Refractory Brick Thermal Conductivity: Match Temperature, Method and Density Before You Compare
Jason Gong
Founder & Sales Director · 10+ Years in Refractory
Two refractory brick datasheets showing different thermal conductivity (k-value) numbers for the same brick type are often not disagreeing — they are reporting different things. A k-value only means something once you know the mean test temperature, the test method (calorimeter, hot-wire or laser flash), and the bulk density of the exact sample tested. Match those three first. Only then compare W/m·K numbers across suppliers or brick types.
Why two datasheets can disagree about the same brick
Thermal conductivity is not a fixed number stamped on a brick. It is the result of a specific test, run at a specific temperature, on a specific sample density. Change any one of those three and the reported W/m·K value changes — sometimes by more than the difference between two genuinely different brick types.
That is why a supplier PDF, a competitor's website and an academic paper can all list "fireclay brick: thermal conductivity" and give three different numbers, without any of them being wrong. Our own insulating refractory brick grade guide and refractory brick properties overview publish typical W/m·K ranges by grade and type for selection purposes — this article does not repeat that table. It exists to teach the skill those tables assume you already have: reading a k-value correctly before you put it next to another one.
The three things that must match before a comparison means anything
1. Mean temperature basis, not "the" temperature
Refractory thermal conductivity rises with temperature for most dense brick, and the physics of heat transfer through a porous brick changes again above roughly 1,000°C, where radiation across the pore structure starts to contribute alongside conduction — not just conduction alone. Thermal Processing Magazine's overview of insulating firebrick describes this shift directly: above that range, "radiation rather than conduction and convection" becomes the dominant heat-transfer mechanism, which is exactly why pore structure (not just density) matters more at high temperature than a single low-temperature reading would suggest.
Every credible conductivity figure is reported against a mean test temperature — the average of the hot-face and cold-face temperature during the test, not the furnace's peak operating temperature. The Edward Orton Jr. Ceramic Foundation's refractory testing documentation states this as a testing precondition: the lab requires "the hot face or mean test temperatures desired" to be specified before the run, up to a maximum hot-face limit of 2,700°F (1,482°C) for its calorimeter method. A k-value quoted with no temperature attached, or a range spanning a datasheet's entire service window, cannot be compared to a competitor's single-point figure at a stated mean temperature.
2. Test method — they do not all measure the same thing the same way
At least six recognized standards cover refractory thermal conductivity testing, and they do not use one universal method:
| Standard | Method family | Typical scope |
|---|---|---|
| ASTM C201 | Calorimeter (steady-state) | Thermal conductivity of refractories generally; establishes the mean-temperature basis |
| ASTM C202 | Calorimeter (steady-state) | Supplements C201 specifically for refractory brick |
| ASTM C182 | Calorimeter (steady-state) | Insulating firebrick specifically |
| ASTM C417 | Calorimeter (steady-state) | Unfired monolithic refractories (castables, not fired brick) |
| ASTM C1113 / C1113M | Hot-wire (platinum resistance thermometer) | Faster transient measurement, different sample geometry than calorimeter methods |
| ISO 8894-1 / 8894-2 | Hot-wire (cross-array, resistance thermometer, parallel) | Applicable up to 1,250°C and below stated conductivity ceilings by sub-method |
This is not a minor technicality. A steady-state calorimeter test (ASTM C201/C202) and a transient hot-wire or laser-flash test are measuring the same physical property through different heat-flow conditions, and published comparisons of insulating refractory materials have found the two families of method disagreeing by a meaningful margin on the same material — differences in the roughly 5–25% range have been reported across brick types and temperatures in method-comparison studies, which is often larger than the real difference between two competing products. ISO 8894-1's own scope statement is explicit that its hot-wire sub-methods only apply below stated conductivity ceilings (roughly under 1.5 W/m·K for the cross-array variant, under 15 W/m·K for the resistance-thermometer variant) — meaning the method itself is only valid for part of the conductivity range a refractory line can cover, from insulating brick through dense silicon-carbide grades.
Practical rule: if a datasheet does not name the test method, treat the number as directional only. If two datasheets name different methods, do not treat the numbers as directly comparable without a stated correction.
3. Bulk density of the exact sample tested
Thermal conductivity through a ceramic brick tracks its bulk density and pore structure far more tightly than it tracks the brick's chemical family name alone. A low-density, fine-pore-structure brick within a given composition class conducts less heat than a denser brick of the same nominal composition — which is why "fireclay" or "high-alumina" as a label is not, by itself, enough information to place a conductivity number. Thermal Processing Magazine frames this as the core selection driver for insulating firebrick: low density and fine, uniform pore structure produce low conductivity within a composition family, and the article's own grade comparison table varies conductivity by product group at matched temperature for exactly this reason. Wikipedia's general reference entry for fire brick likewise records typical bulk density in the 1.88–2.05 g/cm³ range with porosity averaging around 31% for common refractory brick — porosity that additives and firing method can shift, changing the conductivity that goes with it.
Two datasheets stating "fireclay brick, 1.05 W/m·K" and "fireclay brick, 0.85 W/m·K" are not necessarily contradicting each other. If the density figures differ — or are missing entirely — the conductivity difference may simply reflect two different fireclay products, not measurement error. Request bulk density alongside conductivity whenever a datasheet reports one without the other.
A worked comparison: reading two supplier datasheets side by side
Say two suppliers each quote a conductivity number for what they both call "dense fireclay brick" for an RFQ:
| Field | Supplier A sheet | Supplier B sheet |
|---|---|---|
| Stated k-value | 1.05 W/m·K | 0.31 Btu·in/(hr·ft²·°F) |
| Test method stated? | ASTM C202 | Not stated |
| Mean temperature stated? | 800°C | Not stated ("typical service range") |
| Bulk density stated? | 2.15 g/cm³ | Not stated |
| Comparable as-is? | — | No — units, method, temperature and density all need resolving first |
Before comparing these two numbers, an evidence-based buyer would: convert both figures to the same unit (W/m·K and Btu·in/hr·ft²·°F are both common on international datasheets and differ by a fixed conversion factor, not a rounding choice); confirm both are reported at the same or a close mean temperature, since a same-brick reading at 400°C and 1,000°C will differ meaningfully; ask Supplier B which standard produced their number, since an unnamed method is not verifiable; and request Supplier B's bulk density, since without it the two "fireclay" products cannot be assumed equivalent even if the corrected numbers happen to land close together. Only after those four gaps are closed does "which supplier's brick conducts less heat" become an answerable question — and even then, it is one input into a lining decision, not the whole decision.
What a k-value does not tell you
A correctly read conductivity number answers one question: how much heat moves through this brick, under these conditions. It does not answer several adjacent questions that a reader comparing bricks often actually needs:
- Which grade to specify for a hot-face vs. backup position. That trade-off between conductivity, mechanical strength, and slag/abrasion resistance belongs to a separate grade-selection decision, including service-temperature and property tables.
- Which brick survives your process chemistry and wear conditions. Conductivity says nothing about erosion, spalling or chemical attack; those require a full property and wear-condition review.
- Whether a sample matches its own datasheet. Once a supplier's data is read correctly and a product is shortlisted, verifying the physical sample against its paperwork is a separate step — see our sample approval checklist.
- What total heat loss or fuel cost a wall section produces. That requires a full wall build-up calculation (layer thicknesses, all layer conductivities at their own mean temperatures, and boundary conditions), not a single brick's k-value read in isolation.
This article's job stops at making the number itself trustworthy to compare. What you do with a correctly compared number is a separate, and separately owned, decision.
Common mistakes when comparing refractory conductivity data
- Comparing numbers with no temperature attached. A k-value without a stated mean temperature cannot be placed on a curve or checked against another sheet.
- Assuming all datasheets used the same test method. Calorimeter (ASTM C201/C202/C182/C417) and hot-wire (ASTM C1113, ISO 8894-1/-2) methods are not interchangeable without a stated correction.
- Ignoring density when it is the real variable. Two bricks of the same chemical family can carry different conductivity because of density and pore structure, not composition.
- Mixing unit systems silently. W/m·K and Btu·in/(hr·ft²·°F) datasheets both circulate internationally; convert before comparing, never eyeball.
- Reading a single conductivity value as valid across the whole service range. Conductivity is temperature-dependent, and the dominant heat-transfer mechanism itself can shift at high temperature — one point on the curve is not the curve.
- Treating conductivity as the only selection criterion. A lower k-value brick that cannot survive the chemical or mechanical environment is not a viable substitute regardless of its heat-loss advantage.
References & Further Reading:
- ASTM C201 — Standard Test Method for Thermal Conductivity of Refractories
- ASTM C202 — Standard Test Method for Thermal Conductivity of Refractory Brick
- ISO 8894-1:2010 — Refractory materials, Determination of thermal conductivity, Part 1: Hot-wire methods
- ISO 8894-2:2007 — Refractory materials, Determination of thermal conductivity, Part 2: Hot-wire method (parallel)
- Edward Orton Jr. Ceramic Foundation — Thermal Conductivity Testing
- Thermal Processing Magazine — An Overview of Insulating Firebricks