Fire Mortar vs. Refax Refractory Mortar

[southpaw]

Hi all,
I got this fire mortar when I bought the fire bricks to build the oven walls and dome with.

the place i got it from told me you dont need to add anything els to the fire mortar, just mix it with water and a little bit of sea salt (10 fire mortar : 1 sea salt) and it will be ready to use.

so I did just that, but when it dried it didnt seem like it was holding the bricks together, cracks started to show!!!

then I asked a few oven biulders that use the same fire mortar about the cracks, they said its ok and that the mortar will hold together with bricks when you start curing the oven (add heat "fire" to the oven gradually).

My Q's are:

1. the fire mortar i got is it the same as refax, same concept just add water and use it?
2. why do they mix sea salt with it?
3. about the cracks that are apearing, how do i get rid of them, or thats normal?
4. should i spray water on the dome a few times daily so the cracks dont increas?

Insulating the oven:
i was thinking of rapping the oven with a 1" woven ceramic insulating blanket then a loose insulation 4" (5-vermeculite : 1-cement).

iam planning to finish with an igloo design enclosure.

my Q's on this topic are:

1. should i rap the blanket directly over the cracks of the dome?
2. if i couldnt find the woven ceramic insulating blanket, would the loose insulation be enough and how thick should it be?

3. should i start curing the oven before or after insulating the oven, or it dosent matter?

[Chris]

Re: Fire Mortar vs. Refax Refractory Mortar

1. I am not familiar with the fire mortar. Due to difficulties in translation, it is quite possible that you have actually purchased a bag of fire-clay, but this is pure speculation on my part.

2. I have NO idea why you'd mix salt with it. Sand, maybe. Everything I have read says that using saltwater to mix concrete or mortar is very bad. However, I am certainly not an expert. Do these other oven builders use salt with success? Are there any instructions or specifications on the fire mortar?

3. I don't have enough experience to comment about the cracks at this stage. Most of the time, I read about cracks that form upon firing, and that minor hairline cracks are to be expected.

4. If your mortar is hydraulic-setting type, then keeping it from drying out would be a good idea.

I'd like to see what the fire mortar looks like before water is added. Does it feel like fine dust, with a little sandy grit in it? My Heat Stop mortar was like this.

Or, is it like tiny pellets? the fireclay I bought looked like tiny pellets.

What happens if you put water on fire mortar that has cured or dried? If you can turn it back into mud, maybe you have fireclay, or it is a heat-setting cement. If it is impervious to water, it may be a hydraulic-setting type.

Insulating questions:
1. You could try repointing the cracks with fresh mortar, but if they're minor, I'd probably leave them alone. I got hairline cracks all over the outside of my dome (couldn't tell about the inside), and just covered it with loose perlite.

2. I build a square enclosure, and filled with loose perlite. The minimum amount of insulation is 6 inches of perlite between the oven and the wall, with no blanket and it's working fine for two weeks.

3. Here's my reasoning why I cured without insulation.
PRO:
- Easier for water (steam) to escape.
- Can see and repair cracks in dome.
- Can use IR thermometer to measure outer wall temperature.
- I hadn't finished the insulation and the house structure, and I was ready to build a fire! (This is probably the primary reason!)

CON:
- Lack of insulation will cause increased temperature gradient across brick, since outside surface will be subject to cooling. The could lead to cracked bricks.

However, there's a very goood chance that it doesn't matter when you start curing the oven, as long as you do it slowly.

[southpaw]

Re: Fire Mortar vs. Refax Refractory Mortar

I finished building the dome (100cm oven) on the 5th of April & I?m planning to start curing the oven before insulating it.

My Q?s are:

- How long should I wait before I start curing?
- If cracks start to show up, how do I get rid of them?

- Is it ok to insulate the oven with woven ceramic blanket right over the cracks?
- Is there a difference between insulating with (1? blanket : 4? vermiculite-cement) or (6? vermiculite-cement)? If there is which ones better?

Let?s say I want to construct a classical brick & mortar chimney, should I:

1. Use regular or fire bricks?
2. Use regular or fire mortar (same mortar used for building the dome, since it will reach high temp.)?
3. After building the chimney, do you need to insulate it from the inside or outside (if it?s a must then with what)?

[southpaw]

Re: Fire Mortar vs. Refax Refractory Mortar

thanks Chris,

i attached a picture of what the fire mortar looks like before adding watter, it looks and feels like powder.

i also listed a few other Q's

Appreciate your help.

[maver]

Re: Fire Mortar vs. Refax Refractory Mortar

My Q?s are:

- How long should I wait before I start curing?

There is a sticky on oven curing that addresses this. You want your mortar set before curing. If your fire mortar is hydraulic curing, you need to give it time to cure hard before you drive moisture out. If you have any mortar left over you might create a small test piece with some residual fragments of firebrick. You could then subject it to some heat (maybe in your home oven) to check how it performs. You could soak a separate piece to see whether the mortar is heat setting or hydraulic setting.

- If cracks start to show up, how do I get rid of them?

Cracks should show up on the outside - that is normal. If you have any large cracks that seem to pass through the dome, then you probably want to point those once your oven is stable.

- Is it ok to insulate the oven with woven ceramic blanket right over the cracks?

Yes

- Is there a difference between insulating with (1? blanket : 4? vermiculite-cement) or (6? vermiculite-cement)? If there is which ones better?

The blanket should perform better. I used 6" of perlite, and it certainly performs fine.

Let?s say I want to construct a classical brick & mortar chimney, should I:

1. Use regular or fire bricks?

Either fire bricks, or regular bricks with a chimney liner. The latter is preferred, and is a must if you are building inside.

2. Use regular or fire mortar (same mortar used for building the dome, since it will reach high temp.)?

regular mortar is fine if a double layer chimney.

3. After building the chimney, do you need to insulate it from the inside or outside (if it?s a must then with what)?

Air is alone good. A double layer chimney with a chimney liner has an air gap between the liner and the brick exterior that provides insulation. If you build with firebrick (ideally you would use splits) and no liner, then insulate the outside with vemiculcrete. I would use a liner (which is what I did).

Marc