a few questions

[matty]

okay so I am now working on the block stand for my oven and I have a few questions for a little on down the road.


first can someone talk to me about vermiculite? In particular, pouring a insulating hearth out of it. I have heard anything from mixing 6 parts vermiculite to 1 part portland cement, all the war to 3 parts cement and 1 part vermiculite. what did you use? also I have heard that the vermiculite mixture is not very structuraly strong? is it safe to lay the entire mass of the dome on top of it?


also in concern with the fire bricks. I have just purchased 200 fire brick and they are extremely fragile. Just pikcing them up, some of the sides chip away. I was origionally going to create my oven floor out of these bricks but i am sure that the pizza peel will break these bricks down over a short period of time. is there something else you would recommend using for the floor?

[Breven]

Re: a few questions

Matty-

You should download the Forno Bravo instructions James has put together. There have probably been hundreds of ovens built using those plans. I beleive they call for a 5:1 Vermiculite or Perlite to portland cement ratio, which is what I did. It is really weird to work with the stuff. It's super lite, like styrofoam. The idea essentially is to coat the tiny styrofoam like balls with cement. It bonds together creating pores. From what I understand, its the trapped air pockets that create the insulating effect. Don't worry about strength. As some of the more technical folks will tell you, it's all about weight distribution. Since the weight is spread out over the layer evenly, there is no chance for failure. There is plenty of discussion on vermiculite here- I think I even recall seeing a video showing. Do a search.
As for your firebrick question...I think good firebrick weighs about 8 lbs. I'd check the weight of your firebrick and make sure what you think you have is what you really have. I'm not sure why they would be so fragile- mine were really sturdy, if I dropped them they wouldn't break, heck they would barely chip. Check other suppliers in your area and see if they all carry the same stuff. If you can find a better quality brick, might be the way to go.

[egalecki]

Re: a few questions

Breven is right. Vermcrete is weird stuff, but it's very strong in compression. If you pick at it, it will crumble, though, so don't be surprised. And I'd be worried about those brick too.

I did use a much larger proportion of vermiculite to portland, though. CVDukes came up with a way to get a 10 or 12 to one ratio- if you mix the water with the portland and pour it over the vermiculite, you can get more vermiculite bound up and get a better insulator. It's a little more difficult to work with, however. It isn't as sticky so you have to be patient if you're doing the dome insulation. If all you're doing is the hearth insulation, no problem. Just dump it in the form and pat it in- make sure you have a solid layer, no voids. I actually patted it to the sides first to make sure my edges were good and then did the rest. It currently has a big ol' oven on it, no problem at all...

[Breven]

Re: a few questions

Oh yeah...Liz makes reminds me of a good point. It's very loose, nothing like using regular concrete, sand and aggregate. When I shoveled it into the forms, I screeded it, then tamped it down a bit using an extra block I had from the stand construction. I compressed it a couple of inches or so...then I added more, screeded it again, tamped a little more..continued until it was nice and packed and level.

Also, I dry mixed the cement and vermiculite together in a wheel barrow first, then added water until it was a moist consistency. I'm sure I got this advice form the FB PDF Instructions. I printed those instructions out and kept them with me every step of the way (along with pictures of other builds like Les's). I have pictures in my picasa album of the mix and screeding.

[nissanneill]

Re: a few questions

Matty, welcome aboard,
Bevan and Elizabeth support what should happen with the vermiculite.
I mixed mine 5:1, vermiculite : Portland cement in a wheelbarrow (even though I had a mixer at hand) using a broad bladed fork (see my oven build by clicking on the link below). When you add the water, it washes off the cement and is weird stuff to mix but I have pictures, hints and tips so check it out.
You will always find information that contradicts what you believe but that's life, you just need to do a little more research or ask for advice from the forum (or those that have gone through it previously). Ultimately, you make the final decision but there will be plenty of warning if you are misguided!

Happy building.

Neill

[Bandrasco]

Re: a few questions

Hi Matty,

I, like most have had that weird feeling about vermiculite/portland mix. I mixed mine 5:1 as the FB build book suggests. It took a few days to actually set but it has tremendous compression strength even though you can chip it away with very little effort.

The firebricks you have sound like mine. Mine were made by Superior Clay, low duty, and still amaze me how they appear to be so easily damaged. Most of my edges were chipped and seem to have come on the palette already damaged. I did drop a few and believe it or not they didn't chip or break. Once I started my build I was able to compare my low duty bricks to the heavy duty ones; big difference in edge quality and weight but I have forced myself not to attempt perfection with imperfect products. I was told that they were good bricks and most masons don't worry about the ends as they normally are covered by mortar in a fireplace. They weighed right at 6lbs as they are slightly smaller than most bricks. I have them on my floor and I think they'll do fine.

Good luck on your build and you came to the right place for answers; the people in this forum have seen most everything and I trust their advice.