To Build a Brick Oven (2010): Day 5

The day after we poured the slab, we took the frame out and continued to let it cure for about a week, not because it needs to cure that long, but because it has been rainy. Here is a good view of the poured hearth. You can clearly see the rebar grooves in this picture.

In the meantime, I did quite a bit of research on firebrick in the area. For these ovens, it is recommended that you find low-duty firebrick (ASTM C 27). The best way to figure this out is to call your firebrick providers and ask. Many building suppliers will not know... a firebrick is a firebrick to them. But they can find the information from their provider as well. Low-duty firebricks are usually cheaper too (I paid $1.30/brick for mine). Medium-duty and High-duty bricks will be more expensive. Low-duty bricks are also softer and will likely have chips and/or small cracks in them with semi-rough edges. Medium or high duty bricks will be crisper, harder, and less likely to chip or crack when you handle them. So why use low-duty bricks? My understanding is that it is less conductive to heat. What I think of is making a pancake in an aluminum pan (high-duty firebrick) compared to a stainless steel pan (low-duty firebrick). In the aluminum pan, and pancake will burn a lot faster because the heat moves through it so well. In the stainless steel pan it will cook slower with the same amount of heat, it is just transfered at a slower, less intense rate. So with bread in an oven, a high-duty firebrick will burn your loaf before it is really done cooking. If you are making your oven just for pizzas and not for bread at all, maybe medium-duty firebrick will work just fine, if not better. Also, low-duty bricks are safe up to 2000 degrees F. You will never get your bricks to this temperature, so nothing to worry about.

Before I did anything, I laid all the bricks on the hearth and made a mock-up. This is what I came up with, but if you scroll down to the later pictures, I changed it just a bit. Now I know how I want the firebrick laid on the hearth slab, so now once we have a nice day, I now know how exactly to lay.

To lay the firebrick on the hearth, you have to make a 50/50 mixture of sand and fireclay. Add just enough water to make a thick paste.

Sand/fireclay mixture.

Then spread it over the hearth at about 1/4 inch thickness. Make sure it is as level as possible, this will prevent you from adding or taking away the clay with each brick to make it level with the rest. This stuff was actually very hard to spread, but it is doable.

Getting it as level as possible

The sun decided to come out, so I put the canopy over so the clay won't set on me.
It does last a while however. It didn't seem to really set until you lay the water-sucking brick on it,
but what was left in the sun set as well.

I started with the front bricks and worked my way back. You should know the exact distances the bricks should be from all edges because you did your mock-up already. So be measuring the whole time. It is also good to draw a line down the middle for a reference point. Every single brick you lay you should level both ways.

Another thing to remember, especially if you use the recommended low-duty firebrick, is to always lay the neatest, crispest side up. The bricks along the sides and back can be your worst quality bricks because they just get covered up by the wall anyways. If you are using firebrick for your walls and arch, you have more to choose from to get the best ones for the hearth. If you are just using firebrick for your hearth, it would be a good idea to pick out your bricks yourself at your supplier to avoid getting badly chipped ones.

In my plans, the first bricks come all the way to the edge.

Checking for levelness. Notice that the clay was spread with a notched trowel.

All done!

I wanted to make my oven door 9-inches high, so I used firebrick at that point instead of red brick. I will also need to cut myself a slightly longer piece of angle iron. From the first picture, I also extended the oven one brick length, bringing the door all the way up to the granite slab. The next nice day, I will mortar in the side walls