Your tiled stove/tiled fireplace is exposed to intensive variations in temperature, that means is it heated up and cools down frequently. The used materials are exposed to great strain. In order to avoid stress cracks as much as possible we process high-quality fireclays and special plasters. Small cracks may sometimes appear because it is not possible to overrule the laws of physics of the expansion of warm objects. But those cracks do on no account impair the stove’s function. Small hairline cracks are therefore absolutely normal and no reason for a complaint. They will usually shrink up when the tiled stove/tiled fireplace cools down.