Absorption Test

Part of Brick Making

Testing brick quality by measuring water absorption rates to predict durability and structural fitness.

Why This Matters

A brick that looks solid on the outside can be a sponge on the inside. High water absorption means the brick was under-fired, poorly compacted, or made from the wrong clay. Install those bricks in a wall and the first freeze-thaw cycle will shatter them — water expands 9% when it freezes, and a porous brick full of water will crack, spall, and crumble within a single winter.

The absorption test is the simplest quality control method available without a laboratory. It requires nothing more than a scale (or a balance), water, and time. A community producing thousands of bricks for permanent structures needs this test to sort good bricks from bad before they go into walls, foundations, or chimneys where failure is costly or dangerous.

Historically, builders tested bricks by sound (ringing vs. dull thud), scratch resistance, and visual inspection. The absorption test adds quantitative data to these subjective methods, giving you a reliable standard that anyone can apply consistently.

The Basic Absorption Test

This is the standard method used by builders for centuries, formalized in modern standards like ASTM C67 but requiring no modern equipment.

Equipment needed

• A balance or scale accurate to within 5 grams (a simple beam balance works)
• A container large enough to fully submerge a brick
• Clean water (not muddy or salty)
• A cloth for surface-drying
• Paper and charcoal or pencil for recording

Procedure

1. Select test bricks. Pull 5 bricks at random from each kiln batch. Never test only the best-looking bricks — the point is to find the worst ones.

2. Dry the bricks thoroughly. Place them near (not in) a fire or in direct sun for 24-48 hours. They must be bone-dry. If you have a kiln still cooling, place bricks inside at the end of the cooling cycle.

3. Weigh each brick dry. Record this as W-dry. Handle carefully — a chipped brick gives false results.

4. Submerge in water. Place bricks in the container and fill with clean water until bricks are covered by at least 25 mm (1 inch). Ensure no air bubbles are trapped underneath — tilt bricks as you lower them in.

5. Soak for 24 hours. Keep submerged the entire time. If water level drops due to absorption, top it up.

6. Remove and surface-dry. Lift each brick out, wipe all surfaces with a damp cloth to remove surface water, but do not squeeze water out of the pores. The brick should look damp but not dripping.

7. Weigh immediately. Record this as W-wet. Do not wait — evaporation begins at once.

8. Calculate absorption percentage:

Absorption % = ((W-wet - W-dry) / W-dry) x 100

Interpreting results

Absorption %	Rating	Suitable For
Under 5%	Excellent	Foundations, chimneys, water-contact, load-bearing walls
5-10%	Good	Exterior walls, general construction
10-15%	Acceptable	Interior walls, non-structural partitions, sheltered locations
15-20%	Poor	Interior use only, dry climates, non-load-bearing
Over 20%	Reject	Crush and re-use as grog (fired clay aggregate) in new batches

Climate matters

In freeze-thaw climates, never use bricks above 12% absorption in exposed locations. In tropical climates without frost, bricks up to 15% can serve adequately in walls protected by roof overhangs.

The Boiling Test (Accelerated Method)

When you cannot wait 24 hours, or when you need to simulate worst-case saturation, boil the bricks.

1. Weigh dry bricks as above (W-dry)
2. Place in a large pot or metal vessel and cover with water
3. Bring to a rolling boil and maintain for 5 hours (yes, five hours — this forces water into the smallest pores)
4. Remove from heat and let bricks cool in the water to room temperature
5. Surface-dry and weigh (W-boil)

Calculate boiling absorption the same way. This value will be higher than the 24-hour cold soak — typically 1.5-2x higher. The ratio between the two tests gives additional information:

Saturation Coefficient = Cold absorption / Boiling absorption

Saturation Coefficient	Meaning
Below 0.78	Well-fired, dense, frost-resistant
0.78-0.85	Adequately fired, moderate frost resistance
Above 0.85	Under-fired or porous — vulnerable to frost damage

A saturation coefficient above 0.85 means the brick’s pore structure allows easy water penetration even without boiling, which means freeze-thaw damage is likely.

Quick Field Tests (No Scale Available)

When you have no scale or balance, these qualitative tests give useful information:

The scratch test

Drag a steel nail or knife blade across the brick face. A well-fired brick resists scratching — the nail slides with a metallic sound. A soft brick gouges easily, leaving a powder trail.

The ring test

Hold a brick loosely by one corner and tap it with another brick or a hammer. Well-fired bricks produce a clear, metallic ring. Under-fired bricks give a dull thud. This test is surprisingly reliable once your ear is calibrated — test a few known-good bricks first to establish the reference tone.

The tongue test

Touch your tongue to a broken face of the brick. A highly porous brick will stick to your tongue noticeably as it wicks moisture — the same way an under-fired clay pot sticks. A well-fired, dense brick feels smooth and does not stick.

The break test

Break a brick in half (drop it on a hard surface or strike with a hammer). Examine the cross-section:

• Uniform color throughout: Fully fired, good quality
• Dark core with lighter edges: Under-fired — the center never reached full temperature
• Visible layers or laminations: Poor mixing or forming, likely to delaminate under load
• Visible pores or voids: Over-wet mix or trapped air during forming

Using Test Results to Improve Production

The absorption test does not just sort bricks — it tells you what went wrong so you can fix the process.

Test Finding	Likely Cause	Corrective Action
High absorption, uniform color	Clay too sandy, not enough fine particles	Add more clay-rich soil to the mix, or sieve out coarse sand
High absorption, dark core	Under-fired, kiln too cool or firing too short	Increase firing time by 20%, stack bricks more loosely for better heat circulation
Low absorption but cracking	Over-fired or cooled too fast	Reduce peak temperature, extend cooling period (48 hrs minimum)
Inconsistent results in same batch	Uneven kiln temperature	Reorganize kiln stacking, add more fire channels, ensure even fuel distribution
Acceptable absorption but low strength	Wrong clay type (too much calcium)	Source clay from a different deposit, or blend with a more aluminous clay

Track your data

Record absorption results for every kiln firing along with: clay source, mix ratios, firing duration, fuel used, weather conditions, kiln loading pattern. Over 5-10 firings you will identify which variables matter most for your specific materials and kiln.

Setting Community Standards

For a rebuilding community producing bricks at scale, establish clear standards:

1. Test 5 bricks per kiln load — always pulled from different positions (top, bottom, center, edges, near fire channels)
2. Stamp or mark passing bricks with a chalk mark or clay stamp
3. Separate grades physically — stack structural-grade, partition-grade, and reject piles in different areas
4. Crush rejects for grog — ground-up fired clay mixed into new batches improves the next generation of bricks
5. Post standards visibly at the brick-making area so every worker knows the targets

The absorption test takes minimal effort and catches problems before they become structural failures. Five bricks, a bucket of water, and a balance are all that stand between your community and a building that might collapse in its first winter.