Kodak IN-4

The intensifier the 1928 primer preferred. It found increasing favour, the primer says, owing to the ease and certainty of its operation and the permanency of the intensified image — and it says so in the same paragraph that records mercury-intensified images as less stable. Wall’s 1924 handbook calls the method less liable to stain and much less poisonous than many others. Both of those judgements were sound in their own time and neither survives contact with what is now known about chromium(VI).

☠Level D: this page is not a procedure, and the reason is a course-wide rule

Potassium bichromate is potassium dichromate, a chromium(VI) compound. The chromium policy is the single place that rule lives and it is unambiguous: chromium(VI) is never used, at any level, anywhere in this course. That is not a judgement made on this page and it is not restated here in other words — read it there, along with the reason the policy has to separate the two oxidation states, because chromium(III) as chrome alum is permitted at Level B.

So this entry does what a Level D page does: the chemistry, the historical workflow in outline, the image characteristics and the hazards, and no procedure. Nothing below is addressed to a reader as something to do. The composition is published because a formula that cannot be read cannot be identified, and identification is the use a reader has for this entry.

The intensifier this course can publish is IN-5, in the same handbook.

Ingredient	Quantity	Form the source specifies
Potassium dichromate	90 g	printed as potassium bichromate
Hydrochloric acid	64 mL	concentrated
Water	to make 1000 mL	9 per cent w/v of the bichromate and 6.4 per cent v/v of the acid. No temperature is published for making it up, and no order of dissolution is given in either printing, so none is recorded here. Wall's independently published version keeps the two in separate stocks and mixes them at use, which is not what Kodak does.

Purpose

To lift a thin negative more or less all over. Kodak Limited’s header is “Chromium intensifier for approximately proportional intensification of thin negatives whether arising from slight under-exposure or under-development”.

Two things in that sentence are worth stopping on. Approximately proportional puts this formula between the other two the course can compare it with: IN-1 lifts the shoulder and leaves the mid-tones, IN-5 is called proportional outright, and this one is hedged. And slight under-exposure or under-development is one of the few places a manufacturer’s formula names the two causes of a thin negative separately — which matters, because an intensifier can do something about the second and very little about the first.

The 1928 primer supplies the general principle: intensification is done by the deposition of some material on the silver image, and it names silver, mercury and chromium compounds as the three materials in ordinary use. This is the third.

Recommended uses

What the sources record, as history. The course recommends none of them, and the chromium policy forbids the substance outright.

Thin negatives, which is Kodak’s own scope and the only one it gives.

Where permanence mattered. This was the formula’s advantage in period practice: the 1928 primer records increasing favour on the grounds of ease, certainty and the permanency of the intensified image, and says in the same breath that mercury-intensified images are less stable.

Where the negative was not perfectly free of hypo. Wall records that the negatives need not be absolutely free from hypo, because the bleaching bath oxidises it — though if much hypo is present the bath may need to be applied twice. That is a real practical advantage over both the mercury and the uranium treatments, each of which is ruined by residual fixer.

Where the result had to be dialled in. Kodak Limited names the control explicitly: the degree of intensification is varied by the time of re-development, and the whole operation may be repeated.

When another formula is preferable

• Kodak IN-5, the silver intensifier, on the facing page of the same handbook, and the one intensifier of Kodak Limited’s five that this course publishes as a formula to mix. It is proportional rather than approximately so, and every reagent in it has a written encyclopaedia entry at Level A or B.
• The 1928 primer’s re-development route, which it calls perhaps the simplest method of intensification: the rehalogenating ferricyanide-bromide bleach followed by a sulfide bath. It is a bleach-and-redevelop process of exactly the same shape as this one, with a bleach the course can handle at Level B, and it is worth knowing as the structural relative of IN-4 that survives.
• IN-1 and IN-21 are not alternatives. All three are Level D and swapping one for another is not a decision this course offers.
• A longer exposure, where the negative is under-exposed. Kodak’s own header distinguishes the two causes; only one of them is a case an intensifier can help with, because there is nothing to build on where there was no exposure.

Mixing

This section gives no mixing procedure.

What can be described without giving instructions is the shape of the thing. This is a single stock solution, unlike every other intensifier in the handbook, diluted 1 + 10 at the moment of use — which is why it appears above as one table rather than several. Neither printing gives an order of dissolution, a temperature or a keeping figure, and none is invented here.

The historical workflow was bleach, wash, redevelop, and Kodak Limited’s own account of it is four criteria and one number that is not a number: bleach thoroughly; wash until the yellow stain is removed; redevelop with a non-staining developer in artificial light or diffused daylight; wash thoroughly and dry. The 1928 printing gives a five-minute wash where the 1949 one gives the criterion, and offers a brief immersion in a sodium carbonate solution to hasten the clearing. Neither printing says which form of sodium carbonate, and the course keeps three separate encyclopaedia entries for the anhydrous, monohydrate and decahydrate salts precisely because a five per cent solution of one is not a five per cent solution of another, so no link is offered here.

🧮What one part in eleven gives

Per litre of working bleach, at the 1 + 10 dilution both printings give:

90 g/L ÷ 11 = 8.2 g per litre

Potassium bichromate

64 mL/L ÷ 11 = 5.8 mL per litre, or 0.58% v/v

Hydrochloric acid, concentrated

The 1928 primer’s description of the bath is “a solution of bichromate containing a very little hydrochloric acid”, and at 0.58 per cent v/v that is a fair description of the working bath. It is not a description of the stock, which is 6.4 per cent v/v of the concentrated acid and is where the whole hazard of the formula sits.

Wall’s independently published version keeps the bichromate and the acid in two separate stocks and mixes three different bleaching baths from them, ranked by strength; Kodak’s is one bottle at one dilution. Two ways of arranging the same two substances, and Kodak’s is the one that puts a concentrated acid and a chromium(VI) salt in the same container.

Behaviour

Recorded from the sources so that the formula can be understood, not so that it can be run.

Everything is a criterion and nothing is a time. Bleach thoroughly; wash until the yellow stain is removed. The one duration in either printing is the 1928 primer’s five-minute wash, and it disagrees with the 1949 criterion rather than filling it in.

The control is the redevelopment, not the bleach. Kodak Limited states it: the degree of intensification can be controlled by varying the time of re-development. That is the same architecture as R-4b in the opposite direction — a first bath that converts and commits nothing, and a second that decides how much comes back.

It repeats. If greater density is required, the operation may be repeated, in both printings.

The developer matters, and Kodak says which kind will not do. A non-staining developer, redeveloping in artificial light or diffused daylight, and specifically not a fine-grain developer of the borax type carrying a high concentration of sulfite: the note is under Interactions and it is one of the most useful sentences in the handbook.

It tolerates residual hypo where its rivals do not. Wall’s observation, and a genuine advantage.

No capacity and no keeping figure is published, in either printing.

Image characteristics

Approximately proportional, which is Kodak Limited’s own hedge and this page keeps it. The gain follows the silver already present, roughly.

Thin negatives from two different causes, with two different prospects. Under-development leaves silver that was never fully reduced and a scale that is compressed but present; under-exposure leaves shadows with nothing in them. Kodak names both, and an intensifier that builds on existing silver can only do much about the first.

Permanent, by the standards of its rivals. The 1928 primer’s judgement was that the chromium-intensified image is the stable one, against mercury. That is a comparison between two treatments this course does not teach, and it should be read as such rather than as a claim about permanence in general.

No colour is published for the intensified image, in either printing. The yellow both mention is the bleach’s stain, which is to be washed out before redevelopment.

Neither printing publishes a density figure, at any dilution, on any material. The word “approximately” is the only quantification the formula has.

The mechanism

A bleach that makes a halide, and a developer that develops it again.

The 1928 primer’s account is compact: the silver image is bleached with a solution of bichromate containing a very little hydrochloric acid — bichromate being an oxidiser of the same type as permanganate or ferricyanide — and the image is then redeveloped and found to be intensified to an appreciable extent.

⚗What the bleach makes, and what this page will not write down

Kodak Limited’s own note names the product of the bleach without meaning to. Warning against fine-grain developers, it says that the sulfite tends to dissolve the silver chloride before the developing agents have time to act on it. So the bleached image is silver chloride, and the chloride comes from the hydrochloric acid, which is the acid’s second job.

Ag⁺ + Cl⁻ → AgCl

The step the hydrochloric acid performs: the oxidised silver is caught as a halide in place, rather than dissolved away

No equation is printed here for the oxidation itself. The primer classes bichromate with permanganate and ferricyanide as an oxidiser and stops; nothing in this corpus establishes the stoichiometry of a dichromate oxidation of image silver in an acid bath, and the course does not write a plausible one. The potassium dichromate page is where the substance is described.

Where the extra density comes from is not fully established either. The 1928 primer’s general statement is that intensification is performed by depositing a silver, mercury or a chromium compound upon the image, which is the corpus’s only account of what chromium contributes here — and it is a sentence about the class of methods rather than a mechanism for this one. What can be said with confidence is the part that is not in doubt: the silver is converted to a halide and developed again, and a developed silver chloride image is not the same image as the one that was bleached.

Why the redevelopment happens in the light. Kodak Limited says artificial light or diffused daylight, and Wall is more explicit about why it matters: he develops with amidol in white light, and notes that other developers may be used but that the image then requires exposure to diffused daylight. Neither states the reason. What the course can say without adding to them is that the bleached image is silver chloride, and silver chloride is light-sensitive; the 1928 printing, which names two developers and no lighting condition at all, is the odd one out among the three sources.

Why “approximately” proportional and not proportional. Neither source explains it, and this page does not supply a reason. What it can point at is the control Kodak does name — the time of redevelopment — which is the same kind of partial control a bleach-and-redevelop toner has, and which is why the result is a range rather than a fixed multiple.

Function of every ingredient

Potassium dichromate, 90 g to the litre of stock, about 8.2 g/L in the working bleach. The oxidiser, and the reason this page carries no procedure. Both printings call it potassium bichromate, which is the name the course’s page carries as an alias. The 1928 primer’s description of the substance is worth having in one place: orange-red crystals, stable in air, dissolving easily to a yellow solution, used in photography both for bleaching negatives and for sensitising gelatin — the two uses that between them define the whole of chromium(VI) in photography, and both of which the chromium policy removes from this course. Its job here is to take electrons from image silver so that the chloride can catch it; the primer classes it with permanganate and ferricyanide as an oxidiser of the same type. More of it, in Wall’s independently published version, gave the stronger bleaching bath and the greater intensification; less gave the weaker. Its page carries the classification — twelve statements including H340, H350 and H360 — and EH40’s 0.01 mg/m³ as chromium with the Carc and Sen notations, and this page does not restate or soften either.

Hydrochloric acid, 64 mL of the concentrated acid to the litre of stock, about 5.8 mL/L in the working bleach. Two jobs, and the second is the one that is easy to miss. It holds the bath acid, which is the condition a dichromate needs to act as an oxidiser at all — the 1928 primer’s phrase is a solution of bichromate containing a very little hydrochloric acid. And it supplies the chloride, so that the oxidised silver comes down as silver chloride in place rather than dissolving away: that is what makes this a bleach-and-redevelop process rather than a reducer. Kodak Limited’s own note about sulfite dissolving the silver chloride is the evidence for it. More acid would bleach faster; less, past a point, and the dichromate has nothing to work in. At 6.4 per cent v/v in the stock this is a substantial concentration of a corrosive acid sharing a bottle with a chromium(VI) salt, which is a combination the course would decline on the acid’s account even if the dichromate were not there.

Water, to make the stock up to a litre, and ten parts of it in the working bleach. The dilution is the formula’s only published quantity control, at 1 + 10 in both printings. Note what the water is not doing here, by contrast with R-4a, where the dilution is the rate control the operator adjusts: Kodak gives one dilution for IN-4 and puts the control in the redevelopment instead.

Interactions

With sulfite in the redeveloper, and this is the most useful sentence Kodak prints about the formula. The note says that fine-grain developers of the borax type carrying a high concentration of sulfite are not suitable for re-development, because the sulfite tends to dissolve the silver chloride before the developing agents have time to act on it. That is D-76 and its relatives, named by their description — and it is the clearest statement anywhere in this corpus of what a high-sulfite developer is actually doing to a halide. A paper developer like D-72 is what Kodak names instead.

With a staining developer, which Kodak also excludes: the instruction is a non-staining developer, since a stain image added to an intensified one cannot be separated from it afterwards.

With light, deliberately. The redevelopment is done in artificial light or diffused daylight, because the bleached image is a light-sensitive halide.

With residual hypo, tolerantly. Wall records that the bleaching bath oxidises hypo, so the negative need not be absolutely free of it — though much hypo means bleaching twice. This is the opposite of IN-21, which residual hypo ruins.

With metals, to which NIOSH records chromates as corrosive, and which the potassium dichromate page carries with the rest of its incompatibility list.

With combustible and organic material, against the solid dichromate, which is an oxidiser.

Variants

The 1928 Eastman Kodak printing is not a variant: the same stock at the same quantities and the same 1 + 10 dilution. What differs is the directions — a five-minute wash rather than a criterion, and two named redevelopers instead of one.

Nepera Solution at 1 + 4, which the 1928 primer names as one of those two redevelopers. It is a proprietary product whose composition its maker did not publish, so the course can record only the name and the dilution the primer gives it. No reconstruction of it appears anywhere in this course, and the fact that a period source names a bottle rather than a formula is itself worth noticing when reading old manuals.

Wall’s 1924 chromium intensifier, attributed to Eder, Welborne Piper and Carnegie, which is a more informative arrangement of the same two substances: two stock solutions kept separately — bichromate at 50 g/L and pure hydrochloric acid at 100 mL/L — from which three bleaching baths are mixed in stated proportions, ranked so that the first gives the strongest intensification and the third the least. Wall develops with amidol in white light. His arrangement publishes the strength control that Kodak’s single bottle does not.

Wall’s chromic acid version, and the ammonium chlorochromate he describes preparing by evaporating ammonium bichromate with hydrochloric acid. Both are chromium(VI) by another route, and both are named here only so that a reader meeting them in a period manual can recognise what they are.

Kodak’s IN-6, the quinone-thiosulphate intensifier, which is the handbook’s strongest and is also built on potassium bichromate. It is not published in this formulary at all, for two independent reasons: the chromium(VI), and an ingredient that is Kodak’s own wetting agent — a proprietary product whose composition its maker does not disclose. A formula whose composition the course cannot state is not published as a formula, whatever its safety level would have been.

No course variant is offered and none could be. There is no version of a chromium(VI) bleach without chromium(VI), and the chromium policy is a rule about the oxidation state rather than about a concentration.

Safety

Level D, and by a route different from the other two Level D intensifiers in this formulary: not from a substance page’s own reading of the rubric, but from a course-wide ruling. The chromium policy states that chromium(VI) — dichromates and chromic acid — is never used, at any level, anywhere in this course, and is taught as chemistry and history with no procedure. This page is that policy applied to a formula. It does not restate the policy in its own words and it does not carve an exception out of it.

Read the potassium dichromate page and the Level D policy for the classification, the exposure limit and the reasoning. What belongs here rather than there is the shape of the hazard in this formula: a 9 per cent w/v solution of a carcinogen and respiratory sensitiser sharing a bottle with 6.4 per cent v/v of concentrated hydrochloric acid, poured into an open dish, with a negative handled wet in it.

☠Two period judgements that were reasonable and are now wrong

Wall’s 1924 handbook recommends this method as less liable to stain and much less poisonous than many others, and he is comparing it with mercury and cyanide. Against those, in 1924, he was right.

The 1928 primer records increasing favour on the grounds of ease and certainty of operation and the permanency of the intensified image, and it too is comparing with mercury.

Both judgements are relative and both are about the wrong axis. The hazard that decides the modern classification is not acute toxicity, which is where mercury and cyanide win the comparison: it is carcinogenicity and sensitisation, which neither source could have assessed and neither mentions. EH40 sets chromium(VI) compounds at 0.01 mg/m³ as chromium with the Carc and Sen notations and a biological monitoring guidance value. A substance being safer than corrosive sublimate is not the same as it being acceptable, and this pair of quotations is the clearest illustration in the formulary of why a period source establishes what was done rather than what is reasonable.

Nothing on this page is a control and none is offered. The classification does not turn on how hard the controls would be; the ruling is that the oxidation state is not used.

Storage

What the sources record, and what the course’s own pages say.

Neither printing publishes a keeping figure, and none is invented here. Kodak’s single stock is evidently meant to keep, since it is diluted at use.

Wall keeps the two constituents apart, in two stocks mixed at use — an arrangement that avoids storing a concentrated acid with a chromium(VI) salt, whether or not that is why he adopted it.

Nothing on this page is stocked in a home darkroom. The potassium dichromate page’s storage entry says so for the substance and this formula does not create an exception.

An old bottle of bichromate is not a find. It is a hazardous-waste question, and it is a question for a professional and the local authority rather than for a course page.

Incompatibilities

Combustible, organic and readily oxidisable material — paper, wood, sulfur, aluminium and plastics — which NIOSH lists in full against the solid, and which CAMEO’s fire entry adds to by noting that the salt may decompose to give oxygen and so support the combustion of other materials.

Metals, to which NIOSH records chromates as corrosive.

Sulfite, in the photographic rather than the hazard sense: a high-sulfite developer dissolves the silver chloride the bleach made, which is Kodak’s own note.

Hydrazine and anhydrous hydroxylamine, with which CAMEO records violent and explosive reactions.

Strong oxidising agents and basic salts, CAMEO’s two reactive groups for the substance.

Concentrated hydrochloric acid, which in this formula is not an incompatibility but an ingredient — and which brings its own handling requirements with it, on the hydrochloric acid page.

See incompatibilities.

Waste

Nothing here is generated in this course, because the process is not carried out and the substance is not used at any level.

A chromium(VI)-bearing waste is a hazardous waste and there is no dilution, neutralisation or reduction step described on this page that would change that. The potassium dichromate page carries the position for the substance and the chromium policy carries the ruling.

It would also be an acid waste, which rules out combining it with any ferricyanide or thiosulfate stream in the darkroom.

Where such a solution exists, it is a matter for a licensed hazardous-waste route and a professional, and the disposal ruling explains why the course will not go further. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Nothing here is a fix, because nothing here is a procedure. These are the failures the three sources record, kept because each of them says something about the chemistry that the formula alone does not.

“The redevelopment was weak and patchy.” Kodak names the cause and it is a developer choice: a fine-grain developer of the borax type, carrying a high concentration of sulfite, dissolves the silver chloride before the developing agents can act on it. A paper developer such as D-72 is what the handbook names instead.

“The negative kept a yellow stain.” The bleach was not washed out. Kodak Limited’s 1949 criterion is to wash until the stain is gone and it offers a brief carbonate immersion to hasten it; the 1928 printing says five minutes, which is a time rather than a criterion and is the weaker instruction of the two.

“The bleach seemed to do nothing, then worked on a second application.” Wall’s account: the bleaching bath oxidises residual hypo before it gets to the silver, so a negative carrying much fixer may need the bath twice.

“There was not enough intensification.” Both printings say the operation may be repeated, and Kodak adds the finer control, which is the time of redevelopment.

“The negative was thin because it was under-exposed and intensifying did not help.” Expected, and Kodak’s own header is careful to name under-exposure and under-development as two different causes. Nothing deposits on a shadow that has no silver in it.

“There is an old bottle of bichromate in the darkroom I inherited.” Not a troubleshooting question. See Waste.

Experiments

No experiment on this page involves making or using this formula.

Test Kodak’s sulfite warning, without any chromium at all. The note says a high-sulfite developer dissolves silver chloride before the developing agents reach it. That claim can be checked on any bleach-and-redevelop process the course does teach: take the rehalogenating ferricyanide-bromide bleach, bleach two identical prints, and redevelop one in a paper developer and one in a high-sulfite film developer. If Kodak is right, the two will not match, and you will have measured a solvent effect that is usually invisible.

Place the three intensifiers on one curve. Sketch what shoulder-only (IN-1), approximately proportional (this formula) and proportional (IN-5) do to the same characteristic curve. Three manufacturer’s adjectives, one axis, and the differences between them become arguments rather than words.

Read the two period judgements against the modern record. Wall calls this method much less poisonous than many others; the 1928 primer praises its permanency. Both are in the sources above. Set them beside the notified classification and EH40’s entry and write down exactly which hazard each source was and was not in a position to assess. That is the skill Part XXVI’s assignment asks for, on a case where the answer is clear.

Follow the substance rather than the formula. Potassium bichromate appears in this course as a bleach here and as a sensitiser for gelatin in the pigment processes, and the 1928 primer names both uses in one sentence. Trace where each of those goes in the curriculum and note that the same ruling closed both. A single policy that reaches two unrelated parts of a subject is worth understanding as a policy.