Another curing question

[pmgnut]

Thank you Mike. Would you happen to have an example of "buttressed on the outside " so I can see what you mean?

[SableSprings]

I'll look for a good example for you. The flying buttress developed for the large churches in Europe is the classic example...but obviously your oven doesn't have a large, structure weight pressing the sides outward excessively. Generally for a buttress, you're just adding some brickwork along the lower wall sides of the arch to keep those bricks from any lateral movement. Some builders have even used metal strapping to prevent this potential for progressive shifting outward. Again no big worries, just something to keep an eye on and if the oven does start displaying evidence of a consistent & outward movement, adding some buttressing would solve the issue.

[pmgnut]

so far its the same cracks opening and closing. One linear crack about 3 courses down from the top {outer common brick layer} and one smaller one on the other side. Some minor hairline cracks in the joints on the back wall. Overall it cooks and burns well though. I'll get around to grinding them out and filling them in with heat stop soon.

[SableSprings]

Here's a link to Karangi Dudes very well documented barrel vault oven. Some good discussion on buttress use about post 13. Consistent oven cracks that return to their original position is fabulous! Honesty not sure if grinding out the cracks & patching is worth it. When the oven expands during firing, those forces will always cause a break at weak points (consider them expansion joints). In addition, it is almost impossible to get a good bond and a crack patch will very likely crack itself & simply fall out. Better to leave the known cracks alone and just keep an eye on them. Relax! Cracks are a badge of honor for a well used oven!

https://community.fornobravo.com/for...rel-bread-oven

[pmgnut]

Ahhh I see. Thanks for finding that for me. So I'm still trying to understand the science behind cracking - am i correct in saying all cracks in a wfo initially start due to moisture still in the masonry? If that is the case, then grinding and filling in after all moisture has escaped would remedy the problem? Or can a wfo still crack even though all moisture is removed? I'm not really worried about the cracks in ours but just trying to understand the science behind it all.

[david s]

There are two main problems, both associated with the presence of water. Firstly that of steam spalling which occurs when water suddenly turns to steam, expands and blows apart the wall from the middle. Water closer to the surface has a better chance to escape so it's the water in the middle that is the problem. As water's boiling point begins at 100C (212F) do not assume it's safe once the temperature gets to around 150C. In fact as most potters know, most steam spalling occurs at around 250-280C

Secondly because the top of the dome gets hotter faster, primarily because of rising heat due to convection, there is a huge difference in temperature between the crown of the dome and the base, which also leads to a huge difference in thermal expansion. (As an aside heat doesn't care what direction to travel by radiation or convection.) The problem is exacerbated if moisture is present because water will hold the temperature down. A considerable amount of energy is required to convert water to steam. So, this problem is further aggravated as the moisture is driven out of the top of the dome leading to a more rapid rise of temperature there and the resulting increased difference in both temperature and thermal expansion. Allowing the whole oven to return to ambient temperature after a fire therefore allows you to begin with an equal start again. It probably also allows some migration of moisture to even out over the whole dome. This is why a series of fires, usually 7 fires in 7 days is more successful than a continuous long fire.

Once this initial process has been completed, if the oven gets wet again, it is not mandatory to return to such a careful regime. Usually one or two long fires are a sufficient remedy to return the oven to normal working order.