Bottom insulation on corrugated steel dries slowly?

[Peterrr]

Hi,

I have a 10cm (4") layer of vermiculite/cement mixture (5:1) on
corrugated steel as bottom insulation for my oven (pics attached).
There is a layer of about 1 cm (3/8") of concrete to level the surface on which the floor tiles are resting.
The insulation has been drying for 6 weeks until I fired it
the first time last week.
Yesterday I fired up the oven for the second time.
After 1,5 hour the dome became white, burning of the soot.
Everything went fine but I noticed that the
corrugated steel became quit hot at about 70C or 160F...
I assume it is because the insulation is still not completely dry.
Does that make sense?

Should I drill some holes in the steel to help any moisture present escape?
Or should I just be patient and let the moisture escape through the side
and via the floor tiles during regular fires?

Any advice is appreciated.

Regards,

Roeland.

[Dutchoven]

The vermiculite concrete stays wet for a long time...at least mine did...I think it probably lost a good bit of water...I would not drill holes...what you might do is a long slow burn...not to get it white hot but more of a curing fire...the underside of the floor will probably get warm and help to further dry the insulating layer
Dutch

[dmun]

I'm not surprised that the steel plate heats up: After a "white" firing of my oven the bottom of the four inch cast concrete support slab is warm to the touch, and steel conducts heat a lot better than thick concrete does.

As Dutch says, it may be a moisture issue. The first month of firings will drive any residual moisture out of the vermiculite concrete. I don't think you need to drill any drain holes: the firebrick floor is porous, and will let any released water vapor escape.

[Peterrr]

@dmun, But your four inch cast concrete support slab is not
supposed to insulate, is it?
I can imagine that concrete is conducting the heat, assuming you
are using it as thermal mass.
But my four inch of vermiculite is meant for insulating the hot floor tiles.

Regards,

Roeland

[Frances]

Of course! I never thought that water vapour can also escape up through the floor of the oven - kept wondering where it would go once the oven was weatherproofed...

Roeland, I'm pretty sure dmun has insulation under his oven, and the concrete slab underneath that still gets warm (right dmun?) I also think that you're right about the miosture making the corrugated steel hot. Every time I put a new layer of anything with moisture in it on the top of my oven the next firing makes it warm to the touch... and then after a couple of firings (and lots of worrying) it stops again.

[Frances]

Oh btw, since I'm being free with my opinion here and haven't commented so far...

I really like your oven! It looks great and I hope you'll continue telling us how it works out.

[Dutchoven]

Peterr and Frances
Dmun definitely has insulated the bottom quite well...the concrete slab he referred to is beneath the insulation that the floor of the oven rests on...you should look into his oven build pictures...he did a most excellent job!...I really do believe it is a moisture issue...CanuckJim said it very well(and I'm paraphrasing) "a wet insulator is not an insulator at all"
Best
Dutch