Help with Stucco Cracks

[Balty Knowles]

I stuccoed the outside of the dome on Sunday & kept it covered with plenty of wet burlap (25 sacks) until last night. Tonight I noticed a mosaic of hairline cracks everywhere. I used the high temp formula from the chemist at Lumnite. 2 calcium aluminate, 2 sand , 1 Fireclay approx 3/4 thick.

I mixed the dry ingredients well & then added water to make a pretty stiff mortar.

What's wrong?

Is this normal? or shoud I re do?

Theres too many to grind & fill. See pics, I dampened the stucco down to highlight the cracks

[CanuckJim]

Re: Help with Stucco Cracks

Balty,

I'm not familiar with the formula you used, so can't say in that direction. However, you mention that the mortar was stiff. Coatings of this kind should be mixed wet, not stiff. By wet I mean just before the slump stage. Consider this one a scratch coat. Score it a bit, then recoat with a half inch of wetter mortar. You should be fine. Just don't let the first coat cure too much before you do it.

Jim

[nissanneill]

Re: Help with Stucco Cracks

Balty,
I am not too sure of what you call stucco!
My interpretation of stucco is a water proofing cement render with small bits of the same render material thrown onto the smoother renderred surface to make a decorative and waterproof finish.
I renderred (stuccoed?) my oven using a simple mix of 3parts red plaster sand to 1 part portland cement. Been using it after the recommended week of conditioning/drying it out and 3 hefty cook-ups and not so much as a hairline crack. This 3/4" render went over 3 x 1" layers of vermiculite cement (5 vermiculite:1 portland cement) which was coated over a 1" high temperature themal blanket.
I agree with Jim, roughen it up and recoat it, possibly with a more commonly reliable render/stucco formula, because after all it should not be subject to any kind of heat. You may wish to give it a lite wash with a portland cement/water slurry coat (which acts like a primer coat), to improve the bond.Tile stores have primer materials that are used to improve the bond of tile adhesive over existing or older cement surface.

Neill

[Balty Knowles]

Re: Help with Stucco Cracks

Thanks for the response guys,

Niell, I'm a Pommy & familiar with your terminology but have learned to use Americanisms after many years. I guess that would make me a Limey. I'm using stucco as a generic term for the additional high heat mortar coat that covers the dome before insulation.

Jim,

I will score the surface but it's as hard as a rock, I can use a grinder.
I'll continue with a much looser mix but the first coat will have already cured for a week by the time I'm able to re coat, do you think I'm OK?


I got he formula from Calucem, makers of Lumnite calcium aluminate. The chemist was very helpful. Interestingly he said DO NOT use lime with calcium aluminate because it is an accelerator & the mix goes off very quickly anyway, but to use equal parts sand & Lumnite with 1 part fire clay.

Thanks again

[CanuckJim]

Re: Help with Stucco Cracks

Balty,

Depends on what you want to do, add more refractory mass or do a finish coat. If it's a finish coat, stucco proper in fact, I'd go with Neill's idea and use a primer on your first coat to provide the stick you need. That way, you'd be okay for sure.

Jim

[Balty Knowles]

Re: Help with Stucco Cracks

Jim,

I'm laying an extra layer of refractory mass before I install the insulation, Not sure that it's even necessary but I already started. The dome itself will be enclosed in a brick structure & never seen again. (I hope).

I worry about the cracks from a functional aspect, e.g. what will happen if they expand & cause the stucco to loosen in big chunks?

I intend to cover with a 2" insulfrax blanket & then Vermiculite, is this enough to stop any chunks dislodging & compressing the insulation?

As you can see I worry a lot.

Thanks

Balty

[CanuckJim]

Re: Help with Stucco Cracks

Balty,

While the cracks may seem unsightly, from what I can see of the photographs, they aren't structural. What I'd do is give it the necessary cure time, then begin a series of SMALL fires to see if the cracks get any bigger and/or smoke appears through them. If smoke leaks, you will have to seal the dome again. It's unlikely that big chunks of your material will break off.

Try to do your coat in one go, fairly wet for a good bond. Depending on the material already there, you might have to wet what's already dry before trowelling on. You want a good, strong bond. Just a question, but if this is a consideration of added mass, why not build a form and pour into it? That way, all you'd have to trowel out is the top.

Jim

[Balty Knowles]

Re: Help with Stucco Cracks

Thanks Jim, I'll do that. You can barely see the cracks unless I dampen down to highlight them.

With regard to pouring into a form, thanks for the idea, I think it would be great for the insulation as well.

I'm very short on insulation space & only have 3 to 4 inches from the dome to the brickwork. See pics. I will extend the brickwork up from the stand to form a round tower built into the retaining wall.

If I place the insulfrax around the inside face of the brick housing & partially fill the space left in the center with vermiculite concrete I think It'll reduce some heat from the floor leaking under the blanket & also keep the blanket flat up against the face of the brickwork.

Regards

Balty

[CanuckJim]

Re: Help with Stucco Cracks

Balty,

Take maximum advantage of the space you have for insulation. Stuff the gap right full. Don't forget to allow enough room above for even more than you can get on the sides. Retrofitting these ovens is a nightmare: do it from the beginning. Looks to me like you're on the right track. Are you the brickie? If so, nice work.

Jim

[Archena]

Re: Help with Stucco Cracks

Balty,

... Are you the brickie? If so, nice work.

Jim

Um, would it have been bad work if he wasn't?










On a (hopefully) helpful note: Stucco




On a semi-helpful, hopefully humorous note: if the second layer cracks you can stain it and tell everybody you were going for the 'crackled' look. (People actually do that on purpose! )


And on the final note: yes, I'll go annoy somebody else now...

[Balty Knowles]

Re: Help with Stucco Cracks

Thanks for the advice Jim.

No, not a bricky but I did serve a 5 year apprenticeship in engineering. I think I got my admiration for craftsmen & their skills then. I've had several accolades in the many years since but it's my indenture papers I'm most proud of. They hang on my office wall in a more prominent position than my degree.

There's an honesty in hand skills & hard work that's very rare elsewhere. I live in Silicon Valley which is quite a cut throat work place & many of the higher ups here obsess over woodworking projects on the weekend. I think thats why.

I placed this thread in the wrong place so I'm starting a new one for the insulation.

Best Regards

Balty

[Bacterium]

Re: Help with Stucco Cracks

Neil.....or others
just back on the stucco/render

using a mix of 3:1 (plaster sand, cement).....
what would lime do to the mix eg
3:1:1 (plaster sand, cememt, lime)

reduce cracking?

I bought some with the last lot of gear to stucco/render the outside of my dome (final weatherproof coat)......but I forgot why its been a few pizzas since then

[nissanneill]

Re: Help with Stucco Cracks

Damon,
By reducing the cement ratio, you will reduce the water proofing characteristics. This will in fact be the case with the addition of other ingredients such as lime.
Today, we use lime I think to make the mortar a little lighter in colour and more workable, but also because it was used years ago in preference to cement. Let's face it, our pioneers used materials that were found locally to build with and limestone burned in a kiln or oven was easy to produce. Today, cement is easy to source and is easily (although expensive in some instances) to transport and it is also superior to lime in strength and water resistance when used in the correct proportions.
I recall some thread on the forum with reference to a specialist who said that lime has no part in refractory materials, (even though I used it in my mortar for the dome).

Neill

[Bacterium]

Re: Help with Stucco Cracks

ok.... I'll stick with the 3:1

Maybe I'll swap the lime for some oxide instead. I suppose I can then correctly colour match the oven coating (final outer coat) with the general area of my pergola. I've just been looking through the Diggers range of oxide:
http://www.diggersaust.com.au/catalogue/index.html

It talks about putting some sort of "Raincoat" stuff of theirs onto the surface after it all - to seal it - as they say "to protect against fungus decay". I'm guessing this is more about keeping the natural look of the surface......whereas the real water resistance comes from the 3:1 mix.

.....any thoughts?

[Archena]

Re: Help with Stucco Cracks

The sealant doesn't so much increase water resistance as make the surface water repellent. It's basically keeping the water from seeping into the surface (which happens normally in concrete) so that fungus, mildew etc cannot grow and salt won't leech (one surface has to take in water for that to happen - think concrete retaining walls).

At least, that's how I understand it.