Experiment: Maximum Black, Base White and Dry-Down

Part XIX will ask you for a base exposure, and it will assume you know three things about the material you are printing on that nobody has published for it. This session measures them.

Purpose

To establish, for one paper and one developer, the exposure at which maximum black is first reached, the development time at which it stops rising, the reflection density of the untouched processed sheet, and the amount by which a print darkens between wet and dry — each with a stated method and a stated uncertainty.

The hypothesis. Maximum black, base white and dry-down are reproducible properties of a paper-developer-processing chain rather than of a paper alone. Fix the chain and each of the three repeats within the uncertainty of its own measurement; change any link and at least one of them moves. The sharper half of the hypothesis is about the third number: dry-down has a measurable material component, distinct from the viewing-condition effect it is usually confused with, and it is larger on a matt surface than on a glossy one.

The control. One paper batch, one developer batch, one processing sequence, one drying method, one reading geometry, one operator, one evening. A strip carried through every arm unchanged, and an unexposed sheet processed with each arm, are the two references everything else is read against.

The one variable that changes, arm by arm: exposure in arm 1, development time in arm 2, the wet or dry state of the sheet in arm 3, and the paper surface in arm 4. Nothing else moves in any arm, and the arms are run in that order because each supplies a number the next one needs.

=What each outcome would mean, written before the measurement

If Dmax plateaus and base density rises over the development series, the plateau model the previous lesson taught is confirmed on your own material and you can read your usable window straight off the pair of curves.

If Dmax has not plateaued by six minutes, either your developer is weaker than you think — check the dilution and the temperature — or the plateau statement the course borrowed from a fibre-paper datasheet does not transfer to your material, which is a finding worth publishing in your own notebook.

If the wet and dry readings differ by less than your instrument’s repeatability, you have not shown that dry-down does not exist. You have shown that its material component is below your noise floor, and that what you have been seeing in the tray is a viewing-condition effect. Those are different conclusions and the page will not let you collapse them.

Learning objectives

By the end of this session you will be able to:

• Find the minimum exposure for maximum black by an exposure series read through clear film base, and say why the film base is in the path rather than absent from it.
• State what that exposure fixes and what it leaves entirely open.
• Plot Dmax and base density against development time and identify the window between the plateau of one and the lift of the other.
• Read base white as a processed quantity rather than as a property of the raw sheet, and explain why the two differ.
• Measure dry-down as a density and convert it into a fraction of a stop, naming the assumption the conversion rests on.
• Distinguish the material component of dry-down from the viewing-condition component, and design a reading that isolates the first.
• Quote a measurement with its uncertainty, and refuse a comparison the uncertainty cannot support.

Prerequisites

• Paper sensitometry, which owns reflection density, the shape of a paper curve, the reference white and the course’s 45/0 convention. None of it is re-derived here.
• The structure of a printing paper, for what the surface is doing to the light before the silver gets a say.
• Print developer chemistry, for development to completion and the plateau this session measures.
• Calibrating the densitometer, for the reflection head, its anchors and — crucially — the three repeatability figures it made you quote.
• Lab: mixing and comparing print developers, which is where the developer batch and its version number come from.

Safety classification

Level A throughout, with no step raised. This session weighs nothing and mixes nothing from powder: the developer was made on the previous page and comes to the bench in a bottle. What is handled is three working baths at tray dilution, at room temperature, with tongs.

• The developer is alkaline and contains metol and hydroquinone, both skin sensitisers, both classified as eye hazards. Gloves and eye protection whenever a solution is handled, and a print lifted with tongs rather than fingers.
• The stop is a dilute weak acid and the fixer is a non-hardening ammonium thiosulfate bath. Neither is heated and neither is concentrated.
• The real risk of the evening is mechanical and it is the dark. You will be making many small strips, handling them wet, carrying them to an instrument and back, with the lights off for long stretches. The bench is laid out and the trays filled with the lights on, the floor is cleared, and the route from the wash to the densitometer is walked once before it is walked in the dark.
• The instrument is mains-powered or battery-powered equipment built in Part XV, and wet prints are going near it. That is a genuine hazard this page introduces and the procedure below deals with it explicitly rather than hoping.

What is not a hazard here, and why. There is no dust exposure at all: nothing is weighed, no jar of powder is opened, and the whole Level B weighing argument of the previous page is absent because the operation that created it is absent. There is no sulfur dioxide route, because the acid and the sulfite never meet — separate trays, separate tongs, separate waste containers. There is no concentrated silver anywhere: the silver in this session is in hardened gelatin on the sheets, or dissolved in the fixer at the low concentrations Kodak’s own table gives for an amateur bath, and neither is a contact hazard. What those substances are like when they are not locked in gelatin or diluted in a tray is a different matter entirely, and it is why the previous page reads as it does and this one does not.

Hazards

Hazard	Where it arises	Control
Skin sensitisation from metol and hydroquinone	Developer tray, wet strips	Nitrile gloves; tongs; change a glove that is contaminated inside
Eye irritation from an alkaline splash	Pouring the working solution; rocking a full dish	Eye protection from first pour to last rinse; rinse a splash at once and keep rinsing
Slips and spills in near darkness	Carrying wet strips between the wash and the instrument	Floor cleared; fixed route walked with the lights on; nothing left on the floor
Water reaching mains or battery electronics	Reading wet strips on the densitometer	The wet-reading rig below; the instrument on its own dry mat, never on the wet bench; hands dried before touching it
Aquatic toxicity of every spent bath	Clean-up	Collect separately; never a drain; see Disposal considerations

Required PPE

Nitrile gloves throughout the wet half, changed when contaminated. Eye protection with side shields whenever a solution is being poured or a dish rocked. No respiratory protection is among the controls here, and the reason is specific rather than reassuring: nothing in this session is a powder and nothing produces a vapour, so the airborne route that justified a mask on the previous page does not exist on this one. Dry hands and a dry cloth are protective equipment on this page in a way they are not elsewhere, because the instrument is the thing being protected.

Ventilation

An openable window or an extractor giving a through draught, as for any tray session. The controls here are not vapour controls — no bath is heated, no solvent is present and the most volatile thing on the bench is water. What the airflow is actually doing is keeping the room comfortable over two hours with the door shut, and keeping the smell of a working fixer from becoming the reason you cut the session short. Run the ventilation check before the lights go off.

Materials

Item	Quantity	Note
Variable-contrast RC paper, glossy, 8 × 10 in	8 sheets	Cut into strips. The session’s principal material
The same paper in a second surface — pearl or satin	2 sheets	Arm 4’s second surface, at the same price per sheet
A fibre-base sheet, glossy and matt	2 sheets	Optional, and the interesting case. The price file prices the glossy sheet and not the matt
Clear film base	1 piece, 25 × 25 mm	A processed, unexposed, fixed and washed offcut of the same film you print from — its rebate is ideal
21-step transmission step wedge	1	For the wet-and-dry patch series of arm 3. Its increment is nominal, not certified
Developer, D72-EB-001 stock	160 mL	Diluted 1+4 to make the 800 mL working bath. From the previous page, or a bought concentrate — but one of them, all evening
Stop bath concentrate	40 mL	1+19, 800 mL working
Rapid fixer concentrate	160 mL	1+4, 800 mL working
Blotters, lint-free	4	For the wet reading, and they must be identical to each other

Chemicals

Nothing is mixed from powder on this page. The working baths are made from stock or concentrate, in the volumes above, at the dilutions their sources publish.

Chemical	Quantity	Form
Metol and hydroquinone	0.50 g and 1.95 g	Already in solution, as the developing agents of the 160 mL of D-72 stock
Sodium carbonate	10.8 g	Already in solution, as the alkali of that stock
Sodium sulfite	7.2 g	Already in solution, as its preservative
Potassium bromide	0.30 g	Already in solution, as its restrainer
Citric acid	40 mL of concentrate	Stop bath at 1+19, 800 mL working
Ammonium thiosulfate	160 mL of concentrate	Rapid fixer at 1+4, non-hardening

Equipment

The reflection head of the densitometer, with its white reference tile and its black trap, warmed up and anchored. Enlarger, timer, easel, safelight. Four dishes of 8 × 10 in and one pair of tongs each. A contact frame or a sheet of 3 mm glass. Thermometer. A guillotine or a steel rule and knife. A drying rack. A squeegee only if you already own one you trust — see the drying note. A watch with a second hand for the wet readings, which are timed.

Estimated cost

Band £. The instrument is Part XV’s and the developer is the previous page’s; this session’s own outlay is a dozen sheets of paper and three tray-fulls of solution. The planner holds the dated figures and the gaps.

Estimated consumables cost

Consumed	This session	Sourced price	Cost this session
RC paper, 8 × 10 in, glossy	8 sheets	£33.50 for 25 sheets to £96.18 for 100	£7.68 to £10.72
RC paper, second surface	2 sheets	The same, and the price file records glossy, satin and pearl at the same price on the day	£1.92 to £2.68
Fibre-base paper, glossy and matt	2 sheets	£47.04 for 25 sheets to £150.28 for 100, glossy only; matt is a separate product the file’s band does not cover	£1.50 to £1.88 for the glossy sheet
Developer stock	160 mL of D72-EB-001	The previous page’s rows come to about £5.70 to £6.00 for one litre of stock	about £0.90
Stop bath concentrate	40 mL	£10.66 to £12.18 per 500 mL	£0.85 to £0.97
Rapid fixer concentrate	160 mL	£21.05 to £25.98 per 1 L	£3.37 to £4.16
Clear film base	one offcut	Free: it is a rebate you already own	—
21-step wedge	not consumed	The price file names it in its own gap list; no UK listing was found	—

The rows that carry numbers total about £16 to £21, and that is a floor rather than a total: the matt fibre sheet that makes half of arm 4 worth running has no sourced price, because the file’s fibre band is for the glossy surface and a matt paper is a separate product sold at its own price. Leave both fibre sheets out and the priced rows come to about £15 to £19. Equipment is not in the table because a densitometer is not consumed.

Waste streams

Three. Alkaline developer, about 800 mL, negligible in silver on Kodak’s own table and collected for its pH and its aquatic classification. Acid stop, 800 mL, in its own container and never in the developer’s. Silver-bearing: the spent fixer and the first wash, into the labelled container under the silver-bearing waste procedure. Solids — gloves, blotters and the strips you do not keep — bagged, because a processed strip carries silver in hardened gelatin. Everything sits under the disposal caveat; check your local regulations.

Alternative route

Without a reflection densitometer, three of the four arms still work and the fourth changes character.

Print a dried reference scale in the same session as everything else: one sheet exposed through the step wedge at the maximum-black exposure, developed for the session’s time, processed, and dried by the session’s method. That strip is your ruler. Every subsequent judgement is then “which step of the reference does this patch match”, made side by side in even light with both pieces dry, and every result is recorded as a step number rather than a density.

What that route actually gives you, stated honestly:

• Arm 1 works unchanged. Finding the first step that no longer darkens is a comparison of adjacent patches, which the eye does better than almost anything else.
• Arm 2 works. You will see the plateau arrive and you will see the base grey later; what you lose is the ability to say by how much.
• Arm 3, base white, is the weakest. Judging a near-white against a near-white by eye is the hardest visual comparison there is, and small base differences will be invisible.
• Arm 4, dry-down, is where the route bites. You cannot lay a wet strip against a dry reference and call the difference a measurement, because the wet strip’s surface is doing something the dry one is not. What you can do is the paired-print method: two identical prints, one judged wet and matched against the reference, the other dried and matched again, with the difference read in steps.

A step of a nominal 0.15 wedge is about half a stop. That is the resolution of the whole route, and it is roughly the size of the effect you are chasing. Write it beside your result.

!A comparison the course has not made, and will not pretend to have made

The honest thing to do with two routes is to run both and publish the disagreement. This course has not done that. No side-by-side trial of the visual route against the instrument has been carried out here, so nothing on this page claims the two are equivalent, and no correction factor between them is offered.

If you own the instrument, you are in a position to settle it: run both routes on the same strips in the same session, tabulate instrument density against matched step number, and publish the scatter. That is the single most useful thing a reader could contribute to this page.

Preparation

1. Bring the instrument up and anchor it on the white tile and the black trap by the daily check, and write down today’s three reflection repeatability figures. They are the numbers every conclusion tonight will be measured against.
2. Build the wet-reading rig before anything is wet (below). Dry runs first, with a dry strip.
3. Cut the paper in the dark, before the first exposure, by the dim-light handling procedure: eight glossy sheets into quarters gives thirty-two strips of about 100 × 127 mm, which is more than the session needs and removes cutting from the middle of a timed sequence.
4. Fill the four dishes and bring every bath, the wash included, to 20 °C ± 1 °C with the lights on.
5. Set the enlarger up once — head height, aperture, filter 2 — and do not touch it again all evening. Every exposure tonight is a time, on one geometry.
6. Mask the easel so each strip is exposed in a defined position, and mark the strips in pencil on the back before they go anywhere near a solution.

Reading a wet print on a 45/0 head, repeatably

1. 1Surface water is a mirror — a 45/0 head rejects the specular glint off a flat surface, not off a lens-shaped drop
2. 2Two identical blotters, one press, a counted time — the state you are measuring is "surface water removed, gelatin still swollen"
3. 3Glass tile on the dry mat — flat backing, and no water path to the electronics
4. 4Read within a stated delay — the strip is drying while you read it; the delay is part of the method
5. 5The same patch, dried, read again — never a twin strip — the pairing is what removes the sheet-to-sheet variation

Procedure

Arm 1 — the minimum exposure for maximum black (about 20 minutes)

7. Tape the clear film base into the negative carrier so that it fills the aperture. The film base is in the path on purpose. In real printing the light reaches the paper through a negative’s base, which absorbs and scatters a little of it; measure the exposure with an empty carrier and you get a number that does not transfer to any print you will ever make. Use a rebate from the same film stock you print from, processed, fixed and washed with the rest of the roll.
8. Expose one strip as a series in thirds of a stop — ten or twelve steps, spanning three to four stops, so that several of them fall past the plateau and prove it is one. Use the timer’s own division rather than counting seconds: the arithmetic belongs to Part XVII.
9. Develop for the session’s fixed time — 2 minutes at 20 °C, the same figure the previous page defends — then stop 10 s, fix 30 s, wash 2 minutes, and dry by the session’s method.
10. Read every step dry. The minimum exposure for maximum black is the first step whose density is indistinguishable from the step after it, where “indistinguishable” means “differs by less than your quoted repeatability” and not “looks the same”.
11. If the last step is still climbing, extend the series and run it again. A series that never plateaus has not found a maximum; it has found the end of your bracket.

🕰Where this definition comes from, and where it does not

The endpoint is not the course’s invention. Hurter and Driffield defined a paper’s range in 1898 as the ratio between two exposures: one which just falls short of producing any deposit, and one which just suffices to produce the deepest black the paper is capable of recording when viewed by reflected light, and worked an example at five seconds against a hundred and sixty. That is a primary source for the quantity arm 1 measures, and it is the same quantity a modern datasheet calls Dmax.

The clear-film-base technique for finding it is not sourced. The course searched its corpus for a Tier 1 or Tier 2 description of that specific procedure and found none, so this page presents it as the course’s own method, derived from the definition above and from the requirement that the number transfer to real printing. If you know of a manufacturer or standards description of it, the page should carry that instead of this paragraph.

Arm 2 — maximum black against development time (about 30 minutes)

12. Expose eight strips at the arm 1 exposure, all identical, in one run so that no lamp drift or voltage change separates them.
13. Develop them one at a time at 20 °C for 30 s, 45 s, 1 min, 1½ min, 2 min, 3 min, 4½ min and 6 min, and process each identically afterwards. Six minutes is the ceiling because that is where the maker’s own published extension statement stops.
14. Alongside each, develop half an unexposed strip for the same time. This is the base-density series and it is the whole reason the arm is worth running: Dmax alone would tell you the plateau and nothing about its price.
15. Dry everything by the session’s method, then read Dmax and base density on every strip in one sitting.

Arm 3 — base white (about 10 minutes)

16. Read the unexposed, processed, dried strip from the session’s own time. That is base white, and it is not the same thing as the raw sheet.
17. Read the raw paper, straight from the packet, unprocessed. Record both. The difference is what the chemistry did to the sheet — every wet bath the paper passed through, and a little of the optical brighteners’ fate — and it is usually small and never zero.
18. Read the second surface, unexposed and processed identically. This is the first number in the session that belongs to the surface rather than to the silver.

Arm 4 — dry-down, measured (about 40 minutes)

19. Print the step wedge onto two identical strips at the arm 1 exposure and the session’s time. One is the paired strip and one is the spare.
20. After the wash, blot the paired strip by the rig above — the same two blotters, the same single press, the same count — and read five stated patches on it: paper white, and the steps whose dry densities are near 0.3, 0.7, 1.2 and Dmax. Note the delay between blotting and reading and keep it the same for every patch.
21. Repeat the whole wet reading three times, re-wetting the strip between runs. The spread of those three is your wet repeatability, and it will be worse than the dry figure from your calibration. Quote it; do not borrow the dry one.
22. Dry the strip by the session’s method, leave it an hour, and read the same five patches on the same strip. Pairing is what removes sheet-to-sheet variation from the answer.
23. Repeat arms 19 to 22 on the second surface, and on a fibre sheet if you have one. Hold the drying method identical across all of them, and state it.

✕Three ways a dry-down measurement quietly becomes a measurement of something else

Reading a twin instead of the same patch. Two prints from the same negative at the same exposure differ by the process’s own noise, which is the same size as the effect. The pairing is not fussiness; it is what makes the number mean anything.

Squeegeeing one and not the other. A squeegee changes the surface, and on a resin-coated sheet it can leave drag marks that a densitometer will read as density. If you squeegee, squeegee everything, identically, and say so.

Changing the drying method between arms. Air on a rack, heat, a blotter book and a glaze do not give the same dried surface, and the surface is half of a reflection reading. The maker forbids glazing and ferrotyping this paper outright — the polythene sticks — and states that prints dry in 10 to 20 minutes at room temperature, which is the method this page holds constant. Pick one and record it as part of the result rather than as a detail of the housekeeping.

Expected observations

Arm 1. The steps darken steeply, then stop. The stopping is abrupt rather than gradual, which is the practical face of the fact that a paper curve has a ceiling and not a shoulder. Expect the plateau to arrive within about a stop and a half of the first visible tone on a normal grade.

Arm 2. Dmax rises quickly for the first minute, is essentially flat from about ninety seconds onwards, and does not move again. Base density is flat through the recommended range and then begins a slow climb that does not stop. The window is wide — the maker publishes a six-minute extension against a one- to two-minute recommendation — and seeing it on your own strips is the point of the arm.

Arm 3. Base white will read a little higher than the raw sheet. Do not over-interpret a difference of a hundredth or two; compare it with your repeatability first.

Arm 4. The dry patches read denser than the wet ones, and the difference is not the same at every density. Expect the largest difference in the upper mid-tones and the smallest at paper white, where there is no silver to change and the surface change is all there is.

Arm 1: what a maximum-black series looks like, and what you are looking for

• One strip, seven steps a third of a stop apart

Show the numbers behind this plot

Reflection density against relative log exposure for a series of steps a third of a stop apart. The curve begins near a paper white of 0.05, lifts slowly for the first two steps, rises steeply through the middle of the series, then bends over sharply and runs flat at about 2.15 for the last four steps. A vertical guide marks the first step of that flat run and is labelled the minimum exposure for maximum black. A note beside the flat portion states that the steps beyond it are not wasted: they are the evidence that the plateau is a plateau rather than a bracket that ended too early.

Series	Relative log exposure, thirds of a stop	Reflection density
One strip, seven steps a third of a stop apart	0.00	0.05
One strip, seven steps a third of a stop apart	0.10	0.07
One strip, seven steps a third of a stop apart	0.20	0.14
One strip, seven steps a third of a stop apart	0.30	0.35
One strip, seven steps a third of a stop apart	0.40	0.72
One strip, seven steps a third of a stop apart	0.50	1.18
One strip, seven steps a third of a stop apart	0.60	1.58
One strip, seven steps a third of a stop apart	0.70	1.86
One strip, seven steps a third of a stop apart	0.80	2.03
One strip, seven steps a third of a stop apart	0.90	2.11
One strip, seven steps a third of a stop apart	1.00	2.14
One strip, seven steps a third of a stop apart	1.20	2.15
One strip, seven steps a third of a stop apart	1.50	2.15
One strip, seven steps a third of a stop apart	1.80	2.15
One strip, seven steps a third of a stop apart	2.10	2.15

Drawn to show the shape and the reading, not measured from a material. The ceiling is drawn at 2.15 because that is the figure ILFORD publish for one current resin-coated paper; yours belongs to your paper, your developer and your processing, and the number this page wants in your notebook is the one you measured. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

What is happening chemically

Three numbers, three different physical stories, and confusing them is the commonest error in printing talk.

Maximum black is mostly not chemistry. There are only so many developable crystals in the layer, and once every one of them has been reduced no further time or energy adds density — that is the plateau of arm 2, and it belongs to the coating rather than to the bath. But the ceiling you actually read is set by how much light gets back out, and that is set by the surface as much as by the silver. Two published pairs make the point without any inference: Foma print 2.0 for the glossy surface of one warm-tone paper and 1.6 for the matt version of the same paper, and state generally that any surface other than the glossy causes a decrease in the maximum density value; ILFORD publish 2.15 for a current resin-coated paper against 2.05 for the one it replaced on the same base. A four-tenths difference between two finishes of one emulsion is larger than any difference you will find between developers.

Base white is the sheet plus its history. The unexposed processed strip carries the paper’s own base tint, whatever the baryta or the titanium-loaded resin layer reflects, any optical brightening agent that survived the baths, and any fog the developer contributed. That last term is why the unexposed strip has to be processed rather than assumed: a developer that fogs and a paper that is fogged look identical on a print and are separated only by controls.

Dry-down is two effects wearing one name, and the measurement separates them.

The material component is optical and it happens in the layer. Wet gelatin is a swollen network full of water; dry gelatin is a collapsed one. The refractive index of the layer, the path length light takes through it, and the geometry of the surface it enters and leaves through all change as the water goes, and the silver that was distributed through a swollen layer ends up concentrated in a thinner one. More of the light that goes in is absorbed before it comes back out, so the density rises. That is what your paired reading measures.

The viewing component is not in the print at all. A wet print is judged in a tray, under an inspection lamp, at close range, at an angle where the wet surface returns a great deal of light to your eye; the dry print is judged on a wall in room light. Some of what printers call dry-down is that change of circumstances, and no densitometer will ever see it.

⚗What the course can and cannot say about the mechanism

The paragraph above about swelling and collapse is the course’s interpretation, assembled from the physics of gelatin that Part XII established for washing and from the geometry of reflection that Part XIII established for density. No Tier 1 or Tier 2 source in this course’s corpus states the dry-down mechanism for a silver gelatin developing-out paper, and none publishes a magnitude for it either.

What the corpus does contain is a manufacturer’s statement of the effect for printing-out paper, where prints are described as getting significantly darker as they dry. That is a real statement about a different material and the course will not carry its magnitude across.

So: the effect is real and you are about to measure it; the mechanism above is an inference and is marked as one; and the number that comes out is yours, for your paper, your developer, your drying method and your reading geometry.

Data to record

Everything below goes in the notebook before you leave the room. The four headed groups exist because each answers a different question later, and a reading whose conditions were not written down is a reading you will not be able to use.

The chain, once. Paper: maker, product, surface code, weight, size, box code and batch. Developer: version or product, dilution, temperature, mixing date, sheets already through it. Stop and fixer: dilution, temperature, the fixer’s running total. Wash: route and temperature. Drying: the method, by name, and the time between the wash and the dry reading. Enlarger: head height, lens, aperture, filter. Room: temperature and, if you know it, humidity — the drying half of this session is a hygroscopic material losing water, and the room is a variable you are not controlling.

The instrument, once. Its anchors, the tile and the trap; today’s three reflection repeatability figures; the warm-up time you actually gave it; and the sentence the calibration page made you write about what your reflection scale may and may not be compared with.

Every reading, individually. Arm, strip number, patch, wet or dry, the delay since blotting where it applies, and the value. Not a summary — the individual readings, because the spread between them is half the result.

The three headline numbers, with their uncertainties. The minimum exposure for maximum black, in stops from a stated reference and with the timer division it was set on. Base white, as a density, with the raw-paper figure beside it. Dry-down, as a density difference at each of the five patches, and as a fraction of a stop with the conversion’s assumption written out.

Analysis

Convert the dry-down density into an exposure correction, and say what the conversion assumes.

Δ(log H) = ΔD / G

Density difference as a change in log exposure

stops = Δ(log H) / 0.301

And as a fraction of a stop

ΔD is the measured dry-minus-wet density difference at a patch, G is the local gradient of your paper’s characteristic curve at that density in density units per unit of log exposure, and 0.301 is the logarithm of two.

The assumption is the whole of it: that a density difference can be traded for an exposure difference at the local slope. That is true only where the curve is locally straight, only for the tone you measured at, and only for the grade you measured on. A paper curve is steep in the middle and nearly flat at both ends, so the same ΔD is worth a small exposure change in the mid-tones and an enormous one in the highlights — which is exactly why printers say dry-down “costs you the highlights”. Quote your correction with the density it was measured at, or it is not a number anybody can use.

Two more things to do with the data.

Test the hypothesis at the second surface. If the matt or pearl sheet shows a larger dry-down than the glossy one, that is the prediction confirmed on your own material, and it agrees in direction with the published finding that the matt surface of a paper reaches a lower maximum density than the glossy one. If it does not, look at your blotting: a textured surface holds surface water differently and your “wet” state may not have been the same state on both.

Refuse the comparisons your uncertainty cannot carry. If your wet repeatability is ±0.03 and your measured dry-down at paper white is 0.02, you have measured nothing at paper white and you should say so in that patch’s row. A table with one honest blank in it is worth more than a table of numbers of which one is invented.

Arm 4: the shape to expect, and the reason a single dry-down figure is a fiction

• Dry reading (the reference)
• Wet reading of the same patches
• The difference, dry minus wet

Show the numbers behind this plot

Two curves and a difference, plotted against the dry density of each measured patch. The upper curve, drawn solid, is the dry reading and by construction runs along the diagonal from about 0.05 to about 2.15. The lower curve, drawn dashed, is the wet reading of the same patches and lies below it everywhere, with the gap smallest at the paper-white end, widest through the upper mid-tones around a dry density of one and a quarter, and narrowing again as the patches approach maximum black. A third, dotted curve near the bottom of the plot shows the difference itself, rising from almost nothing at paper white to a broad maximum in the upper mid-tones and falling back towards the black end. The shape of that third curve is the argument: a single number for dry-down would have to be read off a curve that is not flat.

Series	Dry reflection density of the patch	Density
Dry reading (the reference)	0.05	0.05
Dry reading (the reference)	0.30	0.30
Dry reading (the reference)	0.70	0.70
Dry reading (the reference)	1.25	1.25
Dry reading (the reference)	1.80	1.80
Dry reading (the reference)	2.15	2.15
Wet reading of the same patches	0.05	0.04
Wet reading of the same patches	0.30	0.26
Wet reading of the same patches	0.70	0.62
Wet reading of the same patches	1.25	1.13
Wet reading of the same patches	1.80	1.70
Wet reading of the same patches	2.15	2.08
The difference, dry minus wet	0.05	0.01
The difference, dry minus wet	0.30	0.04
The difference, dry minus wet	0.70	0.08
The difference, dry minus wet	1.25	0.12
The difference, dry minus wet	1.80	0.10
The difference, dry minus wet	2.15	0.07

Drawn to show the shape of the relationship and the reason the analysis asks for five patches rather than one. The values are illustrative and are not the course's measurement; no manufacturer publishes a dry-down figure for a developing-out silver gelatin paper, and the numbers that belong in this plot are the ones you measure. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Troubleshooting

What you see	Where to look
The arm 1 series never plateaus	The bracket ended too soon, or the developer is under-strength. Extend the series before you blame the paper
Wet readings scatter far more than dry ones	Expected, and it is why you quote them separately. If the scatter is larger than the effect, tighten the blotting and the delay before you tighten anything else
Base density climbing across the whole arm 2 series, including at 30 seconds	Not a development-time effect. Developer fog on paper if the developer is old, paper storage fog if a fresh packet reads clean
Dmax lower than the maker’s published figure by more than a tenth	Check the fixer first: under-fixing leaves undissolved halide that lifts the whole sheet, and over-fixing can reduce the image
Parallel marks in the dried strip that read as density	Squeegee lines on prints
The dry reading is lower than the wet one	Almost certainly a method fault rather than a discovery. Look for free water on the surface at the wet reading, and read the rig diagram again
The instrument drifts between the wet and dry sittings	Densitometer drifting zero. Re-anchor between sittings and record that you did
Readings depend on where on the strip you place the head	Uneven density across a strip, or the enlarger’s own uniformity, which Part XVI mapped

Clean-up

Dishes and tongs washed in the order developer, stop, fixer, rinsed between. The glass tile from the wet rig washed and dried. The blotters kept, labelled, and used again next time, because “the same two blotters” is a condition of the method rather than a matter of thrift. The instrument wiped dry with the mains disconnected and put away in its case. Run lab closing.

Storage

The dried strips are data and are kept: sleeved, labelled on the back in pencil with the arm, the strip number and the date, and stored flat and out of light with the notebook page they belong to. A strip is the only physical evidence that a number in the notebook was ever read off something, and a strip without its label is not evidence.

The developer goes back into its bottle only if it never went into a tray. What was in the tray is spent and is discarded: neither its maker nor Kodak publishes a route by which a tray of paper developer is returned to stock, and a bath topped back into a bottle is a bottle of unknown bromide.

Disposal considerations

Three streams: alkaline developer, dilute acid stop, and silver-bearing fixer with its first wash. The developer carries negligible silver on Kodak’s own table, so what governs it is pH and aquatic classification; the fixer holds dissolved silver complexes and belongs to Part XII’s silver stream. Collect each separately, keep it labelled and closed, and take it to a facility that accepts it — ILFORD’s own advice to domestic users in the United Kingdom is a household waste and recycling centre rather than a drain. Local regulation governs, and it differs by jurisdiction; check your local regulations and read the disposal caveat before deciding anything from this paragraph.

Questions

1. Why is the clear film base in the negative carrier for arm 1, and what would be wrong with the number you got without it?
2. Your arm 2 series shows Dmax flat from 90 seconds to 6 minutes and base density rising from about 3 minutes. What is your usable development window, and what does the upper end of it cost you?
3. You measure a dry-down of 0.12 density at a patch whose dry density is 1.25, where the local gradient of your paper curve is 2.4 density units per log exposure unit. Give the correction in stops, and state the assumption in one sentence.
4. Base white on your processed control reads 0.06 and the raw sheet reads 0.04. Your dry reflection repeatability is ±0.02. What may you conclude?
5. A printer says their paper has a dry-down of a third of a stop. Name three things they have not told you without which the figure cannot be used.
6. Arm 4 on the matt sheet gives a larger dry-down than on the glossy sheet from the same box. Give the interpretation the page predicted, and give the method fault that would produce the same result.

Further experiments

Change one link and re-measure. The hypothesis says these are properties of a chain. Test it: run arm 1 and arm 3 again with the only change being the developer — the bromide-boosted D72-EB-002 from the previous page — and see which of the three numbers moves and by how much. The prediction is that the exposure moves, base white moves slightly and Dmax does not.

Put a number on the viewing half of dry-down. Print two identical sheets. Judge one wet under your inspection lamp and write down the exposure you would give next time; dry the other, judge it under the light the print will hang in, and write down the same. The difference between those two judgements, minus the material dry-down you measured tonight, is the part of the effect that lives in your eye and your lamp rather than in the paper.

Measure how long “dry” takes. Read the same patch at 10 minutes, 30 minutes, 2 hours and 24 hours after the wash. If the density is still moving at two hours, then the hour this page waits is not enough and your dry-down figure is a partial one.

Take it to fibre. A fibre sheet swells more, holds more water and dries slower than a resin-coated one, and the prediction is a larger dry-down and a longer time to reach it. Hold everything else and find out. The paper has no sourced price in this course, so record what you paid.

Recap

• Three numbers, three different physical stories: maximum black is a ceiling set mostly by the coating and the surface, base white is the sheet plus its processing history, and dry-down is a material change plus a viewing change wearing one name.
• The minimum exposure for maximum black is found by an exposure series read through clear film base. The endpoint is Hurter and Driffield’s own definition; the technique is the course’s, because the corpus holds no Tier 1 or Tier 2 description of it.
• That exposure fixes the black end of the scale and settles nothing about the highlights, which is why it is a starting point for Part XIX and not a print.
• Dmax plateaus and base density does not. The window between them is the usable development range, and its price at the far end is fog rather than contrast.
• Surface beats developer. Foma publish 2.0 glossy against 1.6 matt for one paper; no developer difference you measure will be that large.
• Dry-down is measured on the same patch, wet and then dry, with the blotting, the backing and the delay held constant — and it is quoted at the density it was measured at, because the density-to-stops conversion is only valid at the local gradient.
• No manufacturer publishes a dry-down figure for a developing-out silver gelatin paper. The course gives a method and no constant, and the number belongs to your chain.
• The uncertainty is part of the result. Wet repeatability is quoted separately from dry, and a difference smaller than either is a blank in the table rather than a finding.

Check your understanding

Question 1. Why does arm 1 expose through a piece of clear, processed film base rather than through an empty negative carrier?

• To reduce the exposure so the series is easier to bracket
• Because in real printing the light always passes through a negative’s base, which absorbs and scatters some of it, so an exposure measured without the base does not transfer to any print you will make
• Because a bare carrier would fog the paper
• Because the film base filters ultraviolet, which the paper is sensitive to

Show the answer and why

Answer: Because in real printing the light always passes through a negative’s base, which absorbs and scatters some of it, so an exposure measured without the base does not transfer to any print you will make

The point of the number is that it can be used. A negative’s clear base — its rebate — is in the optical path of every print, absorbing and scattering a little light, and leaving it out shifts the measured exposure by an amount that has nothing to do with the paper. Using a rebate from the same film stock you actually print from also removes the base density of that stock as a variable between this measurement and the printing it is meant to inform. The technique is not sourced in this course’s corpus and the page says so; the endpoint it finds is Hurter and Driffield’s definition, which is.

Question 2. You measure a dry-down of 0.12 density on a patch whose dry density is 1.25, where your paper’s local gradient is 2.4 density units per log exposure unit. What is the correction in stops, and what does the conversion assume?

• 0.12 stops, assuming density and stops are the same quantity
• About 0.17 stops: Δ(log H) = 0.12 / 2.4 = 0.05, and 0.05 / 0.301 = 0.17 — assuming the curve is locally straight at that density, so the same figure does not apply at the highlight end
• About 0.4 stops, assuming a gradient of one
• It cannot be converted, because density and exposure are unrelated on a paper

Show the answer and why

Answer: About 0.17 stops: Δ(log H) = 0.12 / 2.4 = 0.05, and 0.05 / 0.301 = 0.17 — assuming the curve is locally straight at that density, so the same figure does not apply at the highlight end

Dividing a density difference by the local gradient converts it into a log-exposure difference, and dividing that by log 2 turns it into stops. The assumption is that the curve is straight over the small interval concerned, which holds in the steep middle and fails badly at both ends: a paper curve is nearly flat in the highlights, so the same 0.12 of density there corresponds to a much larger exposure change. That is the mechanism behind the practitioner observation that dry-down costs the highlights most, and it is why the page insists a dry-down correction is quoted together with the density it was measured at.

Question 3. Your wet reflection repeatability is ±0.03 and the dry-minus-wet difference you measure at paper white is 0.02. What goes in the notebook?

• 0.02, since that is what the instrument displayed
• Nothing measurable at paper white, with the repeatability figure printed in the cell — the measurement cannot resolve a difference that size
• Zero, since the difference is within the noise
• The average of several attempts, which cancels the noise

Show the answer and why

Answer: Nothing measurable at paper white, with the repeatability figure printed in the cell — the measurement cannot resolve a difference that size

A reading smaller than the spread of the method that produced it is not a small result; it is no result, and writing it down as a number invites somebody to use it. Nor is it zero: zero is a claim that the effect is absent, and you have not shown that either. The honest cell says what could not be resolved and what the resolution was, which is also the cell that tells a later reader exactly what a better instrument would need to beat. Averaging more attempts does eventually help, but only if the scatter is random rather than a systematic drift as the strip dries under the head — which on a wet reading it very often is.

Question 4. Foma publish a maximum density of 2.0 for the glossy surface of one of their papers and 1.6 for the matt version of the same paper. What does that pair establish, and what does it not?

• That matt papers use a different, weaker emulsion
• That the surface finish alone can change maximum black by 0.4 density on one emulsion — far more than any developer difference — but it says nothing about what a developer can do to either surface
• That the matt paper is under-developed at the recommended time
• That reflection density measurements are unreliable on matt surfaces

Show the answer and why

Answer: That the surface finish alone can change maximum black by 0.4 density on one emulsion — far more than any developer difference — but it says nothing about what a developer can do to either surface

The two figures are for the same emulsion on two finishes, so the difference isolates the surface. It is a large difference — 0.4 in density is a factor of about two and a half in the light returned — and it dwarfs the developer differences the previous page’s lab measures, which is the practical reason a comparison of two papers with different finishes tells you mostly about the finishes. What it does not establish is any developer claim at all: the maker’s figures are for their own recommended processing, and the effect of a change of developer on either surface is a separate measurement, which is what arm 2 and the further experiment are for.

Question 5. A print is judged wet in the tray under an inspection lamp and looks right; dry on the wall it looks heavy. How much of that is the effect this experiment measures?

• All of it — that is the definition of dry-down
• Only the part that a densitometer reading of the same patch, wet and then dry, can see; the rest is a change of viewing circumstances that no instrument on the print can detect
• None of it — dry-down is entirely a viewing effect
• It cannot be separated even in principle

Show the answer and why

Answer: Only the part that a densitometer reading of the same patch, wet and then dry, can see; the rest is a change of viewing circumstances that no instrument on the print can detect

Two distinct things happen between the tray and the wall. The layer itself changes: swollen gelatin collapses, the silver ends up in a thinner layer, the surface geometry changes, and more of the light entering is absorbed before it returns — that is the material component, it is real, and it is what the paired wet-and-dry reading measures. Separately, the print moves from a bright lamp at close range over a wet, highly reflective surface to room light on a wall — that is a change in the observer’s conditions, it is often larger than the material component, and no reading taken on the print can see it. The further experiment on this page measures the second by difference, and keeping the two apart is what stops a printer applying a lamp problem as a chemistry correction.

Question 6. Arm 2 shows maximum black flat from 90 seconds to 6 minutes while base density begins climbing at about 3 minutes. What has the arm established?

• That development time is a contrast control on paper after all
• That the usable window runs from where Dmax plateaus to where the base lifts, and that going past it costs clean whites rather than contrast
• That the developer is exhausted after 3 minutes
• That the paper is fogged

Show the answer and why

Answer: That the usable window runs from where Dmax plateaus to where the base lifts, and that going past it costs clean whites rather than contrast

The two curves answer different questions and the arm exists to separate them. Dmax saturates because the supply of developable crystals is finite, so past the plateau extra time adds nothing at the black end. Base density has no such limit: unexposed crystals keep turning over slowly for as long as the sheet is in the bath, so the price of a long development is a greyed white rather than a changed contrast. The window between the two is what the manufacturer’s published extension statement is really describing, and measuring it on your own material is what converts a datasheet sentence into a working range you can defend.

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