mortar on firebricks

[fuegos]

On my oven build i have rendered the outside with a clay/sand mix and then a finish coat of lime mortar for waterproofing .There has been some cracking , mainly due to high ambient temperatures during the work and firing the oven too soon.More of a problem is the thin face of firebrick above the oven opening , below the lintel  , about 3.5 cm deep.In normal , non heat exposed construction i would use some metal mesh that's available for corners or edges when plastering but i think that expansion due to heat will just crack the mortar . Has anyone come up with another fix ?

[dcp]

Yess.
1.-Rated up to 500 celsius continuum or 800 short time.
it is basalt fiber mesh if you want to have only supperficial layer. Is about 10$/sq. meter 10/10 mm mesh.
or if you want to replace all basalt fiber dispersable.   This for mixing direct in plaster.About 9$/ kg. 
You can order sample pack or buy from smarter-building-systems.com
I already contact them and orderd sample pack.
2.- linseed oil direct over the plaster.3-4 layers .  This will polimerize in time and made a watreproof surface.

[fuegos]

Thanks dcp I'll check that out or maybe glass fibre ?

[dcp]

I do know a mason specialized in traditional ovens.
he used a glassfiber mesh used for rendering mortar  and is not satisfied with that mesh because is coated with some resin and when oven is hot is giving away bad smelling odours.
those are phaenolic resins ( organic compounds) that are not good to have around you family
the render is not cracking but smells bad.
Smells bad= health problems in future.
I do try to avoid such products because one never knews how it will be affected.
thats why I did speak about uncoated mesh and dispersible fiber. Those are pure stone.
I did a small scale experiment with alumina cement from Kerneos mixed with rockwoll for blowing(not plates) and mixed them with foam from a foam generator( used for making aircrete) 
I got it a heat rezistan ligtweight plate.
I did test the plate over the riser of a 200mm BBR when I tested the core before buliding the stove.
I do know that Rockwoll is coated with som oils for moisture reppelling.
No craks no nothing.
Maybe u can mix Rockwoll with clay/sand and it will not cracking no bad smells 
If you have no problem with that maybe you can try.

[fuegos]

it's outside the oven dcp , on the outer face where the door sits so not exposed to high temperatures.I think it's the different rates of expansion - the brick and the mortar that's covering it - that is the problem .