Firebrick (Fireclay Brick): Duty Classes, Composition, Sizes and Uses

Firebrick is dense fireclay brick: a fired, acid alumina-silica refractory brick with 30 to below 45 % Al2O3 under ISO 836:2001, graded in the US by ASTM C27-98(2022) into super duty, high duty, semi-silica, medium duty and low duty classes. Europe designates the same brick as group FC 30, FC 35 or FC 40 under ISO 10081-1:2003, and India specifies it as moderate, high or super heat duty under IS 6:1983, IS 8:1994 and IS 9010:1978.

The trade calls it fire brick, fireclay brick, furnace brick or, in UK use, chamotte brick. In North America the reference shape is the 9-inch straight, 9 × 4-1/2 × 2-1/2 in (229 × 114 × 64 mm), and fireclay brick is the acid alumina-silica class of refractory materials.

The Indian grades step up from 30 % to 38 % to 40 % Al2O3 minimum, while the US classes rest on test results: PCE, modulus of rupture and reheat change. Industrial firebrick is priced per quote, and its main health rule is the OSHA respirable crystalline silica limit of 50 µg/m³ (29 CFR 1910.1053, effective 2016-06-23).

Every value names its standard and edition, the product data sheet or the dated source it comes from; see our methodology and fact-checking process.

The table below summarizes the class boundaries and designations of firebrick by standard.

What Is Firebrick?#

Firebrick is a fired, acid alumina-silica brick held together by a ceramic bond, and its US class is set by tested properties, not by clay content. Its acid character means silica-rich chemistry that suits acid rather than basic slags.

ASTM C27-98(2022), first approved in 1958, classifies fireclay brick by physical properties because the Al2O3 and SiO2 contents of its classes overlap. The same standard classifies high-alumina brick by Al2O3 content instead.

The chemistry line between the two families sits at 45 % Al2O3 in ISO 836:2001: below it the brick is fireclay, at or above it high alumina, and silica brick starts at 93 % SiO2. Fireclay brick is the first of the alumina-silica classes among refractory bricks.

Super, High, Semi-Silica, Medium and Low Duty Firebrick#

ASTM C27-98(2022) sorts fireclay brick into five classes, super duty, high duty, semi-silica, medium duty and low duty, and splits super duty and high duty into regular, spall-resistant and slag-resistant types. The classifying tests are pyrometric cone equivalent (PCE) to ASTM C24-09(2022), modulus of rupture to ASTM C133-24 and reheat change to ASTM C113-14(2025), with ASTM C16 and C134 also referenced; the inch-pound values are the standard units.

The table below sets the five classes against their types, classifying tests and one named product per class where a data sheet exists.

A duty name on a data sheet is the producer's claim, and the buyer checks it against the PCE, MOR and reheat results the producer reports with their test editions.

ISO 10081 Fireclay Groups and Indian Heat-Duty Grades#

Outside the US, fireclay brick is designated by ISO 10081-1:2003 (EN ISO 10081-1:2005) as group FC 30, FC 35 or FC 40, and India specifies it in IS 6:1983, IS 8:1994 and IS 9010:1978. The ISO designation adds the principal raw material, its state and the bond type to the group name; ISO confirmed the 2003 edition on 2025-06-10.

The table below compares the ISO groups with the requirements of the three Indian standards; psi values are calculated from MPa.

An IS 8 fire brick, the high heat duty grade, needs at least 38 % Al2O3 and PCE cone 32, eight points of alumina and two cones above IS 6, with a load-softening ta of 1400 or 1425 °C (2,552 or 2,597 °F) by type.

What Is Firebrick Made Of?#

Firebrick is made of fireclay grog, that is calcined fireclay or flint clay (chamotte), bonded by plastic fireclay and fired, and its fired chemistry is SiO2 and Al2O3 with smaller amounts of iron, titanium and alkali oxides. Fireclay brick is rarely made from one raw clay: producers blend clays and add grog, according to a peer-reviewed review by Garbers-Craig (Journal of the SAIMM, 2008).

The table below lists the materials of fireclay brick, their role and, for the fired brick, the calcined chemistry of three named products.

Alumina rises with duty class across one producer's range, from 30.78 % in the medium duty Smithfield to 37.63 % in JET D.P. and 44.32 % in the super duty PILOT D.P., while silica falls from 62.93 % to 49.37 %. The super duty value sits just under the 45 % line where high-alumina brick begins, and each grain behind these figures is described under refractory raw materials.

Ceramic Bond, Clay Binders and Additives in Fireclay Brick#

Fired firebrick needs no cement: its permanent bond is a ceramic bond formed by sintering above 800 °C (1,472 °F) under ISO 10081-1:2003, while temporary pressing aids only hold the green brick until they burn out. The same standard also allows an inorganic chemical bond formed below 800 °C (1,472 °F) for products that rely on a chemical binder instead.

Plastic fireclay is the clay binder that holds the grog together in the pressed brick before firing. Kyanite Mining specification literature describes raw kyanite as an expansion agent that offsets the firing shrinkage of clays.

Temporary binders, such as lignosulfonate, dextrin and polyvinyl alcohol (PVA), give green strength for pressing and handling and burn out or decompose during firing without leaving a permanent bond. Their chemistry and handling rules are covered under temporary and pressing binders.

Firebrick Properties: Standard Requirements and Typical Product Values#

Firebrick data come in two kinds: standard requirements that every brick of a graded class must meet, shown here from the Indian standards, and typical values that one named product reached in its producer's tests. US class limits sit in ASTM C27-98(2022) and are not repeated here.

The table below pairs each property with its standard requirement, typical values of three named products (calcined chemistry in the composition table) and the ASTM and ISO methods.

A firebrick's high-temperature limit carries three different numbers, and they answer different questions. The super duty PILOT D.P. carries a producer service rating of 1599 °C (2,910 °F), a PCE of cone 33 equivalent to 1743 °C (3,169 °F), and IS 9010:1978 asks for a load-softening ta of at least 1450 °C (2,642 °F): the rating is a claim, PCE a softening test without load, and RUL the temperature at which the brick starts to deform under load.

Reading a Firebrick Data Sheet: Seven Checks#

Seven checks turn any producer's firebrick data sheet into numbers that compare with another offer. They are listed below in the order a buyer reads the sheet.

1. Confirm the class claim against ASTM C27-98(2022) or the named IS grade.
2. Read chemistry on its stated basis, calcined or as-received.
3. Match strength values to their method, ASTM C133 cold crushing strength and modulus of rupture in the C133-24 edition, and density to C20-00(2022).
4. Check that PCE was measured to ASTM C24 pyrometric cone equivalent, edition C24-09(2022), or to ISO 528:1983.
5. Read reheat change with its temperature and schedule to C113-14(2025).
6. Read hot load with its schedule, since ASTM C16-03(2025) and the ISO 1893:2007 RUL test work on different principles.
7. Compare density, porosity and strength together, for example 2.2 g/cm³ (137 lb/ft³) and 45 MPa (6,527 psi) at 42 % Al2O3 for Belref ARBEL 42, and ask for the test date.

Which Standards Govern Fireclay Brick?#

Fireclay brick is classified by ASTM C27-98(2022) in the US, designated by ISO 10081-1:2003 in Europe and specified by IS 6, IS 8 and IS 9010 in India, and tested with ASTM and ISO methods that are not interchangeable. EN ISO 10081-1:2005 is the European adoption of the ISO designation.

The table below lists the classification and test standards side by side; methods in one row measure the same property, and their results are not interchangeable.

EN ISO 10059-1:2025 and EN ISO 5014:2025 replaced EN 993-5 and EN 993-6 on 2025-12-09, so a European data sheet citing the older parts predates that change. Refinery brick linings follow API Std 975, 2nd edition (April 2025), for quality control of the brick system, and every other classification and test method is indexed under refractory standards.

Where Is Firebrick Used in Industry?#

Industrial firebrick lines hot blast stove cooler zones, coke oven non-silica parts, preheaters, glass furnace regenerators and structures, titanium tetrachloride chlorinators and backup layers behind high-alumina brick. These uses come from the application list of HWI KX-99, a high-fired super duty brick, and the HWI glass brochure (2022), unless another source is named.

The table below maps each industry to the zone where fireclay brick serves and the reason given by the source.

In one plant example, a rotary-kiln incinerator for hazardous waste runs above 1000 °C (1,832 °F) on 300 mm (11.8 in) of high-alumina brick, 60 to 90 % Al2O3 by zone, backed by 50 mm (1.97 in) of hard firebrick (refractories WORLDFORUM paper). The firebrick there sits in the cooler layer, the same role it plays in glass regenerator backup and stove cooler zones.

Bricklaying, Mortar and First Heat-Up of Firebrick Linings#

Firebrick is laid, not cast: courses are set in a fireclay or high-alumina mortar, air-setting or heat-setting, with thin joints and an expansion allowance, and the fired brick itself needs no curing. Joint thickness, bonding pattern and cutting belong to refractory installation methods.

The mortar matches the brick chemistry, fireclay mortar for fireclay brick, and a high-alumina lining needs a larger expansion allowance than a fireclay lining. Industrial practice cuts firebrick with a wet saw under dust control, because dry cutting releases respirable silica.

A first heat-up concerns the mortar and any cast parts of the lining, and it follows their producer's guidance rather than a brick schedule. Refinery brick systems add the quality-control rules of API Std 975, 2nd edition (April 2025). Four related practice topics are linked below.

• refractory mortar: air-setting and heat-setting types for laying firebrick
• refractory dry-out and heat-up schedules: heat-up of cast parts within a brick lining
• refractory lining design: working, safety and insulating layers
• refractory repair and maintenance: relining and hot repair of brick linings

Why Firebrick Linings Fail and How They Are Inspected#

Firebrick linings fail by thermal spalling, by shrinkage and subsidence above the class temperature, and by alkali or slag attack. The data sheets of one producer show how duty class shifts the first two: the medium duty Smithfield loses 9.5 % in a panel spalling test after a 1599 °C (2,910 °F) preheat against 3.3 % for the high duty JET D.P., and it shrinks 2.8 % on reheating at 1399 °C (2,550 °F).

The table below links each failure mode to its cause and to the check that detects it.

Incoming brick is checked for size and warpage to ASTM C134-95(2023), porosity and bulk density to C20-00(2022) and strength to C133-24, with a visual check for iron spots. In-service checks and acceptance sampling for brick linings belong to refractory inspection and quality control.

Crystalline Silica and Other Health Rules for Firebrick#

The health hazard of firebrick is respirable crystalline silica released when brick is cut, ground or torn out, and OSHA limits it to 50 µg/m³ as an 8-hour TWA (29 CFR 1910.1053, effective 2016-06-23). Fired fireclay brick contains quartz and cristobalite, and the OSHA action level is 25 µg/m³, with the construction rule in 29 CFR 1926.1153 (effective 2016-06-23).

In the EU, Directive 2004/37/EC as amended by Directive (EU) 2017/2398 sets 0.1 mg/m³ for the respirable fraction of crystalline silica dust, with transposition by 2020-01-17. The ECHA line for quartz and the other classifications follow in the table below.

The table lists each classification and limit with its instrument and date.

Two related safety topics are linked below.

• crystalline silica and cristobalite in refractories: exposure control during cutting and tear-out
• asbestos in old refractories: materials found in old linings

Firebrick Grades, Packaging and Sourcing#

Industrial firebrick is bought by class and shape on pallets, for example 465 straights or 888 splits per pallet at one US producer (Alsey), and offers compare on class standard, chemistry, test data and shape series. A request lists six items, shown below.

1. Class and standard: ASTM C27-98(2022) class, ISO 10081-1:2003 group or IS grade with edition.
2. Chemistry with its basis, calcined or as-received.
3. Test methods with editions for PCE, strength, reheat change and hot load.
4. Shape series: US 9 × 4-1/2 × 2-1/2 in (229 × 114 × 64 mm) or ISO 5019-1:1984 squares of 230 × 114 × 64 or 76 mm, with arches, wedges and splits per refractory brick sizes and shapes.
5. Quantity in pieces or pallets.
6. Matching mortar for the class.

Firebrick reaches buyers through producers, distributors and the suppliers of complete linings, and a request with class, edition, shape series and quantity lets each one quote the same brick. Producers and distributors by country are listed under refractory manufacturers and suppliers.

What Sets the Price of Industrial Firebrick?#

Industrial firebrick is priced per quote, and seven factors set the price: duty class and grade, raw-material origin, shape complexity, quantity, packaging, freight and whether installation is included.

The table below lists each price factor and what the buyer states in the request.

The US BLS producer price index for clay refractories (WPU1352, 1982 = 100) stood at 405.818 in September 2026, a trend, not a price. The index is not seasonally adjusted and tracks the whole clay refractory group, so a firebrick quote still rests on the seven factors above.

Firebrick Compared With Other Refractory Brick and Lining Materials#

Every firebrick is a refractory brick, but not every refractory brick is a firebrick: fire brick in industrial use means the fireclay class, while high-alumina, silica, magnesia and carbon-containing bricks are other classes of the same family. Castables compete with firebrick where a lining needs no joints or a complex shape, while brick brings factory-set properties and fixed dimensions, as set out in castable vs refractory brick.

Insulating firebrick is a light, porous product for backup layers, classified by ASTM C155-97(2022) (active, 2022-09-27), and it is not a dense firebrick. Its role is to limit heat flow behind the working lining, while dense firebrick carries load, abrasion and slag.

Firebrick Versus Common Building Brick#

Common building brick is a solid masonry unit made from clay or shale under ASTM C62-23, while firebrick is graded by refractory tests such as PCE and reheat change under ASTM C27-98(2022). Brick for residential fireboxes has its own specification, ASTM C1261-22 (active, updated 2022-12-23).

Alternatives to Fireclay Brick by Service#

Four classes replace fireclay brick when a zone runs hotter, needs insulation or has an awkward shape: high-alumina brick, silica brick, insulating firebrick and castables. The table below names the service in which each one takes over.

Firebrick FAQs#

Are Kiln Bricks the Same as Fire Bricks?#

Kiln brick is a usage name, not a class: an industrial kiln is lined zone by zone with fireclay, high-alumina, magnesia-spinel or other classes. Magnesia-spinel brick, for example, serves cement kilns and ladles.

What Does Hard or Dense Firebrick Mean?#

Hard or dense firebrick is ordinary fireclay brick, named to set it apart from insulating firebrick. One high duty product, Alsey JET D.P., has a bulk density of 2.14 g/cm³ (133.7 lb/ft³) on its data sheet.