Buiding of a 60cm(24'') homebrew oven (NL). Vcrete question.

[Matthijs]

Wow, spend hours on this site already and still have tons of questions.

I started my pizza oven project (in the Netherlands). My idea is to logg my progress on this forum/topic.
So my plan (only the dome) in a nutshell:

• 60 cm (24”) internal diameter of dome (iglo and oven mouth in one part)
• 5 cm (2’’) homebrew (with Propylene fibres and Stainless Steel needles)
• insulation blanket
• 5 cm (2’’) Vermiculite

This on a table (~100cm/40 inch square) (walls from sand-lime brick) with lintels (stone 12cm/5’’)
On top of this I made a 11 cm / 4.4’’ Vermiculite thick layer. (in a wooden frame)
I used approx 140 liter (medium grain)vermiculite. Mixed in small batches with Portland cement in a 1:5 ratio. I added ~1.5 part water.
10 l Vermiculite + 2 l portland (mixed slightly) added ~3,5 liter water (slowly while mixing by hand) This mixed till a slightly dry (mudlike) substance was reached. I could make a ball with it that kept its form.
Than dumped the whole on the table (with some small holes for the steam). Continued until my frame was filled.

[Matthijs]

This was last week (7 days ago). I kept it dry the whole time, temperature was ~10deg C. Very wet weather..
Yesterday I removed the frame and noticed the Vcrete is not very hard. A bit crumbly. If I press my finger on the surface in some places I can press-in the surface ~1 mm.

I have the feeling I did something wrong.

Is It normal for vcrete to be like this?

[david s]

That sounds pretty normal. The vermicrete should be just strong enough to build on to. If you make it stronger by adding more cement then it doesn’t insulate very well.The weep holes should be through the supporting slab not through the insulating slab.
Try to allow the vermiculite to dry for a couple of weeks before building over it. That may be hard if you have bad weather.

[sergetania]

David knows so much more than I do but at least l can share my newbie experience. I really freaked out working with vkrete (mine was perlite concrete - my panic is documented in my build thread) and then seeing what it is when it dries. When they say it's very weak they are not kidding. Don't worry about it, just build on top. That's what normal is with that stuff. Honestly, I am not a fan but I guess it works well for certain things.

[Matthijs]

Stainless Steel Needles / melt extract fibers.

Thanks, you are right! it's already a bit more solid than 5 days ago. I had the chance to let it dry in the wind a bit. This seems to help
I Will keep it uncovered to give the water the change to evaporate. I'm thinking of covering the vcrete with lime morter when the dome is competed. (but the idea still needs to develop).

So, now I'm starting the preparations of the dome. I did quite a lot of searching on the 'stainless steel needles'.

In China there are plenty of suppliers for there concrete reinforment (stainless steel fibers only 4 euro / kg) but the shipping costs are very high. The are only willing to ship 5 kg to the Netherlands but by air (Express post, 70 euro shipping) Only If I order 1000 kg they will send this via sea.. So no luck there.

Now I understand these needles are not essential for the homebrew mix.

-Can someone confirm this? (and if possible provide a link to the discussion)

-Is an alternative for the Stainless Steel needles a option? (high carbon steel/ Glassfiber/ Ceramic)?

-Would the risk of cracks be lesser on a smaller dome?

[sergetania]

I have seen David often say that SS needles and not essential but PP needles are.
It's an interesting question about the risk for smaller structures. Quite possible it's less. For practice, I have made a small Japanese style charcoal grill (if interested search for "casting practice - an oven of other type" thread). No SS needles and in some places like the corners the walls are probably less than 1" thick. It's been fine,no cracking anywhere though I have only heated it up to around 500F. It's made of commercial castable refractory,not homebrew, but I don't know how much it matters.

[UtahBeehiver]

Although you are from the Netherlands, Nick JC did a cast build in the UK and used melt extract needles. He lists the supplier in his build so it might be a closer source for you.

[david s]

Randomly distributed stainless steel needles are the recommended reinforcement for castable refractory. If looking for a supplier their real name is "stainless melt extract fibres". Because they are so fine (around 0.5mm) and quite thermally conductive they dissipate their heat easily to the refractory surrounding them. This therefore does not create the expansion issues that thick rebar does in a rapidly heated refractory. It would also therefore increase the overall thermal conductivity of the casting little. Being stainless overcomes corrosion issues.I use them in conjunction with proprietary dense castable refractory because I want to make the strongest product I can. Because my ovens are small the amount required is not cost prohibitive, but most oven manufacturers do not use them, including FB, at least they don't specify that they're included and they would if they were. They are expensive and additionally heavy to freight. They are also difficult to handle. After more than 10 years using them I still get one stuck in my hands for approximately every second casting I make, but never deep enough to draw blood. A bitch to work with for the first time though. For these reasons their use with home-brew which is a cheaper alternative than proprietary castable is highly questionable. The use of the burnout polypropylene fibres however, is far more important.

[Matthijs]

Ah.. Thanks this is very useful !
I have the link Nick JC shared. Still >30 euro for 3 kg (excl shipping). 40/50 euro is quite some money. (especially for Dutchies)

So Non-stainless steal is not okay and a thick reinforcing material is not okay.

Is the length of the fibers very important? I did found some very small 1 cm long (0.3 mm thick) Stainless. I guess they will provide only little reformcent strength.

How about Glass fiber? This is heat resistant, cheaper, better available an not as painful to work with. Or do you think the lesser thermal conductivity and differences in expansion cause issues?

I also found these ceramic (ziconium) needles 0.9 mm (I attach a photo and data sheet)

If this will not work out I continue without the melting fibers. (only apply the PP fibers)

[david s]

AR glass fibres are OK to use for our purposes as glass has a melting point of around 900C which is well in excess of the temperature range we cook in. AR means alkaline resistant and they are coated with zirconium to provide the corrosion resistance required. As zirconium is expensive and the fibres are likely to have been made in China, the amount used varies considerably, so be wary of a cheap price and purchase from a reputable dealer. Look for concrete countertop suppliers. I've been using them in conjunction with stainless fibres and proprietary castable refractory for around three years with good results. There is good evidence for their use in this application.(see attached paper)

Glass fibres in refractory.pdf

[Matthijs]

Thanks for the lecture. Sound like a good substitute. I found 5 kg AR glass fibers for 40 euro. This is enough for a lot of ovens.

[Matthijs]

I would like to lay the oven floor in a 50% fire clay and 50% sand mix. This on top of the vermiculite floor. I can not find out if this mix is a dry mix OR a mix (pasta) with some water included?
What is advised Dry or Wet?

Also "fine sand" would this be silver sand or would just "standard" sand (we call it Metselzand in NL)?

[david s]

You can do it either dry or wet, but IMO it’s better dry to allow movement of the floor bricks. If done wet the mix will act like mortar. Additionally if done dry the removal of a spalled or cracked floor brick is far easier. Opinions and experience may differ though, happy to hear to the contrary.

[Matthijs]

I know about the 'rule' 63% for the door height. How about the door width?

The inner dome diameter is 60 cm (23,6''). I planned the door width on 40 cm (15,7'') (17,5 cm high).

- Would this width be okay?

I understand too wide would cause in too big heath lose. Too small; no room for working.

[sergetania]

I will let David answer about potential heat loss. I have 20" inch wide opening and it's nice and wide (on 30" dome). Thinking about it, anything narrower than 16" would be very very tight. You will probably have to cut a loading peel to 12-13" wide and make it very short (assuming pizza size 12" - downsizing to 10" makes sense with a smaller oven).

One thing I would change in my build is the top curve of the entrance. I suspect because mine is not very curvy I get some smoke coming out of the front of the oven and not through the chimney. The smoke escapes the chimney opening in the center (which I should have also made wider). Am I making sense? I think making the top of the entrance more round and chimney opening wider would help the smoke problem. Not sure how concerned you are about getting some soot on the facade. It's not that hard to wipe the soot off with a soot cleaner so not the end of the world. Then again, making the top more curvy will further restrict the entrance because you will have to tilt the peel up and pull it back. It will be tight. Can you cut out the shape of the opening from a cardboard and see if you can operate through it?