Kiln bricks chip, crack and get damaged by glaze runs, and most of that is repairable with the right refractory materials. The distinction that matters is between cosmetic and structural: a chipped edge is a maintenance job and a crack through a wall is not.
What can be patched
Chips, small cracks, damaged element grooves and glaze splashes can be repaired with kiln cement or a refractory patching compound rated for the kiln's temperature. Done properly a patch lasts years. The essential step is letting the repair dry completely before firing, because trapped water becomes steam and blows the patch back out.
Kiln cement and refractory cement
Both are heat-resistant materials for bonding and patching, sold by suppliers who stock kilns. Match the rated temperature to the kiln and read the product's instructions on curing, which usually specify air drying followed by a slow first firing. Ordinary building materials have no place here at any temperature.
What needs a brick replaced
A crack running through a wall, a brick that has crumbled around an element groove so the element cannot be held, or damage to a structural corner. Replacement bricks are available for kilns from established makers, which is another reason parts availability matters when buying used.
Avoiding the damage in the first place
Most brick damage in a hobby kiln comes from glaze runs and from knocks while packing. Kiln wash on shelves, bare feet on pots, care with props and not dragging pieces across the brick prevent almost all of it. Bricks are soft and are damaged more easily than people expect.
Questions people ask about kiln bricks
Can I repair a cracked kiln brick?
Chips and small cracks, yes, with a refractory patching compound rated for the temperature, dried fully before firing.
What cement can I use in a kiln?
Kiln or refractory cement rated for the kiln's temperature, from a pottery supplier. Ordinary building materials will not do.
When does a brick need replacing?
When a crack runs through a wall, a corner is damaged structurally, or a groove has crumbled so an element cannot be held.