Strength Testing

Part of Soap Making

How to measure lye concentration before soap making — the egg test, feather test, natural pH indicators, and specific gravity — and how to correct lye that’s too weak or too strong.

Why This Matters

Soap making is a chemical reaction: saponification. Fats and oils react with alkali to produce soap and glycerol. For this reaction to go to completion — meaning all the fat reacts and you get a stable, usable soap — the proportions must be correct. Too little lye, and you get soft, greasy soap that goes rancid quickly. Too much lye, and the finished soap is caustic — it will irritate and burn skin rather than clean it.

When you are working with industrially produced sodium hydroxide, this is straightforward: the concentration is printed on the package, and you calculate your recipe by weight. When you are producing lye from wood ash, you have no such guarantee. Every batch of ash is different. Every leaching run is slightly different. You could spend days rendering tallow and mixing a large soap batch only to produce something unusable because you never tested the lye.

Strength testing is what converts lye from a variable, unpredictable output into a known, controllable input. These tests were used for centuries before any scientific understanding of pH or specific gravity existed — the egg test was described in 17th-century English agricultural manuals. They are empirically reliable, require no manufactured tools, and can be performed with materials found in any kitchen. Learning them is non-optional for any community relying on home lye production.

The Egg Float Test

This is the oldest and most reliable field test for lye concentration. It takes about 30 seconds and requires only a fresh egg and a container of lye.

The principle: An egg’s buoyancy in liquid depends on the liquid’s density. Plain water is too light to float an egg; saturated salt brine is dense enough to float it easily. Lye at correct soap-making concentration — specific gravity approximately 1.10 for soft soap, 1.30–1.35 for hard soap — produces exactly the buoyancy needed to float a fresh egg to a specific level.

Procedure:

1. Use a fresh egg. The egg must be at room temperature, not cold from storage. Cold eggs are slightly denser and will give a false low reading.
2. Gently lower the egg into a container of your lye solution deep enough that the egg can float freely.
3. Observe where the egg sits:

Reading the result:

• Egg sinks: Lye is too weak. Not enough dissolved alkali to raise liquid density to target level. Do not use for soap making yet — boil down to concentrate.
• Egg floats completely at the surface with most of the egg above water: Lye is too strong. Dangerous — will produce caustic soap. Dilute with fresh water.
• Egg floats with a coin-sized area of the top showing above the surface (approximately 2–3 cm diameter, roughly the size of a large coin): Lye is at correct strength for soft soap (potassium-based, paste soap). This is your target for traditional wood ash soap.
• Egg floats with a palm-sized area showing (5–7 cm across): This is closer to the concentration needed for hard bar soap with sodium-based lye, or a very strong potash lye that will produce a firmer soap product.

Calibrating your judgment: The first time you do this, cut a coin from bark or use an actual coin as a visual reference. Place the coin at the water surface before testing to see what “coin-sized” looks like. After a few batches, you develop an eye for it.

Freshness matters: As eggs age, the air cell inside grows, making the egg lighter. An old egg will float at lower lye concentrations than a fresh one, giving a false “correct” reading when the lye is actually too weak. Use eggs that are no more than 2–3 weeks old. If eggs are your only testing tool, keep a small separate supply for testing purposes and mark them.

The Feather Dissolution Test

An older test, less precise than the egg method but useful for a quick initial screen.

The principle: Lye at very high concentration will dissolve keratin — the protein in feathers, hair, wool, and fingernails — within a few minutes of contact. At lower concentrations it takes longer or doesn’t occur.

Procedure: Drop a small clean feather (chicken, goose, or similar — remove any large debris) into the lye. Observe for 5–10 minutes.

Reading the result:

• Feather dissolves within 5 minutes: Lye is extremely strong — significantly over target. Must be diluted.
• Feather softens, becomes translucent, and shows clear structural breakdown within 10–15 minutes: Lye is at or above soap-making strength. This is the target zone for a strong potash lye.
• Feather remains largely intact after 20 minutes: Lye is too weak for soap making. Concentrate by boiling.
• No visible change after 30 minutes: Lye is very weak or possibly contaminated. Retest with egg method; consider re-leaching.

The feather test is a rough screen, not a precise measurement. Use it as a first checkpoint — if the feather is unaffected after 20 minutes, you know you have a problem without needing to proceed further. If it dissolves quickly, combine with the egg test to assess whether it is at target or over-concentrated.

Note: the feather test also works with a small clipping of hair or wool yarn as an equivalent substitute.

The Potato Float Test

A variant of the egg test using a potato chunk. Less reliable than the egg method due to variation in potato density by variety and water content, but useful when eggs are unavailable.

Procedure: Cut a uniform cube of raw potato approximately 3 cm on each side. Peel it to eliminate any surface density variation from the skin. Lower into lye.

Reading the result: The potato should float with a small, visible area above the surface — roughly similar to the egg test target. A potato that sinks indicates weak lye; one that rides high with most of its surface exposed indicates over-concentration.

The potato test is a rough approximation at best. Potato density varies enough between specimens that your “test potato” may be misleading. If you have a reliable egg supply, use eggs.

pH Testing with Natural Indicators

Natural pH indicators do not give you a specific gravity reading, but they tell you whether your lye is strongly alkaline, moderately alkaline, or barely alkaline — which is useful for troubleshooting and for checking samples when your egg test results seem ambiguous.

All three of the following produce colored solutions that shift dramatically with pH:

Red Cabbage Indicator

Preparation: Chop a small amount of red cabbage, add to a cup of hot (near-boiling) water, let steep for 10 minutes, strain. The resulting liquid is deep purple-red. Store in a sealed container — it keeps for 1–2 weeks if cool.

Test procedure: Drop 5–10 drops of indicator into a small sample of lye diluted 1:10 with water (undiluted lye degrades the indicator instantly and gives an off reading). Observe color change.

Color scale (approximate):

• Red-pink: strongly acidic (pH 4–6) — not relevant for lye, but confirms your indicator is working
• Purple: neutral (pH 7)
• Blue-green: mildly alkaline (pH 8–10)
• Yellow-green: strongly alkaline (pH 11–12)
• Yellow: very strongly alkaline (pH 13–14) — this is what you want to see for soap-making lye

Soap-making lye should turn cabbage indicator yellow or yellow-green. If it stays blue-green, your lye is only mildly alkaline and far too weak.

Turmeric Indicator

Turmeric (or its active compound curcumin) shifts from yellow to orange-red in strongly alkaline conditions.

Test procedure: Dissolve a pinch of turmeric powder in a small amount of cooking oil or water. Dip a strip of paper (or cloth) in the turmeric solution and let it dry — this is your indicator strip. Touch the dried strip to a small sample of lye.

Reading: Yellow strip turns orange to orange-red in contact with strong alkali. Weak alkali produces little or no color change.

This test is less quantitative than the cabbage test but extremely simple to perform in the field.

Beet Juice Indicator

Fresh beet juice behaves similarly to red cabbage: turns from red-pink in acidic conditions to yellow-brown in strongly alkaline conditions. Beet indicator is less reliable than cabbage (it degrades faster and the color shift is less dramatic), but it works as a rough confirmation.

All natural indicators caveat: These methods confirm alkalinity but do not distinguish between “almost strong enough” and “at target” with the precision of the egg test. Use them as supplementary confirmation, not as your primary measure. Strong alkali = yellow turmeric, yellow cabbage indicator. But “strong” could mean anywhere from pH 11 to pH 14. The egg test is irreplaceable for setting exact concentration.

Specific Gravity Measurement

Specific gravity is the ratio of a liquid’s density to the density of water. Water = 1.000. For potassium lye at soap-making concentration, you are aiming for approximately 1.10 for traditional soft soap from low-water-content recipes, up to 1.30–1.35 for a hard soap lye approach.

Without a manufactured hydrometer, you can make a simple field hydrometer:

DIY hydrometer from a sealed tube:

1. Take a thin glass tube or narrow bamboo section, sealed at one end
2. Add a small amount of lead, clay, or gravel to the bottom to weight it so it floats vertically
3. Mark the waterline on the tube when floating in plain water — this is your 1.000 reference
4. Float in a saturated salt solution (about 350 g salt per liter water) — mark that line (approximately 1.20)
5. The scale between and beyond these marks can be interpolated

This is a rough tool but gives you a quantitative reference point to supplement the egg test. With practice, you can calibrate your DIY hydrometer against the egg test results to establish reliable concentration markers.

What to Do When Lye Is Too Weak

Signs: Egg sinks in test. Feather unchanged after 20 minutes. Cabbage indicator stays blue-green.

Solution: Boil down.

Pour lye into a ceramic or cast iron pot. Apply heat, bring to a low simmer, and evaporate water. Test every 10–15 minutes. As water leaves, potassium concentration rises. Stop when egg test passes.

Approximate guidance: Lye that produces a “just barely floating” egg (egg touches bottom but doesn’t rest there) needs roughly 15–20% volume reduction. Lye that sinks the egg entirely may need 30–50% reduction.

Do not rush by boiling vigorously. A hard boil causes splashing of concentrated alkali and accelerates the process dangerously — approach target concentration from below, testing frequently.

What to Do When Lye Is Too Strong

Signs: Egg floats with most of its shell above water. Feather dissolves rapidly (under 5 minutes).

Solution: Add water.

Remove from heat if concentrating. Add small amounts of clean water, stir, and retest. It takes only a small volume of water to significantly drop concentration near the high end — add in increments of 10% of total volume, test after each addition.

Keep notes on how much water you added — this tells you how far off you were, which helps calibrate future leaching runs.

Documenting Your Batches

Over time, batch records are one of your most valuable assets. A simple log for each lye batch should note:

• Date of leaching
• Ash species (what wood was burned)
• Volume of ash used
• Number of passes and volume of water each pass
• Raw lye volume collected
• Volume after boiling reduction
• Egg test result (how much of egg showed above water)
• Any additional observations (color, smell, feather test result)

After 5–10 batches, patterns emerge. You will learn whether your local oak burns give you a reliable 30% reduction target, or whether your maple ash consistently runs weak and needs 45%. This knowledge reduces testing time and increases confidence in your results.

Common Mistakes

• Using old or refrigerator-cold eggs for the egg test — gives false readings; eggs must be fresh and at room temperature
• Relying on feather or potato test alone without the egg test — neither gives sufficient precision for soap making
• Testing undiluted lye with cabbage indicator — destroys the indicator immediately; always dilute 1:10 first
• Adding water to over-concentrated lye while it is still on the heat — splashing risk; remove from heat first
• Boiling lye unattended near the target concentration — the final stages of concentration happen quickly; constant monitoring required
• Not recording batch data — forces re-learning the same calibrations repeatedly
• Testing with a single method only — use at least two methods (egg + indicator) to cross-check before committing a batch to soap making

Key Takeaways

• The egg float test is the primary tool: a fresh, room-temperature egg should float with a coin-sized area (2–3 cm) visible above the surface for correct soft-soap lye concentration
• The feather dissolution test is a quick initial screen: feather breakdown in 10–15 minutes indicates adequate strength; no change in 20+ minutes means lye is too weak
• Natural indicators (red cabbage, turmeric, beet juice) confirm strong alkalinity — yellow color in cabbage indicator or turmeric strip signals pH 13+ lye
• Weak lye: boil down in increments of 10–20% volume, testing after each reduction
• Strong lye: add water in increments of 10% volume, testing after each addition
• Keep batch logs — species, volumes, reduction amounts, test results — to build calibrated knowledge over time
• Always use at least two test methods before committing lye to a soap batch