Clay Brick Raw Materials: How to Test Whether Your Clay Is Suitable

Clay brick raw materials suit a plant if they shape without cracking, hold their form while drying, fire strong at a workable temperature and stay free of damaging impurities. To find out, test plasticity, grain size, shrinkage, carbonates, sulfur, carbon, soluble salts and fired properties on representative samples. Results decide the forming, drying and fuel route.

Most clay brick raw materials can be made to work by blending, grinding or adjusting the process, so a poor single result rarely ends a project. What the tests give you is the cost of making that clay work, and the risk of finding out too late. This guide explains how to sample a deposit, what each test tells you, how to read the results and what they change in the plant. For the steps that follow, see the clay brick manufacturing process. Tests on finished bricks are covered in clay brick testing methods.

What Clay Brick Raw Materials Must Do

The Brick Industry Association (BIA) Technical Note 9 sets three requirements: plasticity for shaping, enough wet and air-dried strength to keep that shape, and particle fusion at suitable temperatures. The British Geological Survey (BGS) puts the same idea in mineral terms:

• Clay minerals provide plasticity. BGS says they may range from 20% to 80% of the raw material.
• Non-plastic grains, usually quartz, limit shrinkage and deformation.
• Fluxes allow vitrification at 900–1,100 °C. BIA notes that metallic oxides promote fusion at lower temperatures.
• Impurities such as carbon, sulfur and gypsum affect firing performance and emissions.

BGS also warns that many clays are unsuitable for brickmaking on their own, and that blending different clays is common practice. So test each candidate material and also the blend you intend to run.

Brick clay tests at a glance

The reference points come from different countries and different clays, so use them to spot questions. The fired brick has to meet your market’s standard, such as ASTM C62 or C216.

How to Sample a Clay Deposit

A good laboratory result on a bad sample is worthless. BGS says reserves should be proven with an appropriate level of site investigation, and for a brick plant that sampling is the first real project expense. Good practice:

1. Map the deposit first. Use boreholes or test pits across the area you intend to dig, down to the working depth.
2. Sample each layer separately. Clay beds can differ sharply in carbonate, sulfur and plasticity. Note depth and position for each sample.
3. Include the waste. Test overburden and partings if they could end up in the feed, because excavators do not separate perfectly.
4. Make the planned blend. Combine samples in the proportions the pit plan would actually deliver.
5. Keep reference material. Label and store part of every sample so a result can be rechecked.

Sample quantity depends on the laboratory and on how many tests and firings you order, so agree on it before you ship. One 2025 study of a Cameroonian clay deposit started from about 2–3 kg per sample for analysis (Nzeugang et al., Clay Minerals). A pilot extrusion needs much more.

If you are planning a line with us, send samples of your raw materials or an existing analysis. The raw material test decides the process route and the fuel plan.

Plasticity, Grain Size and Drying Shrinkage

These three properties decide how the body behaves between the mixer and the kiln.

Plasticity and Atterberg limits

Plasticity decides whether a body will extrude cleanly. The liquid limit and plastic limit bracket the moisture range in which clay behaves as a plastic solid, and the plasticity index is the difference between them.

• Low plasticity. The column tears or its edges break, and green strength may be low. Options are finer grinding, adding a more plastic clay or more souring time.
• High plasticity. The clay needs more water, sticks, shrinks more and tends to crack in the dryer. Options are adding sand, grog or other non-plastic material.

Nzeugang et al. report that low-plastic materials (index below 10%) and highly plastic materials (index up to 35%) are not suitable for extrusion, and describe 20.4% as plastic behavior in their sample. Treat this as a screen. ASTM D4318 tests only the fraction passing a 425 µm sieve, so it says nothing about the coarse material. ASTM lists D4318-26 as the active edition, so make sure the laboratory reports against the current one.

Results can vary widely within one region. A study of clays from the Sulaimani area of Iraq, Rashed (2017), found plasticity index values from 2.0% to 28.4% and shrinkage values from 4.3% to 15.7% across samples (the paper labels this figure both linear shrinkage and shrinkage limit). That spread is why one sample cannot represent a deposit.

Grain size and the Winkler diagram

Grain size affects shaping, drying and many fired properties. The usual tool is the Winkler diagram, which plots the fractions below 2 µm, 2–20 µm and above 20 µm against fields for different products.

Treat the field boundaries with care. Dondi, Fabbri and Guarini (1998) reviewed 350 Italian clays, found that most bodies fell in the “thin-walled hollow bricks” field whatever the product, and proposed new size ranges based on above 10 µm, 2–10 µm and below 2 µm. Modern plants can process bodies that older diagrams would exclude.

What to take from a grain-size curve:

• Too fine. One clay studied with a 62% fine fraction plotted outside the brick and roof tile fields and suited pottery better (Khalfaoui and Hajjaji, 2010). Too many fines tend to mean high shrinkage and slow drying.
• Too coarse. Thin webs in hollow clay bricks tear easily and need a finer, better-ground body than solid bricks.
• Coarse shale or gangue. The curve depends on grinding, so test material after it passes the crusher and mill you plan to use.

Drying shrinkage and green strength

Drying cracks often trace back to the body as much as to the dryer. BIA TN 9 gives drying shrinkage of 2–4% and firing shrinkage of 2.5–4% for bricks, and says allowances are made in forming. Published papers sometimes apply wider ranges or different test bases, as the Iraqi figures show, so compare only like with like.

Run the test on bars extruded at realistic moisture, because hand-molded bars give misleading results. Record:

• Water needed to extrude.
• Linear shrinkage from wet to dry, and from dry to fired.
• Cracking and warping under controlled drying.
• Dry and wet strength, which decide how high bricks can be stacked.

That last point sets the setting route. On our reference lines, one-setting stacks wet bricks directly on kiln cars, limited to about 7–14 layers because wet bricks are weak, and dries them to 6–7% residual moisture. Two-setting dries first, then stacks 14–19 layers. A clay with low green strength points toward two-setting. A clay sensitive to drying also needs a longer dryer schedule.

Carbonates, Sulfur, Carbon and Soluble Salts

Chemistry is where a promising clay can run into trouble. The responses in this table are our judgment, not published rules; the sourced points follow below it.

Carbonates. Total content is only part of the story. BGS says carbonate minerals must be fine-grained because coarse particles cause lime blowing, and Ceramic Industry makes the same point: particle size, more than the presence of limestone, causes the problem. Bodies with a lot of fine carbonate are used commercially. Dondi, Fabbri and Vincenzi (1992) describe Italian heavy-clay bodies with carbonate up to about 40% overall and note that carbonate-free clays are never used there for hollow products. Rashed (2017) measured 9.3–29.6% CaCO₃ in the Iraqi samples. Carbonates raise the firing loss and affect color, and the test firing shows the outcome.

Sulfur and carbon. BGS gives preferred levels below 0.1% sulfur and 1.5% carbon. Colliery waste can run far higher, with carbon from less than 5% to over 30%. That can be useful fuel, but it shifts the firing and the emissions. US EPA AP-42 reports fluorine at 0.01–0.06% in typical raw materials. Our article on brick kiln emissions shows how to turn these analyses into an emission estimate.

Soluble salts. Rashed classified 0.115–0.371% total soluble salts as low under BS 3921:1985. That standard has been superseded, so check the limit that applies to your market. AP-42 notes that some plants add barium carbonate to stop sulfates rising to the brick surface. Keep in mind that efflorescence also needs water and a path to the surface, and the BIA (TN 23A) says salts more commonly come from other materials in the wall than from the brick itself.

Test Firing: The Decisive Brick Clay Test

Laboratory data predict behavior, and a test firing shows it. Extrude bricks or bars from the planned blend, dry them, then fire them at stepped temperatures. Nzeugang et al. fired at 900, 950, 1,000, 1,050 and 1,100 °C, which brackets the 950–1,050 °C (up to 1,150 °C) used on our reference lines. Measure at each step:

• Firing shrinkage and weight loss.
• Compressive strength and cold-water absorption against your target standard.
• Color and surface.
• Efflorescence and lime blowing.
• Black core and bloating.

Then look for the firing range: the temperature band within which the brick meets the specification. A wide range tolerates normal differences across a kiln car. A narrow range needs tighter kiln control, a modified body, or both. See clay brick firing temperature for how that range shapes the firing curve.

Laboratory firing is faster than a production tunnel kiln and sets far fewer bricks, so it won’t reveal every problem caused by stacking and gas flow. Use a firing cycle that resembles the planned one, and treat borderline results as a warning.

Testing Shale, Coal Gangue and Fly Ash

Modern plants often use hard or waste materials, and they need extra checks. The checks in the table below are our judgment, apart from the BGS point it cites. On our reference lines, shale with coal gangue is the most common body. Gangue typically makes up 30–70% of the body and fly ash can reach 20–40%, with river silt and mine tailings also used.

Calorific value decides how much external fuel the kiln needs. Bodies rich in coal gangue can fire on internal fuel alone on our reference lines, but too much fuel in the body is hard to control. Test the blend.

From Clay Test Results to Plant Design

The table is our engineering judgment, as a starting point for discussion with your supplier.

These decisions feed the equipment choice for the vacuum brick extruder and the tunnel kiln, and they explain why a brick line quoted without a raw material test is only a rough estimate. See clay brick plant cost for the wider set of drivers.

FAQ

What raw materials are used for clay bricks?

Surface clays, shales and fire clays are the three natural families, according to the Brick Industry Association. All consist of silica and alumina with varying metallic oxides. Modern plants also use coal gangue, fly ash, river silt and mine tailings, usually blended with clay or shale, so the blend needs testing as well as each separate material.

What plasticity index is good for brick clay?

There is no universal figure. A 2025 study in Clay Minerals reports that low-plastic materials (below 10%) and highly plastic materials (up to 35%) are not suitable for extrusion, and describes 20.4% as plastic behavior. Use that only as a first screen. Extrusion trials on your own blend give the real answer, because grinding and additives change how the body behaves.

How much limestone can brick clay contain?

Particle size usually matters more than total content. Coarse carbonate particles hydrate after firing and blow out the brick face, while fine carbonate can be tolerated at much higher levels. Italian heavy-clay bodies with substantial carbonate have been documented in published research. Measure both total content and particle size, then confirm lime blowing behavior with a test firing.

Can I use a clay that fails one test?

Often, yes. Blending, finer grinding, sand or grog additions, or a change of drying and setting route can correct many results. BGS notes that blending is common practice. The question becomes whether the fix is affordable at your planned volume, and whether the deposit can supply the blend consistently for the life of the plant.

Do I need a test firing if I already have a chemical analysis?

Yes. Chemistry indicates likely behavior, but it does not measure strength, absorption, color, efflorescence, lime blowing or firing range. Only firing does that. Test at several stepped temperatures with bricks or bars extruded from the planned blend, and compare the results with the product standard your market uses.

Planning a Fired Clay Brick Plant?

Our fired clay brick production line is configured around your raw materials, target output and fuel, starting with the raw material test. Send samples or an existing analysis along with your brick types. For a route that needs no clay, compare calcium silicate vs clay bricks.

Sources

• Brick Industry Association, Technical Note 9: Manufacturing of Brick: https://gobrick.com/media/file/9-manufacturing-of-brick.pdf
• Brick Industry Association, Technical Note 23A: Efflorescence – Causes and Prevention: https://www.gobrick.com/media/file/23a-tn23a.pdf
• British Geological Survey, Mineral Planning Factsheet: Brick Clay (April 2022): https://nora.nerc.ac.uk/id/eprint/532490/1/Brick%20Clay%20Mineral%20Planning%20Factsheet.pdf
• ASTM International, ASTM D4318-26, Liquid Limit, Plastic Limit, and Plasticity Index of Soils: https://store.astm.org/d4318.html
• A. Nzeukou Nzeugang et al., The clayey deposit from Bomkoul (Douala, Cameroon), Clay Minerals 60, 113–126 (2025): https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0009855825000081
• M. Dondi, B. Fabbri and G. Guarini, Grain-size distribution of Italian raw materials for building clay products: a reappraisal of the Winkler diagram, Clay Minerals 33(3), 435–442 (1998): https://www.cambridge.org/core/product/D6756B361C1AB3E0D7180635A59E721C
• M. Dondi, B. Fabbri and S. Vincenzi, Raw materials for the heavy-clay industry in Emilia-Romagna and Marche, Geologica Carpathica, Series Clays 2, 83–90 (1992): https://www.sav.sk/journals/uploads/01131302GCClays-Vol1_No2_83-90.pdf
• A. Khalfaoui and M. Hajjaji, Assessment of the ceramic suitability of a raw clay, Ceramics – Silikáty 54(4), 295–302 (2010): https://www2.irsm.cas.cz/materialy/cs_content/2010/Khalfaoui_CS_2010_0000.pdf
• K. A. Rashed, Study of Raw Materials for Manufacturing Perforated Clay Bricks (Chamchamal and Qaradagh Regions in Sulaimani, Iraq), Sulaimani Journal for Engineering Sciences 4(4) (2017): https://sjes.univsul.edu.iq/SmartJournal_Uploads_sjes/1/articles/38/Study%20of%20Raw%20Materials%20for%20Manufacturing%20Perforated%20Clay%20Bricks%20(Chamchamal,%20and%20Qaradagh%20Regions%20in%20Sulaimani%20%20Iraq)250218035310.pdf
• Ceramic Industry, Identifying and Correcting Clay Body Defects (2003): https://www.ceramicindustry.com/articles/ppp-identifying-and-correcting-clay-body-defects/
• US EPA, AP-42 Section 11.3, Brick and Structural Clay Product Manufacturing: https://www.epa.gov/sites/default/files/2020-10/documents/c11s03.pdf