Tannic acid toner for cyanotype

A toner that begins by destroying the photograph. Everywhere else in this course, alkali is the thing a cyanotype must be kept away from — the reason the print goes into an unbuffered mount, the reason the paper must be free of chalk before it is ever coated. Here it is the first bath, on purpose, and the second bath is what decides whether that was a good idea.

The ammonia solution — the alkaline bleach

Ingredient	Quantity	Form the source specifies
Ammonia solution (ammonium hydroxide)	10 mL of a 28% solution	printed as "Ammonia 28%"; the kit supplies the solution and this is a volume of it
Water	100 mL, added	Distilled water, given as an amount rather than a make-up volume. The 07-0092 printing gives 1000 mL here instead; see Variants.
Mixed separately from the toner and used first. This is the bath the cyanotype literature warns about everywhere else in the course: alkali is what a finished cyanotype must be kept away from.

The tannic acid solution — the toner proper

Ingredient	Quantity	Form the source specifies
Tannic acid	10 g
Water	500 mL, added	Distilled water, so 20 g of tannic acid per litre. The 07-0092 printing gives 20 g in 1000 mL, which is the same strength by a different pair of numbers.

Used in this order — toning a finished, fully processed cyanotype

1. The ammonia solution — until the colour has been bleached out — No time is published. The end point is the disappearance of the blue, and it is not reversible.
2. Water — 10 minutes — Cool water.
3. The tannic acid solution — until the desired colour is achieved — No time is published here either, and the print can be taken out at any point, which is what makes the range of colours available.
4. Water — 15 minutes — Running water, then dry.

Mix both solutions separately. This is a two-step immersion process. Immerse the print in the ammonia solution until the colour has been bleached out. Wash in cool water for 10 minutes. Then immerse the print in the tannic acid solution until the desired colour is achieved. Wash under running water for 15 minutes and dry.

The sheet is explicit that this is done after the cyanotype process has been completed entirely: it is a treatment of a finished print, not a step in making one.

Purpose

To turn a finished cyanotype from blue to brown or black, in two baths. The supplier’s own heading for the pair is Brown to Black Tones, and the sheet is explicit about when it happens: the process is done as a toning after you have completed the cyanotype process entirely.

The sheet also says where the idea comes from, which is unusually candid for a kit instruction: it credits the toning ideas to Jan Arnow’s A Handbook of Alternative Photographic Processes. The formula is therefore a supplier’s published working of somebody else’s method, and this entry says so rather than presenting it as an original.

Recommended uses

A cyanotype that is finished, washed and dry. Not a partially processed one.

Where the whole colour range between brown and black is wanted from one bath. The print can be pulled from the tannic acid at any point, which is the same freedom the bleach-and-redevelop silver toners give.

Where a warm print is wanted from a process that only makes blue ones. That is the whole appeal of toning a cyanotype, and it is why the kit sheet carries three different toners rather than one.

Not on a print you cannot make again. The first bath is irreversible; see Behaviour.

When another formula is preferable

• Where the blue is to be kept, no toner at all. A cyanotype’s blue is the process, and every toner in the kit’s list begins by attacking it.
• Where a violet is wanted, the sheet’s own mild borax bath — and not its lead acetate alternative, which this course will not print as a procedure. See Variants.
• Where a green is wanted, the sheet’s dilute sulphuric acid bath. See Variants.
• Where the print is a silver one, none of this applies: the gold, sulphide and selenium toners in this formulary work on silver and have nothing to do with Prussian blue.

Mixing

Two bottles, mixed separately, used one after the other.

1. The ammonia solution. 10 mL of 28 per cent ammonia in 100 mL of distilled water.
2. The tannic acid solution. 10 g of tannic acid in 500 mL of distilled water.

The sheet’s instruction is mix both solutions separately, and the two are never combined: this is a sequence of baths, not a mixture.

🧮What the two baths carry, and where the printings disagree

10 g ÷ 0.5 L = 20 g per litre

Tannic acid

10 mL in 110 mL = about 1 part in 11

Ammonia, as the 28 per cent solution

The tannic acid is the number both Photographers’ Formulary printings agree on: 10 g in 500 mL here, 20 g in 1000 mL in the other, 20 g per litre either way.

The ammonia is where they part company. This printing puts 10 mL of the 28 per cent solution into 100 mL of water; the other puts the same 10 mL into 1000 mL. That is a tenfold difference in the strength of the bleach, and it is not a rounding. Both are printed here rather than averaged, because an average of two disagreeing printings is a third formula nobody published. What can be said is that the bleach’s end point is visual — until the colour has been bleached out — so the weaker bath is the slower one rather than a different result, and the weaker bath is the one that gives a person time to stop.

Behaviour

The first bath is irreversible and it is fast. The Prussian blue page carries the evidence, from Ware: a buffer at pH 9.4 completely decolourises Prussian blue, by irreversible hydrolysis, in one to ten minutes. Household ammonia is far above pH 9.4. A bath of it does not lighten a cyanotype; it takes the image substance apart.

Neither bath has a published time. The sheet gives two end points — bleached out, and the colour you want — and reserves its numbers for the washes.

The colour is chosen in the second bath. The print can be removed at any point, which is where the range between brown and black comes from.

The ten-minute wash between the baths is the only fixed step in the middle, and it is there so that alkali does not arrive in the tannic acid.

Nothing is published about how long either bath keeps or how many prints it will take, and nothing is invented here.

Image characteristics

Brown through to black, per the sheet’s own heading.

The blue does not come back. A cyanotype bleached in alkali and then under-toned is not a lighter cyanotype; it is a print whose image substance has been hydrolysed. The Prussian blue page states the general rule: alkali is not reversible, where the ordinary weak fading of a cyanotype is, being reversible in a peroxide bath.

Highlights are the fragile part, as in every cyanotype: there was least Prussian blue there to begin with, so there is least for the second bath to work on.

The result depends on the paper as well as the baths, because a cyanotype’s image sits in and on the fibres rather than in a binder. No source read for this course quantifies that, and this page does not.

The mechanism

Half of this is established and half of it is not, and the page says which is which.

⚗The bleach, which is documented

The image substance of a cyanotype is Prussian blue, and its one real weakness is alkali. Ware reports Holtzman’s finding that a buffer at pH 9.4 completely decolourises it by irreversible hydrolysis in one to ten minutes — and the reason that figure matters beyond this formula is that pH 9.4 is the pH of a saturated solution of calcium carbonate, the buffer routinely put into archival board.

So the first bath of this toner is the standard failure mode of the cyanotype process, run deliberately and to completion. That is the whole of what the corpus establishes about it, and the Prussian blue page carries it in full.

What the tannic acid then does is not established by any source read for this course, and no mechanism is offered. The tannic acid page covers its two documented photographic roles — Russell’s tannin dry plate and its action as a gelatin hardener — and says nothing about cyanotype toning. The kit sheet gives an instruction and no chemistry. What can be stated without going beyond the sources is the observable: the print comes out of an alkaline bath with its blue destroyed and iron in the paper, goes into a tannic acid bath, and comes out brown to black.

Naming the product would be easy and would be a guess. A page that wrote “iron tannate” or “iron gall” here would be supplying chemistry the course has not verified, which is precisely what Rule 1 forbids. It is therefore left open, and it is the first of the experiments below.

Why the two baths are never combined. Tannic acid is an acid and the bleach is a strong alkali; mixed, they neutralise each other and neither does its job. The sheet’s mix both solutions separately is the practical form of that, and the ten-minute wash between them is the other half.

Function of every ingredient

Ammonia, 10 mL of a 28 per cent solution. The bleach, and the ingredient that makes this a two-step process rather than a one-bath toner. Its job is to hydrolyse the Prussian blue of the image — the reaction the rest of this course spends its time preventing. More bleaches faster and gives less time to stop, which is the practical difference between the two published printings; less bleaches more slowly to the same visual end point. Note the form: 10 mL is not a quantity and 10 mL of a 28 per cent solution is, which is why the strength travels beside the volume. Its own page carries the classification, the vapour hazard and the handling, and none of it is restated here.

Tannic acid, 10 g in 500 mL, so 20 g per litre. The toner, and the ingredient whose function this page can describe and cannot explain. What it does is put a brown-to-black substance where the blue was; what that substance is, no source read for this course establishes, and the page says so under The mechanism rather than filling the gap. The strength is the one figure both Photographers’ Formulary printings agree on, which is worth something: two sheets that differ by a factor of ten on the bleach give the same 20 g per litre here. More or less is not published as a control by any source read here; the control the sheet does give is time in the bath.

Water, distilled, in both baths. 100 mL in the bleach and 500 mL in the toner, given as amounts rather than as make-up volumes. Distilled rather than tap, which matters more in an iron process than in a silver one: tap water carries its own iron, its own carbonate hardness and, in many supplies, an alkalinity a cyanotype is already vulnerable to.

The two washes, which are the only timed steps in the formula. Ten minutes in cool water between the baths, so that alkali does not reach the tannic acid; fifteen minutes in running water at the end. On a process whose image substance is destroyed by alkali, a wash that leaves ammonia in the paper is not a finished print.

Interactions

With alkali, which is the whole first bath and the whole hazard to the print. After toning, everything the Prussian blue page says still applies to whatever is left: unbuffered mounts, unbuffered enclosures, no chalk-buffered board.

With the print’s own paper. A cyanotype made on a chalk-buffered paper has already lost some of its image; the same alkali sensitivity that makes this toner work is the reason the process demands unbuffered paper from the start.

With the second bath, through the wash. Alkali carried into the tannic acid neutralises it.

With the other toners on the same sheet, which are alternatives rather than additions: acid for green, borax or lead acetate for violet.

With metal, as a general rule for any iron process: an iron-cyanide chemistry is exactly what picks up a trace of iron from a tray and shows it.

Variants

The 07-0092 printing of the same toner. Photographers’ Formulary’s other sheet gives the bleach as 10 mL of 28 per cent ammonia in 1000 mL of water and the toner as 20 g of tannic acid in 1000 mL. The toner is the same strength; the bleach is a tenth as strong. Both are printed here rather than reconciled. Nothing in either sheet says which is intended, and the weaker bleach is the more controllable one.

Green tones, from the same sheet. A 1 per cent sulphuric acid solution — the kit’s acid is supplied at 48 per cent for shipping, and the sheet’s own arithmetic is 2 mL of it into water — with the print immersed until the desired colour is achieved and then washed for 15 minutes. Note the sheet’s own safety line, which is the right one: always add the acid to the water, not the reverse. It is a different formula and would need its own entry.

Violet tones, from the same sheet: one of the two options and not the other. The sheet offers either a mild borax solution or a warm 5 per cent lead acetate solution. The borax route is an alkaline bath on an alkali-sensitive image and would need its own reading before it could be published as a formula. The lead acetate route is not published by this course at any strength. Lead salts sit at Level D — see lead nitrate for the classification of the class — and a soluble lead salt in an open tray in a home darkroom is not something a kit sheet’s single sentence makes acceptable.

No course variant is offered. There is nothing to make safer by reformulating the published bath, and the one figure a course variant might want to settle — which of the two ammonia strengths to use — is a question the sources answer two ways and the reader can test in an evening.

Safety

Level B, from ammonia. Tannic acid is Level A and Prussian blue is Level A; the ammonia is the whole of the classification, and its own page carries the hazard, the vapour and the controls.

The practical points that follow from that page: ammonia solution is worked with ventilation that actually moves air, because the hazard is the vapour rather than the liquid; splash protection for the eyes; gloves; and the bottle opened away from the face, because a 28 per cent solution releases ammonia as soon as it is opened.

Never mix the ammonia with anything acidic, including the second bath of this very formula and including any bleach-type household product. That is why the sheet says to mix the two solutions separately and why the wash between the baths is not optional.

A supplier’s disposal claim that this course does not adopt. Bostick and Sullivan’s comparable cyanotype toning sheet makes the claim that depleted solutions may be poured down the drain and are safe for septic systems and the environment. The course does not repeat that claim: it is a manufacturer’s assertion about an unstated volume into an unknown sewer, and it is not a hazard assessment. Photographers’ Formulary’s own sheet, more carefully, asks the user to consult local sewer and water authorities, which is what the disposal ruling says too. See Waste.

Storage

Two bottles, separately, labelled, and never near each other’s contents.

The ammonia bottle loses strength every time it is opened, because the gas leaves the solution. A bath mixed from an old bottle is weaker than its label says, and there is no published test for it here.

The tannic acid solution is a solution of a plant polyphenol in water, and no keeping figure is published by the sheet or by any source read for this course. Expect it to darken; nothing establishes what that costs.

Neither bath has a published capacity. The sheet says nothing about how many prints either will take.

Dark storage for both, as a general precaution, and dated labels, because neither bath announces its age.

Incompatibilities

Acids of any kind, with the ammonia bath, including this formula’s own second bath.

Household bleach and anything chlorine-releasing, with ammonia, absolutely.

Alkali, with a finished cyanotype — which is the joke of this formula and the rule for everything after it: unbuffered mount, unbuffered enclosure, no chalk-buffered board.

Metal trays, as for any iron process.

Lead acetate, which the sheet offers as an alternative toner and which this course does not use.

Waste

Two streams, neither of which is a silver stream. The ammonia bath is a dilute alkali carrying hydrolysed iron and cyanide as the ferrocyanide of the destroyed Prussian blue; the tannic acid bath is a dilute organic acid carrying iron and tannin.

Not combined, and not neutralised against each other in a bottle.

Not down the drain on a supplier’s say-so. One of the two kit sheets in this corpus says depleted solutions may be poured away and are environmentally safe; the disposal ruling is what this course follows instead, and the standing caveat applies: local regulation decides, and this course cannot tell you what it says where you are. Collect both, label them, and follow the general chemical waste SOP.

Troubleshooting

The print bleached and then would not tone. The bleach went too far: there is nothing left in the paper for the tannic acid to act on. The bleach is irreversible and this is the failure to plan against.

The bleach is taking a very long time. Which printing did you mix? One is ten times the strength of the other. Neither publishes a time.

The tone is patchy. Agitation in the first bath, most likely: an uneven bleach gives an uneven toner.

The colour keeps drifting after the print is out of the bath. No source read for this course addresses this, and the sheet gives no warning about it; treat your own observation as the evidence and record it.

Blue is coming back in the highlights. It should not: alkaline hydrolysis is irreversible, and what looks like returning blue is more likely to be Prussian blue that the bleach never reached.

A sharp smell over the tray. Ammonia, doing exactly what a 28 per cent solution does. That is a ventilation problem rather than a chemistry one, and its own page is where the controls are.

The second bath went cloudy or dark. Nothing published covers it. Record what you saw and mix fresh.

Experiments

Name the product. The gap this page could not fill is what the tannic acid actually makes. A toned print, a control and any elemental or spectroscopic method available — even a careful set of solubility and colour tests — would say more than the literature this course holds. It is the most useful experiment on this page because it is a question, not a demonstration.

Settle the ammonia strength. Two identical cyanotypes, one bleached in each of the two published baths, timed to the same visual end point and then toned together in one tannic acid bath. Two Photographers’ Formulary sheets disagree by a factor of ten; whether the result differs, or only the speed, is a single afternoon.

Map the colour against time. Prints pulled from the tannic acid at intervals and dried together. The sheet promises brown to black and gives no times; the set of reference prints is the useful output.

Bleach partially, on purpose. Pull prints from the ammonia at a quarter, a half and three-quarters of the way to a complete bleach, then tone them all together. This is split toning in an iron process, and it works for the same reason it works in a silver one: the second bath can only reach where the first one went.

Test the alkali claim at the other end. Ware’s figure is that pH 9.4 decolourises Prussian blue in one to ten minutes. A strip of cyanotype in a saturated calcium carbonate solution, timed, is a demonstration of why this course insists on unbuffered mounting board — and it uses the same chemistry as this toner’s first bath.