Hollow Clay Bricks: Manufacturing Process and Where They Fit

Hollow clay bricks are made like solid bricks, except that the clay is extruded through a die fitted with a core bridge and cores, which form the holes as the column leaves the extruder. The column is wire-cut, dried and fired. In ASTM terms a brick becomes hollow when voids exceed 25% of the gross cross-sectional area, up to 60%; the holes save clay, weight and firing energy per unit.

The process steps are the same as for any extruded brick, which our article on the clay brick manufacturing process covers. This guide covers what changes when the unit has thin shells and webs: how hollow and perforated bricks are classified, how the die is built, how void ratio is measured, what drying and firing need, and where hollow clay bricks fit in a building, with their advantages and disadvantages. Figures marked “on our reference lines” come from the fired clay brick lines YuxingCorp supplies; other figures carry their source.

What Counts as a Hollow Clay Brick

Each standard system draws the line differently, so a “hollow brick” in one market may be a “perforated brick” or a “Group 2 unit” in another.

China’s two product standards reflect a common split. Perforated bricks have many smaller vertical holes and are typically used in load-bearing walls. GB/T 13544-2026, issued on 25 May 2026, replaces the 2011 edition and takes effect on 1 December 2026 (until then GB/T 13544-2011 applies). It sets strength classes MU10 to MU30, density classes of 1000 to 1300 for bricks and 900 to 1200 for blocks, and changes the hole-type and hole-size requirements (chinesestandard.net record). Hollow bricks and blocks have fewer, larger holes, and GB/T 13545-2014 describes them as mainly used for non-load-bearing parts of buildings; its 2014 edition dropped the MU2.5 strength class and added a 5-hour boiling absorption requirement. Check the full text of each for the hole-ratio limits.

Hollow Clay Brick Manufacturing Process: What Changes

The line is the same as for solid bricks, but several steps have less margin:

1. Raw material preparation. Thin webs need a finer, more uniform body than solid bricks. Coarse particles or stones that a solid brick tolerates can tear a web or block a gap between core and die.
2. Vacuum extrusion. Hollow and perforated bricks must be vacuum-extruded on our reference lines; two-stage vacuum extruders there reach up to 4.0 MPa extrusion pressure and −0.092 MPa vacuum. De-airing gives the column the density and green strength that thin walls need. See vacuum brick extruder for how the machine is specified.
3. The die and core assembly forms the holes; this is where most hollow-brick defects start (next section).
4. Cutting. Wire cutting must not crush thin webs or drag the column. Cut faces should be checked for torn webs.
5. Setting. Hollow units are lighter but have weaker edges. Grippers and setting patterns are designed for the unit, and the setting decides whether air and kiln gas can reach the inner webs.
6. Drying and firing, covered below, are shorter per unit but need even conditions through the thickness.
7. Inspection. Void ratio, shell and web thickness, and cracks along the core pattern are checked alongside strength and absorption.

Changing the die switches the line from one product to another. Typical products on our reference lines include perforated bricks of 240 × 115 × 90 mm and 270 × 130 × 118 mm and hollow blocks of 390 × 190 × 190 mm, alongside solid standard bricks.

The Die: Die Mouth, Core Bridge and Cores

A hollow-brick die has three parts. The die mouth forms the outer shape of the column. The core bridge, sometimes called a spider, is a frame across the clay flow, upstream of the die mouth. It carries the cores, which project into the die mouth and form the holes.

Four things decide whether the die produces good units:

• Flow balance. Clay at the outer walls is slowed by friction against the die mouth; clay between the cores is not. If the outer shells and the inner webs leave the die at different speeds, the column tears or bends. Die designers balance the flow with the core positions and the shape of the die entry, and use water or other lubrication on the die walls.
• Knitting behind the bridge. The clay splits around each leg of the bridge and must weld together again before it leaves the die. Too little pressure, or too short a distance from bridge to die exit, leaves weak seams that crack in drying or firing.
• Minimum shell and web thickness. The standard sets the floor. ASTM C652, as summarized by the BIA, requires minimum solid face shells of 3/4 in. (19.1 mm) for 3- and 4-in. nominal widths, 1 in. (25.4 mm) for 6 in., 1¼ in. (32 mm) for 8 in. and 1½ in. (38 mm) for 12 in. units (BIA TN 41, 2008; check the current C652 edition).
• Wear. Cores and die liners wear fastest where the clay is most abrasive. As they wear, webs thicken, voids shrink and dimensions drift, so wear parts are measured on a schedule.

The BIA puts the general rule simply: hole size and number depend on the manufacturer’s capability, the clay, the firing process and the intended use (BIA TN 41). A die drawing should be agreed only after the raw material test, because the same core pattern behaves differently in different clays. Die sizes also have to allow for drying and firing shrinkage (BIA TN 9).

Void Ratio: How It Is Measured and What It Changes

Void ratio, or hole ratio, is the share of the gross volume or bed area taken by holes. It is measured, not taken from the die drawing:

• ASTM C67 measures void area in cored units by filling the cores with sand and measuring the sand volume, on a sample of ten bricks (BIA Technical Note 39).
• EN 772-3 determines net volume and percentage of voids of clay units by hydrostatic weighing, and EN 772-9 by sand filling.
• GB/T 2542-2012 includes a method for porosity and pore structure of wall bricks.

Void ratio drives weight. As an illustration, assume a fired body density of 1.7 t/m³ (our assumption). A 390 × 190 × 190 mm block has a gross volume of about 14.1 liters:

These are our calculations, not product data. A real block’s mass depends on its body density, which pore-forming additives can lower further.

Void ratio also changes how strength is reported. The same unit gives a higher net-area strength than gross-area strength, so compare test reports on the same basis. Our guide to clay brick testing methods explains the difference.

Drying and Firing Hollow Units

Hollow units need less energy per kilogram, but a larger share of it goes to drying. In average figures from German brick factories, total energy was about 1,300 kJ per kg for hollow blocks against about 2,000 kJ per kg for solid bricks, with the dryer taking 53.0% of the energy for hollow blocks and 44.8% for solid bricks and pavers (IZF guideline for IDCOL, 2019).

Drying. Thin webs dry quickly, but the outer shells, which face the drying air directly, can dry ahead of the inner webs. The resulting differences in shrinkage crack webs at their junctions with the shells. The IZF guideline prefers chamber dryers for plants that make products of very different weights and dimensions, such as solid bricks and hollow blocks, because each chamber can run its own program. Our article on the clay brick dryer compares chamber and tunnel dryers.

Firing. Heat and air reach the inside of a thin-walled unit faster than the core of a solid brick, so thick solid bricks need longer firing than thin-walled hollow blocks. Peak firing temperatures on our reference lines are typically 950–1,050 °C, up to 1,150 °C, depending on the body. The setting pattern matters more for hollow units: if the holes are blocked by neighboring units, the inner webs see less heat and less oxygen. Our article on clay brick firing temperature covers the firing curve.

Pore-Forming Additives and Thermal Performance

Holes are one way to improve insulation; making the clay itself porous is the other. Combustible additives such as paper waste burn out in the kiln and leave fine pores in the body.

A study of hollow clay bricks made with paper waste shows the size of the effect. Adding paper waste lowered the thermal conductivity of the fired body from 0.68 W/(m·K) without additive to 0.39 W/(m·K), a reduction of about 43% (Sutcu et al., Energy and Buildings, 2014). The same study modeled three hollow brick designs by finite-element analysis, including radiation and convection inside the holes. Lowering the body’s conductivity lowered the unit’s thermal transmittance in every design analyzed, and thermal resistance depended nonlinearly on hole geometry, material properties and temperature. Hole layout and body conductivity work together.

For a plant, pore formers bring trade-offs. They add a material to source and dose, they change the firing because they burn inside the body, and they change kiln emissions: the EU reference document on best available techniques for ceramics gives raw gas values for various pore-forming agents and average clean gas concentrations for porous clay blocks (EU Ceramic Manufacturing BREF, 2007, Tables 3.4 and 3.5). Test both the additive and its effect on the firing curve before committing to it.

Hollow Clay Bricks: Advantages and Disadvantages

The BIA describes the main structural use in the United States: single-wythe walls, reinforced or unreinforced, typically 4 to 8 in. nominal thickness with 35–60% voids, in commercial, residential, school and hotel buildings, sound barriers and retaining walls; hollow veneer units of 3 to 4 in. typically have about 26–35% voids (BIA TN 41). In markets that follow Chinese practice, perforated bricks carry load and larger-void hollow blocks are mostly used for infill and partitions.

FAQ

How are hollow clay bricks made?

The clay is prepared as for solid bricks but ground finer, de-aired in a vacuum extruder and pushed through a die whose core bridge holds cores that form the holes. The column is wire-cut into units, which are dried and fired. On our reference lines, hollow and perforated bricks are always vacuum-extruded, and changing the die switches the product.

What is the void ratio of a hollow clay brick?

In ASTM C652, hollow brick has voids of more than 25% and up to 60% of the gross cross-sectional area, split into classes H40V (up to 40%) and H60V (above 40%). Eurocode 6 groups clay units into four groups by the percentage and direction of holes. Void ratio is measured by sand filling or hydrostatic weighing.

What is the difference between perforated and hollow clay bricks?

Usage varies by standard. In Chinese practice, perforated bricks (GB/T 13544) have many smaller vertical holes and carry load, while hollow bricks and blocks (GB/T 13545) have fewer, larger holes and are mainly used for non-load-bearing parts. In ASTM, any clay unit with more than 25% voids is a hollow brick under C652.

Are hollow clay bricks load-bearing?

Some are. The BIA describes hollow brick used in reinforced and unreinforced single-wythe structural walls, with cells grouted around reinforcement. Chinese perforated bricks are made in strength classes MU10 to MU30 for load-bearing walls. Larger-void hollow blocks under GB/T 13545 are mainly for non-load-bearing parts. The design standard in your market decides.

Do hollow clay bricks need a different machine from solid bricks?

Usually not a different line, but a capable one. The same vacuum extruder can make solid, perforated and hollow units with different dies, provided it has the de-airing, pressure and flow area the hardest product needs. Cutting, setting grippers, dryer programs and kiln settings must also suit the thinner walls.

Planning a Hollow Clay Brick Line?

Perforated bricks and hollow blocks are made with interchangeable dies on our fired clay brick production line, with vacuum extrusion as standard for every hollow product. Send raw material samples with your target product list, so that the die design, drying and firing are tested on your clay.

Sources

• Brick Industry Association, Technical Note 41: Hollow Brick Masonry (January 2008): https://www.gobrick.com/media/file/41-hollow-brick-masonry.pdf
• Brick Industry Association, Technical Note 9: Manufacturing of Brick (December 2006): https://www.gobrick.com/media/file/9-manufacturing-of-brick.pdf
• Brick Industry Association, Technical Note 39: Testing for Engineered Brick Masonry – Brick and Mortar (November 2001): https://www.gobrick.com/media/file/39-testing-for-engineered-brick-masonry–brick-and-mortar.pdf
• B. Haseltine, Information for Training – EN 1996: Compressive Loading, European Commission JRC Eurocodes: https://eurocodes.jrc.ec.europa.eu/sites/default/files/2022-06/3_am_Haseltine.pdf
• GB/T 13544-2026 Fired perforated brick and block, standard record: https://www.chinesestandard.net/Related.aspx/GBT13544-2026
• GB/T 13545-2014 Fired hollow bricks and hollow blocks, standard record: https://www.chinesestandard.net/Related.aspx/GBT13545-2014
• GB/T 2542-2012 Test methods for wall bricks, standard record: https://www.chinesestandard.net/Related.aspx/GBT2542-2012
• EN 771-1:2011+A1:2015 Specification for masonry units – Part 1: Clay masonry units (iTeh Standards preview, normative references): https://iteh.es/catalog/standards/sist/479eafd4-c9f5-44e4-97dd-2bcec02dae0a/sist-en-771-1-2011a1-2015
• E. Rimpel, Institut für Ziegelforschung Essen (IZF), for GFA Entec AG and IDCOL, Tunnel Kiln: Technology Overview and Project Assessment Guideline (August 2019), via the Climate and Clean Air Coalition: https://www.ccacoalition.org/sites/default/files/resources/2019_Tunnel%20kiln%20technology%20overview%20and%20project%20assessment%20guideline.pdf
• M. Sutcu, J. J. del Coz Díaz, F. P. Álvarez Rabanal, O. Gencel and S. Akkurt, Thermal performance optimization of hollow clay bricks made up of paper waste, Energy and Buildings 75, 96–108 (2014): https://doi.org/10.1016/j.enbuild.2014.02.006
• European Commission JRC, Reference Document on Best Available Techniques in the Ceramic Manufacturing Industry (August 2007): https://bureau-industrial-transformation.jrc.ec.europa.eu/sites/default/files/2019-11/cer_bref_0807.pdf