Part XVIII Overview: Printing Chemistry

Part XVI put a print in your hands. This part explains it. Not how to make one — that is already done, and Part XIX will make it well — but why this paper in this developer gives this black, this white and this colour, and what has to change to move any of the three.

It is the first part that gets to use everything at once. The paper developers are already in the formulary, the stop baths came from Part X, the fixers from Part XI, the washing from Part XII, and the curve arithmetic from Part XIII. Almost nothing here is new chemistry. What is new is the material.

Why paper is not film in a different shape

Four differences drive the whole part, and every page below is one of their consequences.

The four things that make a printing paper its own material

1. 1It reflects rather than transmitsso its density is measured in light that goes in and comes back out, and its whole scale fits in about two density units instead of a film’s three or four
2. 2Its support can drinkfibre lets solution into the base and polyethylene does not — which is why one paper fixes in a minute and washes in an hour, and the other fixes in thirty seconds and washes in two minutes
3. 3Its emulsion is designed for a different jobcrystals far smaller than a film’s, a chloride-rich halide, blue sensitivity only, and a speed measured in single-figure film equivalents
4. 4It is developed to completiona film is pulled from the developer at a chosen contrast; a print is left until nothing more happens, so development time is a plateau rather than a control

The map

Four lessons and three practical pages, in that order. The sheet first, because the support decides the processing. Then the emulsion, which decides speed and colour. Then contrast control, which is the emulsion trick that gives seven grades from one box. Then the developer, which is where Part VIII’s chemistry meets a new material. The lab mixes print developers from raw chemicals and compares them; the experiment measures the part’s three numbers; the break/fix page takes the print that will not go black and finds out why.

That order is not a preference. You cannot reason about a fixing time until you know whether the support drinks; you cannot reason about image colour until you know what the crystals are; you cannot choose a filter until you know what the filter is selecting between; and you cannot judge a developer until you know what the paper it is developing was designed to do. Each page hands the next one the fact it needs, and the three practical pages then use all four at once.

What this part does not own

ISO(R), paper speed and grade matching belong to Part XIII and are used here, not re-derived. Safelight fog and enlarger uniformity belong to Part XVI. The f-stop exposure arithmetic belongs to Part XVII. Washing, permanence and silver recovery belong to Part XII. Toning belongs to Part XX. Where a page here needs one of those, it links it.

What you produce

A print developer mixed from raw chemicals rather than bought; a measured maximum black, base white and dry-down figure for one paper-and-developer pair; and a diagnosis worksheet. The dry-down number has to exist before Part XIX and not after it, because fine printing means choosing an exposure for a print you are looking at wet and will hang up dry. Without your own figure every base exposure in Part XIX carries the same unknown error, and no amount of careful dodging corrects a systematic one.

!Dry-down is measured here, not quoted

No manufacturer sheet in the course’s corpus publishes a dry-down figure for a silver gelatin developing-out paper, and Part XIII refused to carry a printing-out paper’s figure across to one. Nothing in this part will give you a percentage to apply. The experiment gives you a method, and the number it returns is yours, for your paper, your developer and your drying method.

Safety framing

Every practical page in this part is Level A: standard darkroom chemistry, at tray dilutions, below 25 °C, with no volatile solvent and nothing heated. The one operation that rises above the background is weighing developing agents as powders — metol and hydroquinone are skin sensitisers, and dust control at the balance is the control that matters. The lab states it inline where it happens, under the weighing procedure. Waste is spent alkaline developer, dilute acid stop and silver-bearing fixer, kept apart; local regulation governs disposal.

What you need first

A processed negative. A working enlarger or contact printer from Part XVI, with its safe working time measured. The developer chemistry of Part VIII — agents, alkalis, preservatives, restrainers and superadditivity — which is assumed in full here and never re-taught.

Alternative route

The three practical pages need a darkroom, and there is no substitute. An enlarging paper is a developing-out material at a film-ISO equivalent of 3 to 6 on ILFORD’s own comparison, sensitive to the blue in ordinary room light, and it cannot be handled outside a dark room at any speed — which is exactly what the hand-coated ultraviolet-printing materials of Part XXI are not.

What a reader without a darkroom can still do is the whole of the first four lessons, which are reading and reasoning, and the datasheet exercises in them — a paper datasheet is free, and reading one properly is a skill the part teaches on its own terms. The experiment’s visual-comparison route also needs no densitometer, only a room that goes dark.

The pages of this part

Part18 of 28Level2 — PractitionerPages7Estimated time9.5 hoursHighest safety levelLevel A

✱3 of 7 pages in this part need a darkroom, a UV source or mains-powered equipment, marked below. Each says what can be improvised and, where one exists, gives an alternative route.

0 / 7 lessons in this part completed

• LessonThe Structure of a Printing Paper: Base, Baryta, Emulsion and Supercoat45 min
• LessonPaper Emulsions: Halide, Grain, Image Tone and Speed50 min
• LessonVariable-Contrast Papers: Three Emulsions, One Sheet55 min
• LessonPrint Developer Chemistry and the Developer-Paper Pair60 min
• LabLab: Mixing and Comparing Print DevelopersA✱ Darkroom · Mains150 min
• ExperimentExperiment: Maximum Black, Base White and Dry-DownA✱ Darkroom · Mains120 min
• Break/fixBreak/Fix: The Print That Will Not Go BlackA✱ Darkroom · Mains90 min