement and water to the vermiculite.

[szv9n5]

Thanks David.... very helpful!