I have an older baking oven where some parts of the mortar/plaster on the outer edge above the firebox have come loose. The mortar is about 4 cm thick there and underneath there are bricks. What type of mortar should I use? The mortar around the damage is quite smooth today, but for me, it's most important that the mortar doesn't come loose again.
Mortar mixed with Gullfiber? It's not the material (glass) itself that insulates but that in its original fluffy form, it creates many pockets of still air. If you mix (crumbled? cut?) Gullfiber into mortar, you won't get any insulating effect, and furthermore, glass wool melts at high temperatures. Not a good idea.yxet said:
How does glass crunch contribute to preventing it from cracking or coming loose? It may counteract crack formation to some extent because it is made up of threads and thus has a reinforcing effect like fiberglass reinforcement in thin plaster and other special uses, but that effect is probably vanishingly minimal due to the brittleness of the glass wool. I don't think it would counteract loosening in any way. Your repair most likely wouldn't have come loose or cracked anyway, that's my thought...
Mixing fiberglass into the mortar for a chimney was quite strange. Fiberglass is not allowed to be used as insulation around a chimney precisely because it cannot withstand high heat (it should be stone wool). Mixing it directly into the mortar is not likely to impress any chimney sweep...
Back to TS's question. The golden rule for all plastering is to never apply a stronger mortar over a weaker one. This only leads to cracks, and the risk is that a new strong layer also destroys what is behind it. If the baking oven is old (and has not been renovated in modern times), it is most likely both plastered and built with lime or clay mortar. Both of these mortars are significantly weaker than modern cement mortars (and clay mortar is weaker than lime mortar). So you should not use any standard fireproof cement mortar, which I am afraid they might recommend at a fireplace store or similar place. You can recognize clay mortar by how it crumbles apart and dissolves in water. However, the fact that a mortar is weak also means that it is better able to absorb the movements that often occur in an older chimney—instead of cracking in large slabs (especially applies to clay mortar). We have renovated our own baking oven and found that it was built with clay mortar—which is also what we used. There are several threads about this already on the forum. Search in the building restoration section.
Back to TS's question. The golden rule for all plastering is to never apply a stronger mortar over a weaker one. This only leads to cracks, and the risk is that a new strong layer also destroys what is behind it. If the baking oven is old (and has not been renovated in modern times), it is most likely both plastered and built with lime or clay mortar. Both of these mortars are significantly weaker than modern cement mortars (and clay mortar is weaker than lime mortar). So you should not use any standard fireproof cement mortar, which I am afraid they might recommend at a fireplace store or similar place. You can recognize clay mortar by how it crumbles apart and dissolves in water. However, the fact that a mortar is weak also means that it is better able to absorb the movements that often occur in an older chimney—instead of cracking in large slabs (especially applies to clay mortar). We have renovated our own baking oven and found that it was built with clay mortar—which is also what we used. There are several threads about this already on the forum. Search in the building restoration section.
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