The safety classification

Every lab, build, experiment and break/fix page carries one of four letters. The letter is not a label of how the process is usually described; it is a hazard assessment of the procedure as written on that page, made against the criteria below. A page states which criteria applied.

The levels answer one question: what does a reader need to have, and be able to do, for this page to be reasonable? Level A assumes a standard home darkroom with gloves, eye protection, ventilation and dedicated utensils. Level B assumes more: splash goggles, stronger ventilation, eyewash within reach, experience with concentrated reagents or with UV and low-voltage electronics. Level C assumes a fume cupboard, controlled waste and supervision, which a home does not have. Level D assumes nothing, because it gives no procedure.

A Standard home darkroom

Suitable with ordinary darkroom controls: nitrile gloves, eye protection, a well-ventilated room, dedicated utensils and correct labelling.

When a page is classified at this level

Only substances whose GHS classification is at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled, and in the quantities of a home darkroom.
No heating of solutions above 50 °C, no open flames, no volatile organic solvents beyond isopropanol in small quantities.
No mains-voltage construction or modification; purchased, certified equipment only.
Waste is silver-bearing fixer or spent developer that can be collected for recovery, or dilute rinse water; nothing that requires specialist neutralisation.

Controls the classification assumes

Nitrile gloves and safety glasses whenever solutions are handled.
Ventilation: an openable window or an extractor; no processing in a sealed cupboard.
Dedicated, labelled containers and utensils never used for food.
The safety data sheet for every product on hand and read before first use.

B Advanced home laboratory

Needs additional controls, experience and precautions beyond the standard darkroom: stronger ventilation, splash protection, careful handling of concentrated reagents or of energies such as UV and low-voltage electronics.

When a page is classified at this level

Handling of concentrated acids or alkalis, silver nitrate, sensitisers, dichromate-free but still toxic or corrosive reagents, or fine powders that must not be inhaled.
Heating or holding solutions above 50 °C, as in emulsion making, or procedures that generate mists or vapours.
UV exposure units, high-brightness LED sources, or low-voltage electronics builds where a certified power supply provides isolation from the mains.
Procedures whose failure mode is a splash, a burn or a spill of something corrosive rather than merely a spoiled negative.

Controls the classification assumes

Everything in Level A, plus chemical splash goggles and an apron for concentrated reagents.
Local extraction or work near an open window with a fan; no enclosed spaces.
Eyewash provision within reach and a spill kit appropriate to the reagents in use.
Weighing of powders with a mask rated for fine particulates where the safety data sheet requires it.
For UV: enclosures, interlocks or timers that prevent direct exposure of eyes and skin.
For electronics: certified, isolated low-voltage supplies; no exposed mains conductors at any stage.

C Specialist or supervised laboratory

Requires facilities a home darkroom does not have: a fume cupboard, controlled waste streams, or supervision by someone trained in the specific hazard.

When a page is classified at this level

Reagents classified as acutely toxic, carcinogenic, mutagenic, reproductive toxins or sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control.
Chromium(VI) compounds, formaldehyde and similar substances in any quantity.
Mains-voltage construction: switching enlarger lamps or heaters from a home-built controller, unless a certified, enclosed relay module is used as directed and the mains side is never opened.
Any procedure whose waste cannot lawfully enter a domestic drain and needs a licensed waste contractor.

Controls the classification assumes

A fume cupboard or equivalent engineered extraction.
Written procedure, spill and exposure response plan, and a second competent person present.
Waste collected, labelled and disposed of through a licensed route according to local regulations.

D Historical study only

Taught for its chemistry, history and significance. No actionable home procedure is given, because the original method depends on substances or conditions that are unacceptable outside a professional laboratory.

When a page is classified at this level

Mercury vapour development, cyanide fixing or toning, uranium and cadmium compounds, and any process whose historical practice is documented mainly through the harm it caused.
Processes for which a safer modern route exists and is taught instead, with the historical method described for understanding only.

Controls the classification assumes

The page describes chemistry, workflow in outline, image characteristics and hazards, and states explicitly that it is not a procedure.
Where institutions or conservators demonstrate the process under controlled conditions, the page points to their published accounts rather than reproducing them.

How the assessment is made

For each practical page the assessor lists every substance and its concentration, every energy source (heat, UV, mains), every step that could produce a splash, a vapour, a dust or a shock, and every waste stream. The GHS classification of each substance is read from a named source and dated. The highest criterion met sets the level. Where a safer route through the same learning exists, it is given and the page notes which level the alternative route earns.

The assessment is recorded on the page in its Hazards, Required PPE, Ventilation, Waste streams and Disposal considerations sections, which the lab template requires and a build-time check enforces.

The policies this rubric hands off to

The rubric says what each level assumes. Five questions recur on so many pages that the course settles each of them once, in its own place, and every affected page cites that page rather than restating the rule.

The Level D policy — what Level D means, the rule that a Level D page carries no Procedure, Preparation or Build section, and the process families the level covers.
The chromium policy — chromium(VI) never used at any level; chromium(III), as chrome alum, permitted at Level B. Where a page mentions either oxidation state, this is the ruling it cites.
Glove selection — what the published permeation data say, and the substances for which the course could find none, which is what a Required PPE line has to work with.
Chemical incompatibilities — the pairs that must never meet and the reaction behind each.
The disposal caveat — what the course can and cannot tell a reader about waste, and why the answer is jurisdictional.

Beside them sit reading GHS and CLP, the lookup for the codes every hazard claim in the course is quoted from, and the first aid quick reference, which is what the assumption “an eyewash provision within reach” actually means.

When one step of a session exceeds its level

A session’s safety level is its baseline, not its ceiling. A single step may be declared at a higher level, provided the page names the step, names its level, and states the controls that step requires — in the Safety classification section, where a reader meets it before the procedure rather than inside it.

This is the rule because the alternative costs more than it buys. Weighing one reagent that meets a Level B criterion would otherwise push an entire Level A session to Level B, putting an ordinary lab behind equipment a home student may not have, and pushing the course toward mixing historical formulas in reduced form when Rule 6 asks for them complete.

Two limits, and they are what make the rule safe rather than a loophole:

A raised step is a step, not a session. If most of the work meets the higher criterion, the session is at the higher level and saying otherwise is a misclassification.
The controls are stated where the step is, not left to the reader to infer. A declared Level B weighing inside a Level A session carries the Level B controls for that weighing, written out.

Where a raised step can be avoided entirely — by buying a solution ready made, or by preparing a stock once outside the session — that is preferred, and the sepia ruling of 5 September 2026 turns on exactly this: the operation sets the level, not the substance.