Castable WFO - Cure with insulation on?

[WoodywWun]

I cast my dome/gallery with homebrew yesterday and I'm planning to let it cure naturally for a week. I then plan a series of curing fires over a week, gradually building the size of the fire. The finished oven will have 50mm (2") of the thermal blanket and the same of 8:1 Vermicrete as insulation.

Should I light my curing fires with the insulation layers on (both?) or add these after my week of firing?

Any advice, thoughts or experience would be appreciated.

[SableSprings]

The general consensus is to put on insulation layer first and then start the curing fires process. The insulation helps moderate the extremes of heat and makes for a more even curing. Also, the vermicrete mixture for insulation is normally 10:1 but with 8:1 and a blanket you're probably golden. Remember...GO SLOW on that curing process. I'd actually just do the blanket, then cure, then add the vermicrete simply because I'd like to cure the casting first and not have the extra moisture of the vermicrete on top. Are you adding a moisture vent?

Good luck...kind of exciting when you get to this stage, but again...now's not the time to rush

[WoodywWun]

Mike,

Thanks for your reply. I'm happy to switch to a 10:1 Vermicrete mix (more insulation) and I'll hold fire on putting it on until the curing fire process is done. Are you an advocate of putting chicken wire or wire ties over the blanket to hold it down? OK so go easy, I will!

I used the sand-dome method for shaping things and covered the sand with wet newspapers. Unbeknown to me the wet paper did not go flat as the home-brew went on so I've a slightly ridged effect inside the dome in some areas. Also, whilst doing my best to get rid of air pockets, some have occurred, inside the dome, which are sufficiently deep to be concerning. Guessing that the newspaper ripple effect might not be a big problem but can I satisfactorily make-good the air pockets or will any attempt to fill these just result in the fill dropping out later? If I don't fill them satisfactorily will I potentially introduce a weakness?

I'm going to fit a moisture vent and I'm looking at options now. One suggestion has been a simple length of copper-tubing, bent with a plumber's spring at an angle of over 90 degrees so as it doesn't catch falling rain (which we do get now and again in the UK!). I see some higher-tech one-way valve ideas come forward but wonder if this might be over the top?

[david s]

You can fill the voids with some homebrew, but sieve a small quantity of the dry mix to take out coarse sand grains then add water to a peanut butter consistency. This must be done while the casting is still moist either too wet or too dry and it won’t work too well. The bent tube idea should work ok but wasps love to use narrow tubes to make nests so check occasionally to ensure they haven’t blocked it.

[SableSprings]

Mike,

Thanks for your reply. I'm happy to switch to a 10:1 Vermicrete mix (more insulation) and I'll hold fire on putting it on until the curing fire process is done. Are you an advocate of putting chicken wire or wire ties over the blanket to hold it down? OK so go easy, I will!
...
I'm going to fit a moisture vent and I'm looking at options now. One suggestion has been a simple length of copper-tubing, bent with a plumber's spring at an angle of over 90 degrees so as it doesn't catch falling rain (which we do get now and again in the UK!). I see some higher-tech one-way valve ideas come forward but wonder if this might be over the top?

I used chicken wire to give the outer coat of render something to grab onto and to keep it in place until after the curing. Others have been successful simply applying the outer render directly onto the batting after the curing process. Do wear gloves and a mask when working with the batting, the fibers come off easily and can be irritating or easily inhaled...

As David noted, bugs love access to tubes & insulation (as do squirrels!). Glue some insect screen over the bottom of any weep holes you have and attach some to the outside end of your venting tube. Some advantages of using the breather cap (from the auto supply store) over the copper tubing is that the cap is already bug/water proof and has a lower profile (height).

Hope that helps...looking forward to your build

[WoodywWun]

Thanks for the posts, I started filling the voids today, What a pain of a job reaching to the back of the dome. I ended up just grabbing handfuls of home-brew and spreading it with the palm of my hand or even fingertips. It seemed to stick well though. Smoothing with a wet paintbrush gave a good even finish. Wish I'd made myself more aware of the potential for this when I made the cast! My back should be OK in a day or two!

For the air vent through the final render coat, is this the sort of thing? I can't seem to find any automotive related vents here in the UK.

https://www.bes.co.uk/automatic-air-...EaApDzEALw_wcB

...I'm sure I can taste the pizza already!


Devon UK New Castable::
https://community.fornobravo.com/for...w-some-options

[SableSprings]

Here's a link to a thread with a lot of info on the automotive vent. The picture looks a little different in the thread than from your link, you might want to look at the following links and then talk to someone in an auto parts store. The most important factor is that you are looking for a very low pressure vent that will allow moisture to escape after the dome has been rendered (and not allow backflow).

https://community.fornobravo.com/for...ge2#post397592

Here's a another link to several types available in the US. Note that you should be looking at a relatively inexpensive item.

https://www.mscdirect.com/browse/tnp...CABEgI4f_D_BwE

I believe the Alemite, 1/8 Thread, 0.25 to 1 psi Relief Pressure, Steel, Zinc Plated, Relief Vent costs less than $3 US and is what Gulf used in his build.

Hope that helps.