refining preliminary design - home brew cast Neapolitan with a cantilevered vent

[CoastalPizza]

I've been following many of the build threads for the last couple of years and am now about ready to start my WFO. I want to thank all the folks here on this site - it is a rich source of information.

I have a basic design and am in the process of sourcing materials. My site has its challenges in that the hearth level is 6' above sea level in an area that is subject to hurricane-related flooding - at least I don't have any frost issues as water pipes in the area are typically only buried about 6".

The preliminary design is a 32" home brew cast Neapolitan with a cantilevered vent. I'll include 2% SS needle for strength, and it will be cast in place but with several seams (using clay slip per David S). I'll use a form for the lower veritical section and then sand (plenty of that around here!) for forming the dome. I really like the easy access of the commercial Neapolitan designs and hope to reproduce that by casting in place a set back vent arch that places the forward edge of the 6" chimney at the oven's front edge. Thus, the legs of the arch are behind the face of the oven and don't add any depth to the oven's entry.

I'm in Colorado right now and will likely source the hearth insulation before I head out to NC in late July. The plan is to use 3" of FoamGlas covered by ~1.5" of 6:1 perlcrete. The reason for the perlcrete is that the oven will likely be under water some time in the next 10 years so I wanted to stay away from CalSil after reading that it "gets mushy" when wet.

[david s]

That seems like a workable plan. I'm all for a shallow entry to provide better access to the oven, but one that does not have sides and is to be used in an outdoor situation, looks to me like a recipe for smoke to escape.
Regarding the calsil getting mushy, that is not my experience with the stuff. It gets wet, but remains solid. I guess there are many different kinds of calsil board. One of my ovens went through a cyclone about 8 years ago and was in fact totally under water from tidal surge. After drying out it still performed ok and is still in regular use.

[Gulf]

..................... I wanted to stay away from CalSil after reading that it "gets mushy" when wet.

If it is this post that I think that you are referring to, imo the jury is still out on exactly what type of insulation is pictured. I assure you that it is not any type of CalSil that I have ever seen. My experience with CalSil goes into the uses of it in industrial applications. The way that the post was tacked on to another thread, you would think that it was a complaint about FB's ceramic fiber board. Ceramic fiber board is an excellent floor insulation, however there are different types for different uses. The poster has not yet stated exactly what type of insulation is pictured, the source, nor has the poster produced a product data sheet so that we can compare apples to oranges .

[CoastalPizza]

That seems like a workable plan. I'm all for a shallow entry to provide better access to the oven, but one that does not have sides and is to be used in an outdoor situation, looks to me like a recipe for smoke to escape.

I’ll need to think about this, maybe adding back shallow side walls plus I’ll certainly need a plan for how to provide some added wind protection when needed.

Regarding the calsil getting mushy, that is not my experience with the stuff. It gets wet, but remains solid. I guess there are many different kinds of calsil board. One of my ovens went through a cyclone about 8 years ago and was in fact totally under water from tidal surge. After drying out it still performed ok and is still in regular use.

This is good to hear - from talking with my neighbor, the last time the water was higher than the planned hearth elevation was 1995. Not something that will happen every year, but something worth considering.

If it is this post that I think that you are referring to, imo the jury is still out on exactly what type of insulation is pictured. I assure you that it is not any type of CalSil that I have ever seen.

It was that post as well as these:

johnrbek: "“Not sure about cal sil board, but the insblok 19 seems to get pretty mushy when wet... I accidentally got my wet and was surprised at how mushy it was.. “"

UtahBeehiver: “"As you know CaSi gets mushy when it gets wet."

The CalSil that I was considering until I read the above was John Mansfield’s Thermo-12® Gold. After reading both of your comments it sounds like it will be just fine so I’'ll be headed down to Denver to Distribution International (formerly E.J. Bartells) to pick up Foamglas and Thermogold CalSil for the hearth. Thanks!

[CoastalPizza]

I made the drive from Longmont to Denver to pick up the FoamGlas and Thermo-12 Gold calsil. I got them at Distribution International (thankscnegrelli). I plan to cut off a small piece of the Thermo-12 Gold calsil and do some testing - I'll post the results with pictures back here. The price was really good - not more than 2x the cost of perlcrete given you'd need about 2x the thickness to get the equivalent insulation.

[UtahBeehiver]

As you are doing cast, take any surplus material and coat the exposed sides of the FoamGlas and CaSi. It will act as abrasion protection while you work on the oven.

[CoastalPizza]

I'll be using the DIY castable recipe from these forums: 3 sand : 1 portland cement : 1 type S lime : 1 fireclay - this is by volume, right?

I can get Type I portland cement (Lowes) or Type I/II (Home Depot) - I've read that some portland cement has added lime - is that the case for either of these? If there is added lime, should I adjust the ratios to bring it back to the 1:1 portland to lime ratio?

I have the stainless steel 'needles' - given the amount, I'll be able to add ~2% by weight. I've read about people using as much as 5% by weight - is 2% enough?

I also can get polypropylene fibers - is the recommended 1% by volume?

For the fireclay, I can get either Hawthorn bond, Lincoln 60, or Newman #7 --- is one any better than the other?

Thanks for all the help!

[david s]

I'll be using the DIY castable recipe from these forums: 3 sand : 1 portland cement : 1 type S lime : 1 fireclay - this is by volume, right?

I can get Type I portland cement (Lowes) or Type I/II (Home Depot) - I've read that some portland cement has added lime - is that the case for either of these? If there is added lime, should I adjust the ratios to bring it back to the 1:1 portland to lime ratio?

I have the stainless steel 'needles' - given the amount, I'll be able to add ~2% by weight. I've read about people using as much as 5% by weight - is 2% enough?

I also can get polypropylene fibers - is the recommended 1% by volume?

For the fireclay, I can get either Hawthorn bond, Lincoln 60, or Newman #7 --- is one any better than the other?

Thanks for all the help!

Yes recipe is by volume.
Get the cement that doesn't contain lime as you'll be adding it anyway. A product that already contains lime will throw out the recipe proportions. Not sure about the ones you mention, they're not available to me. Take a look on the bag.
Going up to 5% for ss fibres makes the mix more difficult to work. Stick with 2% (by weight)
Polypropelene fibre addition is 1% (by volume), but requires about double the mixing time to ensure proper dispersal.
Any fireclay should do. Any clay, for that matter should do, but avoid bentonite as the extremely fine particles promote shrinkage.

[CoastalPizza]

Here are the results of the hearth insulation tests.

When I poured water on the FoamGlas (FG) it beaded up on the surface - even after a few minutes, the water just wet the surface and really didn't penetrate. When I did the same thing to the Thermo-12 Gold calsil(CS) the water immediately soaked into the surface.

Next I cut a small piece of both the FG and CS. Each piece was weighted and then placed in a saucer filled with a small amount of water. After five minutes I weight them each again. They were then immersed in water (the FG needed some weight on top to hold down, the CS sank!) for another five minutes, then removed and weighted again. I let them sit on the counter for another 5 minutes (to see if they would drain) and then weighted them each for the last time.

For the saucer test where they were wet only from the bottom, the FS only absorbed about 0.02 grams of water per cm^3, contrast this with the CS that absorbed 0.8 grams of water per cm^3. Since water is 1.0 g/cc that would indicated that 80% of the volume of the CS is water filled - CS is basically a sponge!

Even after immersion, the FG only absorbed about 0.06 grams of water per cm^3, the CS only increased a few tenths of a gram indicating that it was already saturated. After allowing them to sit out on the counter, the FG lost 15% of its absorbed water and the CS only 2%.

The last test I did was a load test. I placed a full gallon jug on top of the CS - this is equivalent to about 10 PSI. The CS seemed to hold up well; not at all squishy or mushy.

[UtahBeehiver]

You would not expect the FoamGlas to absorb water since it is really aerated glass where a gas is injected into molten glass to aerated the product, hence the name FoamGlas. But your home science project is interesting. BTW, did you notice when cutting the FoamGlas it smells like rotten eggs which is the sulfur in the injection gas.

[CoastalPizza]

I just check with the local supplier for Whitacre Greer light duty buff firebricks (Palmetto Brick, Hampstead, NC) and they only carry the 9"x4"x1.25" size. Is the 1.25" thickness going to be too thin for the hearth? Should I double it up to get 2.5"?

[UtahBeehiver]

1.25" thick are known as splits. You will need to use regular 2.5" thick or double up splits in order to get the thermal mass you need for the floor.

[CoastalPizza]

Would adding a 1.25" thick concrete slab (using the DIY castable recipe w/o the SS needles) under these thinner bricks work as an alternative?

[UtahBeehiver]

Umm, good question, right of the top of my head, I would say it would, but I would like to see David S chime in on this one since he is our casting expert. I am assuming you are still using CaSi and FoamGlas underneath.

[CoastalPizza]

Yes, no changes to the sub-hearth insulation.