Kodak SB-4

A bath that tells you when it has stopped working. Kodak Limited prints SB-4 with a colour attached — violet-blue when it is fresh, yellow-green when a partly used bath has stood a few days — and in a subject where keeping figures are usually absent or nominal, a signal the worker can see across the room is worth more than a date on a label.

Ingredient	Quantity	Form the source specifies
Chrome alum	30 g	potassium chrome alum, the dodecahydrate crystals
Sodium sulfate (anhydrous)	60 g or 140 g (crystalline, which is the handbook's main column)	anhydrous
Water	to make 1000 mL	The handbook gives no temperature for making the bath up. Cold water is the sensible choice because a warmed chrome alum solution turns from violet to green, and Kodak attributes that change to the composition of the salt rather than to the temperature alone.

Purpose

To harden the gelatin layer between development and fixing, so that a film or plate can be worked at temperatures at which an unhardened layer would soften, frill or reticulate. Kodak’s heading is a hardening bath for use in tropical processing, and its stated working range is 75 to 95 °F — 24 to 35 °C, which is to say from a warm English summer up to conditions in which no unhardened emulsion is safe.

It is a processing bath and not an emulsion additive. The same salt appears in this formulary twice more doing entirely different jobs: on bare glass as a subbing dip, and inside a molten emulsion as an in-emulsion hardener at a fraction of the gelatin weight. Three uses, three quantities, and only this one is a tray.

Recommended uses

Processing above about 24 °C, which is what the formula was published for. The bath goes in after the developer and before the fixer.

A negative that must survive further chemical treatment. Intensification, reduction and stain removal all soften a layer, and a hardening step before them is the standard protection. Kodak’s own formalin bath SH-1 is recommended for exactly that case, and this bath occupies the same slot in the sequence without formaldehyde in it.

A hand-coated plate that frills, which is the reason this entry exists in a course about making your own emulsions rather than a course about processing films. Part V’s plates are unhardened by default and a frilling plate has three possible remedies: a sub under the layer, a hardening bath after development, or a hardening fixer.

Where the fixer must stay plain. A hardening fixer puts alum and acid in the same bottle as the hypo and shortens its life; a separate hardening bath keeps the fixer simple at the cost of one more tray.

When another formula is preferable

• In this course, nearly always — and the alternative is potassium alum. Part V’s hardening rule is three-tier: no hardener in the emulsion by default, a potassium alum acid hardening fixer where processing needs the layer to hold, and chrome alum in the emulsion only where a lesson genuinely requires it. The published baths for the middle tier are F-5 and the F-53 stock made up as F-54. Three reasons decide it, and they all point the same way: potassium alum carries no GHS hazard classification at all, where chrome alum carries three irritation statements; a hardening fixer is an item the student is buying anyway; and this bath uses 30 g of chrome alum per litre against the 50 to 200 mg an in-emulsion dose puts in a whole batch.
• When the negatives are not going to be worked warm, no hardening bath at all. Duffin’s warning runs the other way from the one people expect: excessive hardening interferes with developer penetration, and on a thick hand coat the fixer is the bath that struggles most.
• When the softening comes from a chemical treatment rather than from heat, Kodak’s SH-1 is the bath the handbook recommends by name — and this course does not use it, because it is a formalin bath and formaldehyde is excluded from Part V on two independent grounds. Its quantities are under Variants.
• When a single bath must do both jobs, the chrome alum fixing bath F-16, with the sharp caveat that Kodak itself prints beside it: it rapidly loses its hardening properties, with or without use.

Mixing

Two salts into cold water, made to a litre. The handbook prints no mixing order for SB-4 and this page does not invent one; where Kodak thinks the order is critical it says so, as it does at length for the acid hardening fixing baths, and it says nothing here.

1. Dissolve the chrome alum, 30 g, in most of the water. Cold. A warmed solution goes green.
2. Dissolve the sodium sulphate, 60 g of the anhydrous salt or 140 g of the crystals.
3. Make up to 1000 mL.

🧮The two forms of the sulphate, and how nearly they agree

Kodak prints the sulphate twice, as it prints every hydrated salt twice, and the two figures are worth checking against each other because a reader with only one form on the shelf has to trust the conversion.

140 g ÷ 322.19 = 0.4345 mol

The crystals, as the decahydrate at 322.19 g/mol

0.4345 mol × 142.04 = 61.7 g

The anhydrous salt at 142.04 g/mol

Kodak’s anhydrous figure is 60 g, about three per cent below the exact equivalent, which is the handbook’s ordinary rounding and is far inside the tolerance of a bath whose function nobody has quantified. What is not inside tolerance is using one figure for the other salt: 140 g of the anhydrous powder would be more than twice the intended quantity.

One curiosity of this particular salt is worth knowing even though it does not bite here. Above about 33 °C, hotter water dissolves less sodium sulfate rather than more, which is the temperature range this bath is used in. At 60 g to the litre the solution is nowhere near saturation, so the retrograde curve is a fact about the chemical rather than a constraint on the bath — but it is the reason to make the bath up cold and let it warm to the room, rather than making it up hot.

Agitate on immersion, and treat that as part of the formula. Thirty to forty-five seconds of movement as the negatives go in, to avoid unevenness, then three minutes standing. The 1928 primer gives the reason the handbook leaves out: alkaline developer carried in on the film precipitates chromium hydroxide as a scum at the emulsion surface, and once the film has dried that deposit cannot be removed.

The rinse before it is optional and worth doing. The primer’s position is that where the hardening bath is used the water rinse may be skipped, but that a few seconds of rinse still prolongs the hardener’s life. Everything carried in on the film is what kills this bath.

Behaviour

It dies by sulfite, not by time. An unused bath keeps indefinitely on Kodak’s own statement. What shortens its life is developer carried in on the negatives, and the developer’s preservative is sodium sulfite, which destroys chrome alum’s hardening power at a rate that depends on both concentrations. That is why the keeping note distinguishes a fresh bath from a partially used one and gives the used one a few days.

It announces both states by colour. Violet-blue fresh, yellow-green spent. Kodak’s 1928 rule for the same salt in a working bath is flat: once the colour has gone yellowish-green the bath ceases to harden and should be replaced.

Capacity is stated and is generous. Twenty 10 × 8 inch films per gallon — about 4.5 litres — before scum markings appear. Per litre that is between four and five sheets of that size.

The hardening is fast. Duffin’s account of the same salt in an emulsion is that the reaction is a rapid one involving ionic changes, and Kodak’s plain bath SB-3 reaches maximum hardening in three to five minutes in a fresh bath. Three minutes is not a soak; it is close to the whole reaction.

It is pH-sensitive. Duffin records that chrome alum’s action is effective at about pH 6 and far less effective at higher pH values. A bath that has taken in a great deal of carbonate developer is losing hardening power for two reasons at once.

Image characteristics

A hardening bath is not supposed to have any, and if it does, something has gone wrong. Kodak states no change of density, contrast or tone for SB-4, and none is claimed here.

What can appear is a defect rather than a characteristic. The chromium scum from unagitated immersion is a greyish veil at the emulsion surface, and the handbook’s own scum markings from an exhausted bath are the same failure arriving by a different route. Both are irreversible once dry.

And there is a second-order effect that the emulsion literature is clear about. A layer hardened hard enough fixes and washes slowly; the fixer struggles to penetrate, and a thick patch that fixed incompletely darkens later in room light. That is the argument Part V’s coating lesson makes for not hardening by default, and it applies to a bath just as it applies to an in-emulsion dose.

The mechanism

Chromium(III) tans gelatin, and tans it hard. The comparative figures are on the chrome alum page: the weight that renders 100 parts of dry gelatine insoluble in hot water is 6 parts of potash alum and 2 parts of chrome alum, and a gel with potassium chrome alum in it is described in the conservation literature as completely insoluble rather than merely harder.

How the ion binds the protein is not settled here. The encyclopaedia entry records that the course could not verify the molecular mechanism from a source meeting its standard, and this page does not settle it in passing. Duffin’s manufacturing text offers a hedged proposal and the chemical page declined to adopt it as fact; a formulary entry is a worse place to make that decision than the encyclopaedia was.

⚗What the colours are telling you

Three chromium species and three appearances, and the bath moves between them in a direction that only runs one way.

Violet-blue is the fresh bath: chrome alum in cold solution, the colour Wall identifies as purple by reflected light and reddish by transmitted light.

Green is the same solution warmed, and Kodak attributes the change to the composition of the salt rather than to heat alone — the coordination around the chromium(III) centre changes, and with it the light it absorbs. In a working bath the same signal arrives slowly, as yellow-green, and means the bath has stopped hardening.

A grey scum is chromium hydroxide, thrown out where alkaline developer carried in on a negative met the bath at the emulsion surface. It is the one of the three that lands on the picture.

Why the sulphate is in there is not established. See Function of every ingredient; the honest answer is that Kodak does not say, and this course does not guess on a manufacturer’s behalf.

Function of every ingredient

Chrome alum, 30 g to the litre. The hardener, and on the handbook’s own account the only ingredient with a job. Thirty grams per litre is a three per cent solution, which is worth setting beside the other two quantities of the same salt in this formulary: the subbing dip is 2 per cent and puts its chromium on the glass rather than in the layer, and an in-emulsion dose is 0.5 to 2 per cent of the dry gelatin weight, which for a student batch is 50 to 200 milligrams in the whole make. This bath carries thirty grams in every litre, which is one to two orders of magnitude more chromium in the room, and that difference is the single strongest argument for the course’s preference for a potassium alum fixer instead. More chrome alum does not harden further in any useful sense — Kodak’s own F-16 doubles it to 60 g per litre and gets a bath that will not keep the day out. Less shortens the working life rather than the hardening, because what the surplus is really buying is capacity against the sulfite coming in on the negatives. The form is the dodecahydrate at 499.4, not the anhydrous salt at 283.22, and no photographic formula in this corpus asks for the anhydrous one.

Sodium sulfate, 60 g anhydrous or 140 g crystalline to the litre. Twice the weight of the hardener, and Kodak assigns it no function anywhere on the page. That is worth saying in the section that exists to say what every ingredient is for, because the alternative — writing a confident sentence about anti-swelling agents — would be this course putting words into a manufacturer’s mouth about the bulk of a formula by weight. What can be said is bounded and is on the encyclopaedia page: this is the third Kodak formula in the corpus to carry the salt in quantity without explaining it, its best-documented photographic appearance is a tropical developer that carries 105 g of it per litre, and concentrated neutral salts are a classical way of restraining the swelling of gelatin. That last is a general statement about gelatin and is not a claim about Kodak’s reasoning. More or less of it has no published consequence, which is precisely the problem: an ingredient nobody has quantified cannot be adjusted on evidence. The one thing that certainly matters is which form is on the balance, because the crystals and the anhydrous powder differ by a factor of 2.3. Do not confuse it with sodium sulfite, whose name differs by one letter and one digit and which would destroy this bath rather than fill it out.

Water, to 1000 mL. Cold, and made up to volume rather than added, because the strength of both salts is what the three-minute time was published against. Warm water is the one thing to avoid: it turns the solution green before a single negative has been through it.

Interactions

With the developer immediately before it, which is the interaction that governs the bath’s whole life. Carried-over alkaline developer throws the chromium scum; carried-over sulfite destroys the hardener. A few seconds of rinse buys both.

With sodium sulfite in any concentration. Kodak’s figures for the same salt in a fixing bath give the scale of it: a bath carrying 1 to 2 per cent chrome alum with sulfite in it is useful only immediately after mixing, and a 5 to 10 per cent one holds up for two or three days. That is the reason a separate hardening bath outlives a chrome alum fixer.

With the fixer after it. The negatives go on into an acid fixing bath, and nothing about this bath requires that fixer to be a hardening one. Doubling up wastes alum and hardens further in the direction Duffin warns about.

With hot water at any stage, which turns the bath green.

With potassium alum, in the sense that they are alternatives rather than partners. Kodak’s own comparison in 1928 already favoured the potash alum bath for a working darkroom: weaker, but stable.

With anything that has held ferricyanide or acid, in the general darkroom sense — one tray, one job, and a chrome alum bath is not a dish to improvise with.

Variants

Kodak SB-3, the plain chrome alum bath. Potassium chrome alum 30 g in a litre of water and nothing else. The direction is to agitate the negative for a few seconds immediately after immersion, and maximum hardening takes three to five minutes in a fresh bath; the 1928 primer places it after development and before fixation in hot weather, either alongside an alum fixer or where a chrome alum fixer does not harden enough. It is the same hardener at the same strength without the sulphate, which makes the pair a ready-made experiment on what the sulphate is for — see Experiments. These are two Kodak formulas rather than two printings of one, and this course does not merge them.

Kodak F-16, the chrome alum acid hardening fixing bath. Solution A is hypo 320 g and crystalline sodium sulphite 40 g per litre; solution B is 60 g of chrome alum with 8 cm³ of concentrated sulphuric acid, made to a litre; one part of B goes into three parts of A with A stirred rapidly. Kodak’s own warning is the reason it is not recommended here: it rapidly loses its hardening properties, with or without use. It is also the formula on these pages that requires concentrated sulphuric acid, poured slowly down the side of a stirred vessel, never the reverse.

Kodak SH-1, the formalin hardener. Formalin 10 cm³ and crystalline sodium carbonate 13.5 g — or 5.0 g anhydrous — made to a litre; three minutes, then a rinse and five minutes in a fresh acid fixing bath before any further chemical treatment. Kodak recommends it where the emulsion would otherwise be softened considerably by stain removal, intensification or reduction. This course does not use it. The exclusion of formaldehyde in Part V rests first on the emulsion literature’s own photographic objection and only second on the hazard record, and the coating lesson sets out both. It is printed here because a reader who meets SH-1 in the same handbook should find out from this course why it is absent rather than assume it was overlooked.

No course variant is offered. There is nothing to make safer by weakening this bath: the course’s answer to the same problem is a different salt in a different vessel, and it is a formula rather than a modification.

Safety

Level B, set by the powder rather than by the tray. Thirty grams of a fine violet crystal per litre is a real weighing, and it is a larger one than any other chrome alum operation in this course.

The classification lives on the chrome alum page and is not restated in different words here: Warning, GHS07, skin and eye irritation with possible respiratory irritation, and — the part that decides how a page like this reads — no sensitisation and no carcinogenicity statement notified. The UK limit is 0.5 mg/m³ as chromium for chromium(III) compounds.

This is chromium(III). The chromium policy holds the distinction between the two oxidation states once, for the whole course, and it is the reason a hardener that shares a syllable with dichromate is permitted at all.

Controls: nitrile gloves and eye protection while the powder is open and while the bath is mixed; weigh over a tray in still air with no fan running, because the hazard is dust and the limit is a dust limit; buy the smallest pack that will serve so the powder is weighed as few times as possible.

Tongs, not fingers. This is a tray between two others, and the general rule about one pair of tongs per tray applies with more force here, because developer on a pair of tongs is exactly what makes the scum.

The bath is warm by design. Thirty-five degrees is not a burn hazard, but it is a temperature at which a solution evaporates and concentrates over a session.

Storage

A stoppered, labelled bottle, and the unused bath is one of the best keepers in the formulary. Kodak’s statement is unqualified: freshly made, it keeps indefinitely while unused.

A partially used bath is not stored, it is used or discarded. The handbook gives it a few days standing, and the colour change to yellow-green is the endpoint. There is no way to restore it; adding fresh chrome alum to a spent bath adds hardener to a solution that already contains what destroyed the last lot.

Label the substance, the strength and the date, per the labelling SOP, and record the date the bath first had work in it as well as the date it was made. Those are two different clocks and only the second one matters.

Keep it out of the way of the fixer. Not because of a hazard, but because a splash of hypo into the stock bottle is a silent loss of the whole bottle’s hardening power.

Incompatibilities

Sulfite, in any form and at any concentration — the fixer’s preservative, the developer’s preservative, a washing aid. See incompatibilities.

Alkaline developer, which throws chromium hydroxide as an irremovable scum.

Concentrated sulphuric acid, which appears in the neighbouring formula F-16 and must be added slowly down the side of a stirred vessel, never the reverse.

Heat, which turns the solution green.

Ammonium chrome alum as a substitute, which Kodak warns evolves ammonia if the bath turns alkaline and puts a greenish dichroic sheen on the emulsion surface. Pay for the potassium salt.

Chromium(VI) in any form, which is not an incompatibility of the substance but a rule of this course. Dichromate and chrome alum must never share a bench, a bottle or a waste stream where they could be confused for one another.

Waste

A chromium(III)-bearing solution in quantity, and this is the largest chromium stream in Part V. A litre of spent SB-4 carries 30 g of chrome alum, against 6 g in a 300 mL subbing dish and 50 to 200 mg in a whole student emulsion.

Bottle it, label it, keep it out of the drain, and follow the chromium(III) rinse route and the disposal ruling. The rinse that follows the bath carries chromium too.

It is not silver-bearing — it sits before the fixer, not after it — so it is a separate stream from the silver waste and should stay in a separate bottle. Two identified wastes are easier to hand over than one unidentified mixture.

Local regulation decides. How a chromium-bearing waste classifies is a matter for the classification method named on the chrome alum page, and this course states the chemistry without computing a threshold.

The option that produces none of this is the course’s own default: no hardener, and a potassium alum fixing bath where the layer needs to hold.

Troubleshooting

A grey veil or a scum over the whole negative. Chromium hydroxide, from alkaline developer meeting the bath without agitation. Agitate for the full thirty to forty-five seconds on immersion, and rinse for a few seconds before the bath. Once the film has dried the deposit cannot be removed.

Scum markings after a long session. The bath is past its capacity. Kodak’s figure is twenty 10 × 8 inch films per gallon; replace it rather than trying to revive it.

The bath has gone yellow-green. It has stopped hardening. This is not a warning that it is about to fail; on Kodak’s own account it is the failure.

The bath was green the day it was made. It was mixed in warm water. Make it again cold.

The emulsion still frilled. Three possibilities in order of likelihood: the bath was spent, the layer is a thick hand coat whose edges are thicker still, or the problem is adhesion to the support rather than cohesion of the layer — in which case the remedy is a sub under the emulsion, not a bath over it.

The negatives now fix and wash slowly, and a thick area has darkened weeks later. Over-hardening. The fixer could not penetrate, and what is darkening is unfixed silver halide. Shorten or drop the hardening step; this is Duffin’s own warning about developer penetration arriving one bath further on.

A violet stain on the bench. Expected of the solution, and also the visible record of where it went. Treat a stain as a control that failed rather than as housekeeping.

Experiments

Find out what the sulphate does, since Kodak will not tell you. SB-3 and SB-4 differ by one ingredient and nothing else. Make both, harden matched strips from one film in each at 30 °C, and compare: the wet layer’s resistance to a fingernail at the edge, the time to dry, the tendency to reticulate when moved between baths at different temperatures, and the capacity of each bath over a session. Nothing in this corpus answers the question; a controlled pair on your bench is a better answer than a guess.

Read the death of a bath. Photograph the bath in a white dish against a grey card after every four sheets, and plot the colour against the sheet count. Kodak gives you two colours and a capacity; you can turn that into a curve for your own developer and your own carry-over.

Test the rinse. Two litres of the same bath, one fed by negatives taken straight from the developer and one by negatives given five seconds of running water. Run both to the yellow-green endpoint and count the sheets. The 1928 primer says the rinse prolongs the hardener’s life; this measures by how much.

Harden against not hardening, at temperature. Four hand-coated plates from one melt, processed at 30 °C, two through this bath and two straight from developer to fixer. Look at the edges, and then look again a month later in room light for the bronzing that says a thick area fixed incompletely.

Compare the two alums honestly. This bath against a potassium alum acid hardening fixer such as F-5, matched strips, same film, same day. The course prefers the potassium alum route on hazard and convenience rather than on performance, and it is worth knowing for yourself what, if anything, that preference costs.