Why a new oven holds moisture

Your oven arrived dry to the touch, yet the maker wants a slow first fire. The reason is hidden water. It sits inside the dome, the floor and the joints, and it has to go somewhere.

Where the water comes from

Start with how the oven was made. One oven builder says the masonry absorbs water during construction and manufacture¹. Masonry here means brick, block and concrete parts. Cement and mortar are mixed with water in the first place, so they begin life wet.

Then there's the slow part. One maker points out that concrete items always have excess water evaporating off to the outside air over time. In ordinary concrete, it can take up to 360 days to get the very inside fully cured². An oven isn't ordinary concrete, but the idea carries over. Water leaves thick material gradually, not all at once.

Finishing work adds to it. After installation, the same maker says there's still a great deal of moisture in the mortars, hearth concrete, vermiculite, and the oven chamber and manifold². Vermiculite and the manifold are parts of that maker's own oven, so check your manual for what yours has.

Wet weather can add more. An oven sitting unused will also collect moisture¹. That's why curing isn't only for the first week of ownership.

Why it matters

Picture a wet sponge on a hot plate. The water doesn't politely drift off. It rushes. In an oven the same rush happens inside the walls. One guide says that if the oven gets too hot too fast, water seeks to escape the entire dome all at once¹.

The result, according to the maker, can be cracking in the dome. Failing to follow the curing steps can result in damage up to and including cracking the dome, severely impacting heat retention². Heat retention is how well the oven holds its heat after the fire.

There's a second trap. If you cure the dome bare and add an outer finish later, water in the new finish has to escape too. One maker warns that water trying to escape the stucco will crack it². Stucco is the rough outer coating. That maker advises curing with the insulation and facade in place, so check your own manual for the order it wants.

Checks you can make

You can't see moisture, but you can look for signs that an oven is holding some:

• The inside feels cool and damp when you reach in¹.
• The ash feels heavy when you clear it out¹.
• The oven has stood unused through damp weather¹.

None of these tells you how much water is left. They only say a gentle fire is a good idea. One clay oven maker advises progressive heating so any humidity can disappear before use³.

What to do about it

Don't try to speed things up. The fix is a slow, planned series of small fires, with each day a little hotter than the last. The schedule lives in your manual. For one maker's layout, read the day-by-day schedule.

Not sure which steps apply to you? The manual guide shows where to look. Already seen smoke or dark patches? Try this page on soot and damp.