Assignment: Cyanotype at Scale, on Fabric, and the Botanical Sequence

Every cyanotype you have made so far has been one sheet of coated paper, small enough to fit a tray, a frame and a hand. Four habits were built into that sheet without your noticing: that a rod can reach across it, that a tray can hold it, that the light source is bigger than the print, and that the wash is over in twenty minutes. All four break when the work gets larger, moves onto cloth, or has to survive being handled rather than framed.

This assignment breaks them deliberately, in four directions. Nothing here is a new formulation and nothing is a new hazard. What is new is that the process has to be scaled, and scaling a process is where you find out which of the things you were taught were chemistry and which were the size of your tray.

The brief

Produce, over about four hours of working time spread across at least two days because of drying:

1. One large print, on a sheet whose shorter side is at least 400 mm — bigger than your usual tray and probably bigger than your ultraviolet unit.
2. One botanical sequence of six or more photograms that reads as a set rather than a collection, made and recorded the way Anna Atkins made hers.
3. One print through a paper negative, with the negative’s own density range measured rather than guessed.
4. One piece of cloth, coated, exposed, washed, dried — and then wash-tested against a swatch you kept back.

And one record covering all four, in the form set out at the end, plus the wash-test data, which is the only original measurement this page asks for.

The formulations are not yours to invent. Every sensitiser this part uses is written out in full, with provenance and the function of every ingredient, in the formulary: the classic sensitiser, the New Cyanotype, the Simple cyanotype, Mike’s cyanotype, Herschel’s 1842 formula, the positive-working Pellet process and the two bought kits. Choose from those pages and cite which you used, because part of what this assignment tests is whether you can pick a formulation for a reason.

Learning objectives

By the end you should be able to:

• coat, expose, process and dry a sheet larger than your equipment was bought for, and say which step set the size limit;
• predict how a three-dimensional specimen will draw itself on a photogram, from the source’s geometry rather than from experience;
• make a paper negative, measure it, and say whether it fits the process you are printing;
• choose a sensitiser for cloth on the mordanting evidence rather than on price;
• design and run a wash-fastness test whose result you would defend, and state what it does and does not license you to claim;
• specify a mount and an enclosure, and explain why the ordinary conservation answer is wrong here;
• state, in one sentence that could be wrong, why this picture is blue.

Prerequisites

The coating and printing lab is assumed entire: you have mixed a sensitiser, recorded a batch, coated by rod and by brush, exposed to a measured dose and processed to a stable blue. The exposure-scale experiment is assumed for its numbers — you need your own exposure scale and maximum density — and the toning lab for what alkali does to a finished print, which is the laundry problem below in a different costume.

Part I’s photogram assignment is where the botanical work began, on bought pre-coated paper in sunlight. This page is that assignment made adult: your own coating, your own sensitiser choice, a sequence rather than three prints, and a written argument about why they belong together.

Part XIX supplies the critique method and the mounting practice — and half of Part XIX’s mounting advice is actively dangerous to a cyanotype. The section on presentation says which half.

Safety classification

Level B, from the classification rubric, and the letter comes from where it came from in the coating lab: potassium ferricyanide as a solid and as a stock solution. Nothing here raises the level; four things about it change with scale.

• Substances and quantities. The same two solids and the same two stocks, in larger volume. An immersion bath for cloth is the largest single quantity of mixed sensitiser anywhere in this part, and a spilled litre is a different cleaning problem from a spilled 2 cm³.
• Energy. Ultraviolet, and more of it, because a large print takes a long exposure and a project week is many exposures. The controls are Part XVI’s and are not restated. Outdoors, the outdoor exposure session SOP governs, and ICNIRP’s advice for outdoor workers — shade, clothing, headwear, and staying out of the hours around solar noon — is what you have instead of an enclosure.
• Waste. Substantially more than any previous page in this part, which is a covers item of its own below rather than a footnote here.
• Incompatibilities. Unchanged and absolute. Nothing acidic and nothing containing bleach comes near the sensitiser, the baths or the waste bottle. The one exception remains the deliberate dilute weak-acid development bath published for particular formulations.

What is not a hazard here, and why. There is no inhalation route in normal work: once the two solids are dissolved, everything on this page is an aqueous solution or a damp sheet, and the only powder-handling step is the weighing you already did in the lab. There is no heated bath, no solvent and no flame; the one heat source anywhere on this page is a domestic iron used dry on cloth, at the maker’s own instruction, and its hazard is a burn rather than a chemical one. And there is no fixing bath, so there is no thiosulfate, no silver and no silver-bearing waste.

⚠Two things this page forbids that the historical literature recommends

No dichromate bath, on cloth least of all. A potassium dichromate bath was once commonly recommended for the wet processing of fabric cyanotypes, both to force the re-oxidation and reputedly to improve light-fastness. Ware records that the toxic, allergenic and carcinogenic hazards now recognised in dichromates make this practice particularly inadvisable for prints on fabrics, which may come into contact with human skin, and that there is at least one well-documented case history of an artist working with cyanotype on textiles who suffered considerable ill-health as a direct consequence of using dichromate baths. Ammonium dichromate and potassium dichromate are chromium(VI) compounds under the course’s chromium ruling. Use air, or the dilute hydrogen peroxide bath the lab already gave you.

No arsenate fixing. Mercer’s 1857 cloth prints, which survive in the Lancashire archive, were fixed in sodium arsenate, then a common calico-printer’s material. Ware’s own comment is that it can hardly be recommended for general use today; the course records the chemistry and gives no procedure.

Materials

Everything here is already yours from the coating lab and Part XVI except the substrates.

Item	For	Note
Sensitiser stocks, freshly made and batch-numbered	all four projects	The formulation is your choice from the formulary; the batch SOP applies
Unbuffered 100 per cent cotton paper, 8 × 10 in	botanicals, paper-negative print, test strips	about twelve sheets including spoilage
Unbuffered cotton paper, at least 400 mm on the short side	the large print	two sheets: one is a rehearsal
Natural-fibre cloth	the fabric project	about 0.5 m², plus ten swatches of the same cloth: six for the scouring trial, four for the wash test
A wide, soft brush kept for this process alone	large print, cloth	a rod cannot span the large sheet; see below
A sheet of plate glass, or the contact frame	all exposures	the frame’s limit is one of the four size limits you are looking for
A felt blanket or other porous backing	all contact exposures	Ware’s remedy for trapped carbon dioxide; not optional at this scale
Pressed and living plant material	the sequence	prepared days ahead
A photographic print on thin fibre-base paper	the paper negative	made in Part XVIII or XIX
Two large trays, a clean bath or a photographic print washer	processing at scale	see project one

Chemicals are exactly those of the coating lab — ammonium iron(III) citrate or ammonium iron(III) oxalate with potassium ferricyanide, plus citric acid and 3 per cent hydrogen peroxide. No quantity is restated here. The formulary pages carry the strengths and the mixing; the arithmetic below works in cubic centimetres of mixed sensitiser, which is the only quantity that changes when the sheet does.

Estimated cost

Band ££, and the split has moved. In the coating lab the chemistry was almost free and the paper was the cost; here the paper is four times the size, the cloth is a new purchase, and the sensitiser is still almost free. A single 400 mm sheet of the cotton papers Ware names costs more than the ferricyanide you will use in a month. Dated figures live in the planner, and for this part most of them are gaps.

Estimated consumables cost

Every quantity comes from the Materials table and the arithmetic below; every price with a number is the planner’s own dated UK figure. Two rows out of eight carry a number, and that is the finding rather than an omission.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	4 pairs	£6.64 to £14.99 per box of 50 to 100	£0.53 to £1.20
Mixed sensitiser, paper side	about 40 cm³	Both solids of the classic sensitiser are now in src/data/prices.json at £17.99 per 230 g each; the formulation here is yours to choose from the formulary and the oxalate routes are unpriced, so a mixed volume carries no figure	—
Mixed sensitiser, cloth immersion	unknown until you weigh it; see project four	Not priced	—
Unbuffered cotton paper, 8 × 10 in	about 12 sheets	Not priced. Hot-pressed cotton paper is already a named gap in the price file	—
Unbuffered cotton paper, 400 mm or larger	2 sheets	Not priced. Both of the file’s paper gaps are written for smaller sheets than this, so this row names a further one	—
Natural-fibre cloth and swatches	about 0.6 m²	Not priced	—
Hydrogen peroxide, 3 per cent, pharmacy	about 200 mL	Not priced	—
Distilled or purified water	5 to 10 L, for stocks and for the wash test	Not priced	—
Pre-coated cyanotype paper, if you take the alternative route	8 sheets	£18.95 for 20 sheets to £36.95 for 50, 8 × 10 in	£5.91 to £7.58

The priced rows come to about £0.53 to £8.78, a floor rather than a total: six of the nine consumables have no sourced price, and two of the six are the substrates that make this assignment what it is. What the table does establish is the ratio the consumables calculator needs. Against Bergger’s own published yield of about three hundred 8 × 10 sheets from a 2 × 300 mL set, a whole project week’s paper coating is about a fifteenth of one set of bottles. Equipment is absent from the table, because a brush, a tray and a sheet of glass are not consumed.

Alternative route

This page declares requiresUV, and at this scale that declaration bites harder than it did on a single sheet. Four obstacles, four answers, and one thing with no answer at all.

No large ultraviolet unit, or none at all — print in the sun, and for this page it is the better source. Bostick and Sullivan list sunlight beside a 1000 W metal halide bulb without apology, and the sun has an advantage here no home unit can match: it is very nearly a point source, and the botanical project is exactly the case where that decides the picture. The section on sun printing below sets out the geometry and the record it demands. A reader with no unit is not making a compromised version of this assignment; for two of its four projects they are making the better one.

No unit big enough for the large print, but a small one you trust. Expose the sheet in sections, with the frame re-positioned and the boundary hidden along a compositional edge — and know before you start that this is the least reliable thing on the page, because the joins record every difference in dose and every minute of drift in the source. The honest alternative is to print the large sheet outdoors and keep the unit for the small work.

No running water at scale. A 400 mm print will not fit most sinks and a metre of cloth will not fit any of them. Three routes, in descending order of how well they work. A clean domestic bath, filled and drained, is a tray of any size you like; it must be rinsed before and after, and must never have had a bleach or a limescale remover in it that day, which is the acid rule wearing a domestic hat. A standing bath changed on a schedule is what Bostick and Sullivan already publish for trays — a change every two minutes — and it works at any volume on far less water than a running tap. And for cloth, a lidded bucket agitated by hand moves more water past the fibres than a shallow tray does. What none of these gives you is the running-water end point, so judge the wash by the water and by the highlights: it is complete when a change comes out colourless and the paper white shows no yellow through a blue viewing filter.

No space to dry a coated textile. Cut the project down rather than compromising the drying: a 300 mm square over a clothes airer in an unlit room is a complete version of project four, and the wash test that is the real content of it works on swatches the size of a beer mat. Coated cloth also keeps — Jacquard state that sensitised paper or fabric can be stockpiled, used within six months for best results, cool and dry in a sealed bag — so the drying and the exposure need not happen in the same week.

What has no alternative. There is no version of any of these four projects that makes a picture without ultraviolet light, and no version that stays dry. The wash is both the development and the fixing, and at this scale it is the largest single requirement the page makes. If neither a bath nor a bucket is available, the honest thing to say is that this page cannot be run, and the substitute is not a smaller print — it is the reading route the part overview sets out, in which the chemistry, negatives and variants lessons and the formulary carry the argument with no wet work at all.

Project one — the large print

Four things change when the sheet gets big, and only one of them is the one people expect.

Coating: the rod runs out before the paper does

Ware specifies the coating rod as a glass rod or thick-walled capillary tube, 6 to 12 mm in external diameter, whose straight central section is equal in length to the width of the coating area. That sentence is the size limit. A 200 mm rod is stiff; a 500 mm rod of the same diameter is a lever, and it will bow in the middle under the light pressure the technique asks for. A bowed rod meets the paper at the ends and not in the centre, and lays down a coating that is thin down the middle of your picture.

So the large print is a brush print, and the trade is known: the negatives and papers lesson puts a rod at about 30 cm³/m² and a brush at 40 to 60, so brushing costs about half as much again per unit area. On a cyanotype that is pennies, and it buys the visible brushed border a rod cannot produce.

Three things carry over from the rod technique unchanged, and they are what usually goes wrong at size. Tape the sheet to a surface you have checked with a spirit level — Ware says this is critical, and on a large sheet a slope of a degree runs the sensitiser into one corner. Mark the coating area in pencil, 1 to 2 cm larger than the negative. And do not touch up: the sensitiser starts soaking in immediately, and a second application over a partly absorbed first one prints as a hard edge.

🧮How much sensitiser a project week actually takes

Take the rod figure of 30 cm³/m² and the brush figure of 45 cm³/m² from the negatives lesson, and work out the areas. This is the course’s own arithmetic from those published specific coating volumes, not a figure from a source.

• 8 × 10 in = 0.2032 × 0.254 m = 0.0516 m². By rod: 1.55 cm³.
• 16 × 20 in = 0.4064 × 0.508 m = 0.2065 m². By brush: 9.3 cm³.

A project week is then roughly: two large sheets at 9.3 = 18.6 cm³; eight 8 × 10 sheets by rod for the botanicals and their spares = 12.4 cm³; two more for the paper-negative work = 3.1 cm³; six quarter-sheet test strips ≈ 2.4 cm³. About 37 cm³, call it 40.

Check it against a maker’s yield: Bergger’s 2 × 300 mL set coats about three hundred 8 × 10 sheets, or 2 cm³ a sheet — close to the brush figure, and it puts a week of paper work at a fifteenth of one set. The paper costs many times what the chemistry does, and that stops being true the moment you leave this part.

Drying: cockling, and why it matters more than it looks

A sheet of cotton paper that has taken up 9 cm³ of water over a quarter of a square metre swells unevenly as it dries, and a large sheet has more room to do it in. The result is cockling, a shallow permanent waviness, and its consequence is not cosmetic: a cockled sheet cannot be held in contact with a negative. Blur is gap times aspect value, so a 1 mm ripple under a diffuse source of aspect value 4 puts 4 mm of blur into the picture there — more than ten times the 0.3 mm Ware takes as the onset of a visibly soft image.

Dry flat, in the dark, hanging only if you have nothing better and then from two corners. Do not force it with heat: the coating lab establishes that a heat-dried sheet does not behave like an air-dried one, and a large sheet dried unevenly cockles more rather than less.

Exposure: when the negative is bigger than the light

Two problems hide in this one. Coverage: a source characterised for uniformity over an 8 × 10 area is not characterised over a 16 × 20 one, and irradiance from an array falls off towards the edges of its own footprint. And the frame: a contact frame built for 8 × 10 in Part XVI does not become a 16 × 20 frame. Plate glass on a board is the usual answer at size — Ware’s own simplest option, on a baseboard covered with a felt blanket, held with strong clips.

The felt is not upholstery. Ware’s outgassing work shows that every siderotype evolves carbon dioxide as a product of the photochemistry itself, that a full-density coating of a 0.7 molar iron(III) sensitiser releases about 392 cm³ of gas per square metre, and that if the gas cannot escape through the back of the paper it collects into bubbles that lift the paper off the negative by an estimated 0.1 to 0.5 mm. Backing the paper with impervious plastic made the blurring glaringly apparent in his own step-tablet tests; a papermaker’s felt blanket cured it. A large print is exactly where this bites, because the area is large, the exposure long and the gas has further to travel to an edge.

⚗Where the fizz goes, and why the back of the paper matters

The classic cyanotype sensitiser is nowhere near 0.7 molar in iron, so 392 cm³/m² is an upper bound for a stronger oxalate coating rather than a figure for your sheet. The mechanism is the same whichever formulation you use, because it is the photochemistry itself: the light reduces the iron and the carboxylate pays for it in carbon dioxide.

The practical consequence is that a paper’s permeability is a printing characteristic and not only a coating one. Ware notes that gelatine sizing may block the pores, that parchmentised papers are less porous, and that plastic substrates and papers mounted on aluminium are impermeable. So a heavily surface-sized sheet can be good to coat and poor to expose — and if your large print is soft in the middle where the small ones were sharp, test that before blaming the frame.

Processing: when the sheet will not fit a tray

The three routes are in Alternative route above, because they are the same three a reader without a sink needs: a clean bath, a large tray on a change schedule, or a print washer if Part XII left you one.

The one thing to add here is handling. Support a large wet sheet from underneath, on acrylic or a sling of polyester mesh. Wet cotton paper of that size tears under its own weight if you lift it by a corner, and the tear goes through the picture.

Project two — the botanical sequence, after Atkins

In October 1843, Anna Atkins distributed more than a dozen copies of the first fascicle of Photographs of British Algae: Cyanotype Impressions to her botanical friends. She issued it privately as a part-book over the decade to 1853 — twelve parts, approximately 420 plates or more, the number varying between the known copies — and Ware’s estimate is that the edition demanded the hand-printing of over five thousand cyanotypes. Talbot’s Pencil of Nature, the first commercially published photographically illustrated book, did not appear until June 1844.

🕰What Atkins actually did, and what nobody knows

No evidence has come to light of the method or formula she used. Ware infers from the colour of the plates and from the sheer productivity of the work that she probably used Herschel’s rapid standard negative-working method, and notes that everything she needed was to hand: her father had built galvanic batteries and could have made the potassium ferricyanide electrolytically, ammonium ferric citrate was becoming a common pharmaceutical preparation, Whatman’s Turkey Mill paper was sold as a superior writing paper, and the processing needed nothing but the pure water of the well at Halstead.

What is documented is her working method, and it is the part you are copying: the plates are negative cyanotypes printed directly from the seaweeds themselves, dried and pressed, and usually unsupported — laid straight on the paper with no glass over them — each labelled with its Linnaean binomial in her own hand on a translucent slip. Talbot, writing in 1864 after decades of watching his own silver prints fade, singled the work out: the whole of these prints, he wrote, remain unaltered after the lapse of several years.

The course reproduces none of it. There is no image pipeline here and no photographs, and a reproduction of a museum object is a licensing matter rather than a design one: the Library of Congress state the standard plainly, that in all cases it is the researcher’s obligation to determine and satisfy copyright or other use restrictions before publishing or distributing an image. What is offered instead is where to look. Ware records nine substantially complete copies in British collections — Chatsworth, the Royal Society, Lacock Abbey, the British Library, the Horniman Museum, the Linnean Society, the British Museum, the Royal Botanic Gardens at Edinburgh and Kelvingrove — others at Detroit, Tel Aviv, the Jardin des Plantes and the Université de Montréal, and Herschel’s own copy at the New York Public Library, whose entire set of images Ware states may be freely browsed online through the library’s digital collections. Go and look before you make yours.

Living against pressed, which is a chemistry decision as well as a picture one

Atkins’s specimens were dried and pressed. Yours may not be, and the difference shows up in three places.

Flatness. A pressed specimen lies in the plane of the paper; a living one does not. That is the whole of the line quality question and it has an equation, below.

Water. A living leaf transpires and a cut stem weeps. Jacquard’s troubleshooting attributes blotches to moisture, including moisture produced by leaves or design elements during the exposure, and their instruction is explicit that paper, hands, printing surface and objects must all be dry. On a ten-minute exposure a fresh specimen is a slow leak onto a sensitised sheet, and the mark is a hard-edged pale blot that no washing removes. Print living material by all means, but expect that mark and use it.

Colour, which does nothing. A red leaf and a green one of the same thickness print almost identically, because the sensitiser responds in the near ultraviolet and blue rather than to the pigment your eye is reading. Tone is set by ultraviolet transmission and thickness, which are only loosely related to how dark a specimen looks. It is worth predicting one specimen wrongly on purpose to feel it.

The translucent leaf and the opaque stem

A photogram has two quite different kinds of subject in it, and most beginners treat them as one.

An opaque element — a stem, a seed pod, a thick rhizome — is a mask: it transmits nothing, gives paper white inside its outline, and everything interesting about it happens at the edge.

A translucent element — a thin leaf, a frond, a petal — is a negative. It transmits some ultraviolet, so it prints as a tone that varies with thickness: a leaf’s veins print darker than its blade, and two overlapping fronds darker again. A photogram of opaque material is a silhouette; a photogram of translucent material is a photograph. Atkins’s algae are the second kind, which is why her plates carry structure and not only outline.

The composition problem this sets is real. A sequence built only from silhouettes has no tonal range and reads as clip art; one built only from thin fronds has no anchor. Most good sets have both, with the opaque material the smaller part of the area.

Contact pressure decides the line, and here is the arithmetic

The uneven contact print entry derives this relation and Part XIX re-applies it; this page applies it to a plant.

blur = gap × aspect value

Ware's penumbra relation

The gap is the distance between the specimen and the sensitised surface, in millimetres — for a photogram, essentially the thickness of the specimen, since the shadow-casting edge is at the top of it. The aspect value is the source’s largest linear dimension divided by its distance from the print. Ware gives 0.01 for the sun, 0.1 for a NuArc 2125 and 4 for a light bed or open sky, and takes 0.3 mm of blur as the onset of a visibly soft image.

The same specimen under two sources

1. 1Gap g — the thickness of the specimen; for a pressed frond, a few tenths of a millimetre, for a stem, several millimetres
2. 2Penumbra — gap times aspect value; the width of the soft zone at every edge in the picture
3. 3Density trace — what a densitometer would read scanning across the edge of the shadow

Drawn from Ware's published relation and his tabulated aspect values; the two annotated blur figures are the course's own arithmetic for a 2 mm specimen and are not measurements.

Three consequences follow, and they are the practical content of this project.

Press the specimens properly. A pressed frond a few tenths of a millimetre thick gives a line you can call sharp under almost anything. A dried but unpressed stem 4 mm thick, under a diffuse bank of tubes, throws 16 mm of penumbra, which is not an edge at all.

Weight the glass, or the gap is the glass’s rather than the specimen’s. Plate glass resting on a thick specimen is a tent, and the paper under the rest of it is nowhere near contact — which is where clips and a felt backing earn their place, and where a thick specimen forces a choice between crushing it and accepting a soft picture. Atkins’s answer was to press everything.

Choose the source for the line you want. For a line like Atkins’s, use the sun. For a soft, diffuse, blooming line — a legitimate picture and not a failure — use the open sky or a bank of tubes deliberately, and say in your notes that you did.

What makes a set a sequence

Six prints on a table are a collection. What makes them a sequence is holding something constant so that a difference means something.

Atkins held four things constant across five thousand prints: one process, one paper, one presentation format, and one labelling convention. What varied was the specimen. That is why the plates function as a scientific publication rather than as decoration.

Decide and write down, before you coat anything, which one thing varies. Species is Atkins’s answer and the obvious one. Others work: one species at six stages of decay, one specimen at six exposures, one specimen printed six ways from the formulary. What does not work is varying three things at once, because then no two prints are comparable and the set has no argument in it.

Project three — the paper negative

An enlarged negative on paper is the cheapest route there is to a large contact print, and the course has been circling it since Part I: Talbot’s own negatives were paper, and his patent of 1 June 1843 covers waxing them to make the sheet translucent so that more light gets through and the fibres show less. That is the whole of this project’s problem in one clause.

Making it

You need a negative the size of the finished print, and a camera negative enlarged onto photographic paper gives you a positive. So there are two routes and they are not equivalent. Two generations from a camera negative: enlarge onto thin fibre-base paper to make an enlarged positive, then contact-print that positive onto a second sheet, emulsion to emulsion. Each generation adds contrast and each adds a second sheet’s fibre structure to the first’s. One generation from a positive: if your original is a transparency or an in-camera positive, one enlargement gives the negative directly, with one paper’s structure in it rather than two.

Use fibre base, not resin-coated, and the thinnest stock you can get. The reason is transmission: you are going to print through this sheet, so its base density is a straight tax on every exposure.

✕What people get wrong about waxing

Talbot’s waxing is a real, dated technique and it does what he says. It is also hot work with a flammable material, and the course gives no home procedure for it: the hazard is a fire rather than a chemical one and it is out of proportion to the benefit here. What is in proportion is to test the idea rather than assume it — make the negative, measure it, and if its base density is what is killing your exposure times, that is a finding, and it belongs in the record as a measured limitation of the route rather than as a step you skipped.

Measuring it, because no source will tell you

No source read for this course publishes a density range for a paper negative used with any process in this cluster. That is a real gap, stated rather than filled: the course establishes it by measurement instead, and the measurement is one you already know how to make.

Two cautions from the negatives and papers lesson apply with force here. Ultraviolet density is not visual density, and a paper base is exactly where the two diverge, because optical brightening agents fluoresce under the ultraviolet you print with. And the Part XV densitometer is a transmission instrument built for film; reading a sheet of photographic paper in it is at the edge of what it can do.

So measure by printing. Contact-print a calibrated step tablet through a blank, processed, unexposed sheet of the same paper, alongside the same tablet printed directly. The difference in the exposure required is the base’s transmission loss in stops; the difference in the number of steps rendered is what the sheet’s structure has done to the scale. Record both.

What it gives that a film negative does not

Three things, and the third is the interesting one.

Size, for almost nothing. A 400 mm negative on photographic paper costs a sheet of paper. On transparency film it costs considerably more, and on sheet film it is not a route most readers have.

Diffusion, as a picture quality rather than a fault. Light passing through paper is scattered by the fibres — the same scattering Part I attributes to Talbot’s negatives and the reason he waxed them. The result is soft, slightly luminous edges and suppressed fine detail, and no digital filter reproduces it convincingly because the scattering happens in three dimensions inside the sheet.

A possible match of scale, offered as a hypothesis and not a fact. The classic cyanotype’s exposure scale is unusually short — Ware puts it at about 0.9 to 1.2, less than half of what platinum wants — and that short scale is its main handicap with ordinary negatives. A paper negative, whose transmitted density range is limited by its base and compressed by fibre scattering, may be one of the few negatives that fits the classic process rather than overwhelming it. The course does not assert that; it is a plausible reading of two established facts, and it is exactly what your measurement can settle. Print one subject as a paper negative on classic cyanotype and on your longest-scale formulation, and report which placed the tones where you wanted them.

Project four — cloth

Which fibres take it, and why

Jacquard’s list of surfaces that can be treated with cyanotype sensitiser is silk, cotton, wool, hemp, linen, canvas, paper, leather and wood, and their statement about the exception is careful: as polyester is not a natural fibre, it generally cannot be used. Those materials work for the reason paper works. There is no emulsion and no binder: the sensitiser soaks into the substrate and the image substance is precipitated among and inside the fibres themselves, so a fibre that will not absorb an aqueous solution has nowhere to hold the iron — and a synthetic monofilament is exactly such a fibre.

Ware makes the positive case sharply: of all the possible methods of printing photographic images, the iron-based processes would seem to be the best-suited for delicate textiles, because they do not employ a sizing agent or binder medium such as gelatin or gum, that would stiffen the flexibility and spoil the drape of the fabric. A cyanotype on silk is still silk. A silver gelatin emulsion on silk would be a stiff sheet that happens to be silk underneath.

⚗Which sensitiser for cloth, and the reason is mordanting rather than contrast

This is the one place in the whole part where the choice between the citrate and the oxalate formulations is settled by something other than exposure scale, and the evidence goes back to a calico printer.

John Mercer, the Manchester colour chemist who gave his name to mercerisation, worked out a negative cyanotype in 1847 and built it on a ferric oxalate sensitiser rather than the universally available ammonium ferric citrate. Ware’s explanation is that his interest was in printing on textiles rather than paper, that he had already shown ferric oxalate to be a good mordant for dyeing wool and cotton, and that the citrate is ineffective in not being absorbed by the cellulose fibres. Mercer’s cloth prints from 1857 survive in the Lancashire archive.

Wendy Wilson, one of the contemporary British textile artists Ware names, has found the same thing in modern practice: the mordanting of sensitiser to cotton and silk fibres appears to be better with ferrioxalate than with ferricitrate sensitizers.

So for cloth, the oxalate formulations have an argument the classic sensitiser does not — and Ware’s own workshop notes on the New Cyanotype say the inexpensive sensitiser may also be applied to fabrics and other surfaces. Two independent observations pointing the same way is not a controlled comparison, and the honest label for it is strong practitioner evidence rather than measurement. Which makes it a thing to test, and the formulary has both sensitisers written out for you to test it with.

Scouring, and an honest gap

New cloth off a bolt is not bare fibre. It carries mill finishes — sizing to help it through the loom, softeners, and often optical brightening agents, which are fluorescent dyes that absorb ultraviolet and re-emit it as visible blue to make white cloth look whiter. Every one of those is between your sensitiser and the fibre, and the brighteners are competing for exactly the photons you are printing with. The generic textile answer is to scour the cloth — a hot wash with a detergent, and a thorough rinse — before doing anything else.

No Tier 1 or Tier 2 source read for this course publishes a scouring procedure for cyanotype, and this page will not invent one. What the sources say is narrower. Jacquard sell pre-sensitised cotton sateen and say nothing about preparing your own cloth; their washing instruction is about the finished print; and their troubleshooting warns of discoloration to brown or yellow when a print meets phosphates, soap or dirty objects, which is a caution about residues rather than a recipe. Ware names optical brighteners among the additives that cause degradation, but he is writing about paper.

So run it as a test rather than a recipe, which is what this course does whenever the evidence stops:

1. Cut six swatches from one piece of cloth.
2. Wash three — hot water, a detergent, and a rinse until the rinse water runs clear — and dry them. Leave three untreated.
3. Check both for brighteners before coating: hold them under your ultraviolet source in a dark room, where cloth carrying them glows a strong blue-white. The negatives lesson gives the same test for paper and the same caveat: it says brighteners are present, not how much.
4. Coat all six identically, expose them together under one negative or step tablet, process and dry them together, and read them.

If the scoured swatches are denser, more even or cleaner in the highlights, you have your answer for that cloth, as a measurement rather than as a rule you were told.

Coating, drying and exposing

Jacquard’s instruction is that fabric may be coated or dipped, and the choice is evenness against economy. Immersion gives the most even result and uses far the most sensitiser, because the bath has to cover the cloth and most of what the cloth does not retain stays behind in it; immerse, lift, and even the loading out by drawing the cloth between two fingers, since a cloth left to drip dries with a gradient down it. Brushing uses less and leaves a visible mark, and on an open weave it goes uneven because the sensitiser wicks along the threads.

Measure what the cloth actually took. Nobody publishes a specific coating volume for fabric, and it depends on the weave and the weight. So weigh the cloth dry, immerse and even it, and weigh it again immediately. The mass gained, divided by the area, in grams per square metre is your figure for this cloth and this sensitiser. Record it as a mass rather than converting to a volume: the mixed sensitiser is denser than water and the course has no measured density for it, so the conversion would be a guess dressed as arithmetic.

Then dry in the dark, flat or evenly hung, with no fold touching another — a fold that dries against a fold prints as a line. Iron it if you want a flat surface for the negative: Jacquard say cyanotype fabric can be ironed before exposure with a dry iron that does not spit or leak water, working on the back. And do not wet it before or during the exposure, which matters more here than on paper because cloth holds water in the weave.

Expose it as you would a print, with two differences. Cloth is far more permeable than paper, so the carbon dioxide problem of the large print does not arise. And Jacquard’s print-out signal is that the fabric looks bronze once fully exposed, with over-exposure almost always preferred to under-exposure — which is the peptisation allowance in plain language, since Ware records that the traditional process obliges as much as two or three stops of it to make up for what the wash removes.

The wash that cloth needs

Longer than paper’s, for a mechanical reason rather than a chemical one: a woven cloth holds unreacted sensitiser between the threads and inside the twist of each thread, where water reaches it only by diffusion and only if the water is moved. Jacquard’s instruction is at least five minutes in cool water changed periodically until it runs clear, and — twice, in two documents — not to use soap.

Take the end point rather than the time. The wash is finished when a change of water comes out colourless and the cloth shows no yellow. Squeeze and open the cloth between changes; a folded cloth in a bucket is washing three of its surfaces. And note the failure mode Jacquard record: a print that darkens as it dries was not washed thoroughly, and what is darkening is sensitiser still printing.

Wash-fastness, treated as an open question

This is the question everybody asks about cyanotype on cloth, and here is what can honestly be said.

The maker’s own comparative statement is the most useful sentence in the literature, and it is not reassuring: in Jacquard’s document comparing cyanotype with their dye product, item three of the list of differences is that cyanotype is not as washable and permanent as SolarFast is on fabric. Their positive instructions are correspondingly hedged — keep washing to a minimum if possible; hand-wash in cold water with a non-phosphate detergent; do not use Synthrapol; over-washing may cause the print to fade; sweat and hand oils may cause discoloration; a faded print can usually be restored by washing in a dilute bath of hydrogen peroxide.

Ware publishes no laundering guidance at all, and the course will not manufacture one from his conservation data. What that data does establish is the mechanism behind every one of Jacquard’s cautions, which is worth more than an instruction.

Why detergent is the problem, in one equation

Laundry detergents are alkaline by design, because alkali saponifies fat, and the classical laundry alkali is sodium carbonate, washing soda. Prussian blue is hydrolysed irreversibly by alkali:

Fe[Fe(CN)₆]⁻ + 3 OH⁻ → Fe(OH)₃ + [Fe(CN)₆]⁴⁻

Alkaline hydrolysis: half the image stays behind as iron, half leaves as hexacyanoferrate(II)

The numbers are Ware’s, by way of Holtzman’s investigation, and they are fast. A buffer at pH 9.4 completely decolourises Prussian blue by irreversible hydrolysis in 1 to 10 minutes, depending on how the pigment was prepared. A 0.25 molar solution of sodium carbonate, at about pH 10.7, destroys the Prussian blue of a cyanotype in less than half a minute. And what is left behind is not nothing: the hydrolysis product is a yellow-brown iron(III) oxide-hydroxide that binds strongly to cellulose substrates — paper or fabric. A cyanotype washed in an alkaline detergent does not fade towards white; it goes yellow-brown and stays that way.

Can it be recovered? A freshly bleached cyanotype can be partially restored by dilute acid with added ferrocyanide, but over time the hydrolysis product becomes insoluble and cannot regenerate the blue; Ware calls this an area needing further research. Alkaline damage is a one-way door with a short grace period, and the bleached-by-alkali troubleshooting entry has the diagnosis.

Why plain water is a problem too

Peptisation is the second mechanism and it needs no alkali at all. Prussian blue disperses into water as a colloid, and the wash carries some of the image away with it. Three independent measurements exist and they agree on the shape of the answer:

Study	Conditions	Result
Ian and Angela Moor, 1989	15 minutes	tap water, pH 7.5 to 8.5: 18 per cent average image density loss. Distilled, pH 6 to 6.5: 4 per cent. Deionised, pH 6.3 to 6.6: reported as 0.00 per cent
Sarah Wagner, Library of Congress, 1991	1.5 hours	dramatic lightening in tap water, slight lightening in deionised water; worse for cyanotypes of recent manufacture
Ware, Cyanomicon Table 9.6	5 min to 4 hours, 20 °C	tap water at pH 7.2: Herschel sensitiser loses 0.139 in density over 4 hours, Ware’s own loses 0.290. Purified water at pH 6.5: 0.032 and 0.022 respectively

Read that last row carefully, because it holds the useful finding. In purified water at pH 6.5 the four-hour losses are small — 0.032 and 0.022 for the two formulations Ware carried to four hours in both waters. In tap water at pH 7.2 the same two lose 0.139 and 0.290, four times and thirteen times as much. A difference of 0.7 of a pH unit, which is the difference between purified water and an ordinary mains supply, changes the loss by most of an order of magnitude. That is not peptisation alone; it is peptisation plus the beginning of hydrolysis, at a pH most people would call neutral.

The test this assignment actually asks for

You are not asked to believe any of the above. You are asked to run the experiment on your cloth, with your sensitiser and your water, and Ware’s own method scales down to a kitchen.

1. Coat, expose and process four swatches identically, from one piece of cloth, in one session, to a solid mid-tone rather than a maximum black, because a mid-tone is where a density change reads most easily. Dry them together and leave 24 hours for the aerial re-oxidation to finish.
2. Read all four, by the method the exposure-scale experiment established for you. They will not be identical; the spread between them is your noise floor, and no result smaller than it means anything.
3. Keep swatch C back untouched. It is the control and it is the whole experiment.
4. Treat the other three for a stated time at a stated temperature: W in purified water, T in your own tap water, D in tap water with the detergent you would actually use, at the dilution on its packet.
5. Rinse all three in purified water, dry them with the control, and read all four again.
6. Report the change as a density difference against the control, not as a percentage of an initial value you measured once.

Two rules about what you may then say. You may say what happened to that cloth, with that sensitiser, that water and that detergent, for that time. You may not say that cyanotype is or is not wash-fast, because one cloth and one detergent do not establish it, and because the artificial ageing that would connect a thirty-minute soak to five years of use is beyond this course. Ware’s alkali test at pH 9.4 ran ten minutes; IPI’s enclosure test ran fifteen days at 70 °C and 86 per cent humidity. Those are what a permanence claim needs, and yours is not one of them.

📓What the wash-fastness data sheet must contain

Cloth: fibre, weave, weight, scoured or not. Sensitiser: formulary page and batch number. Water: source and pH if you can measure it. Detergent: product and dilution. Temperature and time for each swatch. Four initial densities and four final ones, with the reading method and its spread. And one sentence saying what you would now tell somebody who bought the piece — which is the only part of this that a buyer will ever read.

Scale changes chemistry

A project week is not four times one print’s chemistry, and two of the differences are worth arithmetic.

Sensitiser. About 40 cm³ for the paper work, from the calculation above, plus an immersion bath for the cloth whose size you will only know once you have weighed it. The bath is the interesting number, because unlike every other quantity in this part most of it is not consumed — it stays in the tray, it has stood in the light and air while you worked, and the coating lab established that a mixed sensitiser deteriorates within hours. An immersion bath is a mix-it-and-lose-it operation, and the way to make it cheap is to keep it shallow and small and work the cloth through it.

Water, which is what people underestimate. The wash volume scales with the tray and the tray with the sheet. A 16 × 20 print has four times the area of an 8 × 10 and needs a tray holding roughly four times the water, so Bostick and Sullivan’s schedule of a change every two minutes for 12 to 15 minutes moves roughly four times the volume for one print. Add a metre of cloth washed in a bucket until the water runs clear, and a project week’s effluent is measured in tens of litres rather than in the two or three the coating lab produced.

⚠What that effluent is, and what does not change at scale

It is the same three streams the coating lab named — spent sensitiser and coating residue, the first bath and wash water carrying unreacted citrate or oxalate and hexacyanoferrate plus peptised Prussian blue, and the peroxide bath if you used one — in a much larger volume. Bottle and label under the labelling SOP and the general chemical waste SOP.

Dilution changes nothing about the one absolute rule. Nothing acidic and nothing containing bleach goes into any of these containers, ever. The classic sensitiser page sets out both Ware’s well-founded argument that this effluent is environmentally unproblematic and the reason the course still publishes no jurisdiction-specific instruction. Check your local regulations; they govern, and a project week is exactly the volume at which a domestic assumption stops being defensible.

One genuinely new consideration at this scale, and it is mundane: a bath used as a processing tray. Rinse it before and after; never use it on a day when a bleach-based or descaling cleaner has been in it; and do not let the wash water stand in it, because the pigment settles and stains.

The blue is not neutral

This is the question the assignment exists to force, and it is on the art track for a reason: it is the only one here with no measurable answer.

Every other process in this cluster produces something in the grey-to-brown family that the eye reads as a photograph — a rendering of tone, notionally colourless, in which the material of the image does not announce itself. Cyanotype does not. Prussian blue is a colour that barely occurs in nature at the intensity a cyanotype reaches, and the eye does not accept it as a neutral rendering of a grey world. A cyanotype is always partly a picture of its own blue, and every subject you put into it is read through that.

Ware devotes the whole first chapter of the Cyanomicon to it, and the range of his headings is the argument: equivocal blue images, acceptance of blue photographs, a rare colour in nature, symbolic blue in art and religion, liberation from blue preciousness, the aesthetics of the colour blue. He is recording a real historical resistance — the blue was long held to disqualify the process from serious pictorial use — and a real modern reversal.

Why did Atkins choose it? The evidence supports a plain answer and not a romantic one. She was making a scientific publication needing over five thousand identical impressions from pressed specimens, and cyanotype was, in Ware’s words, altogether more expeditious and inexpensive than Talbot’s silver processes. The blue came with the chemistry; there is no evidence she chose it as a colour. What she could not have known, and what Talbot noticed twenty years later, is that she had also chosen the permanent process — and the blue is now inseparable from what the work means to a modern viewer. That is the difference between an artist’s reason and a work’s meaning, and it is worth knowing which one you are giving when you answer for your own.

▷The paragraph you have to write

Two hundred words, containing a claim that could be wrong.

Not “I chose cyanotype because I like the blue”, and not “the blue evokes water and memory”, which is true of any blue and so says nothing about yours. Something shaped like: this subject is about X; a grey print would have said Y about it; the blue says Z instead; and the cost is W.

The cost clause is what makes it an argument, because every choice in printing gives something up and a paragraph that names nothing lost has not made a decision. Then test it: read it with your six botanicals in front of you and ask whether it describes the prints on the table or the prints you meant to make.

Sun printing, and the record that makes it repeatable

Sunlight is the alternative source for this whole part, and for two of these four projects it is the preferred one. Judging the exposure is a skill rather than a number, and here is what it consists of.

Judge by inspection, through a split-back frame. Cyanotype is a printing-out process: the image forms visibly during the exposure, so the frame can be opened at a hinge, half the print lifted while the other half stays registered, and the progress read directly. The colour sequence is the one the coating lab set out — yellow-green through grey-blue to a bronze-grey in the deepest shadows — and Jacquard’s signal for cloth is the same word, bronze.

The variability is real and no source will estimate it for you. Ware’s figure of 30 to 40 W/m² is for average sunlight; what reaches your print varies with latitude, season, hour, cloud and the direction the frame faces, and Jacquard’s own published range for sunlight is 1 to 30 minutes — a factor of thirty, which is a manufacturer correctly declining to give a number. No published guidance read for this course lets you predict a sunlight exposure from conditions. The honest answer is a test strip every session, and it is not a lesser answer: a test strip measures the sky you are actually printing under, which no forecast does.

Two geometric facts to work with rather than around. Direct sun has an aspect value of 0.01, the sharpest contact-printing source there is — but it moves through its own diameter in about two minutes, so a long exposure is made by a source that is not in one place, and a frame propped to catch the morning sun is at a different angle to it an hour later. The north summer sky is the diffuse alternative: no shadow problems, an aspect value like a light bed, and about three stops weaker. Two different lights, two different pictures from the same specimen.

The record is what makes a sun exposure repeatable, and there is nothing else. Date, clock time, sky, the frame’s orientation and tilt, elapsed time, and the print’s appearance at each inspection, in the daylight exposure SOP and the exposure log. A session recorded that way can be repeated to within the weather; one recorded as “about twenty minutes, sunny” cannot be repeated at all.

Sequencing and presentation

Six prints, a large one, a cloth and a paper-negative print have to be edited, ordered and housed, and the last of those is where a cyanotype differs from everything else in the course.

Editing. Cut before you order. Six prints that all work beat eight of which two are weak: a weak print tells the viewer the set is unedited, and they then look at the strong ones with less trust.

Order. A sequence has a first and a last print and the space between them is the argument. Two orders work most often: the one holding everything constant except the single declared variable, so the set reads as a series, and the one running from most legible to least, so the viewer learns the vocabulary before it is complicated. Say which, in the record.

Mounting, where the conservation advice you have been given elsewhere reverses.

!Why the standard mount board is the wrong board here

This is the single most consequential practical fact in this part, and it is worth the space.

The problem. Conservation-grade board and paper are buffered: they carry an alkaline reserve of calcium carbonate to neutralise acid and protect cellulose. ISO 9706 for permanent paper requires an alkali reserve corresponding to at least 2 per cent w/w calcium carbonate and a pH between 7.5 and 10.0. A saturated calcium carbonate solution sits at about pH 9.4 — the pH at which Prussian blue is destroyed in one to ten minutes.

The evidence that this is not theoretical. Ware coated processed cyanotypes of all five sensitisers with a paste of calcium carbonate and distilled water, left a control area on each, and held them above 95 per cent relative humidity. Within 48 hours the coated areas had lost 0.40 to 0.60 in density against their own controls. And the Image Permanence Institute — Held, Burge and Nishimura — contacted cyanotype test prints with papers at pH 3.1, 6.4, 9.3 and 10.3 and aged them at 70 °C and 86 per cent relative humidity for 15 days. All specimens in contact with alkaline and even neutral papers were seriously faded. Only acidic paper almost fully protected the images. Their conclusion is quoted here because its honesty matters: alkaline enclosures should not be used; the data also precludes recommending neutral pH enclosures, as they too caused significant damage; and acidic enclosures cannot be recommended either, because they may deteriorate the paper substrate.

Where the standards sit. ISO 18902 recommends buffered papers for the storage of all photographic materials, and Ware notes that cyanotypes are neither discussed nor excluded in it. The Library of Congress are the exception and they are explicit: buffered paper at pH 7 to 9.5 for acetate and nitrate film, platinum, silver and chromogenic prints — and unbuffered paper, tending to pH 6 to 7, for cyanotypes and architectural drawings. That is the sentence to take to a supplier, along with IPI’s warning that acid-free, archival, museum-quality and conservation board are marketing terms which are neither standardised nor legal. Ask for board and enclosures that state unbuffered, expect a photographic retailer not to stock them, and note that Ware’s preferred enclosure is a non-buffered archival wrapping paper in a traditional four-flap folder.

And no adhesive on the back of the picture area. Many adhesives are alkaline and hydroxide migrates through plain paper. Hinge or corner-mount from the margins, as Part XIX already teaches for a different reason.

=Let it breathe, and do not seal it

The second unusual instruction, and it runs against ordinary photographic practice.

A cyanotype that has faded in light regains density in the dark by aerial oxidation, and it needs air to do it. Ware calculates that less than one cubic centimetre of air suffices to re-oxidise a typical 10 × 8 inch cyanotype — tiny, but not zero — and he demonstrates that cyanotypes stored in anoxic housings with an oxygen scavenger are very susceptible to light-fading, the exact opposite of usual conservation practice.

So archival polyester sleeves are not the right enclosure, because they admit light but not air; a relatively porous, fairly opaque, unbuffered wrapping paper is better on both counts. For a framed print, use a deep window mat so there is a reservoir of air behind the glass, and do not seal the package. Ware’s own summary is three words: let your cyanotypes breathe.

For display the general guidance holds, and is worth repeating because the pigment is light-sensitive: the Library of Congress recommend against permanent display, put exhibition light levels at 30 to 100 lux, and cap ultraviolet at 75 microwatts per lumen.

The cloth is not mounted. It is the one piece here that is meant to be handled, and the honest way to present it is as cloth — hung, draped or folded, with its wash-test data sheet beside it. If it is going to be mounted behind glass, it should have been a print.

The record required

For every piece submitted, on one sheet:

Field	Note
Sensitiser	formulary page, batch number, date mixed, and why that formulation
Substrate	paper make and weight, or cloth fibre, weave and weight; scoured or not
Preparation	coating method, and the measured loading — cm³ per sheet, or g/m² for cloth
Light and dose	unit, distance and minutes; or date, time, sky, orientation and elapsed time
Processing	first bath, wash regime and end point, peroxide or air, drying
Result	your own reading of maximum density, plus one sentence of what you would change

The lab notebook and the exposure log carry most of those fields; the loading figure and the wash-test table are what this page adds.

What to submit

• The large print, dry and flat, with sheet size and coating method pencilled in the margin.
• The botanical sequence, six or more prints in a stated order, with the declared variable named.
• The paper-negative print, with the paper negative itself, and its transmission loss and step count.
• The cloth, plus the untouched control swatch and the three treated swatches.
• The record above, the wash-fastness data sheet, and the paragraph on the blue.

One row of the process comparison atlas comes out of this if the standard negative appears anywhere in the four projects. If it does not, print it once more before you leave the part.

Recap

Scaling a process is a diagnostic. The rod bows at 400 mm, so the large print is a brush print and costs half as much again in sensitiser. The tray stops working, so the wash becomes a bath and the water tens of litres. The specimen stops being flat, so the source’s aspect value decides the line. The substrate stops being paper, so the sensitiser choice becomes a mordanting question that Mercer settled in 1847 and Wilson confirmed in modern practice, both pointing at the oxalate.

Two things do not scale at all. Alkali destroys this image — in the laundry at pH 10.7 in under half a minute, in a buffered mount at pH 9.4 in one to ten minutes, and in a neutral enclosure slowly enough that a fifteen-day accelerated test was needed to see it. And the image needs air, which is why the enclosure that protects everything else in your portfolio is the wrong one here.

Atkins made five thousand of these with pressed seaweed, well water and sunlight, and they are unaltered. The chemistry has not changed. What you are adding is the record.

Check your understanding

Question 1. You are printing a dried but unpressed fern stem, about 3 mm thick, and you want an edge that reads as sharp. Your options are direct sun (aspect value 0.01) or a bank of UVA tubes 150 mm above the frame whose array is 600 mm across. Which do you choose, and what is the arithmetic?

• The tubes, because a larger source gives more even illumination and evenness is what sharpness depends on
• The sun: blur is gap times aspect value, so the tubes at an aspect value of 4 give 12 mm of blur against the sun’s 0.03 mm, and 0.3 mm is already the onset of a visibly soft image
• Either, provided the glass is weighted hard enough to close the gap, since pressure and not geometry sets the line
• The tubes, and then compensate by shortening the exposure so that the penumbra does not have time to record

Show the answer and why

Answer: The sun: blur is gap times aspect value, so the tubes at an aspect value of 4 give 12 mm of blur against the sun’s 0.03 mm, and 0.3 mm is already the onset of a visibly soft image

The aspect value is the source’s largest dimension divided by its distance from the print: 600 divided by 150 is 4, which is exactly the figure Ware tabulates for a light bed or open sky. Multiply by the 3 mm gap and the penumbra is 12 mm wide — forty times the 0.3 mm at which the eye starts calling an image soft. The sun subtends about half a degree, giving an aspect value of 0.01 and 0.03 mm of blur for the same specimen, which is below what the eye resolves at its near point. The third option is the interesting wrong answer: pressure can close a gap only down to the thickness of the object, and a 3 mm stem holds the glass 3 mm off the paper no matter how hard you clamp it, tenting the sheet over a wide area as well. The fourth misunderstands what a penumbra is — it is a spatial distribution of illuminance, not a time-dependent effect, so it scales with the exposure rather than being outrun by it.

Question 2. A friend asks you to make cyanotype cushion covers they can put through a normal wash. What do you tell them, and on what evidence?

• That it will be fine if they use a non-phosphate detergent, since that is the maker’s published instruction
• That you cannot support it: the maker’s own comparison says cyanotype is not as washable as their dye product, laundry alkali hydrolyses Prussian blue to a yellow-brown iron oxide-hydroxide that binds to the fibre, and a 0.25 molar sodium carbonate solution destroys the image in under half a minute
• That machine washing is fine as long as the print is re-oxidised in hydrogen peroxide afterwards, which restores the density each time
• That it depends entirely on the fibre, and cotton will survive a wash where silk will not

Show the answer and why

Answer: That you cannot support it: the maker’s own comparison says cyanotype is not as washable as their dye product, laundry alkali hydrolyses Prussian blue to a yellow-brown iron oxide-hydroxide that binds to the fibre, and a 0.25 molar sodium carbonate solution destroys the image in under half a minute

Jacquard do publish the non-phosphate instruction, but they publish it alongside "keep washing to a minimum if possible", "over-washing may also cause the print to fade" and, in their comparison document, the flat statement that cyanotype is not as washable and permanent as their dye product on fabric. That is a manufacturer hedging, and reading only the first half of their advice is how the folklore claim gets made. The chemistry says why: detergents are alkaline because alkali saponifies fat, and Prussian blue is hydrolysed irreversibly by hydroxide to iron(III) oxide-hydroxide, which is yellow-brown and binds strongly to cellulose — so the failure is a stain rather than a fade. Holtzman’s figures put complete decolourisation at pH 9.4 in one to ten minutes and, in 0.25 molar sodium carbonate at pH 10.7, in under thirty seconds. The peroxide answer confuses two different mechanisms: peroxide re-oxidises Prussian white, the reduced form, and does nothing for a molecule that has been hydrolysed and half washed away. The fibre answer is a real variable but not the governing one; the alkali reaches the pigment whatever is holding it.

Question 3. You have hinged your best botanical print onto four-ply board whose packet states that it meets ISO 18902, sealed it behind glass in a shallow frame, and hung it in a lit hallway. Name the two independent mistakes.

• The frame is too shallow for the print to lie flat, and the hallway light will fade the paper base
• The board carries an alkaline reserve of at least 2 per cent calcium carbonate, which hydrolyses Prussian blue, and the sealed package denies the print the air it needs to regain density in the dark
• ISO 18902 board is unbuffered and therefore too acidic for the paper, and the glass will trap moisture against the print
• The hinges should have been adhesive across the whole back for support, and the frame should have been sealed more thoroughly to exclude pollutants

Show the answer and why

Answer: The board carries an alkaline reserve of at least 2 per cent calcium carbonate, which hydrolyses Prussian blue, and the sealed package denies the print the air it needs to regain density in the dark

Both mistakes come from applying good general photographic practice to the one process it does not fit. ISO 18902 asks for buffering, and the related ISO 9706 puts the alkali reserve at at least 2 per cent calcium carbonate; a saturated calcium carbonate solution is about pH 9.4, which is the pH at which Holtzman found Prussian blue completely decolourised in one to ten minutes. Ware’s own paste test lost 0.40 to 0.60 in density in 48 hours at high humidity, and IPI’s accelerated test found serious fading even against neutral papers. The second mistake is subtler and runs against instinct: a cyanotype that has faded in light regains density in the dark by aerial oxidation, needing less than one cubic centimetre of air for a 10 by 8 print, and Ware demonstrated that prints in anoxic housings are unusually susceptible to light-fading. So a sealed package is actively harmful here, which is why the recommendation is a deep window mat and an unsealed frame. Option four gets both halves exactly backwards, which is the point: adhesive on the verso within the picture area is a third mistake, since many adhesives are alkaline and hydroxide migrates through plain paper.

Question 4. Your paper negative gives beautiful soft prints but needs eight times the exposure of a film negative of the same subject. Before changing anything, what is the measurement that tells you where the eight times went?

• Read the paper negative on the Part XV densitometer and compare its density range with the film negative’s
• Contact-print a step tablet directly, and again through a blank processed sheet of the same paper, and compare the exposures required and the number of steps rendered
• Expose a test strip at eight different times and pick the one that matches the film negative’s result
• Measure the reflection density of the paper negative’s darkest and lightest areas and subtract

Show the answer and why

Answer: Contact-print a step tablet directly, and again through a blank processed sheet of the same paper, and compare the exposures required and the number of steps rendered

The blank sheet separates the two things that are confounded in the finished negative. Base transmission loss is a flat tax that costs exposure and nothing else; density range and fibre scattering change what the print looks like. Printing a step tablet through a blank, processed, unexposed sheet isolates the first — the shift in the exposure needed is the base loss in stops — while the change in the number of steps you can still tell apart measures what the sheet’s structure does to the scale. The first option fails for two reasons the negatives lesson already establishes: the Part XV instrument is a transmission densitometer built for film, and ultraviolet density is not visual density, which matters most for a paper base that may carry fluorescent brighteners. The third option finds a working exposure without explaining anything, which is what you would do next rather than now. The fourth measures the wrong quantity entirely: you are printing through this sheet, so its transmission is what matters and its reflection density is not related to it in any usable way.

Question 5. You are choosing a sensitiser for a silk scarf. Which argument decides it, and what is its status?

• The classic citrate sensitiser, because it is the cheapest and silk is expensive enough already
• An oxalate sensitiser, because Mercer built his 1847 cloth process on ferric oxalate having shown it to be a good mordant for wool and cotton while the citrate is not absorbed by the fibres, and Wilson reports the same in modern practice on cotton and silk — two independent observations, not a controlled trial
• An oxalate sensitiser, because its longer exposure scale is what silk requires
• Either, since the sensitiser only has to dry on the surface and the fibre plays no chemical part

Show the answer and why

Answer: An oxalate sensitiser, because Mercer built his 1847 cloth process on ferric oxalate having shown it to be a good mordant for wool and cotton while the citrate is not absorbed by the fibres, and Wilson reports the same in modern practice on cotton and silk — two independent observations, not a controlled trial

The evidence is real and its status should be stated precisely, which is the point of the question. Mercer chose ferric oxalate deliberately for textile work and Ware gives his reason: he had already introduced it into his own dyeworks as a mordant for wool and cotton, whereas the citrate is ineffective in not being absorbed by the cellulose fibres. Wendy Wilson, working today, finds that mordanting to cotton and silk is better with ferrioxalate than with ferricitrate. Two independent observations 170 years apart pointing the same way is strong practitioner evidence, and it is not a measurement — nobody read for this course has run the controlled comparison, which is precisely why the page suggests you run it on swatches. The exposure-scale answer is true of the oxalate sensitisers but irrelevant here, because scale is a property of the negative-to-process match and not of the substrate. And the last option contradicts the central fact of the whole part: there is no binder, so the fibre is not a passive support but the thing that has to hold the iron and then the pigment.

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