As wood fired ovens heat-up, they are prone to expansion. And that expansion typically occurs between the firebrick and the mortar joint which may leave a small hair-like or hair-line crack in the mortar.
Generally, the hairline crack will not affect the performance or structural integrity of the oven… but it will bug you for a while. Do not try to fix the crack by grinding out the mortar and reapplying fresh mortar.
The easiest way to fix a hairline crack is to fill the void with a small amount of flexible fireplace mortar that you can apply with a caulk gun. Super easy! Simply visit the Rutland store on Amazon.com and purchase a tube of high-temp mortar.
The BEST WAY to prevent visible hairline crack on the outside of the oven is to wrap the oven with Ceramic Fiber Blanket during the initial oven construction. If a hairline crack does occur, it will stop once it hits the blanket and will not transfer to the outside of the oven.