Make the bricks or "biscotti" floor oneself using fire clay

[ugnsnisse]

The floor is supposed to be heat resistant but not too conductive, The fire bricks commonly available here in Sweden are high alumina ones meaning they are great for a furnace but not so good for an hearth.

The italian made ovens use a pie shaped clay-based floor that is divided in to 4 pieces. They make the pieces by hand and fire them in a kiln. I'm thinking, why not make such a floor myself for the next build?

Why not just air dry them and put them into place and fire them in the kiln that oven is?

What I have in mind is fire clay that can be bought here at the cost of 0.7 euros€/kg, but what else should be in the mix to make a solid nice cake that will make it work excellent?

Sand? Steel needles? Chamotte? Lime? Any ideas??

[david s]

Good luck with your idea. It would be far simpler to use fire bricks. If you could successfully fire clay wares in a WFO don't you think that with a couple of thousand years experience that the Italians would have tried this method? It won't work and I'll tell you why. Controlling the temperature rise by firing with wood is very difficult and that temperature rise is crucial for unfired clay. It must be held down to 100 C/hr or steam spalling will occur below 300C. In addition in the range of 500 - 650C complex changes occur to the clay body and this stage must also be taken very slowly. Because a WFO, unlike a fully loaded kiln, is empty, the temperature rise fluctuates and because of the huge air intake it is difficult to get the thing over 500 C let alone 1000C (biscuit temp). Therfore, you will also need a digital pyrometer to measure this temperature rise. A heavily grogged clay body is also helpful to produce a suitable clay body with good thermal shock characteristics.
As an example of the difficulty you face, and evidence of what I'm talking about, I once fired a wood fired kiln that I had built and was not careful enough during the firing to keep its temperature rise slow and even (because I was teaching a classes at the same time). Compounding the problem was the fact that it was only partially loaded with wares. The result was that all the wares had fine cracks in them making them useless for glazing and further firing. If you want to try making your own floor sections it would be better to seek out a potter or a brickworks who may be able to fire them for you.

[ugnsnisse]

Thanks for the advice. You seem to know a few things about pottery.

I don't necessarily need to fire it in the oven, I'd rather find a potter that fires it for me if that is better for the result.

But what material should I use for the biscotti? Is pure clay the way to go? Clay contents below:


SiO2% 67

K2O% 3,5

CaO% 0,9

Fe2O3% 5,6

MgO% 1,6

Al2O3% 14,6

P2O3% 0,1

edit: fire bricks would indeed be the easiest way but we don't get the "medium duty" ones here. Other than for my first build when I happened upon them by chance, it isn't until now that I understand how lucky that was. Thought they were lower grade = worse in the long run but the pizza crust is excellent.

[david s]

Surely people in Sweden use fire bricks in their fireplaces. Do you have an aversion to high duty bricks if they're available? They would be quite suitable IMHO but harder to cut. If you are still set on making your own you could try 60% ball clay 40% grog mixed dry, then add water until you get a plastic consistency, then press into forms. One big problem is uneven drying which leads to warping so turn them regularly as they dry. Fire to 1100C.(there is evidence that fired at a lower temp gives better thermal shock resistance as in the Sth American pottery (fired to only 700 or so) which can be placed directly on a fire. But the lower firing leaves it softer. Maybe you can find out what temp the Italian biscotti ovens are fired to. Expect around 8% shrinkage from plastic to fired state so make your forms 8% bigger than required size. The smaller the units are the less likely they are to crack.

[ugnsnisse]

Have you got experience in baking directly on stones with high alumina content? (50-70% or more, high duty)

I don't but from what I've heard from folks who do, it is not good. But you seem certain it is.

High duty bricks are readily available here, but medium duty are not. Thats why I want an alternative for the floor.

I'm pretty sure it is possible to make some pieces just need to know what to use and as you already mentioned, the specific temperatures involved.

[david s]

Have you got experience in baking directly on stones with high alumina content? (50-70% or more, high duty)

I don't but from what I've heard from folks who do, it is not good. But you seem certain it is.

High duty bricks are readily available here, but medium duty are not. Thats why I want an alternative for the floor.

I'm pretty sure it is possible to make some pieces just need to know what to use and as you already mentioned, the specific temperatures involved.

I used high duty bricks for the first oven I built and never felt that they were a problem, cooked wonderful pizzas, bread etc.

[Toomulla]

if you want to build a WFO stick to what the majority have used successfully i.e fire brick. if your intention is experimental construction go for it but don't be suprised when the end result does not perform as well as the proven materials. we will follow your construction with interest