Facade Brick Develop a hairline crack
Question for those on the forum who have built a pompei style brick oven. I fired mine up for the 3rd time this past weekend. I built a somewhat larger fire this time but still not hot enough to cook in it. Oven got to about 450F.
I notice the brick facade developed a hairline crack in the mortar. This crack travels horizontally across the facade on both sides of the oven door and is at the level of the brick lintel holding up the bricks for the chimney. I was surprised to see it as I have been building up the fires. My first oven had done the same thing but I had an arched entry way in the oven and a square one on the facade. The arched entry way had a thin piece of metal bent in the shape of the arch and was permanent in the oven. When this had happened that time I exchange emails with Alan Scott and he thought maybe the heat expanding the metal that was arched caused it to lift the facade and develop the crack. This time I have a square oven entry with brick lintel and an outer facade that is arched with no metal (self standing) and again I get this crack. It never amounted to any more than that the first time but it sure takes away from the all the hard work. Anyon else have similar experiences? Thx.
in my (long) thread containing all of my building pictures, i posted about this same thing happening. the header over my doorway opening is archbrick, rather than a solid lintel, and there is no metal involved. when my oven fired up and moved around with expansion/contraction, a crack developed all the way around the dome, ending up at either corner of my arch, right where it turned sharply downward.
i think the primary thing happening here, is that when initially fired, the bricks expand and move around a bit, and decide that they would prefer a different orientation than how we oh-so-carefully laid them. the refractory masons will tell you this is normal, and is one of the reasons for curing the oven. however, what worried me, is that my crack found a clear path all of the way around, therefore allowing that section of the arch header to creep forward a bit.
in your design, if i'm understanding correctly, the expansion/lateral force of the top section of the dome could do the same thing with your lintel---the bricks heat up and expand and they all push against each other. the weakest point is going to give, and that point in this case appears to be where the lintel ties in with the rest of the dome, and the result is that the lintel/adjoining facade breaks loose from its surroundings.
unless it seems to have expanded to the point where something may be structurally compromised, i wouldn't worry about it. apply fireplace cement...but keep an eye on it!
overdo it or don't do it at all!
This also came up in the brick-oven group and I posted a note about it in answer to a query. (Alan seems to have disappeared from the group again leaving me as the only other brick designer -- Kiko's an earthenware guy.)
Here's the relevant msg thread:
--- In firstname.lastname@example.org, "ovenenth" <ovenenth@y...> wrote:
> The facade is common clay brick (not fire brick). How is the thermal
> break introduced? It is too late for this one but interesting to know
> how one does this?
### You use either an expansion joint or a bond break. The expansion
joint uses an elastic medium like a neoprene or rubber pad in place of
the mortar & then a flexible sealant to keep the water out. The bond
break places a separation layer between the disimilar materials,
usually something like building paper or flashing, to allow for a slip
plane. In this case you'd keep an air gap between the firebrick & the
common brick to act as the thermal bond break. Some folks will also
use a different joint called a "control" joint that is designed to
crack (it uses inflexible joint materials instead of rubber or
neoprene). The idea being that you put this type in a place within the
structure where it won't affect the structural integrity or aesthetics
- kind of "it's going to crack anyway so I'm going to make it crack
where I want it to crack and no one will see it."
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