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Old 07-11-2011, 04:39 PM
lt4827's Avatar
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Default ready for the insulating hearth...

First, let me start off by telling you that I am BLONDE and have no affinity for anything involving numbers. That said, we are ready to pour our insulating hearth. I have read the Pompeii instructions over and over, still unable to find the answer to my question. I have scoured the forum and still have come up short. The issue that I have is how does one calculate the amount of portland cement and vermiculite......I understand that it is a 6:1 ration, no problem, I understand that in cooking terms. But how does one know "how much" will be needed? The island hearth for the insulating slab is 50" X 50" by 3.5".....I have plugged it in to the calculator and gotten numbers for cu ft, cu in, but don't know where to go to further the equation......I don't want to start the pour and find out that I need more of everything!!!!! I am 45 min away from the nearest Lowes. HELP!!!
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Old 07-12-2011, 02:54 PM
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Default Re: ready for the insulating hearth...

Quote:
Originally Posted by lt4827 View Post
First, let me start off by telling you that I am BLONDE and have no affinity for anything involving numbers. That said, we are ready to pour our insulating hearth. I have read the Pompeii instructions over and over, still unable to find the answer to my question. I have scoured the forum and still have come up short. The issue that I have is how does one calculate the amount of portland cement and vermiculite......I understand that it is a 6:1 ration, no problem, I understand that in cooking terms. But how does one know "how much" will be needed? The island hearth for the insulating slab is 50" X 50" by 3.5".....I have plugged it in to the calculator and gotten numbers for cu ft, cu in, but don't know where to go to further the equation......I don't want to start the pour and find out that I need more of everything!!!!! I am 45 min away from the nearest Lowes. HELP!!!
There are 27 CF in one CY. I believe vermiculite is sold in CYs. So - get enough CYs of vermiculite to fill the hearth forms. The portland will really not add much, if any, volume - as it will mostly fill the voids in the vermiculite.

one 94 lb bag of Portland cement = 1 cubic foot. So - 27 bags of Portland in 1 CY of Portland cement.

If you were making one CY of vermicrete - I would use one (1) CY (or 27 CF) of vermiculite and 4.5 bags (4.5 CF) of portland cement.

Last edited by Cheesesteak; 07-12-2011 at 03:01 PM.
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Old 07-12-2011, 03:14 PM
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Default Re: ready for the insulating hearth...

I'll give a bit of an alternative opinion. I've built two ovens now using perlcrete under the floor and my recommendation is based on that. I can't say if the same applies to vermiculite, but would assume it does. Perlite is cheaper then vermiculite here, so that is what I use.

I buy my Perlite in 4 cubic foot bags for about $18. Since the perlite is already in cubic feet I stay there for all my calculations. Calculate the volume of the hearth you are going to poor in cubic feet. Everything must be in feet, so a standard 4 in. slab would be 1/3 of a foot ect. I would then multiply that by two and buy that many cubic feet of perlite. Once you wet the perlite down and mix it with the portland it shrinks pretty dramatically. You might not lose a full half the volume but it will be close and it never hurts to have a little spare insulation around. On my recent oven I ended up with about a third of a bag too much using this method. Close enough for me.
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Old 07-12-2011, 04:12 PM
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Default Re: ready for the insulating hearth...

Buy some calcium silicate board, its way easier to use.
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Old 07-12-2011, 06:54 PM
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Default Re: ready for the insulating hearth...

Thanks for the replies. Have not found the calcium silicate board around here, but will keep looking until we are ready to do something.....maybe 2 weeks or so.

Shuboyje, thanks for the breakdown .....since my area to put the insulated hearth into is relatively small, your explanation makes sense to me. We will see if we can make it work.

Cheesecake, since I am math challenged, cf / cy / just confuse the issue. Keeping the computations within the same frame of reference works for me. Thanks for your input.

I will post pictures on the progress, nothing this weekend as we are attending a Wedding out of town.....no rest for the weary.
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Old 07-21-2011, 09:55 AM
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Default Re: ready for the insulating hearth...

Not sure if you poured your hearth yet, but I just did mine and thought I'd chime in.

Perlite and Vermiculite are similar, but different products. Both have insulating qualities.

Overall Perlite is a better insulator and can withstand higher temperatures. However, for our application either would suffice.

I chose to go with Perlite because it does not absorb water. You may hear otherwise from others, but check it out it floats. Perlite "adsorbs" water. Vermiculite "absorbs" water. Since I am going with an Igloo design I wanted to have an insulator that allowed water to pass through in case my cover developed a hairline crack. Now onto the mix...

If numbers give you a headache think in parts by volume to help you visualize. For a 1:6 perlite mix you should use 1 part portland cement (make sure it is portland and not ready-mix) 6 parts perlite and 1.6 parts water. The parts can be whatever you have lying around.

If you are going to use Vermiculite remember that it "absords" water so the ratio is different. For Vermiculite you should use 1:6:2.4 (portland/vermiculite/water).

Additionally, I mixed my water and portland into a soupy slurry berfore I added the aggregate. I mixed well until there were no lumps. I tried to mix the water last and ended up with a lot of clumps of cement that will decrease the thermal efficiency and strength of the final mix.

Don't take my word for it mix a couple of sample batches and see for yourself

Hope this helps
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Old 07-21-2011, 10:45 AM
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Default Re: ready for the insulating hearth...

Dottavio, Thank you for your thoughts on the hearth mix. No, we have not poured the insulating hearth yet, busy, busy, busy here. Out of town on work related issues have slowed us done a bit.

My husbands' sister is a rock head for New Mexico Tech in Socorro and she told us pretty much the same thing in regards to Perlite vs Vermiculite. They do testing of perlite at the lab there, so she is saving bags of it for us.....it will arrive here in Texas next weekend. So, some time in August, we will organize ourselves for the pour. I will post pics as we work it. Have you posted any of your pour?

Thanks again for your input.
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Old 07-21-2011, 12:41 PM
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Default Re: ready for the insulating hearth...

Get more perlite/vermiculite and cement than you calculate you will need. Allow for about 50% for shrinkage and compaction.

Finding that you are running short of material half way through your pour is not good.

You can always return unused unopened bags to the supplier. Most suppliers will do this as a matter of course with no re-stocking charge.
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Old 07-21-2011, 01:16 PM
Peasant
 
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Location: New York
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Default Re: ready for the insulating hearth...

I have not posted any pics. It has taken two years of site preparation to get to this point. My brick oven project has turned into an outdoor kitchen project.

However, I am finally at the point were I am going to build my dome.

I should start a thread and post some pics if I can get my act together.

One more thing... the consistency will look like a grainy oatmeal and if you grab a handful and squeeze only the slightest amount of water should drip out.

Again, try a sample. It will give you the confidence to mix the larger batch

Good Luck
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