Kiln bricks are the refractory blocks that line a kiln and hold the heat in. In a home electric kiln they are almost always soft insulating firebrick, cut with grooves that hold the heating elements. This guide explains insulating firebrick vs hard brick, what grade names like K-23 and K-26 mean, why thickness matters, which cracks are normal, and how kiln makers say to handle repairs.
Home Pottery HQ is a research site. We did not repair any kilns for this guide; the facts below come from kiln maker support pages, manuals and refractory supplier listings named in the Sources line.
What kiln bricks do
Kiln bricks have three jobs: hold heat in, keep the outside of the kiln cooler, and support the elements in their grooves. Good insulation means faster heating, less electricity per firing and more even temperatures.
They also wear out. Every firing expands and contracts the brick, and loading pots, brushing past walls and dropping posts all chip the soft surface. Knowing what is normal wear and what needs a fix saves both money and elements.
What is firebrick?
Firebrick, also called fire brick, fireclay brick or refractory brick, is a block of ceramic material made to line furnaces, kilns, fireboxes and fireplaces, according to Wikipedia's entry on fire brick. The same entry puts typical refractory bricks at roughly 25% to 45% alumina with about 60% silica, plus smaller amounts of other oxides. Refractory simply means the material keeps its shape at temperatures that would melt or slump ordinary building brick.
"Firebrick" covers a wide family. The firebrick sold for fireplaces, wood stoves and pizza ovens is usually dense, heavy brick. The kiln brick inside a home electric kiln is a different product: light, porous insulating firebrick. Sellers also use names such as ceramic fire brick, lightweight fire brick and kiln fire bricks, so read the grade and density on the listing rather than the name. The rest of this guide explains how the two main types differ and which one a home kiln uses.
This guide covers understanding, repairing and replacing the brick in a kiln you already own. We do not cover building a kiln from firebrick; buy a commercial kiln and follow its maker's instructions.
Insulating firebrick vs hard fire brick
There are two broad families of kiln brick.
Insulating firebrick (IFB, soft fire brick or lightweight fire brick) is full of tiny air pockets. It is light, easy to cut and carve, and insulates very well, but it is fragile. Sheffield Pottery describes it as pale yellow and soft enough to mark with a fingernail. Electric kilns from Skutt, L&L, Olympic and Paragon use it for walls and lids.
Hard fire brick (dense brick) is heavy and strong. It handles abrasion, flame, ash and slag well but insulates much less. It is used in wood, salt and soda kilns, gas kiln fireboxes and floors, and industrial furnaces. Reference sources note that dense brick suits harsh conditions like wood firing, while porous kiln brick suits electric and gas kilns.
| Feature | Insulating firebrick (soft) | Hard firebrick (dense) |
|---|---|---|
| Weight | Light | Heavy |
| Insulation | High | Low |
| Strength and abrasion | Fragile, scratches easily | Strong, resists abrasion |
| Typical ratings | 2,000°F to 2,800°F (1,093°C to 1,538°C) grades, G20 to G28 at Sheffield | High Heat Duty cone 31 to 32; Super Duty cone 33 to 34 (Sheffield) |
| Typical use | Electric kiln walls and lids, raku and gas kiln linings | Wood and salt kilns, fireboxes, gas kiln floors |
| Cutting | Hand saw or rasp | Masonry saw |
K-23, K-26 and other brick ratings
Insulating firebrick is sold in grades named by a number that matches its approximate maximum service temperature in hundreds of degrees Fahrenheit. Brand prefixes vary (K, G, JM), but the number tells you the rating.
| Grade | Rated temperature | Where you see it |
|---|---|---|
| 20 | 2,000°F (1,093°C) | Backup insulation, low-fire uses |
| 23 | 2,300°F (1,260°C) | Standard brick in most electric pottery kilns; L&L calls K23 its standard brick |
| 25 | 2,500°F (1,371°C) | L&L uses K25 on JH models and on the front edge of some front-loading kilns |
| 26 | 2,600°F (1,427°C) | Hot faces in gas kilns and high-fire linings |
| 28 | 2,800°F (1,538°C) | Very high-temperature hot faces |
Kiln makers use K-23 brick in many electric kilns, including models rated to cone 10, and some use a higher grade in spots that take more wear or heat, as L&L does with K25. Higher grades are generally denser and insulate a little less. If you replace bricks, use the grade and thickness your kiln maker specifies, not whatever a refractory supplier has in stock.
Brick thickness: 2.5 in vs 3 in
Electric kilns usually come with 2.5 in or 3 in walls. Thicker brick holds heat better and keeps the outside cooler, at the cost of weight and a slightly smaller chamber for the same outside size. L&L sells standard K23 brick in both 2.5 in and 3 in thicknesses, each 4.5 in high and 9 in long. In our product research, the Paragon Caldera uses 2.5 in brick and the Skutt KM-614 uses 3 in. Our best kilns for home pottery guide lists brick thickness for each model.
Are kiln brick cracks normal?
Mostly, yes. L&L states that firebrick cracks as it heats and cools and that this is expected, and that the cracks close up when the kiln is hot. Olympic's manual says hairline cracks after a few firings are expansion cracks and do not harm how the kiln works.
What is not normal:
- A crack that goes all the way through a brick, so pieces move.
- Bricks that sag, bulge or have fallen out of place.
- Broken element grooves where an element hangs loose.
- A lid crack that keeps growing or lets you see light.
- Large chunks missing from the floor.
Kiln brick repair with maker cement
Small damage is a normal owner repair, done with the cement, mortar or repair kit your kiln maker sells or recommends. Kiln makers publish repair steps for their own models, so start with your manual or the maker's support pages. L&L, for example, has support articles on repairing small holes and chips, fixing the lid seal and stopping heat leaks.
What the makers' guidance has in common:
- Power off first. Unplug the kiln or switch off its breaker, and work only on a cold kiln.
- Clean the damaged area of loose crumbs with a soft brush or vacuum.
- Use the maker's product. Olympic's manual says kiln mortar can be spread thinly over exposed brick to seal it and stop dusting, and that floor holes can be patched with kiln wash mixed to a paste, scraped flush and dried before firing.
- Keep cement out of element grooves and off the elements.
- Let it dry fully before the next firing.
Replacement bricks can be cut to fit, but swapping a wall or floor brick means removing elements or the steel jacket on many kilns. That is where most owners hand over to a technician.
What maker repair kits contain
L&L's Soft Brick Repair Kit, as listed by The Ceramic Shop, holds a half pint of phosphate-bonded cement, a half pint of brick facing, a small firebrick piece, a quart of firebrick dust and instructions. The listing says it is for small to moderate surface cracks, not deep or through cracks, and that it is meant only for L&L kilns. Other makers sell their own cements and parts, so buy the product made for your brand.
What a maker's brick replacement procedure looks like
Skutt publishes a brick replacement sheet for its kilns. It starts with "Always Unplug The Kiln Before Working On It!", tells owners to order bricks from the parts list through a Skutt dealer, and warns that the elements are brittle and must be handled gently when they are lifted out of the grooves. It also says replacing the bricks that hold element terminals involves cutting and renewing element connectors and should normally wait until that element needs replacing. If any of those steps sound beyond you, that is the point to call a technician.
Brick facing and hardeners
L&L sells a brick facing that hardens the inside surface of the brick, mostly for lids. L&L says to dilute it to about the consistency of milk and apply one thin, nearly transparent coat, and warns that too much can soak in deep and flake off after a few firings. Use only what your kiln maker recommends.
Where to buy replacement kiln fire bricks
Start with your kiln maker, because the right brick for your kiln is the grade, thickness and cut the maker specifies. Skutt's replacement sheet points owners to the parts list and a Skutt dealer. L&L bricks are sold by kiln type at dealers such as The Ceramic Shop, with or without hard element holders, so you can reuse parts you already have. L&L also lists its standard K23 brick in 2.5 in and 3 in thicknesses, 4.5 in high and 9 in long.
Refractory suppliers are the second source. Sheffield Pottery, for example, lists insulating firebrick in grades from G20 to G28 and dense refractory bricks in High Heat Duty and Super Duty grades. Online marketplaces such as eBay also list insulating fire brick, for example cases of 9 x 4.5 x 2.5 in K23 brick, but those listings rarely say which kilns the brick suits.
Before you order a fire brick for a kiln from anyone other than the maker, check four things: the grade number (it should match or exceed the original), the thickness, the dimensions of the brick you are replacing, and whether it is insulating or dense. Dense fireplace firebrick is the wrong product for an electric kiln wall, because it insulates poorly and is far heavier than the brick around it.
When to call a kiln technician
Call a qualified kiln technician, or the kiln maker's service line, when:
- Bricks are loose, broken through, or falling out of the wall.
- An element groove is broken and the element sags or touches the shelf.
- The lid or floor needs replacing rather than patching.
- You see scorch marks on the steel jacket, or wiring and terminals are exposed.
- The kiln keeps losing heat after you have patched the obvious gaps.
Never work inside a kiln that is plugged in or hard-wired and powered. Electrical repairs, element terminals and any work behind the switch box belong to a qualified technician or licensed electrician.
Element condition is a separate topic; see our kiln elements guide. For routine care that protects bricks, see kiln maintenance.
Brick dust safety
Kiln brick and kiln wash can release fine dust when crumbled, cut or vacuumed. Treat it like clay dust: wet-wipe or use a HEPA vacuum, do not dry sweep, and wear a NIOSH-approved respirator when cutting brick or cleaning out debris. Our clay dust safety guide covers the same habits for the whole studio.
Common kiln brick mistakes
- Dropping posts and shelves against the walls while loading.
- Leaning on the lid edge or slamming the lid.
- Letting glaze drips sit on the floor, where they eat into brick. Protect the floor and shelves with kiln wash on the right surfaces.
- Using generic mortar instead of the maker's cement.
- Replacing brick with a different grade or thickness than specified.
What you need
For most owners, brick care is a small repair kit from the kiln maker, a soft brush and a HEPA vacuum. If you are shopping, brick thickness is worth comparing; see our best kilns for home pottery guide. If you are looking at a used kiln, brick condition is one of the first things to inspect, covered in our kiln for sale guide.
Sources: L&L Kilns parts listings for 2.5 in and 3 in K23 and K25 firebrick; The Ceramic Shop listings for L&L brick by kiln type and the L&L Soft Brick Repair Kit; Skutt brick replacement instructions (10.2 Brick Replacement, hosted by Archie Bray Clay); L&L knowledgebase articles on firebrick cracks, small hole and chip repair, lid seal, heat leakage and brick facing; Olympic Kilns electric kiln manual; Sheffield Pottery refractory brick and kiln cement listings; Wikipedia, Fire brick (definition, composition, dense vs insulating); eBay listings for cases of K23 insulating firebrick (sizes only); products table data for Paragon Caldera and Skutt KM-614. Specs checked October 2026.