Neapolitan sand-salt mix under the oven floor?

[pacoast]

I've worked out most of the details on my next oven build, which is going to be a Neapolitan type oven, built as authentically as possible. This is not for a commercial venture, so it's out of my budget to hire some masons from Napoli to come build it for me & remains a DIY project.

I'm interested in the traditional "insulation" or bedding that Napoli builders use under the terra cotta cooking floor. Which is a sand-salt mixture, perhaps with ash or crushed volcanic glass as well. Yes, I know that modern insulation like CaSi boards are superior insulators. And a better way to go for most ovens.

But in the interests of doing this build the traditional, old fashioned way, does anyone here know more about the composition of this bedding or how it is traditionally mixed? Presumably the sand is not intended to be an ideal insulator, but rather to slowly absorb some heat. It might increase the oven warm up times, but still absorbs heat slowly enough to not really cool a fully heated floor. But enough that it can help moderate temperature swings in the floor and possibly even help recharge the floor temperature from the bottom when you are making a lot of pizzas. I've also heard that something similar was traditionally used in Spain (& probably a lot of other places) as well.

Any thoughts on how this might be optimized? Or has anyone here seen the floor of an authentic Neapolitan oven being laid?

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[david s]

Re: Neapolitan sand-salt mix under the oven floor?

Anything that will create lots of air spaces would work. I think that is the theory behind the broken glass idea and probably the same for the salt if you used rock salt. Sand would also create spaces between the grains. The denser the material the poorer the insulation. Sand salt and glass are all way heavier than vermiculite or perlite. I think folk just used whatever was available and cheap. We now have access to all kinds of materials that the ancients never had. It would be a pity if you found that your oven lost heat out of the floor easily because it will be impossible, or extremely difficult to replace your floor insulation once your oven has been built. Stick with vermiculite or perlite

[ugnsnisse]

I'm also interested in the theory of using salt. They use it as a layer between the terra cotta floor and bricks underneath. My guess is that it helps to distribute heat from the magazine that is the bricks. What happens if water leaks in though?
Now that I think of it I'll be using rebar and metal in my current oven so involving salt would be in unnecessary risk. I'll level my floor bricks with a centimeter of 0-2 mm regular sand.
Underneath the floor I've got 15 cm of leca/cement mix.

[david s]

Salt is used in solar thermal plants to store heat, so no it's not a particularly good insulator. Where we live salt will take up water from the atmosphere which turns it to mush. Folk often put rice in the salt shaker in an effort to tkeep the salt dry and help it flow. But even that doesn't work when the humidity is over 90%. Sand, glass and salt were used by people who didn't have access to better materials. Here's a useful resource.
http://www.engineeringtoolbox.com/th...ity-d_429.html

On using rebar in your build I'd advise against it as its way more conductive than any refractory that surrounds it, leading to greater expansion with fast heat rise and resulting in cracking. As heat also accelerates corrosion only stainless should be used. The standard reinforcing for castable refractory is stainless steel needles, which can conduct their heat to the surrounding refractory more easily because of the greater surface area they present. If rebar worked, industry would use it, and they don't.