Mortar Question

Just sitting here looking forward to spring for the opportunity to finish my pizza oven I started last fall. Living in Minnesota has put things on hold for a few months. I got my base done last year and will move forward from there when weather allows. Anyways my question is when I mortar the picture frame or edge bricks for the hearth slab can I just use regular mortar or does it need to be high temp mortar? I plan on using the mix your own recipe provided for the high temp mortar but was hoping to just use regular type s for that picture frame that way that mortar would be a good color match to the mortar I end up using for the brick veneer. I see the directions call for the high temp but then I read somewhere that doesn’t set up without a hot fire which these edge bricks would never see so I guess I’m just a little confused here.

Hello, Adam, and welcome to the BrickWood forum!

I know it’s tempting to downgrade the mortar for the hearth frame brick, but there’s a good reason they’re spec’ed for high temperature mortar.

When you complete your arch, you will find that the arch bricks rest partially on the insulated base and partially on the frame brick. That may seem to suggest that the hearth frame is out of the line of fire (so to speak).

BUT… in fact, those brick will be absorbing heat from the base row of refractory brick, as well as heat “rejected” by the insulative material in the hearth base. For the most part, they are inside the envelope created by the insulation surrounding the firebrick, so they are still part of the oven.

Here are some observations:

  • The high temperature mortar does set very hard in about a day. It has to, or your structure would just fall apart brick by brick before you ever got to the firing stage.
  • When you see references on these forums to firing the mortar to set it rock-hard, that doesn’t imply that it stays soft until it’s fired. There are other mortar materials that act like that, but you won’t be using those in your Mattone Marile build.
  • By observation, I’ve found that the hearth frame’s exposed surface does get noticeably warm when the oven is operating. It’s not hot enough to require firebrick, which you are using both for structural reasons and for its refractory properties. But you do want high-temp mortar because it will handle expansion and contraction better than Type “S”.

You might consider a veneer on the vertical faces of the hearth frame—it could be the same as what it sounds like you’re using for the arch, or it could be complementary. If you are using a double layer of insulation (as I’d expect in Minnesota, and so glad I did that here in chilly Connecticut as well), you might find that your stucco brings you close to the edge of the hearth frame, and the veneer brings you all the way to the edge. Everyone’s oven varies a bit in this regard.

Hope this is helpful, Adam, and that you’ll come back often as you build up your oven.

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