Laying Out the Laboratory: Wet Areas, Dry Areas and Contamination Control

A laboratory is not a room with equipment in it. It is a set of decisions about where things happen, in what order, and what can reach what. Most of those decisions cost nothing, and they are much cheaper to make now than after a graduate that smelled faintly of fixer has ruined three films and you have spent a month blaming your developing times.

By the end of this page you should be able to draw your own space to scale with a dry area, a wet area, a ventilation path, a waste station and an eyewash point marked on it, say which operations belong on which side of the line, and name the three contamination routes your drawing still leaves open.

The division everything else follows from

Two kinds of work happen in a photographic laboratory and they are hostile to one another.

Dry work is weighing powders, cutting and handling film and paper, loading a tank, writing labels and reading a data sheet. It needs a surface that is dry, a light you can read by, and no splashes.

Wet work is mixing, diluting, pouring, processing and washing. It produces splashes, drips, condensation, and a fine mist whenever a solution is poured from any height.

Kodak’s guidance for amateurs puts the division at the centre of both its permanent and its temporary darkroom plans: a dry bench for enlarging, printing and handling films, negatives and paper, and a wet unit for mixing and all processing operations, either on opposite sides of the room with space between them or side by side with a splash guard in between. The AJ-3 leaflet says the same thing to somebody working on a kitchen counter: kitchens and bathrooms nearly always make the best places for a temporary darkroom because they supply running water, electrical outlets and a work surface; separate the room into a wet area and a dry area; and put a plastic sheet under the trays. It is worth noticing that the manufacturers endorse the domestic kitchen explicitly rather than tolerating it, provided the food is out of the way.

The reason is not tidiness. It is that the two kinds of work fail each other in both directions. A drop of fixer on the dry bench becomes a mark on the next negative laid there. A dusting of sodium carbonate carried from the weighing area into an open developer tray becomes an alkalinity you did not intend. Kodak’s own list of the causes of an out-of-control process names both directions: dry chemicals that become airborne during mixing and settle in an adjacent solution and solution splashed or dripped into another solution.

A kitchen laid out as a laboratory

1. 1Dry bench — balance, weighing boats, spatulas, labels, notebook, data sheets; stays dry
2. 2The break — a physical gap, or a splash guard if the benches must adjoin
3. 3Wet bench — graduates, mixing vessel, trays; a tray or bin under everything that holds liquid
4. 4Sink and water supply — rinsing, washing, and the eyewash point; not a disposal route
5. 5Extract at the wet end — openable window plus fan, so air leaves where the vapour is made
6. 6Air path — in at the door, across the dry bench, over the wet bench, out; never the reverse
7. 7Waste station — one labelled container per stream, each standing in a containment tray
8. 8Spill kit — absorbent, heavy gloves, bags, within reach without crossing the room

The plan is a kitchen, but the argument is the same for a spare room, a garage bench or a folding table.

The one-way workflow

The division becomes useful when you add a direction to it. Kodak’s account of why the benches are arranged as they are is a description of a one-way path: paper comes off the storage shelf, goes into the enlarger, then to the developer, then through the rest of the solutions, then to the wash. Nothing goes back.

One session, one direction

1. 1Dry: read, weigh, recordthe data sheet, the balance, the label written before the bottle is filled
2. 2Dry to wet: carry the solid acrossin a covered weighing boat, at bench height, not over the trays
3. 3Wet: dissolve, make up to volume, labelthe vessel stands in a containment tray from the moment it holds liquid
4. 4Wet: processdeveloper, stop, fixer, wash — in that order and never back up the line
5. 5Wet: collect waste by streameach solution to its own labelled container, never combined
6. 6Wet: rinse, then dry and put awaygraduates rinsed in the order they will next be used

The rule that does the work: nothing that has touched fixer goes anywhere upstream of the fixer until it has been washed.

Read the last step again. The failure mode is not the tray sequence, which nobody gets wrong; it is the equipment sequence, which everybody does. Tongs, graduates, funnels, thermometers and hands all travel freely if you let them, and they travel backwards as easily as forwards.

Ventilation as a design decision

Ventilation is the one thing on this page that is difficult to add later, so it is decided first.

What it has to achieve

Three things get into the air in a photographic laboratory. Dust, from weighing and pouring powders. Vapour, from acid and alkaline solutions — Kodak names acetic acid, sulfur dioxide and ammonia as the common indoor air contaminants of photographic processing. Mist, from pouring, from vigorous agitation and from any solution being splashed. The job of ventilation is to dilute and remove all three, and to do it before they mix with the air you are breathing.

That last clause is the whole of HSE’s guidance on extraction: control should contain and capture a vapour-air mixture before it can mix with the workroom air. It is also the reason a fan in the wrong place makes things worse. A fan at the door, blowing in, pushes tray vapour past your face on its way to the window. A fan at the window, drawing out, pulls it away from you. Same fan, opposite result.

What the sources actually say

Nobody publishes an air-change rate for a home darkroom. Four documents give figures for adjacent situations, and they agree more closely than you might expect.

Source	Situation	Rate
Kodak AK-3, Darkroom Design for Amateur Photographers	amateur darkroom, chemical handling area	change the whole volume of air 5 times per hour
Kodak J-300, Environmental Guidelines for Amateur Photographers	manual sink-line or open-tray processing	exhaust to outdoors at a minimum of 10 room volumes per hour
HSE COSHH essentials P1	manual film and plate development, commercial	powered wall or window fans, more than 5 air changes per hour, with a through draught
CIBSE Guide B §2.3.24.4, quoted by ILFORD	photographic darkrooms	6 to 8 air changes per hour, natural ventilation through a light trap acceptable for small occasional-use rooms

!Where these figures come from and what they are not

Three of the four are written for a workplace and the fourth for a hobbyist darkroom of the 1990s and 2000s. None of them is a measurement of a kitchen with three trays in it, and this course has found no source that gives one. What the convergence tells you is the order of magnitude: a darkroom needs its air replaced several times an hour, not once a day and not continuously. Treat 5 to 10 changes per hour as the target band, decide by how much vapour your work actually makes, and note that CIBSE’s own advice is to size a general-purpose darkroom from its heat gain rather than from an air-change figure at all.

Kodak J-300 gives the arithmetic, and it is worth doing once for your own room, because the numbers are smaller than people expect.

Extract rate a room needs, against its volume

• 5 air changes per hour
• 10 air changes per hour

Show the numbers behind this plot

Two straight lines through the origin showing the extract flow a room needs at five and at ten air changes per hour. At five changes per hour a 10 cubic metre room needs 50 cubic metres per hour and a 40 cubic metre room needs 200. At ten changes per hour the same rooms need 100 and 400. A marker at 25 cubic metres, which is a small bathroom or a large kitchen corner, shows 125 cubic metres per hour at five changes and 250 at ten; domestic extract fans are commonly rated in this range.

Series	Room volume (m³)	Extract rate (m³ per hour)
5 air changes per hour	0.00	0.00
5 air changes per hour	10.00	50.00
5 air changes per hour	20.00	100.00
5 air changes per hour	30.00	150.00
5 air changes per hour	40.00	200.00
5 air changes per hour	50.00	250.00
10 air changes per hour	0.00	0.00
10 air changes per hour	10.00	100.00
10 air changes per hour	20.00	200.00
10 air changes per hour	30.00	300.00
10 air changes per hour	40.00	400.00
10 air changes per hour	50.00	500.00

The two rates are published — Kodak AK-3 gives five air changes an hour and Kodak J-300 gives ten room volumes an hour — and the lines are exact arithmetic from them, volume times rate. Nothing here is a measurement of any actual room. Room volume is height by length by width in metres; a fan's rating is printed in m³/h or l/s, and 1 l/s is 3.6 m³/h.

A room 2.4 m high, 3 m long and 2.5 m wide is 18 m³, and needs about 90 m³/h at five changes an hour, or 180 m³/h at ten. Domestic extract fans sold for bathrooms and kitchens sit squarely in that range, which is the practical answer to the question this page is really being asked: no, you do not need laboratory extraction.

The home baseline, and what is not acceptable

The workable arrangement in most homes is an openable window at the wet end with an extract fan in or beside it, and a source of make-up air at the other end of the room — an open door, a trickle vent, a gap under the door. Air has to be able to get in or the fan simply strains against the room. Kodak’s version of this is a fresh-air supply positioned so it does not disturb the capture of vapour at the exhaust, with the exhaust set slightly higher than the supply so the room runs at a slight negative pressure and vapour does not escape into the rest of the house.

Three arrangements are not acceptable, and it is worth being explicit about why.

• A sealed cupboard or closet. There is no supply of air and no path out, so every vapour the work makes stays with you and rises in concentration for as long as you work. Kodak’s own closet darkroom plan is worth reading here for what it does not say: it praises a closet for being easy to make dark and warns that it will have no running water and possibly no electrical outlets, and it does not address ventilation in a closet at all. The classification rubric is explicit where the plan is silent — Level A requires an openable window or an extractor, and no processing in a sealed cupboard.
• An internal bathroom with only a mechanical extract and no window. No source in this course’s corpus addresses this case, so what follows is the course’s own reasoning rather than a citation. It is better than a cupboard, because there is a fan; but the fan is sized for shower moisture, in many installations runs only while the light is on, and draws its make-up air from the rest of the house — which is the direction you least want vapour to travel. If it is the only room you have, work with the door open, run the fan throughout, keep the trays covered, and keep the sessions short.
• A room with a fuel-burning appliance in it. An extract fan can pull combustion products back down a flue. This is a building-services question, not a photographic one, and the answer comes from whoever is qualified to look at the appliance.

✦The cheapest ventilation improvement is a lid

Before you buy a fan, cover the trays. Kodak’s guidance is explicit that covers on tanks and trays, made of a durable non-reactive material and covering as much of the open surface as possible, are an effective control, and that good general ventilation plus effective covers is often the whole answer. HSE lists the same move — fitting lids to equipment — among the simple controls that come before extraction. A tray of stop bath with a sheet of acrylic over it between prints is emitting a fraction of what an open one does.

Surfaces, containment and light

The surface

The wet bench needs a surface that a spilled solution cannot soak into and that will survive being wiped down after every session. Kodak recommends laminated plastic of the kind used for kitchen worktops, or linoleum or sheet vinyl carried up the back wall to the shelf so that the splash zone is covered and there is no dust-catching corner at the back; where no covering is fitted, the wood is sealed with a chemical-resistant paint or lacquer. HSE’s sheet for manual film development adds the detail that matters most: work surfaces should be easy to clean and lipped, so that a spill runs along the bench rather than off it into your lap.

What has to be between a spilled solution and the floor

Drawn for clarity, not to scale. A tray under everything that holds liquid is the cheapest single improvement on this page. Layer depths are drawn to be readable, not to scale: on real film the base is roughly a hundred times the emulsion, and drawn honestly the emulsion would vanish. Any thickness given in the labels is the real one.

Secondary containment is the same idea applied everywhere: a tray or a shallow bin under every vessel that holds liquid, sized to hold the entire contents of the largest thing standing in it. HSE’s version is to keep developing solutions in shallow trays to contain spillage; the storage page extends the rule to the shelf.

Floors and splashes

Whatever the floor is, assume it will be splashed. A sealed hard floor can be mopped; a carpet cannot, and a rug in front of the wet bench is a wick. If the floor is not sealed, a sheet of polythene or a plastic tablecloth under the wet end is a two-minute answer. Closed shoes are on the protective equipment page for the same reason.

Light

Two different lighting requirements coexist and people usually solve only one.

Working light has to be bright enough to read a label, read a meniscus at eye level and see a splash on a pale surface. If you cannot read the fine print on a bottle where the bottle actually stands, the light is not adequate, and a task lamp on the dry bench is cheaper than a mistake.

Safelight is a different problem with different rules, and it belongs to the parts that handle sensitive material. Kodak’s constraint that shapes the layout is that a safelight is kept at least 1.2 m from the working surface, which sets a minimum ceiling height over the trays and rules out putting a lamp on the shelf directly above them.

⚠Water and electricity in the same room

Enlargers, timers, fans and task lamps are mains equipment standing near trays of liquid. Princeton’s guidance for photographic studios is to fit residual-current protection wherever an outlet or a piece of electrical equipment is within six feet (about 1.8 m) of the risk of water splashes. Nothing in this course asks you to wire anything: mains work is Level C in the classification, purchased and certified equipment only, and a residual-current device that plugs into a socket needs no wiring at all. Dry your hands before touching a switch.

Water: a supply, and a drain that is not a disposal route

You need water for four things: making up solutions, washing film and prints, rinsing equipment, and rinsing yourself. They are not the same water and confusing them causes trouble.

Clean water is what goes into a solution. It comes from the tap into a clean vessel and touches nothing else on the way.

Rinse water is what comes off equipment and off your gloves. It carries whatever was on them, which means the first rinse of a fixer graduate is dilute fixer and belongs with the fixer waste, not in the sink. Kodak’s AJ-3 leaflet asks for a dedicated container of water for rinsing hands, kept expressly so that other solutions are not carried into the developer.

Wash water is what runs over film and prints. It is a large volume of dilute everything, and what happens to it is a question the waste page answers.

The drain is a route out of the building, not a disposal method. Kodak’s handling guidance carries one instruction that belongs here rather than on the waste page, because it is about the plumbing rather than about the chemistry: never pour photographic processing chemicals into a drain where chlorine-containing cleaning agents are present unless the drain has been thoroughly rinsed, and run plenty of water down the drain before and after. Household bleach in a trap is an acid-chlorine reaction waiting for a stop bath. Kodak’s darkroom design sheet adds that some photographic solutions, acid ones in particular, corrode a sink and its drainage, so the sink is washed and the drain flushed with cold water after every session.

Vessels that never go back to the kitchen

Every source in this course’s corpus that addresses domestic photographic work says the same thing in the same tone. ILFORD: do not use utensils meant for preparing food to make up and store photochemicals, keep foodstuffs away from where photographic chemicals are prepared and used, and do not store chemicals in soft-drink bottles. Kodak: do not eat, drink or smoke in chemical handling areas, and store concentrates away from food. It is one of the few rules on which the manufacturers, the regulator and the university safety offices are unanimous, so it is stated here as a rule rather than argued.

Two reasons stand behind it, and they point in opposite directions. A vessel that has held fixer and goes back to the kitchen carries thiosulfate into food. A vessel that has held food and comes to the bench carries fat, sugar and detergent into a developer. The second is the one nobody expects.

Colour coding, and one graduate per family

ILFORD’s leaflets give the working scheme: buy plastic beakers and bottles in different colours and use a different colour for each solution; mark each dish or tray developer, stop or fix and store its chemistry in a matching container; use a separate pair of tongs for each dish. Kodak’s process manual reaches the same conclusion from the industrial end: use separate mixing vessels for developers and fixers.

Colour is a shortcut, not the system. Nothing in this course is identified by colour alone, because colour vision varies and darkroom light is not white. Write the family on the vessel as well, in the same words every time: DEV, STOP, FIX, WASH, SILVER. A strip of tape and a marker pen is the whole implementation.

The families that must not share a vessel are:

Family	What it must not meet	Why
Developer	fixer, stop bath, any acid	thiosulfate and acid both destroy developing activity; the acid also lowers pH, which the developer depends on
Stop bath and other acids	sulfite, thiosulfate, hypochlorite	acid on sulfite or thiosulfate gives sulfur dioxide; acid on hypochlorite bleach gives chlorine
Fixer	developer	carried-over developer stains and exhausts the bath
Silver nitrate and silver-bearing rinses	everything	silver is precipitated by chloride and reduced by almost anything organic, and it is collected rather than discarded

The chemistry behind the second and third rows is the storage page’s business, and the silver row is the silver nitrate page’s. The point here is that the row you cannot see coming is the first one, because its failure is silent.

Cross-contamination: the classic failure

How a trace of fixer reaches next week's developer

1. 1Pour fixer with a shared graduate
2. 2Put it down wet — a film of thiosulfate stays on the wall of the vessel
3. 3Quick rinse — removes most of it, and most is not all
4. 4Dries invisible — nothing to see, nothing to smell after a day
5. 5Measure developer with it next week
6. 6Contaminated stock — every film or print made from it is affected
7. 7Interruption A: one graduate per family — the only one that removes the failure rather than reducing it
8. 8Interruption B: wash, do not rinse — detergent, brush, then a final rinse in clean water
9. 9Interruption C: developer first — measure developer at the start of a session, before anything acid is opened

Only interruption A removes the pathway. B and C reduce the probability, which is not the same thing.

Both manufacturers state the effect and neither states a threshold. ILFORD: even a trace of fixer can contaminate the developer, and possibly ruin your next film, and, for prints, a trace of fixer or stop bath can contaminate the developer, leading to inconsistent results or — at worst — completely blank prints. Kodak’s diagnostic chart lists “developer contaminated with fixer or stop bath” among the causes of an abnormally thin negative, and gives the remedy as replacing the developer and washing the mixing equipment thoroughly.

!A number this course could not find

How much thiosulfate does it take to damage a developer? This course searched its manufacturer, regulatory and conservation corpus and found no published figure — no concentration, no parts per million, no threshold of any kind. What the sources give is a direction and a severity: any trace is undesirable, the effect is a loss of density and contrast, and the failure is progressive rather than sudden. Treat “keep it out entirely” as the working rule, not because a small amount is certainly ruinous, but because nobody has published the amount that is not.

The other two routes the layout leaves open are worth naming, because a student who has fixed the graduate problem often has not noticed them. Airborne powder: weighing sodium carbonate near an open tray, which Kodak names explicitly. Hands and tongs: a hand that has been in the fixer and then adjusts the developer tray, or one pair of tongs doing all three dishes. ILFORD’s answer is tongs per dish and the explicit warning not to contaminate the developer tongs with the stop bath; Kodak’s is a dedicated bowl of rinse water and a clean towel.

Setting up and packing down

If the space is shared, the design problem is different: the laboratory has to appear in ten minutes and disappear in ten.

The box kit. Kodak’s suggestion for a temporary darkroom, and it still works: keep all the equipment in one or two boxes, which saves both the time spent collecting it and the chance of losing something. Two boxes is the better number, and the division is the same one as the benches — a dry box for balance, spatulas, labels, notebook and data sheets, and a wet box for graduates, funnels, trays, tongs and bottles. The wet box lives inside a lidded plastic crate that doubles as its own containment tray.

The folding bench. A folding table with a wipeable cover over it, set up away from the sink for dry work, is the cheapest dry bench there is. The wet side stays at the sink where the water and the drain are.

The pack-down sequence matters more than the set-up. In order: seal the waste containers and put them where they live; wash the wet vessels in family order, cleanest first; wipe the surfaces down with a damp cloth, which is Kodak’s instruction as well as the obvious one; return the dry box to its shelf; wash your hands. HSE’s sheet for manual development asks for the work area to be cleaned daily and the room weekly, and for lids to go on containers immediately after use, even empty ones. Then, and only then, the room is a kitchen again.

Accessibility

Nothing above requires standing, and most of it does not require reaching.

Seated working. Kodak’s own dimensions help: the tray shelf in its closet darkroom sits 3 feet (0.9 m) from the floor, which suits a standing worker, and its dry bench and wet unit are each 26 inches (66 cm) wide. For seated work, a lower surface and a shallower bench are better on both counts, and a folding table at desk height with the trays on it is a legitimate wet bench. What must not change is the separation and the containment.

Heavy containers. A five-litre container of concentrate is difficult to pour accurately and dangerous to pour tired. Decant once, on the day it arrives, into bottles you can lift with one hand, and label each with everything the original carried. Kodak’s storage guidance asks for containers to be positioned where they can be reached without stretching, which is the same rule seen from the other end.

Smaller volumes. Nothing in this course requires a large batch. Making 250 ml of a working solution instead of a litre reduces the lifting, the spill size, the waste and the cost of a mistake. The concentration page shows that the arithmetic is identical at any scale.

Not handling powders at all. Buying liquid concentrates rather than mixing from dry chemicals removes the dust route entirely, and Princeton’s guidance for photographic studios recommends it as the first choice rather than as an accommodation. What it costs you is the ability to make historical and modified formulas, which arrives later in the course; several parts state a liquid-concentrate route alongside the from-scratch one.

Pouring without lifting. HSE’s sheet for manual film development asks for pumps or squeezable bottles in place of hand pouring. A pump dispenser on a five-litre container is a laboratory control that happens to be an accessibility one.

The list to complete before the first chemical arrives

This is the page’s output, and it becomes the lab-opening procedure you run at the start of every session.

1. The dry area and the wet area are identified, physically separated or split by a splash guard, and each has a written list of what happens there.
2. The wet surface is non-absorbent and lipped, or covered so that it is.
3. A containment tray stands under everything that will hold liquid.
4. The window opens, or the extract runs, and you know which way the air moves through the room.
5. There is a source of make-up air at the far end from the extract.
6. Clean water is available, and a separate rinse container for hands and equipment.
7. Every vessel is labelled with its family in words, and none of them will return to food use.
8. The waste containers exist, are labelled, are empty, and stand in a tray.
9. The eyewash point is identified and reachable in a few seconds without crossing the room.
10. The spill kit is within reach of the wet bench.
11. The data sheet for every product you will open has been read, and its version and date recorded.
12. Someone in the house knows what you are doing and roughly how long you will be.

📓Lab notebook

Draw your room to scale, once, and keep the drawing in the front of the lab notebook. Mark the two areas, the air path, the waste station, the eyewash point and the spill kit. Then write underneath it the three contamination routes the layout still leaves open, and what you do about each. Both halves get revised; the second half is the one that earns its keep.

Recap

• A laboratory is a set of decisions about where things happen and what can reach what. The first is the wet and dry division, which both manufacturers put at the centre of their darkroom advice.
• The workflow runs one way, from reading and weighing through mixing to processing to waste to clean-up, and equipment travels the same way. The failure is the vessel that goes backwards.
• Ventilation has to dilute and remove dust, vapour and mist, and capture them before they reach you. No source gives a figure for a home darkroom; four adjacent sources converge on 5 to 10 air changes per hour, which for a domestic room is an ordinary extract fan. Covering the trays comes before buying the fan.
• The wet surface is non-absorbent and lipped, everything liquid stands in a tray that would hold all of it, and the light is bright enough to read a label and a meniscus.
• The drain is a route out of the building, not a disposal method, and never receives photographic chemistry where chlorine-based cleaner may be sitting in the trap.
• One vessel per solution family, labelled in words as well as colour, never returning to food use. Even a trace of fixer damages a developer; no published threshold exists, so the rule is total exclusion.
• A shared room works through a box kit and a pack-down sequence, and every accessibility route on this page — seated heights, decanting, smaller volumes, liquid concentrates, pump dispensers — is a control that a laboratory would recognise on its own terms.

Check your understanding

Question 1. You have one room, one bench and no possibility of a second surface. What is the minimum acceptable version of the wet and dry division?

• Do dry work in the morning and wet work in the evening, on the same clear bench
• Physically separate the two ends of the bench with a splash guard, and keep the dry end dry throughout the session
• Abandon the division; it only matters in a purpose-built darkroom
• Put everything on trays and work anywhere on the bench

Show the answer and why

Answer: Physically separate the two ends of the bench with a splash guard, and keep the dry end dry throughout the session

Kodak allows the two units side by side provided a splash guard separates them, and that is what a single bench can reproduce: a guard, or a real gap, with the balance and the paper at one end and the trays at the other. Time separation fails because the failure is residue, not simultaneity - a dry bench that was wet an hour ago is still contaminated. Trays are secondary containment, which is a different control and does not stop airborne powder reaching a tray.

Question 2. A room is 2.4 m high, 3.0 m long and 2.5 m wide. Roughly what extract flow does it need at eight air changes per hour?

• About 18 m³ per hour
• About 60 m³ per hour
• About 145 m³ per hour
• About 1,150 m³ per hour

Show the answer and why

Answer: About 145 m³ per hour

The volume is 2.4 × 3.0 × 2.5 = 18 m³, and eight changes an hour is 18 × 8 = 144 m³/h. That is an ordinary domestic extract fan, which is the point of doing the arithmetic: darkroom ventilation figures sound industrial until you multiply them by a small room. Note that 8 ACH is the upper end of the CIBSE range quoted by ILFORD for photographic darkrooms, and that CIBSE itself says a general-purpose darkroom should be sized from its heat gain instead.

Question 3. A student's negatives are thin and their prints will not reach a full black. Two of their four graduates are unmarked and one smells of fixer. What is the cheapest first test?

• Buy a new box of paper and a new film, and repeat the whole sequence
• Send the film to a laboratory to confirm the camera exposure
• Mix fresh developer in a vessel that has never held fixer, and process a test film with everything else unchanged
• Increase the development time by fifty per cent and see whether the negatives improve

Show the answer and why

Answer: Mix fresh developer in a vessel that has never held fixer, and process a test film with everything else unchanged

Thin negatives and prints that will not reach black are both a loss of developing activity, and Kodak's own diagnostic chart lists developer contaminated with fixer or stop bath as a cause, with the remedy of replacing the developer and washing the mixing equipment. Changing one variable - the developer and the vessel it was mixed in - while holding film, exposure, time and temperature still is what makes the test diagnostic. Increasing development time would mask a contaminated developer rather than identify it, and would then mislead you for months.

Question 4. Which of these is a reason to keep a dedicated container of rinse water on the wet bench, rather than rinsing hands under the tap?

• Tap water contains chloride, which would precipitate silver from your gloves
• It keeps the transfer of one solution to another off the equipment and out of the sink, and it is available without crossing the room
• Rinse water must be collected as waste and cannot be allowed to enter the drain at all
• The tap is part of the dry area and must not be used during processing

Show the answer and why

Answer: It keeps the transfer of one solution to another off the equipment and out of the sink, and it is available without crossing the room

Kodak asks for a container of water for rinsing hands expressly so that solutions are not carried into the developer, and it is on the bench so that it is used at the moment it is needed rather than three steps later. The chloride answer is a real effect but belongs to silver nitrate work, not to hand rinsing; and while the first rinse off a fixer vessel does belong with the fixer waste, hand-rinse water is not a stream this page asks you to collect. The sink is part of the wet area, not the dry one.

Question 5. You are told that colour-coding graduates by solution family is enough to prevent cross-contamination. What is wrong with that as a complete answer?

• Nothing; ILFORD recommends colour coding and that is sufficient
• Colour coding is unreliable in darkroom lighting and for readers whose colour vision differs, so each vessel also carries its family in words
• Coloured plastics leach dye into photographic solutions
• Colour coding only works if the vessels are glass

Show the answer and why

Answer: Colour coding is unreliable in darkroom lighting and for readers whose colour vision differs, so each vessel also carries its family in words

ILFORD does recommend different colours for different solutions, and it is a good scheme - but a scheme that encodes safety-relevant information in colour alone fails under safelight, fails in low light, and fails for a substantial minority of readers. The course's rule throughout is that nothing is identified by colour alone. Write DEV, STOP, FIX on the vessel as well; the tape and marker cost nothing and survive both problems.

Question 6. Why does this course say that an extract fan blowing into the room is worse than one drawing out of it?

• A fan blowing in raises the room pressure and forces vapour into the rest of the house, and it pushes tray vapour past the operator on its way to the opening
• Fans are less efficient when blowing than when drawing
• Blowing in raises the humidity, which fogs photographic paper
• There is no difference; only the number of air changes matters

Show the answer and why

Answer: A fan blowing in raises the room pressure and forces vapour into the rest of the house, and it pushes tray vapour past the operator on its way to the opening

HSE's guidance on extraction is to capture a vapour-air mixture before it mixes with the room air, and Kodak asks for the exhaust rate to exceed the supply rate so that the room sits at a slight negative pressure and vapour does not escape into adjoining rooms. Both point the same way: take air out at the wet end, let it in at the clean end. The number of air changes matters too, but a correct rate in the wrong direction still delivers the vapour to your face.

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