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Best mix the perlite cover on top of the ceramic blanket

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  • Best mix the perlite cover on top of the ceramic blanket

    I've been searching around this massive forum regarding what perlite mix that works best for people, it seem to be a mix of mixed up messages (pun intended nted)

    I got these ingredients at hand:
    - Perlite 3-6mm
    - Portland Cement
    - Lime
    - Fireclay

    Thank in advance
    You give Americans an inch...

    And they'll invent their own metric system..

  • #2
    The standard mix for perlcrete insulation over the ceramic batting is 10:1 (perlite: cement). These are volume ratios, not weight. David S has noted that a handful of clay helps the mixing process. Also, you want to mix by hand. The mix is very crumbly and difficult to apply, especially in the lower (more vertical) area of the dome. Using a form to hold it in place without forcing a pack makes the application much easier for the lower dome half. This also helps to have a more consistent thickness. Once you start applying the perlcrete over the top half, the job becomes much easier. Some of us recommend putting a wire mesh (chicken wire or metal lath) over the batting to reduce compression and provide a little bit of "grab" for the perlcrete. Be patient & go slow.

    The perlcrete will have a tendancy to slump if too much is applied without support or a partial curing set. Some folks have chosen to do incremental rings, allowing each ring to set (a day or two) before doing the next ring on top. Lots of ways to do this...just relax & don't try to rush the process. Once you've got the dome covered, let it cure & dry before you start firing. After curing the oven, you can smooth out and reshape with stucco/render over the perlcrete. Hope that helps.
    Mike Stansbury - The Traveling Loafer
    Roseburg, Oregon

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    • #3
      A curved shape trowel will help with holding pcrete and shape. As Mike says, take your time, it is not something you can do in a day. Click image for larger version

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      • #4
        As the attached table shows the addition of cement to the mix drastically reduces insulation value. I find a 10:1 mix is about as lean as I can go and still have the stuff reasonably workable and resulting in a substrate with sufficient strength to render against. It results in a layer about as insulating as blanket. If doing an enclosure a dry mix of perlite is the best insulator and there's no problem about having to drive off the water. However, for an igloo style, with no walls to contain loose material it requires something to make it stick (cement) and water so the cement will react.
        I do the whole oven in one go, to a thickness of around 35mm (one and a half inches). Any thicker and drying the inner part of the layer becomes much slower, better to do subsequent layers with a week of drying between layers.I use just hands to place the mix , always starting from the base and making a flat ledge on the top which allows you to judge the thickness as well as having a platform for the next row to sit on. Although I do it in one go I do recommend that first time vermicreters just do a row at the base about 150mm (6" high) and allow this to set for 24 hrs. This makes the next lot to build on top of far easier. Most folk swear that the mix won't work when doing it for the first time, but it does. Apart from reducing the cement as much as possible, the correct amount of water is also imperative. Too much and the water will wash yje cement off the grains leaving an inconsistent mix. To little and the mix is too dry and crumbly. As a general rule 3 litres of water to every 10 litres of vermicrete is about right, but the finer grade grains require a little more. If water pools in the bottom of your barrow wait a few minutes as the stuff absorbs the water and if this condition persists add a little more dry material in the same proportions.
        I find a little powdered clay, around a handful for every 10 litres (2gallons) of vermiculite does wonders to make th mix more workable. Also I find a mix of 50/50 perlite/ vermiculite works better than either of them alone. I use a medium grade of both. I use just a flat trowel to tap the outer surface which results in a good surface when just eyeballing the profile. It is also essential to wear rubber gloves, otherwise you'll regret it the following day.

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        Last edited by david s; 11-26-2021, 02:26 AM.
        Kindled with zeal and fired with passion.


        • #5
          SableSprings, UtahBeehiver and david s - A big thank you for your comprehensive replies, very helpful, this is why I love this forum! I feel much more confident at laying the perlcrete now, thanks again amazing people
          You give Americans an inch...

          And they'll invent their own metric system..