Paper Emulsions: Halide, Grain, Image Tone and Speed

A sheet of enlarging paper and a roll of fast film are made of the same three things: silver halide crystals, gelatin and a support. Everything else about them is different, and the differences are all deliberate. The paper is six or seven stops slower than a fast film — ILFORD put their enlarging papers at a film-ISO equivalent of 3 to 6. It shows no grain at any enlargement you will ever make on it. It can be handled under a lamp you can see by. And it is brown, or neutral, or faintly blue, depending on a choice made in a coating plant that never involved a dye.

This page is about the emulsion that produces those four facts, and about which of the usual explanations for them survive contact with a source.

The halides, and the names still printed on boxes

A paper emulsion is precipitated from silver nitrate and a soluble halide, exactly as a film emulsion is, and the halide chosen sets the family.

Family	Halide	What it was for	Speed
Chloride paper	silver chloride	contact printing by artificial light	slowest
Gaslight paper	chloride, or chloride-rich chlorobromide	contact printing by domestic gaslight	slow
Chlorobromide	both, in a stated ratio	general printing, warm tones	between
Bromide paper	silver bromide	enlarging	fastest

Kodak’s 1928 primer sets out the division in two sentences that have not needed correcting: a slow bromide emulsion coated on paper “is known as bromide paper and is used for printing, and especially for making enlargements”, while “the less sensitive papers which are commonly used for contact printing by artificial light contain silver chloride in the place of silver bromide”. The names are a statement about the halide, not about the speed — the speed follows.

🕰Gaslight paper, and what the name actually meant

The name is a claim about the light you could print by, and it dates a domestic technology. A printing-out paper needed sunshine. A chloride developing-out paper was fast enough to be printed by a gas mantle indoors, in the evening, by an amateur — Getty record that many so-called gaslight papers could be exposed in-house rather than by sun exposure, which is the whole of what the name asserts.

E. J. Wall, writing in 1929 from commercial practice, draws the emulsion-maker’s division rather than the customer’s: gaslight or development papers are “as a rule unwashed emulsions containing varying ratios of bromide and chloride of silver”, while bromide papers are always washed emulsions. The course’s formulary carries one of each of his and Baker’s — Wall’s soft-working gaslight paper, Trumm’s bromide paper emulsion and the chloride contact emulsion — so the quantities behind this paragraph can be read rather than taken on trust.

What current makers actually say. ILFORD describe their MULTIGRADE emulsions as “chloro-bromide” once, in the contrast-control sheet, and publish no halide ratio anywhere. Foma label FOMABROM, FOMASPEED and FOMATONE MG Classic all as silver chlorobromide — the neutral paper, the resin-coated paper and the warm-tone paper alike. Hold on to that: it is the first crack in the story most printers are told about where warmth comes from, and the section on image tone below returns to it.

Crystal size: the smallest crystals in the course

A printing paper’s crystals are far smaller than a film’s, and that single fact accounts for two of its most obvious properties.

It has no grain you will ever see. A film crystal in a modern emulsion is about 0.2 µm across, and a 35 mm negative gets enlarged twelve times or more on the way to a print, so those crystals become visible structure. A print is viewed at life size. Its own crystals are smaller than a film’s and they are never magnified, so a printing paper resolves far more than the job asks of it and its granularity never enters the argument.

It is very slow. Small crystals catch few photons, and a crystal needs a minimum number of silver atoms at a trap before it is developable — that is Part IV’s latent image, unchanged. Small crystals mean low speed, always.

!A figure the course does not have

No manufacturer sheet, standard reference or conservation publication in the course’s corpus gives a crystal size for a modern printing paper. The course therefore states the direction and not a number, and offers the three sourced pieces of evidence it does have.

Silver load. Baker gives paper emulsions 15 to 25 g of silver nitrate per litre against 40 to 50 for negative emulsions — roughly half to a third of the silver in the same volume of emulsion.

Halide practice. Duffin records that chlorobromide emulsions gain speed and lose contrast as their bromide content rises, and that they are frequently made by reversed precipitation; both are techniques for controlling crystal growth.

Getty’s account of graded papers, which is a statement about crystal size in paper explicitly: graded papers “were manufactured with silver halide particles of different sizes”, uniform sizes giving high contrast and mixed sizes lower contrast.

None of those is a micrometre figure, and this page does not supply one.

Speed, and why slow is the design and not the compromise

Paper speed is published, to a different standard from film speed, as an ISO Speed (P) figure. Part XIII owns what those numbers are and how they are read; what matters here is the size of them.

Paper	ISO P, filters 00–3	ISO P, unfiltered	Maker’s film-ISO equivalent
MULTIGRADE RC DELUXE	240	500	3 to 6, for the RC range
MULTIGRADE FB CLASSIC	230	500	about 6
MULTIGRADE FB COOLTONE	250	590	about 6
MULTIGRADE FB WARMTONE	100	200	about 3
HARMAN DIRECT POSITIVE FB	— (fixed grade)	—	1 to 3

ILFORD’s own comparison is the useful one and they print the warning with it: ISO paper speed is not film speed, and MULTIGRADE RC papers correspond to a film ISO of roughly 3 to 6. Against HP5 Plus at 400 that is between six and seven stops. Against a film at 100 it is four to five.

Why would anyone want that? Because the paper is exposed for seconds under a lamp you control, not for a fraction of a second in a world you do not. A slow material buys:

• Room to work. Dodging and burning need an exposure long enough to move a hand through. A ten-second base exposure can be shaped; a tenth of a second cannot.
• A safelight. A material fast enough to matter in dim amber light would have to be handled in the dark, and the whole craft of printing depends on seeing what you are doing.
• Latitude in the enlarger. ILFORD note that MULTIGRADE RC papers show no significant change in picture quality if left for 24 hours between exposure and processing, so an exposed sheet is not a perishable thing.
• Contact-speed versus enlarging-speed as a real choice. A bromide enlarging paper is fast enough for a dim projected image. A chloride contact paper is not, and does not need to be, because the negative is pressed against it under a bright lamp. Foma make the point about their slowest current paper: FOMATONE MG Classic is “designed primarily for contact work”, its speed is considerably lower than their other papers, and long exposures on it show reciprocity failure — Foma name the Schwarzschild effect and put the onset above fifteen minutes.

📷The extreme case, in one paragraph

HARMAN DIRECT POSITIVE FB is the slowest and hardest paper a reader can buy: a fixed-grade paper of a contrast similar to MULTIGRADE grade 3½ to 4, at a stated ISO 1 to 3, on which more exposure gives less density. That is slow enough to load into a camera, which is exactly what Part VI and Part VII do with it. It also needs a deep red ortho safelight rather than the amber that serves ordinary papers, which is the next section’s subject arriving early.

Spectral response, and the whole reason a safelight exists

Where a printing paper responds, and where its safelight is allowed to emit

• Unsensitised response (340–500 nm) — silver chloride and chlorobromide, near-UV and blue
• Green-dyed component (480–555 nm) — only on a variable-contrast paper
• ILFORD safelight cut-off, 580 nm and above (580–660 nm) — dark brown or red
• Foma, VC paper: 625 nm and above (625–700 nm) — orange or red

Band edges are the published safelight cut-offs; the response bands are drawn from the makers' own descriptions and from their wedge spectrograms, which are printed without a numbered sensitivity axis. Not a measurement. The coloured strip approximates where the visible spectrum falls and is a reading aid only; the wavelengths in the labels carry the information. The bands and curves are drawn to show the relationship, not measured.

ILFORD state the underlying fact in one sentence: “All chloro-bromide (black and white) emulsions are blue sensitive with a slight sensitivity to green light. To make an emulsion sensitive to colours in addition to blue, sensitising dyes need to be added.” Silver chloride and silver bromide absorb in the near ultraviolet and the blue and are essentially blind to red — which is Part IV’s spectral sensitivity and is not re-derived here.

Two consequences follow, and both are practical.

An amber safelight is possible. Kodak list their OC light amber filter for contact and enlarging papers. ILFORD tell you to use a safelight with a cut-off no lower than 580 nm, of a dark brown or red type. Foma give 575 nm for FOMABROM and FOMASPEED, 610 nm for the slow warm-tone FOMATONE, and 625 nm for FOMABROM VARIANT, which they describe as orthochromatically sensitised. That last number is worth noticing: the same maker asks for a deeper safelight on their variable-contrast paper than on their graded one, because sensitising dye has been added. The measurement of what your own lamp does to your own paper belongs to Part XVI’s fog test, and no published cut-off substitutes for it.

The blue end of the enlarger’s output is the working part of its spectrum. A tungsten lamp puts most of its energy in the red and infrared, where the paper is blind. The paper is exposed by a fraction of the light in the cone, and this is why a condenser lamphouse that seems bright can still give long exposures, and why the LED head of Part XVI is designed around blue and green emitters rather than white brightness.

Image tone, and the mechanism the sources actually support

A silver gelatin print has a colour, and no dye is involved. Developed silver is a metal, and the colour you see is a property of the physical form of the deposit — how finely divided it is, and therefore how it absorbs and scatters the light that reaches it.

That much is established by more than one Tier 1 source and in more than one process.

In printing-out papers, Getty state it flatly: photogenically developed silver particles are much smaller than chemically developed ones, and “the color of the silver image of a POP silver chloride photograph relates to the size of the silver particles”, running from light yellow-brown through red to darker brown; silver bromide printing-out images are cooler and greyer.

In developing-out papers, Getty are equally direct about the modern warm-tone products: the emulsion chemistry of warm-tone photographic papers “was slightly modified to restrict the growth of silver particles during development. The smaller size of the developed particles created the warmer tonality of the image.”

So the mechanism is developed particle size. Small, finely divided silver scatters and reads warm; larger, more filamentary silver reads neutral or blue-black. That connects directly to covering power, which Part VIII established: the same mass of silver in a different physical form gives a different density, and here it also gives a different colour.

⁇What makes a paper warm — and the source that says it is not the halide

The common assertion is that warm equals chloride: more silver chloride in the emulsion, warmer print. The sources do not line up behind it.

For the halide route. Baker (1941) states that if the bromide is much in excess of the chloride “the tone will be colder but the gradation will usually have a longer scale”, while an excess of chloride tends to higher contrast and a shorter scale “but will more readily yield colour on suitable development”. Wall himself, elsewhere in the same book, writes that the more bromide an emulsion contains relative to the chloride, the faster it is as a rule, the longer its scale, and the less easy it is to obtain warm tones. Duffin (1966) agrees on speed and contrast.

Against it. Wall (1929, pages 95–96) ran the experiment. He made a series from pure chloride through 5, 10 and 20 per cent bromide to pure bromide, exposed them under one test plate to magnesium ribbon, a Nernst lamp and incandescent gas, and developed them all in one metol–hydroquinone developer. “In every case the same colored image was obtained from the lightest to the deepest deposit.” His conclusion: it is the rate of deposition of the silver that determines the colour of the image, and not the composition of the emulsion.

And the modern evidence. Getty attribute warm tonality to restricted particle growth during development, not to halide ratio. Foma, the only maker in the corpus who publishes a halide at all, call their neutral paper, their RC paper and their warm-tone paper all silver chlorobromide. ILFORD publish no halide ratio for any MULTIGRADE product.

What the course states. The established mechanism is developed particle size. Halide ratio is one of the levers a manufacturer has on that size, and the historical literature says so — but it is a lever on the mechanism, not the mechanism, and Wall’s own controlled series is a published Tier 3 result that a change of halide alone did not change the colour. A claim that a named modern paper is warm because it contains more chloride is not supported by any source in the corpus, because no maker publishes the ratio. What the makers do publish is the second lever, and it is the one you can actually operate.

The lever you can operate: development

If the colour is set by the rate and form of silver deposition, then everything that changes the rate changes the colour — and manufacturers publish exactly that, in their own words, for their own papers.

• Developer choice. ILFORD state that the choice of developer affects the image colour of MULTIGRADE FB WARMTONE, and that the warmest results come from HARMAN WARMTONE and ILFORD BROMOPHEN developers. Their paper-developer sheet says BROMOPHEN is “particularly recommended” for the Warmtone papers to get the warmest image tone. Foma recommend Fomatol PW, formulated for FOMATONE, for its warm-brown result.
• Dilution and time together. Foma state that further diluting Fomatol PW and lengthening development gives a stronger warm tone still on FOMATONE. ILFORD go the other way for cool: MULTIGRADE RC COOLTONE wants approximately double the normal development time to reach its coolest image colour, and they note the price — approximately half the developer capacity, because each print sits in the bath twice as long.
• Fixing. ILFORD state that a hardening fixer gives a cooler image tone on FB WARMTONE, and that long fixing times affect image colour.
• Washing. This one surprises people. ILFORD state that short washing times give FB WARMTONE a cooler image colour, and that for the warmest results prints should be washed for at least thirty minutes.

Four processing steps, each with a published effect on colour, from one manufacturer, about one paper. The chemistry of why is discussed on the print developer page — the additives that slow deposition are the same ones Kodak’s D-156 and D-166 are built around.

🧪A test you can run in one session

Take one packet of warm-tone paper and one negative. Make four prints of equal density: at the recommended development time; at twice it; at the recommended time in a developer diluted twice as far, with the time lengthened to match; and at the recommended time in a neutral developer. Dry all four, and view them together on a white card in daylight.

The prediction the sources support is that the differences will be visible and will be small compared with the difference between the warm-tone paper and a neutral one. The emulsion sets the range; the tray moves you within it. If your four prints separate more than that, you have found something worth writing down.

Developer-incorporated papers

Some papers carry a developing agent in the coating, so that development begins the moment the sheet meets an alkaline bath rather than when the agent diffuses in.

This is not a historical curiosity. Foma state it of a paper on sale now: FOMASPEED’s “developing agents incorporated into the emulsion layer facilitate rapid machine processing and a shortening of development times in manual processing to 60 – 90 seconds at 20 °C”. That is a manufacturer’s own statement about a named current product, and it is the only one the course has found.

The consequence matters more than the fact, and it is a consequence for experiments:

✕Why this ruins a developer comparison

On a developer-incorporated paper, part of the developing chemistry is in the sheet. Compare three developers on it and their differences will be flattened, because they are not doing all the work. The reader gets a null result and concludes that developer choice does not matter — which is a property of the paper being reported as a property of the developers.

The lab in this part is a developer comparison, and it specifies a paper that carries no incorporated agent for exactly this reason. Before any comparison, check the datasheet for the phrase.

Base fog, paper age, and what the storage advice is protecting

An unexposed sheet does not stay unexposed for ever. Heat, humidity, time and stray radiation all produce developable silver in crystals that were never struck by light, and the result is base fog: a base that is not white, highlights that are not clean, a maximum black that is a little lower, and — because fog silver is finely divided — a tone that drifts warmer.

The makers’ storage advice is aimed at exactly this, and its conditions are the interesting part. ILFORD ask for a cool, dry place in the original packaging, preferably below 20 °C, with high temperature and high humidity avoided, and then state the keeping life subject to that condition: up to three years for MULTIGRADE FB CLASSIC, up to two years for the RC range. Read the sentence in that order — the condition first, the number second — and it is a useful claim rather than a guarantee.

Diagnosis belongs to the atlas, not to this page. If your whites are grey and a sheet from a different box comes out clean, the fault arrived with the material, and the entry that works through it is paper storage fog; the room half of the same symptom is safelight fog and Part XVI’s fogged paper page.

Three ways a print can be warm, and the words that keep them apart

By this point in the page there are three entirely different mechanisms in play, and they are routinely confused in print.

What is warm	Where it comes from	When it is decided
Emulsion tone	the size and form of the developed silver	at the coating plant, then moved by your development
Base tint	a coloured stock or a colourant in the baryta	at the coating plant, and not movable at all
Toning	a chemical conversion of the image silver into another compound	by you, in Part XX, after the print is fixed and washed

ILFORD’s own product table separates the first two by printing base tone and image tone as different columns: MULTIGRADE RC WARMTONE is warm in both, FB CLASSIC is white base and neutral image, FB COOLTONE is cool base and cool-of-neutral image. Foma tell you that FOMATONE’s base “is coloured in compliance with the tone of the developed silver” — the tint chosen to agree with the emulsion.

!Toning is not tinting

The course keeps these apart deliberately, and the distinction is not pedantry. Toning converts the image silver into a different chemical compound — silver sulfide, silver selenide, a gold-silver deposit — so the image is no longer the same substance. Tinting colours something that is not the image, usually the base. A warm-tone paper is neither: it is silver, in a different physical form.

Why it matters practically: a toned image has different permanence, because it is a different compound; a tinted base has the permanence of a dye; and a warm emulsion tone has exactly the permanence of silver. Three routes to a brown print, three different answers about what happens to it in fifty years.

The art decision, made before any toner

The last choice on this page is the first one you make at the counter, and it is an aesthetic decision informed by everything above.

A neutral paper gets out of the way. It is the right default for work where the subject and the tonal structure are the argument, and it is what most reproduction and most documentary printing wants. A cool paper puts a faint blue in the deepest tones, which reads as clinical, modern, metallic — and ILFORD’s COOLTONE will only give it to you if you develop for twice as long as usual. A warm paper puts brown in the mid-tones and shadows, which reads as older, softer, more intimate — and it costs you speed: MULTIGRADE FB WARMTONE runs at ISO P100 where FB CLASSIC runs at P230, a little over a stop.

The order to think in is: emulsion first, then developer, then toner. An emulsion whose native colour already serves the picture needs the least done to it, and every later step is a smaller correction on a better starting point. A neutral paper pushed brown by a toner is a different object from a warm paper that was brown to begin with — the first has had its silver converted, the second has not — and the two do not look the same to a careful eye or to a conservator.

Part XX will offer the toners. This page’s argument is that you should arrive there having already chosen the paper whose own colour you wanted.

Recap

• The halide names the family: chloride for contact, bromide for enlarging, chlorobromide between. Kodak’s 1928 division still holds. Current makers publish little or nothing about halide ratios; Foma call their neutral, RC and warm-tone papers all chlorobromide.
• Paper crystals are much smaller than film crystals, which is why paper has no visible grain and is very slow. The course has no sourced crystal-size figure for a modern paper, and gives the indirect evidence instead — Baker’s 15–25 g of silver nitrate per litre against 40–50 for negative emulsions.
• Slow is the design. ILFORD put their RC papers at a film-ISO equivalent of 3 to 6. Slowness buys a safelight, dodging time and latitude.
• Unsensitised chloro-bromide responds in the near-UV and blue only, with slight green sensitivity — ILFORD’s own words — which is what makes an amber safelight possible and what makes the blue end of the enlarger’s output the working part of its spectrum.
• Image colour is developed particle size, established for both printing-out and developing-out papers. Halide ratio is a lever on that size, not the mechanism, and Wall’s own controlled series found no colour change across a chloride-to-bromide range in one developer.
• The lever you operate is development, and makers publish its effects: developer choice, dilution, time, fixing and even washing all move the colour of a warm-tone paper.
• Emulsion tone, base tint and toning are three different things. Only the third is a chemical conversion of the image.

Check your understanding

Question 1. Give two independent reasons why an amber safelight is possible for printing paper and impossible for panchromatic film.

• Paper is slower, and paper is thicker
• Paper emulsions respond only in the near-UV and blue while panchromatic film is sensitised across the visible spectrum, and paper is orders of magnitude slower so a given illuminance takes far longer to register
• Paper has a supercoat and film does not, and film is developed to a chosen contrast
• The safelight filter blocks ultraviolet, and film is more sensitive to ultraviolet

Show the answer and why

Answer: Paper emulsions respond only in the near-UV and blue while panchromatic film is sensitised across the visible spectrum, and paper is orders of magnitude slower so a given illuminance takes far longer to register

Both halves are needed. Spectrally, an unsensitised chloro-bromide emulsion is blind above about 500 nm — ILFORD say it is blue sensitive with slight green sensitivity — so a lamp emitting above 580 nm falls in a gap that a panchromatic film, sensitised deliberately across the visible, does not have. Photometrically, paper at a film-ISO equivalent of 3 to 6 accumulates exposure some six or seven stops more slowly than an ISO 400 film. Note that the gap narrows on a variable-contrast paper: Foma ask for 625 nm on their orthochromatically sensitised FOMABROM VARIANT against 575 nm on the graded FOMABROM.

Question 2. The same warm-tone paper is developed for one minute and for three minutes at the same temperature and the same final density. What happens to image tone, and by what mechanism?

• Nothing changes, because development time affects density only
• The tone moves, because the rate and form of silver deposition change and image colour is a property of developed particle size and shape
• The tone moves, because longer development dissolves silver chloride and leaves silver bromide behind
• The tone moves, because the developer oxidises and stains the gelatin

Show the answer and why

Answer: The tone moves, because the rate and form of silver deposition change and image colour is a property of developed particle size and shape

Image colour in a silver gelatin print is the physical form of the metallic silver, not a dye and not the residual halide — the halide is dissolved away in the fixer. Getty attribute warm tonality in warm-tone papers to restricted particle growth during development, and Wall concluded from his own controlled series that the rate of deposition determines the colour. Manufacturers publish the effect directly: ILFORD want approximately double the development time on RC COOLTONE to reach its coolest colour, and Foma get a stronger warm tone from a further-diluted developer with the time lengthened to match.

Question 3. A datasheet for an RC paper states that developing agents are incorporated into the emulsion layer. What does that invalidate?

• Its ISO(R) figures, because the paper develops itself
• A comparison of three print developers made on that paper, because the developers are no longer doing all the work
• Its safelight recommendation, because the incorporated agent is light sensitive
• Its maximum black, because incorporated agents cannot reach full density

Show the answer and why

Answer: A comparison of three print developers made on that paper, because the developers are no longer doing all the work

Foma state this of FOMASPEED, and give the practical sign of it — manual development shortened to 60 to 90 seconds. Part of the chemistry has moved into the sheet, so differences between developers are damped and a comparison run on such a paper reports a property of the paper as though it were a property of the developers. Its own sensitometry is unaffected: ISO(R), speed and Dmax are all measured on the paper as sold, processed as recommended.

Question 4. Warm emulsion tone, a warm base tint and sepia toning all give a brown print. Which of the three changes the chemical identity of the image, and why does that matter?

• All three, since all three change what the print is made of
• The base tint, because a colourant is added to the sheet
• Toning only — it converts the image silver into another compound, so the print’s permanence is the permanence of that compound rather than of silver
• Warm emulsion tone only, because the halide ratio was changed at the coating plant

Show the answer and why

Answer: Toning only — it converts the image silver into another compound, so the print’s permanence is the permanence of that compound rather than of silver

A warm emulsion is silver in a finer physical form; a warm base is a colourant in the stock or the baryta, with the image sitting on top of it unchanged; toning converts the silver into silver sulfide, silver selenide or a gold-silver deposit. Only the third is a change of substance, which is why only the third changes the permanence argument. The course keeps toning and tinting as separate words for exactly this reason, and Part XX will do the conversion deliberately.

Question 5. Baker states that an excess of bromide over chloride gives a colder tone and a longer scale. Wall, running a series from pure chloride to pure bromide in one developer, got the same coloured image in every case. How should a page treat the two statements?

• Follow Baker, since a later source supersedes an earlier one
• Follow Wall, since a controlled experiment beats a general statement
• State that the established mechanism is developed particle size, name both positions, and decline to assert a halide explanation for any named modern paper, since no maker publishes the ratio
• Omit both, since the sources disagree

Show the answer and why

Answer: State that the established mechanism is developed particle size, name both positions, and decline to assert a halide explanation for any named modern paper, since no maker publishes the ratio

The two positions are not equally reachable and neither is discardable. Both authors agree that halide ratio governs speed and contrast; they disagree about colour, and Wall’s disagreement is backed by a described series rather than an assertion. The modern Tier 1 statement — Getty’s, that warm-tone emulsions are modified to restrict particle growth during development — is about particle size, which is the mechanism both halide practice and development rate act through. So the honest page names the mechanism, records the disagreement, and stops short of a claim about a named product that no datasheet supports.

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