Hairline cracks in a new oven: what they usually mean

You've found a thin line across the dome or floor of your new oven, and you're wondering if you've wrecked it. Often not. But a crack deserves a careful look, so here are the causes in the order to check them.

Cause 1: normal heat movement

Materials grow a little when they heat and shrink as they cool. One clay oven maker says heating a wood-fired oven will create small natural cracks on the interior. It calls this a completely normal and natural process due to thermal expansion of structures, and compares it to buildings, bridges and chimneys¹. The word it uses is "dilatation", which here means the same thing as expansion.

The same maker says the most typical cracks after curing are fine hairline cracks on the base and walls. These, it says, are only cosmetic and will not impact on the performance of the oven¹. Outside, it describes them in the grout joints and the dome². Grout is the filler between bricks or tiles.

That's one maker's view of its own clay ovens. Other materials and other makers may see things differently, so find the cracks section of your manual.

Cause 2: heating too fast

Cracks that are wider than a fine line are a different matter, and I can't tell you their cause. Makers do warn about heating too fast. One says fast curing makes the water escape all at once and may result in cracking the dome³. Another guide says the same thing about water seeking to escape through the whole thickness of the dome if the oven gets too hot too fast⁴.

Follow your manual's schedule, and ask the maker about any crack that worries you.

Cause 3: sudden cooling

Never use water to lower the temperature inside the oven or to put out the fire. One maker says it increases wear and tear and can lead to spalling, pitting and cracking³. Spalling means flakes breaking off the surface.

Why gentle curing helps

The reason makers like a slow start is that the joints and structures adapt better to changes in temperature when the heat is brought up gradually². A gentle cure won't promise a crack-free oven. It just gives the oven the best chance to move smoothly.

What to do about a crack you've found

1. Look at it once the oven is cold. Never poke at a hot oven.
2. Take a photo with something for scale and write down the date.
3. Read the cracks section of your manual.
4. Keep to the curing schedule. Don't add extra heat to "dry it out".
5. Check the crack again after the next fire and compare it to your photo.

Some makers ship repair materials. One clay oven maker says it includes a repair kit with every oven for cracks on the outside². Use only the materials and steps your own maker provides for repairs. Don't improvise with household filler.

When to ask the maker

Write to the maker, with your photos, if any of these apply.

• The crack is wider than a fine line, or grows between fires.
• A piece is loose or has fallen out.
• Your manual says nothing about cracks.
• The crack is in a place the manual calls out, such as a joint or the floor.
• You aren't sure.

I haven't saved a source that gives a width above which a crack matters, and it would differ between oven types. So the maker's number, in writing, is the one to trust. And if the oven is under warranty, keep the photos and dates with your receipt.

For why the water is there at all, see why new ovens hold moisture. For soot, see smoke and dark patches.