11.4 Developer Chemistry, Vegetable Dyes and Metallic Salts

Developer Chemistry, Vegetable Dyes and Metallic Salts

Exam Focus: Developer volume and its lift capability is the single most-used piece of knowledge in a colour salon, and the volume-to-percentage conversion is examinable arithmetic. The metallic salts material is safety-critical: a positive test means refusing every oxidative and restructuring service, not reducing the developer volume.

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1. What a Developer Actually Does

Hydrogen peroxide (H₂O₂) is the oxidising agent — the developer — in every permanent and demi-permanent haircolour and in every on-scalp lightener. Candidates lose marks here because they learn the volume chart without ever learning what the chemical is doing, and then cannot answer a question phrased as a reason rather than a number.

A developer does two jobs at the same time, and both are oxidations:

1. Lift. Released oxygen diffuses the client's natural melanin, breaking the large pigment granules into smaller, colourless fragments. This is what raises the level.
2. Deposit. The same oxygen converts the small, colourless aniline derivative dye precursors in the colour into large coloured molecules that couple together inside the cortex and are then physically too big to wash back out.

The alkalising agent in the colour — ammonia, or monoethanolamine (MEA) in low-odour lines — is not the lifting agent. Its job is to swell the cuticle so the mixture can enter, and to raise the pH so the peroxide decomposes quickly enough to be useful. This is a favourite exam trap: "ammonia lifts and peroxide deposits" is wrong in both halves. The alkalising agent opens the door; the peroxide does both the lifting and the depositing. Chapter 5.2 covers the underlying oxidation-reduction chemistry; here the practical point is that a developer is useless on its own and dangerous when mismatched.

Why the bottle is acidic

Straight out of the bottle, peroxide sits at a pH of about 2.5 to 4.5. That acidity is a deliberate stabiliser: at low pH the molecule holds its extra oxygen atom and the product keeps its labelled strength for months. The moment it meets an alkaline colour or lightener at pH 9 to 10.5, decomposition accelerates and the service begins. That single fact explains every storage rule you will be asked about.

• Store in the original opaque bottle, tightly capped, in a cool dark place. Light and heat both catalyse decomposition, and a developer that has lost strength under-processes without any visible warning.
• Never mix colour in a metal bowl and never use metal clips on processing hair. Metal ions catalyse peroxide decomposition and can distort the result.
• Never pour unused mixed colour back into the developer bottle. The mixture continues to release oxygen gas, and a sealed bottle can build pressure. Mixed product is discarded, not stored.
• Buy and rotate stock so that developer is used well inside its shelf life; a bottle that has been opened repeatedly and stored warm is no longer the volume printed on the label.

The New York rules that attach to your developer shelf

New York regulates the bottle as much as the technique, and these three citations are examinable together:

• 19 NYCRR § 160.26 — every product used in the business must keep its original manufacturer labelling intact, and poisonous or corrosive substances must be distinctly marked. Decanting developer into an unlabelled squeeze bottle at the station is a violation on its own.
• 19 NYCRR § 160.25 — a Safety Data Sheet must be available on the premises for each product used, developer included. Section 2.3 covers the full hazard-communication duty.
• 19 NYCRR § 160.12 — restrictions on open sources of ignition. Peroxide is an oxidiser: it does not burn by itself, but it feeds a fire that has already started, which is why storage away from heat is a regulatory matter and not merely good housekeeping.

2. Volume, Percentage and the Arithmetic

Developer strength is printed in two systems, and an exam question may give you either one and ask for the other.

1. Volume (vol) measures the volume of oxygen gas that one volume of the solution can release. One litre of 20-volume developer can liberate roughly twenty litres of oxygen gas.
2. Percentage (%) measures the actual proportion of pure hydrogen peroxide in the water.

The two are linked by a constant you should memorise: volume ÷ 3.3 ≈ percentage, and percentage × 3.3 ≈ volume.

• 20 ÷ 3.3 = 6.06, so 20 volume ≈ 6%.
• 30 ÷ 3.3 = 9.09, so 30 volume ≈ 9%.
• 9% × 3.3 = 29.7, so a 9% developer is a 30 volume.
• 12% × 3.3 = 39.6, so a 12% developer is a 40 volume.

Detailed Developer Comparison Table

Diluting a developer

Because volume scales linearly, equal parts of two developers give you the average of the two.

• Equal parts 20 volume and 10 volume produce a 15 volume.
• Equal parts 40 volume and 20 volume produce a 30 volume.

Worked example. You need 2 ounces of 30 volume, and the shelf holds only 40 volume and 20 volume. Measure 1 ounce of each: the result is 2 ounces at 30 volume. Dilute only with developer of the same brand and system, or with distilled water if — and only if — the manufacturer's directions permit it. Never thin a developer with tap water, conditioner or shampoo to "make it gentler"; you change the chemistry and the processing time in ways you cannot predict, and you leave the manufacturer's directions, which § 160.26 assumes you are following.

3. Choosing the Developer: a Repeatable Decision

Work the same five steps every time, and the choice stops being guesswork.

1. Establish the natural level at the scalp (Chapter 11.2).
2. Establish the target level.
3. Subtract: levels of lift required.
4. Match the lift to a volume from the table above.
5. Decide on-scalp or off-scalp, because that constrains the answer independently of the arithmetic.

Example one. Natural level 5, target level 7. Two levels of lift, so 20 volume with a permanent colour.

Example two. Level 6 natural with 80% grey, target the same level 6 with full coverage. This is a deposit-heavy job, not a lift job: 20 volume with a permanent colour containing enough natural or neutral base. Going up to 30 or 40 volume here makes coverage worse.

Example three. Level 7 natural, target a level 10 high-lift blonde. Three to four levels, so 40 volume with a high-lift permanent — and off-scalp in foils, because 40 volume on the scalp is outside almost every manufacturer's directions.

Four traps built into these decisions

• More volume does not mean more colour. As volume rises, lifting increases and depositing decreases. That is the mechanical reason 40 volume gives poor grey coverage.
• You cannot rescue under-processing by adding volume mid-service. If the formula was wrong, the answer is to stop, rinse, reassess and reformulate at the next appointment.
• The mixing ratio is part of the formula. Permanent colour is commonly mixed 1:1, while high-lift shades and many demi-permanent lines call for 1:1.5 or 1:2. Changing the ratio changes both the consistency and the amount of peroxide available, so follow the ratio the manufacturer prints.
• Heat is not a shortcut. Added heat accelerates peroxide decomposition, and most manufacturers prohibit supplemental heat with 30 and 40 volume on the scalp. Chapter 11.6 covers the on-scalp lightening rules in full.

4. Vegetable and Natural Dyes

Not every colour a client has used is a professional oxidative product, and the two families below are the reason the consultation question "what have you put on your hair at home?" is a safety question rather than a courtesy.

Henna is a vegetable dye made from the dried, powdered leaves and stems of Lawsonia inermis. Its active molecule, lawsone, stains the keratin of the cuticle and outer cortex a characteristic reddish-orange. Henna is a coating dye: it does not lift, it cannot be lightened predictably, and it builds up with every application into a dense layer that physically blocks oxidative colour, lightener and permanent wave solution from reaching the cortex. Heavily hennaed hair feels stiff and coated, processes unevenly or not at all, and the only reliable remedies are to grow it out or cut it off. Because henna is not an oxidative dye, an oxidative colour remover has nothing to act on and will not strip it.

Other traditional plant dyes behave the same way. Chamomile deposits a pale yellow stain, indigo deposits blue-black and is frequently layered over henna to create a "black henna" effect, and sage darkens gradually. All of them coat, none of them lift, and all of them interfere with subsequent chemical services.

Compound henna is the dangerous member of the family: plant dye that has been adulterated with metallic salts to make the colour build faster and last longer. A client who says "it was just henna, it's natural" may be describing a compound product, which is exactly why you test rather than take a history at face value.

5. Metallic Salts and Progressive Colour Restorers

Metallic dyes are sold to consumers as gradual, progressive or colour restorer products, most often marketed to men for reducing grey. They are formulated with metallic salts such as lead acetate, silver nitrate, copper sulfate or bismuth citrate.

The mechanism is not oxidation at all. The metal reacts with sulfur in the hair's keratin to deposit dark metal sulfides on and just inside the cuticle. Each application adds another layer, so the hair darkens progressively over a week or two — and the result is a characteristically dull, flat, opaque colour that often turns greenish, purplish or brassy in daylight. Metallic build-up also makes hair feel harsh and brittle and blocks penetration in the same way henna does.

Current federal status. The FDA terminated the colour additive listing for lead acetate in hair dyes (formerly 21 CFR § 73.2396). The rule was published in 2018, stayed while objections were considered, and the stay was lifted effective 6 January 2022, so lead acetate is no longer a lawful colour additive in hair dyes sold in the United States. This does not make the topic historical: old bottles remain in clients' bathrooms, and imported or online products still contain lead, silver, copper and bismuth compounds. Chapter 5.3 covers the wider FDA framework for hair care products.

Why this is a safety emergency, not a formulation problem

Critical safety rule: Never apply permanent haircolour, lightener or permanent wave or relaxer solution over hair carrying metallic salts.

When hydrogen peroxide or a persulfate booster meets metallic salts — copper and lead especially — the metal catalyses a violently fast, exothermic decomposition. The mixture can heat rapidly, steam or smoke, and give off a foul odour, while the hair swells, turns dark or gummy and can break off at the scalp or disintegrate in the bowl. Clients have been burned. A milder but still service-ending version of the same problem occurs in permanent waving, where the metallic layer causes wildly uneven processing and discoloration.

6. The Metallic Salts Test

Any doubt about home colour is settled with a strand test before a chemical service, not after.

1. In a glass or plastic bowl — never metal — mix 1 ounce (30 mL) of 20-volume hydrogen peroxide with 20 drops of 28% ammonium hydroxide (ammonia).
2. Cut about 20 strands of hair from an inconspicuous area, bind one end, and immerse the strand fully.
3. Leave it at room temperature for 30 minutes, then read the result.

Read that table carefully, because two of the four results are counter-intuitive and both are examined. Lead lightens quickly — it does not turn the strand black. Silver produces no reaction whatsoever, and it is precisely that absence of change that identifies it; a candidate who has been taught "positive means bubbling" will call a silver result negative and then destroy the client's hair.

If the test is positive

• Decline the oxidative colour, the lightener, and any permanent wave or relaxer. There is no reduced-volume version of this service that is safe.
• Explain plainly what was found and why, without diagnosing the product the client used.
• Offer what is genuinely available: a non-oxidative temporary or deposit-only rinse where the manufacturer's directions allow it over coated hair, conditioning treatments, and a cutting plan that removes the coated hair over time.
• Record the test, the observation and the refusal on the client card, along with the recommendation given. Chapter 11.5 covers the documentation standard for preliminary procedures.

7. Exam Traps in This Section

• 20 volume is 6%, not 20%. The conversion is divide by 3.3.
• Grey coverage is a 20 volume job; raising the volume reduces deposit.
• Peroxide is stabilised acidic; the alkalising agent in the colour triggers it.
• Henna is a coating dye and cannot be stripped with a colour remover.
• Metallic salts darken the hair through metal sulfides, not through oxidation.
• A silver result in the strand test is no reaction — still a refusal.