38" Pompeii build

[david s]

I use a similar method using a template. The whole back of the oven is then pretty much covered in one single piece, using the off-cut triangles to fill any gaps at the base. Very little wastage and the blanket sits in place by itself.

[Frenchie]

Very nice, david s!

[Frenchie]

I managed to find a couple of hours a couple of days ago, wired the insulation, fitted mushroom cowl, and first layer vermicrete. Since lit a hotter curing fire (dome getting to 250C now).

JRPizzas suggestion about lath was good, but the lead time was long for me by mail order and I already had chicken wire and vermiculite so I just cracked on with it.

feels like progress.

vermicrete is weird stuff isn’t it! Got a bunch more to do.

[david s]

Yes it is weird stuff. Because perlite and vermiculite are so hygroscopic, a large amount of water is required to make up a workable mix. I usually use 4 litres of water for every 10 litres of vermiculite/perlite, but the amount required does also depend on the grade used (finer grades require more water) as well as the amount of moisture thats already contained in the dry material. The drying time is also dependant on how thick the layer is, but when it has dried it will go white and appear to be dry, but be careful because while it may appear dry on the surface it will be moist deeper in. Judging from your pic, yours still appears moist. If the temperature gets higher, say around 350 C internal, the layer will sometimes crack from the expanding steam. Try throwing a sheet of plastic over the oven during your drying fires to see if there's any condensation on the underside of the plastic. I prefer to allow sun and wind to do much of the drying and for a layer of around 40mm thick this usually takes a week, weather dependant. Sun and wind will dry the layer from the outer surface while drying fires will push moisture out from the inside to the outer surface.

Your oven looks fabulous and I understand your enthusiasm to get it completed.

[Giovanni Rossi]

Thank-you for this reply, good insights and your approach sounds like something I will replicate - although I only managed 3" blanket vs your 4 - 2" of P/V'crete on top sounds achievable, both in terms of the materials to hand and space remaining on the slab! Out of interest, how long did you wait between pericrete layers? Given the rapid decline in temperatures and availability of dry days, it would be good to get all the work done in 1 day, although it is perhaps more realistic to expect to lay 2 layers is 1 day and not 3!

Sorry, I've been out of country and connection challenged. You're probably finished by now but, you just need to wait for the layer to cure enough to provide a solid surface for the next one. I didn't have time constraints so I did a layer per day.

[david s]

Because both perlite and vermiculite are so hygroscopic, to get a workable mix it requires around double the quantity of water than the hydration process will use (depending on the grade used appropriately 4 litres of water for every 10 litres of vermiculite or perlite) so if you used a 100 litre bag there will be 40 litres of water added, which still leaves 20 litres of free water to be eliminated. Any water at the surface will dry fairly quickly which is indicated by the layer turning white, usually after around 3 days ,in ideal conditions. However, moisture deeper in will still remain. A similar experience is observable with a pile of sand left outside. (in ideal drying conditions). After a week the top inch will be dry, but the rest remains damp. After a month the next inch may be dry but the rest damp. The deeper in the greater the difficulty in removing moisture. This is the reason for applying the stuff in layers with drying periods in between. You haven’t posted how thick the layers that you applied are, or the daily weather conditions so it’s difficult to tell. Try the plastic sheet over the oven during firing to test for the presence of retained moisture. The heat from within will eventually push the moisture out, but this can take months of firings.