39 inch - 1000mm corner brick WFO build - Newcastle au

[david s]

Hi David, The percrete would be approx 50mm (2 inches) thick with another 10-15mm of builders render over. So far I have done 7 days of 3-3.5hr curing fires with gas burner on lowish flame, plus 2 wood fired curing fires of 3.5 hours so far. Hoping after a few more days of curing fires getting slightly bigger each time the moisture should be driven out of the oven. Thanks for the tip re plastic during firing to check moisture. Will test before sealing dome.

I'm not sure how much perlite you ended up using but at a guess of 3 X 100 litre bags, that would be around 120 litres of water added to the mix. About half will be chemically used up in the hydration process as it chemically combines with the cement, but it still leaves about 60 litres which is a lot to dry out. Covering it with a cement render reduces its ability for theeither the weather or heat from the fires to push it out. You will eventually get it dry, but it may take a long time. You should follow the instructions provided by the manufacturer of your kit. As an explanation of the drying of a p/v crete slab, read the attached experiment, which relates to a 5:1 underfloor slab, but the same principle applies. Water in the insulation layers drastically reduce the insulating ability as heat is easily conducted by moisture. If the outer surface of the oven's rendered shell feels hot then there's moisture there. It should only ever feel barely warm. With experience you'll find a hand held to the outside a particularly good indicator.

Vermicrete insulating slab copy.doc.zip

[Newie newby]

Hi David, The percrete would be approx 50mm (2 inches) thick with another 10-15mm of builders render over. So far I have done 7 days of 3-3.5hr curing fires with gas burner on lowish flame, plus 2 wood fired curing fires of 3.5 hours so far. Hoping after a few more days of curing fires getting slightly bigger each time the moisture should be driven out of the oven. Thanks for the tip re plastic during firing to check moisture. Will test before sealing dome.

[david s]

I ended up getting a few coats of the perlite/mortar mix on and then straight mortar mix over the dome to get a slightly better finish. It's still a bit rough, but after the curing process I am going to paint / roll on a few coats of acrylic render which should hide most of the rough bits. The weather this last week has been against me with storms and light rain, so have done curing fires for around 3-3.5 hours each day with my gas burner on low. Got myself an infrared thermometer which is handy for checking temps. When weather clears, small curing fires with wood will begin! I also ended up adding a small steam vent on the side of the dome with some left over plumbing fittings and a brass threaded cap. The fittings go in as far as the insulation blanket and I wired onto the chicken mesh then rendered around to hold it in.

Not sure how thick your perlcrete layer is, but it does take a lot of water to make up the mix. Obviously the thicker it is the more difficulty moisture has of escaping to the surface. I suggest you delay the sealing up of the dome until all moisture has been purged. Try throwing some plastic sheeting over the dome during firing to test if there’s still moisture condensing under it.

[Newie newby]

First cook up in the oven too, some chicken thighs and veges, came up a treat. Looking forward to lots more tasty meals!!

[Newie newby]

First timber curing fire last night, will do a few more over next couple of days and then get the dome sealed up.

[Newie newby]

Weather has cleared so the temporary awning has been put away and the chimney installed. Ready to light a curing fire..

[Newie newby]

I ended up getting a few coats of the perlite/mortar mix on and then straight mortar mix over the dome to get a slightly better finish. It's still a bit rough, but after the curing process I am going to paint / roll on a few coats of acrylic render which should hide most of the rough bits. The weather this last week has been against me with storms and light rain, so have done curing fires for around 3-3.5 hours each day with my gas burner on low. Got myself an infrared thermometer which is handy for checking temps. When weather clears, small curing fires with wood will begin! I also ended up adding a small steam vent on the side of the dome with some left over plumbing fittings and a brass threaded cap. The fittings go in as far as the insulation blanket and I wired onto the chicken mesh then rendered around to hold it in.

[david s]

Ok, if your layer is between 1mm and 25mm thick, then it will dry really fast, maybe in a few days.
FWIW I like to maximise either strength or insulation in particular layers, you can't have both.
Once the p/v layers are done and dry and the 7 fires in 7 days completed, I cover the insulation layers with a matrix of random wires like a spider web to give the outer rendered shell some extra strength and reinforcement. I then apply the mix, with AR reinforcing fibres added to around 12-15mm thick, wet sponge finish to remove tooling marks, then wrap the whole oven in clingwrap to hold moisture in that layer for a week. This also compensates for the loss of moisture back into the dry p/v crete layer.

This of course is not the only way to do it, just what I've arrived at from building lots of ovens attempting to minimise materials and weight at the same time as maximising
insulation and strength

Ha Ha I just noticed the light coming through where the removable flue fits. It spolights my pliers left inside the oven.

[Newie newby]

Yeah sorry it was a bit late when I posted last night so sorry for confusion. The first coat (1-25mm thick) has dried out heaps more in the last few hours, but am going to leave it to dry out a bit more before I do the next coat. This kit supplier recommended just a sand/cement mix and a few handfuls of builders clay in the mix but I wanted to try for a bit more insulation with the perlite mix.

[david s]

Hi David, the mix was 5 parts perlite, 1 part mortar mix (made up of 10% lime, 20% cement, 70% sand), 1-1.5 parts water. The purpose of this layer was to provide a bit more insulation along with the 50mm blanket, plus fill up bumps and lumps from the insulation layer and make it easier to do add final render after another coat or 2 of this mix. Should I wait until totally dry between coats or is ok to do another layer today if still a bit soft and crumbly?

I'm still not sure what mix you used. You did say in your previous post that the mix was made up of (perlite/builders mortar/cement/water mix). I took this to mean that you added more cement as well as the cement that's present in the mortar mix. but the last post you said “5 parts perlite, 1 part mortar mix (made up of 10% lime, 20% cement, 70% sand), 1-1.5 parts water."
Because lime is a cementious ingredient that would equate to an 8:1 mix.(check table again for strength/insulation value)
You didn't say how thick the layer was that you applied, so I can't guess how long it will take to dry, which of course is weather dependant too.

Just rereading your thread and noticed that you bought a kit. You should follow the manufacturers recomendations and their instructions (if provided).
Your build is looking great by the way.

[Newie newby]

Hi David, the mix was 5 parts perlite, 1 part mortar mix (made up of 10% lime, 20% cement, 70% sand), 1-1.5 parts water. The purpose of this layer was to provide a bit more insulation along with the 50mm blanket, plus fill up bumps and lumps from the insulation layer and make it easier to do add final render after another coat or 2 of this mix. Should I wait until totally dry between coats or is ok to do another layer today if still a bit soft and crumbly?

[david s]

Also got the first insulating layer (perlite/builders mortar/cement/water mix) rendered on the dome. Some sections I had to work hard to keep the render on the dome, just wanted to slump off...Got it done with 5 parts perlite, with my limited rendering skills there was no way I could have made it stick and stay on at 10 parts perlite - a bit rough and a few hollows to fill in with next layers. I was a bit scared to work the first layer to smooth it as it looked like it might fall off if I played with it too much. I'll get another layer on tomorrow and spend a bit more time as it goes off to smooth it off for a better finish with a damp sponge.

I'm not sure of the mix and its proportions, that you've used, or quite what it is that you are trying to achieve with this layer, Is it strength or insulation? The layer also works to restore a nice shape and works to provide a firm substrate for a strong impervious rendered outer shell. The addition of mortar, which is a mixture of cement and sand increases strength, raises the proportion of cement in the mix and probably places your brew somewhere near the bottom row.

The table shows the relative strength and insulating value of different insulating concrete mixes which should be useful.(vermiculite and perlite are interchangeable with similar characteristics)
A 5:1 mix is neither particularly strong or insulating. Because perlite is so water absorbent there's tons of free water left that can become a probem if not removed. Just like a pile of sand that will stay moist for many months evn though the surface dries quite quickly, so does a thick perlcrete layer. You don't say how thick the layer is that you've applied. From experience a 10:1 35mm layer takes around a week in good drying conditions to eliminat most of the free water. A thicker and much richer mix (5:1) will take longer. Pushing heat into it from the inside will also drive this moisture out.

I'm not saying what you've done will not work, you don't say how thick the layer you'va applied was, but it is helpful to understand what's going on and the qualities of the materials applied in different proportions. With 50mm of blanket insulation, the need for more insulation is not really a high priority.
Throw some plastic over the dome if you are doing any drying fires. This will indicate the moisture elimination.

[Newie newby]

Also got the first insulating layer (perlite/builders mortar/cement/water mix) rendered on the dome. Some sections I had to work hard to keep the render on the dome, just wanted to slump off...Got it done with 5 parts perlite, with my limited rendering skills there was no way I could have made it stick and stay on at 10 parts perlite - a bit rough and a few hollows to fill in with next layers. I was a bit scared to work the first layer to smooth it as it looked like it might fall off if I played with it too much. I'll get another layer on tomorrow and spend a bit more time as it goes off to smooth it off for a better finish with a damp sponge.

[Newie newby]

Big day today. 2 layers of the 25mm 1200 degrees c rated insulation and chicken wired over dome

[Newie newby]

Tiling landing and around WFO done