Alan Scott oven project begins

[david s]

Apologies, I think it may be that my Microsoft Office is out of date and it won't let me export docs. I'm feeding it money, hope that will fix it. Get back to you in a few days.

While i'm here this thread of an Alan Scott oven should be of interest to you.
#1

[astroinfidel]

David it opens as code..gibberish to this code Luddite. I tried notepad and Explorer to open it with the same results.

Here are a few pics of the beginning of the walls this afternoon. The Heat Stop 50 is actually very nice stuff to work with. I laid the first 4 angled bricks horizontally on the top of the foundation wall. I let them set up, then turned them 90 degrees and laid the 4 bricks as one big brick. I was going to use that method for all the walls, to avoid having to try to butter the vertical surfaces. But the Heat Stop 50 sticks so nicely to the vertical brick faces that even a rookie like me has no trouble buttering them. So I will do the rest of the wall bricks one at a time using a line as in the pic.

Dean

[david s]

Try this

Vermicrete insulating slab copy.doc.zip

[astroinfidel]

Walls are laid out, and I will start mortaring them into place tomorrow. I have a rather unconventional plan for doing so that I am hoping is going to let this rookie bricklayer get nice thin joints, and a nice straight wall. More to come...

Dean

[astroinfidel]

And David the Perlite slab wept water for 2 days. I kept it wet and covered for 7 days, and will let it start slowly drying out now, simply because I want to start working on the walls and will have it uncovered for most of the day. I of course can't dry the Perlite part of the slab without also drying the hearth slab, and I am in no hurry to have the hearth slab lose water any faster than necessary. I will probably try to keep it from drying out completely for a few more weeks to let it gain maximum strength. I can't soak it with the hose anymore though, as the hearth bricks are laid on the fireclay/sand mix, and that hasn't even begun to dry yet either. No rush. Now that I am not keeping it covered, it will start to dry faster. I will cover it before rain though. By the time I am ready to use the oven the Perlite part of the slab will likely be close to dry, and some gentle firings should take care of driving out the remaining moisture.
And unfortunately I wasn't able to open that document you linked to. Likely my own lack of office tool knowledge...
Dean

[astroinfidel]

Hi David,
Yes it was articles like you provided about Portland losing strength that caused me to consider alumina concrete instead. But alumina concrete only appears to be beneficial above 1500F, and I am hoping to keep the temperature of the slab less than 1000F, and ideally not much over 800F. This will require one or more long slow firings, to pump the heat gently into the slab rather than quickly. I used a cement to sand/aggregate ratio of 1 to 3, and poured at between a 75 and 100 mm slump. Combined with the crushed granite only aggregate, this should make for some pretty strong concrete. Granite is not the very best, but is better than most for lack of expansion as it heats. And I poured the slab at 4.5 inches instead of 3.5 inches. This should give me 28% more resistance to flexing. So I am hoping that I can stand to lose 50% or even more of the strength of the slab and still have enough. I am not hearing of people having problems with the slab with this kind of oven, so it must look worse on paper than it works in reality. Or so I am hoping.
Dean

[david s]

One big drawback of casting your own vermicrete slab is that it takes a very long time to dry it out by firing the oven. Builders have reported that it takes months to eliminate and their ovens continue to improve long after they think it must be dry. Water in insulation reduces its ability to insulate because it is so much more conductive than air. If possible allowing it to remain exposed and allowing sun and wind to do much of the drying for you makes the process easier as does adding weep holes under the insulation for moisture to escape once you do start the drying fires.
I did a little experiment to test the water elimination by natural drying that you might find instructive. Probably too late if you have already built over it, so don't expect the oven performance to be as expected for some time.

Vermicrete insulating slab copy.doc.zip

[david s]

Portland cement is generally considered unsuitable and confirmed in practice to fail at the temperatures we use.

https://www.concreteconstruction.net...temperatures_o

https://www.civil-engg-world.com/200...-concrete.html

Regarding aggregates this is pretty good information
https://link.springer.com/article/10...069-016-0175-2

“When the temperature exceeded 150 °C, Portland cement paste contracted with increasing temperature. The thermal shrinkage of the cement paste and expansion of aggregates have potentially resulted in thermal stresses in the cement pastes causing micro-cracking. “

[astroinfidel]

Here are a few more pics, including a dry run with no mortar. I have just set the hearth bricks in a 50/50 fireclay/sand mixture, and will soon start the walls. I did not pour the slab with alumina cement after all. The day before while doing some last minute reading I found the following on an engineering site:

"High alumina cement concrete is one of the foremost refractory materials but its performance varies with the range of temperature. Between room temperature and about 500°C, concrete made with high alumina cement loses strength more than concrete made with Portland cement, then up to 800°C the two concretes are comparable but above 1000°C, high alumina cement gives excellent performance."

Here is the link: http://www.engineeringenotes.com/con...chnology/31082

So it appears at the temperatures that the slab is likely to encounter, Portland cement is as good or better than alumina cement. And alumina cement starts out weaker than Portland to begin with. And after reading about the process of "conversion" that degrades and weakens alumina concrete, I decided to stay with Portland. As aggregate I am using only crushed granite to reduce or hopefully eliminate spalling. Not as good as traprock or some other igneous rocks, but hopefully will be able to take the heat that it will see in the slab and cladding.

I poured 5 3/4 inches of Perlite concrete insulation, then 4 1/2 inches of granite aggregate concrete for the hearth slab. That gives me a cool 9 inches of thermal mass when I include the 4 1/2 inch firebricks on their side. That should store enough heat for a loaf or two.

Dean

[astroinfidel]

Hi Mike,
And sorry to be so slow getting back to the thread myself. My cousin doesn't have his oven permanently set up yet, and it is just the slab with the oven on some stacked blocks. I think he has 2 inches of ceramic insulation under the hearth firebricks, but then I think it is just the thick concrete slab. So this may be robbing him of some heat. He also does not have a very efficient door, so that likely doesn't help either. I think he has 2 inches of ceramic blanket and a few inches of Perlite concrete on top, and that is all.
I have been busy with the oven build. Here are 6 pics and I'll follow with 6 more in a new post as 6 seems to be the limit per post.

And I love your temperature data! I will be installing two thermocouples in the dome cladding so I can speed the learning curve and just geek out keeping track of it. .
Dean

[SableSprings]

Hi JR pizza,

I guess how much mass is considered excessive depends on a few things. I have free unlimited firewood, and I am going to be baking mostly bread. I think if you are going to be mainly using the oven for pizza the hearth mass would be considered excessive. But I am building this for bread, and for the long term. It is located on land that has been in my family for over 100 years, and I hope it is still being used here for baking in another hundred or two. I likely won't use it commercially, but I want the option of someone being able to if so desired sometime in the future. And I love the idea of firing it once for the day for bread, and then being able to cook dinner without refiring it days later. So I will be happy to have the hearth mass. I am retired and have lots of free wood and the time to let it heat up, so it just isn't an issue for me.

I have been using my cousin's Pompeii style oven that he built from FB plans. It is a big one..48 inches, and when I fill it and get good steam from the loaves, it produces fantastic bread. But even when fired for about 5 hours, I can't get a second bake out of it. I am down to 400 F by the time the first batch comes out, and it doesn't recover. So for my purposes, I did not find it had enough thermal mass.

Dean

Welcome Dean! Sorry to come in a little late to this thread, but I do find it interesting that your cousin's oven doesn't store enough heat for a second bread bake. I have a 39" Pompeii style oven with 3.5-4" of 5:1 perlcrete insulation under my cooking floor and normally bake about 20 loaves (in 3-4 loads). I have a full firebrick cooking floor and wish I could have put in the superior CaSi insulation board instead of the perlcrete. I bake my baguettes at 575F and finish with a load of whole wheat type breads at 475-500F an hour or so later. I now fire my oven late in the evening (overnight-slow burn/reduced draft) before baking and clear any remaining coals/ash mid-morning the next day. The dome is always cleared by morning and I usually have to open the oven for a time to cool it down to bread baking temps. That cool down period for me can take 1-2 hours (low draft) and I'm sure you will have to work out a firing schedule that allows for the cool down (and includes the seasonal outside air temps ). I've attached a temp profile below from my first method of firing in the morning of bake day, where I don't believe I was saturating my oven bricks to full thickness. I think you will want to take similar readings of your cooking floor after the oven has been cured to develop a similar temp graph to settle on your cooking/baking windows.

Normally, I throw in a chicken or dutch oven full of potatoes & onions after bread bakes and have had no problems having a fine meal from them. I know that others with much better insulation (the newer materials) and normal full brick thickness, have reported cooking temps several days after initial firing. Insulation & an insulated door are MAJOR factors in retaining heat in any WFO.

As others have said, I'm looking forward to following your build!

[JRPizza]

It's awesome that you are going into your build eyes wide open. We will all watch your build with great interest and hopefully you will continue to post/contribute after the oven is in use so we can all learn about how oven's perform with copious amounts of thermal mass .

[astroinfidel]

Hi Joe,

I just happen to be at a time and place in life where I have unlimited firewood, and lots of time at hand. That changes the equation a bit I think. Let it take 10 hours to saturate! From what I am reading of those who have built this oven, they are able to do 6 loads of 10 to 12 loaves of bread on one firing once saturated, then use it for cooking dinner days later, and as a slow cooker later still. Without the luxury of time and lots of wood literally laying around, the thermal mass I am building to probably wouldn't make as much sense. I also like the idea of a family member of the future having the option to use the oven for a small bakery if desired. So bring on the concrete and thermal mass.
And Russell, I have asked for a quote from the manufacturer of Thermo Gold 12 to cover the 56x48 inch under hearth area that I need. I asked for a quote for 4 inches thick, and told them I want to pour a 4 inch slab over it for a hearth for a bread oven. I'll let you know what they say when I hear back from them. And yes I expect it to be a lot pricier than vermiculite and cement!

Dean

[Gulf]

Dean,

The bricks will be on edge, and the plans do call for refractory mortar for the hearth slab. What exactly do you feel I need to rethink?

Imo 8" of floor seems like a bodacious amount of real estate to heat saturate. But, it sounds like you've got this. Let us know how it performs.

[astroinfidel]

Hello Joe,

The bricks will be on edge, and the plans do call for refractory mortar for the hearth slab. What exactly do you feel I need to rethink?

Dean