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Does clay vitrifiy over use, or only if it reaches certain temperature? - Forno Bravo Forum: The Wood-Fired Oven Community

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Does clay vitrifiy over use, or only if it reaches certain temperature?

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  • Does clay vitrifiy over use, or only if it reaches certain temperature?

    Hello FB Pompeii gang, I have a question. When using 'fire clay', not grog or fire brick shavings, does it vitrify over many firings? or does it just dry out? I am wondering because most low fire clay seems to be above the temperature the ovens get too when referring to cone temperature conversion charts from a quick google search. The lowest temperature low fire clay I have seen available is cone 17 which roughly translates to 1360 -1400 degrees Fahrenheit. I feel like this has been answered in the past, so feel free to post links to other posts. I'm about to lay a hearth and I was wondering about the mix of fire-clay and sand for the hearth paste as seen in the Pompeii oven building instructions. It would seem to me that the clay would generally just dry out. Are there hardened stages between vitrification and dry 'hearth paste' aka 1 part sand and 1 part fire-clay? I think I have split this into more questions than I had when I started writing, but I'm curious what the general consensus has been.

    Thanks, James

  • #2
    Hello James, I don't believe any part of your oven will reach the temps required for vitrification, especially anywhere on the "outside" of the brick. The bricks reflect a great deal of the energy and will be vastly different temps on the fireside than they are on the "outside". There's a chart in the instructions that highlights this. Have fun building.

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    • #3
      Originally posted by JayMez View Post
      Hello FB Pompeii gang, I have a question. When using 'fire clay', not grog or fire brick shavings, does it vitrify over many firings? or does it just dry out? I am wondering because most low fire clay seems to be above the temperature the ovens get too when referring to cone temperature conversion charts from a quick google search. The lowest temperature low fire clay I have seen available is cone 17 which roughly translates to 1360 -1400 degrees Fahrenheit. I feel like this has been answered in the past, so feel free to post links to other posts. I'm about to lay a hearth and I was wondering about the mix of fire-clay and sand for the hearth paste as seen in the Pompeii oven building instructions. It would seem to me that the clay would generally just dry out. Are there hardened stages between vitrification and dry 'hearth paste' aka 1 part sand and 1 part fire-clay? I think I have split this into more questions than I had when I started writing, but I'm curious what the general consensus has been.

      Thanks, James
      Hi James,

      Vitrification is when the materials, primarily silica begin to turn to glass. There can be partial or full vitrification. Silica's melting point is somewhere around 1500C from memory, but can be encouraged in the presence of fluxes to melt at lower temperatures. This is how pottery glazes are created, by adjusting the silica, flux alumina balance .However there is no vitrification below around 1000 C. Clay will however turn permanent, ie not turn back to mud, north of 573C. This is not vitrification but is known as the alpha change. This is all academic, but your sand/clay levelling mix under your floor bricks will get nowhere near any temperature that would either vitrify it or render it permanent. Also the number of times you fire it up will also not have any bearing on the state of the materials at those temps.

      Dave
      Last edited by david s; 09-26-2017, 01:47 PM.
      Kindled with zeal and fired with passion.

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      • #4

        Originally posted by JoesDreamHomes View Post
        Hello James, I don't believe any part of your oven will reach the temps required for vitrification, especially anywhere on the "outside" of the brick. The bricks reflect a great deal of the energy and will be vastly different temps on the fireside than they are on the "outside". There's a chart in the instructions that highlights this. Have fun building.



        Thanks very much.


        Originally posted by david s View Post

        Hi James,

        Vitrification is when the materials, primarily silica begin to turn to glass. There can be partial or full vitrification. Silica's melting point is somewhere around 1500C from memory, but can be encouraged in the presence of fluxes to melt at lower temperatures. This is how pottery glazes are created, by adjusting the silica, flux alumina balance .However there is no vitrification below around 1000 C. Clay will however turn permanent, ie not turn back to mud, north of 573C. This is not vitrification but is known as the alpha change. This is all academic, but your sand/clay levelling mix under your floor bricks will get nowhere near any temperature that would either vitrify it or render it permanent. Also the number of times you fire it up will also not have any bearing on the state of the materials at those temps.

        Davey
        Both explanations help a lot.

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