The Hearth Slab

So, like I said in the last post think of the oven as a chamber made of brick wrapped in a layer of thick concrete wrapped in a layer of insulation. The brick and concrete layers are what hold the heat from the fire and then radiate it back into the chamber for the bread baking.
The hearth slab is the bottom of this concrete layer and the platform on which the oven itself is built. It is made from a concrete made of the high temp "ciment fondu" mixed with sand and 1/4" crushed rock to make a high temp concrete. It is reinforced and in fact, held up by a grid of 5/8" re-bar. The slab does not extend all the way to the walls but "floats" suspended by the re-bar supported on the outer foundation walls. This construction allows for the oven to expand when heated without cracking the foundation walls.
We built forms from 1x6 lumber that had slots cut out so that it would fit over the re-bar. Here Sarah is cutting out the slots.





Here's the form in place ready for the pour.







We rented a small concrete mixer although I could have used a big one. We would mix up a batch, dump it into a mortar pan and then heave it up and tip the mortar pan into the form, over and over again. It was exhausting.

The next day after the forms were removed I filled the gap with the vermiculite/ cement mixture to prevent junk from falling into it in the meantime.

Insulation Layer

I was eager to get going now that I finally was in possession of the cement.
The oven can be thought of as a brick chamber surrounded by a thick layer of high temperature concrete and a blanket of lightweight insulation.
The insulation consists primarily of vermiculite. Vermiculite is a mineral that when heated up expands into puffy worm like (thus the "verm-") particles, sort of like rice krispies. It is also used as a soil additive to loosen up clay soils or retain water in sandy ones. Once the oven is complete loose vermiculite is poured over the oven as a loose fill insulation. Beneath the oven it has to adhere to itself and the bottom of the oven and so is mixed with a bit of portland cement (the primary binder in concrete) and poured onto the forms that carrie helped me make last time i did any real work on the oven.
Before the vermiculite is poured 6x6 mesh is put down with some of the struts bent up. The mesh will help to hold the vermiculite layer up. The bent struts will hold fast into the concrete layer above it.
Once the mesh is in place the vermiculite cement layer is poured wall to wall to a depth of about 3 inches.
Sarah (my sister) regards the pour of the first batch of vermiculite as we both realize that this is going to take a while.

Cement!

Finally, the Calcium Aluminate Cement!
On my way back from finishing my trucking job I stopped by Kerneos in Chesapeake Virginia, importer of Ciment Fondu, a high temp calcium aluminate cement that will make up the bulk of the "thermal mass' of this retained heat oven.
I trucked it up the Delmarva and up the driveway. Finally!

Uggh

I wish I had more to post here but last week when I called up Lynn at Kerneos (the north American distributor of LaFarge's Ciment Fondu (the high temp cement)) she said that since it was the end of the month they did not have any pallets that they could split(??) and so we must wait, til when I do not know. One of these days we'll get that god damn cement and finish this oven, one of these days.