Experiment: The Image That Would Not Stay

In 1802 Thomas Wedgwood and Humphry Davy published an account of images made on paper and leather soaked in silver nitrate. It worked. Shadows came out white against a darkening ground, in two or three minutes of direct sun. And then, in the same paper, they wrote the sentence that stopped photography for a generation: no attempt to prevent the uncoloured parts from being acted on by light “as yet” had succeeded, and the picture could only be examined in the shade, for a few minutes, or by candlelight. You are about to repeat that, with better paper and the same result.

Purpose

To produce a photographic image using no chemistry whatever, and then to watch it destroy itself, so that the word fixing means something physical before Part XI explains it.

Hypothesis. Commercial silver gelatin paper, given enough daylight, will form a visible image without a developer; and because nothing has removed the silver halide that light did not act on, that image will keep changing in room light and will not change in the dark.

The control. A twin sheet, made from the same packet, carrying the same objects, exposed for the same time in the same light, and then sealed in an opaque envelope from the moment the exposure ends. Every part of its history is identical to its twin’s except the one thing being tested.

The single variable. For the permanence test, the only variable is exposure to room light after the exposure. Two further tests on this page each change exactly one thing as well: in the exposure series, only the time changes; in the spectral test, only the colour of the filter changes.

!Important

The course has not yet published its own measured timings for this page. The exposure brackets below are a starting range, argued from published paper speeds, not a result - and the numbers you write down are the data. If your first bracket lands entirely dark or entirely blank, that is not a failure; it is the first result, and it tells you which way to move the bracket.

Learning objectives

By the end of this page you should be able to:

• describe what a printing-out image is and how it differs from a developed one, in terms of who does the work and what you see while it happens;
• predict, before you look, the order in which red, green, blue and unfiltered light will darken a silver gelatin paper, and say why in terms of photon energy;
• state what a fixing bath would have to remove, and what it would cost the image;
• design and read a stepped exposure series, and explain why visual ranking is a weak measurement;
• keep a dated observational log that another person could use to repeat what you did.

Prerequisites

Silver salts and light is assumed: photolysis, printing-out versus developing-out, and why violet acts and red does not. The manifest also places Wedgwood, Davy and the permanence problem before this page. No chemistry, darkroom or camera is needed.

Safety classification

Level A. The criteria of the classification rubric that decided it:

• Substances and quantities. No substance is weighed, dissolved, diluted or poured on this page. The only material is a finished, coated commercial paper, handled dry and kept dry.
• Energy. Nothing is heated, there is no flame, no solvent and no ultraviolet lamp. The only energy source is sunlight, outdoors, which is a real hazard and is controlled below.
• Electrical. Nothing is built, wired or plugged in.
• Waste. No liquid waste is produced at all. The output is four sheets of dry paper, which you keep.

The one hazard that the Level A criteria do not name explicitly is solar ultraviolet, because most darkroom work happens indoors. It is named here instead.

Hazards

Solar ultraviolet, on skin and eyes. This experiment asks you to stand outside for up to an hour in the strongest sun you can find, which is precisely the exposure ICNIRP’s guidance for outdoor workers is written about. Its points that matter here: the four-hour period around midday carries the greatest risk; sunburn occurs on cloudy days as well as clear ones, and breaks in cloud can push the ultraviolet level to or above the clear-sky value; and a useful field test is that when your shadow is shorter than you are, the ultraviolet is strong. Do not look at the sun, with or without a filter, at any point in this procedure - nothing here requires it.

Cuts. Masks, stencils and cut-down sheets mean scissors or a craft knife. Cut on a mat, away from your body, with the blade retracted whenever you put the knife down.

Glass. A sheet of picture glass holds the objects flat. Tape all four edges before you start, carry it flat, and do not lean it against anything.

What is not a hazard on this page, and why. No solution is handled, so there is nothing to splash, inhale or swallow, and no waste stream that needs neutralising. That is a statement about this specific procedure, not about photographic materials in general: silver bromide and silver chloride each carry aggregated ECHA classifications on PubChem as very toxic to aquatic life, and the silver nitrate you meet in Part II is classified as causing severe skin burns and eye damage. Locked into a hardened gelatin layer on a plastic-coated sheet, they are not something you handle as a substance. Loose in a bottle, they are.

Required PPE

The protective equipment for this page is sun protection, not chemical protection.

• A wide-brimmed hat and sunglasses that block ultraviolet. ICNIRP gives the ocular exposure ratio as about 0.5 for sunglasses alone and about 0.02 for sunglasses worn with a brimmed hat, so the combination is worth far more than either.
• Long sleeves; ICNIRP records that most summer clothing gives a protection factor above 10.
• Sunscreen on what is left uncovered, reapplied, and shade between exposures.
• Nothing about the paper requires gloves. Clean, dry hands are what it needs, for the reason in the next section.

Nitrile gloves and splash goggles arrive in Part II with the first solution, and stay for the rest of the course.

Ventilation

Nothing on this page produces a vapour, a mist, a dust or an aerosol, so ventilation is not one of the controls here. You will work outdoors for the exposures because that is where the light is, and indoors, in dim light, to load and unload the frames.

Materials

Item	Quantity	Notes
Resin-coated variable-contrast black-and-white paper, glossy	4 sheets, about 13 × 18 cm	Cut from larger sheets in dim light. Any grade or surface works; glossy shows subtle tone best.
Picture-frame glass or a clean sheet of window glass	1, slightly larger than the paper	Edges taped. A clip frame is ideal.
Rigid board	2, larger than the glass	Backing for the sandwich; hardboard or thick mounting card.
Objects with varied opacity	a handful	A leaf, lace or net, a coin, a key, a feather, a paperclip, a piece of tracing paper.
Opaque card	2 sheets	One for carrying boards face-down, one cut into the sliding mask.
Opaque envelope, light-tight	1	The kind photographic paper is sold in is ideal; otherwise doubled black card taped shut.
Coloured lighting gel or coloured acetate: red, green, blue	1 small piece of each	Swatch books from a stage-lighting supplier are cheap. See the caution under Procedure.
Masking tape, pencil, timer	—	Pencil, not pen: ink will not survive the storage envelope.

Chemicals

None is handled. No substance is weighed, opened, dissolved or poured on this page, and there is no chemical inventory to keep. For completeness, this is what is present in the material and stays there:

Substance	Where it is	Handled as a substance?
Silver halide (silver bromide, silver chloride, or both)	Microscopic crystals suspended in the gelatin emulsion layer	No
Gelatin	The binder holding the crystals, plus a hardened supercoat over it	No

The manufacturer’s technical sheet does not state which halides its emulsion contains, and this course does not guess. Both are named above because the behaviour you will see is consistent with either, and because their identities are worth knowing: silver chloride is a white solid darkened by light, silver bromide a yellowish one, and both are recorded on PubChem with aggregated ECHA classifications including “very toxic to aquatic life”.

Equipment

Scissors or a craft knife and a cutting mat; a steel rule; a timer or a phone clock you can read in sunlight; a notebook and pencil; a table or paving slab you can put the boards on flat; and somewhere dim to load and unload them. A darkroom is not needed. A room with the curtains drawn and one low lamp well away from the work is enough, and a changing bag is better if you have one.

Estimated cost

Cost band £: one packet of paper, a piece of glass you may already own, and a gel swatch. Paper is the only recurring cost, and this experiment uses four sheets. Dated price ranges live in the laboratory planner rather than in the lesson text, so that they can be updated without rewriting the page.

Estimated consumables cost

One run consumes four sheets of paper and nothing else. The glass, the boards, the objects and the envelope survive the session, and no chemistry is handled at all, so this is the smallest consumables figure in the course.

Consumed	This session	Sourced price	Cost this session
Variable-contrast RC paper, 5 × 7 in	4 sheets, about 13 × 18 cm	£16.06–£44.71 per 25 to 100 sheets, 5 x 7 in, variable contrast RC (£0.45–£0.64 a sheet)	£1.79–£2.57
Coloured lighting gel: red, green, blue	one swatch of each; swatch books are often free	Free from a stage-lighting supplier	—
Pencil and record sheet	one page	None. notebook carries a cost band and no dated figure	—

The priced rows come to £1.79 to £2.57 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 1 of the 3 rows carries no dated price, so it is counted as nothing here and is certainly not free. A priced entry is a dated range to plan against, never a quotation.

Nothing else is used up. The paper figure is per run, and the exposure comparison in Further experiments doubles it.

Waste streams

None that is liquid, and none that is hazardous under the Level A criteria. The output is four sheets of dry, exposed paper, all of which you keep - two of them for months, because Part XI comes back for them. Offcuts and any spoiled sheets are dry paper carrying a small quantity of silver halide in gelatin. See Disposal considerations below.

Preparation

1. Pick the day and the hour. Direct sun makes this experiment quick and the differences obvious. Bright overcast works and takes longer. Check the ultraviolet index forecast for your area and read the Hazards section again before you go out around midday.
2. Write the log before you expose anything. Rule up the tables from Data to record in your notebook. A result you did not write down is a result you will have to obtain again.
3. Cut and label the paper in dim light. Draw the curtains. Open the packet, take out only what you need, close the packet, and return it to its black bag before you do anything else. Cut four sheets of roughly 13 × 18 cm. On the back of each, in pencil, write A, B, C or D and the date. The emulsion side is the glossier, slightly curled-inward side.
4. Handle it as a photograph, not as paper. The Library of Congress’s handling guidance for photographic collections is freshly washed hands, no touching of the image surface, and holding by the edges. Do that here: your fingers carry moisture and grease, and this sheet will never be washed.
5. Build the sandwiches indoors, in the dim light. For each: board, then paper emulsion side up, then the objects, then the glass, and tape or clips at the corners. Put the opaque card over each finished sandwich and leave it there.

🛠Build it

Sheet A and sheet B are twins and must be built to match: the same objects in the same places, or as close as you can manage. If your objects cannot be duplicated, use two halves of the same leaf and say so in the log. The whole permanence argument rests on the two sheets differing in one thing only.

Procedure

Stage 1: the twin photograms, A and B

1. Carry boards A and B outside, still covered.
2. Note the time, the date, the sky (clear, hazy, broken cloud, overcast) and whether your shadow is shorter or longer than you are.
3. Lift both cards together and start the timer.
4. Watch. Note the time at which you can first see any tone at all outside the objects, and then the time at which the open ground stops visibly changing between one check and the next.
5. After your chosen exposure - see Stage 2 for how to choose it - re-cover both boards and carry them indoors to the dim room.
6. Unload both. Put B straight into the opaque envelope, seal it, and write on the outside: UNFIXED PRINT - DO NOT OPEN IN LIGHT, with the date and time. Put A somewhere in ordinary room light where you will see it every day, out of direct sun.

Stage 2: the exposure series, C

7. Cut a piece of opaque card slightly larger than sheet C and mark it in seven equal bands.

8. Build the sandwich for C with no objects, or with one small flat object, and lay the mask over the glass so that it covers all seven bands.

9. Outside, start the timer and slide the mask back by one band at the times below, so that band 1 receives the total and band 7 the last interval only.

   Starting bracket, direct summer sun: cumulative 32, 16, 8, 4, 2, 1 minutes and 30 seconds. Starting bracket, bright overcast or winter sun: multiply every figure by four and re-judge.

   This bracket is an estimate, not a measurement. It is argued from two published facts: ILFORD gives Multigrade RC papers an approximate equivalent film speed of ISO 3 to 6, which is slow but not nearly as slow as Wedgwood’s silver-nitrate paper, and Wedgwood and Davy reported that theirs reached its full effect in direct sun in two or three minutes. Printing out, however, needs roughly a million times the exposure that a developed image needs, so minutes to tens of minutes is the right order of magnitude to start from. Prove it or move it.

10. Record the seven cumulative times, and rank the bands by eye from lightest to darkest.

Stage 3: the spectral test, D

11. Cut the red, green and blue gels into strips that cover about a fifth of sheet D each. Leave one fifth open as the control, and cover the last fifth with a piece of ordinary window glass.
12. Build the sandwich with the strips laid directly on the paper under the cover glass. Note which strip is where, on the back of the sheet, in pencil.
13. Expose for a time from Stage 2 that gave a clear mid-tone on the open ground - not the maximum. A fully black control tells you nothing about the filters.
14. Record the order in which the five areas darken, and their final ranking.

✕A coloured gel is not a filter with a clean edge

Lighting gels are made to colour a stage light, not to define a passband, and few makers publish a transmission curve. A “red” gel may pass appreciable deep blue, and almost all of them pass ultraviolet. So this stage gives you an ordering, not a spectral sensitivity curve, and a strip that darkens under red tells you about that gel as much as about the paper. Record the maker and the swatch number in your log; without them your result is not repeatable.

Stage 4: the week

15. Every day for seven days, look at sheet A in the room where it lives and record what you see: the tone of the ground, the tone of the shadows, whether the edges of the shapes are still sharp, and the overall colour. A sketch with a pencil-shaded tone scale beats an adjective. If you photograph it, use the same light, the same distance and the same camera settings each day, and say so.
16. Do not open envelope B until day 7. Every inspection costs it exposure. If you cannot resist, allow yourself one look on day 3, in the dimmest light you can read by, for under a minute, and write down that you did it - it is now part of B’s history.
17. On day 7, open B in the dim room, put it beside A, and record the comparison in the same terms you have been using all week. Then re-seal B, with the date, and keep it. Part XI wants it.

Expected observations

Stated as predictions, because the course has not published its own log for this page. Where a prediction has a source behind it, the source is named; where it does not, it is marked as a prediction to be tested.

• During exposure. A visible tone in the open areas, appearing without any bath. This is what distinguishes a printing-out image, and conservators define printing-out papers in exactly those words: the image appears gradually and visibly during exposure. (Sourced.)
• Colour. Something in the range of dull violet, grey-violet or lavender-brown rather than neutral black. Ware records that a silver halide holding excess halide - which is the condition of every developing-out emulsion - prints out to dull violet for the chloride and pale grey for the bromide. (Sourced.)
• A ceiling. The ground darkening quickly at first and then slowing to a near-stop. Senebier saw this in 1782, and the mechanism is in What is happening chemically below. (Sourced for the observation; the timescale on your paper is a prediction.)
• Speed. First tone in direct sun within a minute or two; a full-looking image in tens of minutes. (Prediction, from the paper speed and Wedgwood’s figures. Test it.)
• The filters. Blue and the open control fastest, green slower, red slowest by a long way, if at all. (Prediction from photon energy; the direction is well founded, the ratios are not.)
• Window glass. A small slowing, not an abolition. ICNIRP records that window glass transmits some radiation down to about 310 nm, so most of the near-ultraviolet and all of the violet and blue get through. (Sourced for the transmission; “small” is the prediction.)
• The week. Sheet A darkening overall, its highlights closing up on its shadows, and the image becoming harder to read as the difference between the two shrinks. Sheet B, in the envelope, close to how it looked on day 0. (Prediction. This is the result the page exists for.)

What is happening chemically

Three things are going on at once, and they explain everything you will see.

Light is doing the whole job, badly. A photon absorbed in a silver halide crystal frees an electron; the electron is trapped and reduces a silver ion to a silver atom; the hole left behind travels to the surface and takes an electron back from a halide ion, leaving a halogen atom. Four silver atoms are enough to make a development centre that a developer could find and amplify. But there is no developer here, so the specks have to grow until they are big enough to see, which takes something like a million times the exposure. That factor of a million is the whole reason this experiment is measured in minutes and a darkroom print is measured in seconds.

2 AgCl + light → 2 Ag + Cl₂

Net photolysis, written for the chloride

The halogen has nowhere good to go, which is why the paper is slow. A printing-out paper of the 1890s was made with an excess of free silver nitrate, and Kodak’s 1928 primer is explicit that this is what caused its rapid darkening in light. A developing-out paper, the kind in your packet, is the opposite by design: it contains no free silver, and is made with an excess of halide so that it does not fog in the developer. Ware describes exactly this condition. The excess negative charge on the crystal surfaces repels the photoelectrons, so silver forms in the interior rather than at the surface; and the liberated halogen, with no free silver ions to help water carry it away, hangs about and re-oxidises the photolytic silver. Photolysis and its own back-reaction fight to a standstill.

That is the ceiling. It also explains the colour: with the silver confined inside the crystals in very small particles, you get the dull violet or pale grey Ware describes rather than a rich brown.

🔬Why the colour is not black

Print-out silver is 10 to 100 nm across; developed silver is micron-sized filaments. The Getty atlas states the size difference directly, and Ware attributes the colour to surface plasmon resonance - conduction electrons oscillating collectively in particles smaller than the wavelength of light. Small particles absorb selectively and look coloured; large filamentary ones absorb everything and look black. Your photogram is warm or violet because of a length in nanometres.

Nothing has been removed, so the exposure never really ends. Every part of sheet A that light did not reach is still loaded with unchanged silver halide. Room light is weak compared with sunlight, but it is not zero, and the same photolysis proceeds at a crawl in your living room. The shadows darken, the ground darkens further, and the difference between them - which is the image - shrinks. Sheet B, in the envelope, receives no photons and does not change.

⚗What a fixing bath would have had to do

Fixing dissolves and washes away the silver halide that light did not act on, leaving only the image silver. Scheele did it with ammonia in 1777 without noticing; Herschel published the modern answer, thiosulfate, in 1819 and applied it to photographs in 1839; Part XI is where this course teaches it.

There is a cost, and this page states it without doing it. A print-out image loses density when it is processed: the Getty atlas notes that printing-out papers require deliberate overprinting because the image intensity falls during processing. Some of the image silver is very small and some of the apparent density comes from silver in the halide crystals themselves, and both are attacked. So a photogram like yours, dropped in a fixing bath, would come out weaker and a different colour. It would also come out permanent. The course does not fix these two sheets, because the point of them is what happens when you cannot.

Resin-coated paper in cross-section

1. SupercoatHardened gelatin, a few micrometres thickprotects the image
2. EmulsionSilver halide crystals suspended in gelatinthe only light-sensitive layer
3. Pigmented polyethyleneExtruded PE filled with titanium dioxidethe white you see
4. Paper baseSealed on both sides by extruded polyethylenewhy it dries in minutes
5. Matte polyethyleneReverse side, made to take pencillabel here

Layer structure as described by the Getty Conservation Institute's atlas. Only the third layer is photographic; everything else is there to hold it flat, white and undamaged.

Data to record

Three tables and a log. Rule them up before you go outside.

Table 1. Conditions. One row per exposure session.

Sheet	Date	Start time	Sky	Shadow shorter or longer than you	Air temperature	Notes
A and B
C
D

Table 2. Exposure series, sheet C.

Band	Cumulative exposure	Time first tone seen	Tone rank, 1 = lightest	Description
1	32 min
2	16 min
3	8 min
4	4 min
5	2 min
6	1 min
7	30 s

Table 3. Spectral test, sheet D. Record the gel maker and swatch number in the notes column, or the result cannot be repeated.

Area	Time first tone seen	Final tone rank, 1 = lightest	Colour of the tone	Notes
Open control
Blue gel
Green gel
Red gel
Window glass

Table 4. The seven-day log. One row per day, for sheet A; the last row also for sheet B.

Day	Date and time	Ground tone, 0 to 5	Shadow tone, 0 to 5	Difference	Edges still sharp?	Colour	Where it has been
0
1 to 6
7 (A)
7 (B)

Score tone 0 for paper white and 5 for the darkest tone the paper reached on day 0, and keep the scale physically in front of you: shade a strip of card in six steps on day 0 and compare against that card every day. The Difference column - ground minus shadow - is the number that matters, because it is the image.

📓Lab notebook

Visual ranking is a weak measurement and you should treat it as one. Your eye adapts, judges relative rather than absolute tone, and is influenced by the surrounding paper. It cannot tell you that band 3 is twice as dark as band 5. What it can do reliably is order things, which is why every table above asks for a rank rather than a value. Part XV builds a densitometer, and the first thing it will do is show you how far off your rankings were.

Analysis

Work through these with your own tables in front of you.

1. Plot difference against day for sheet A, from Table 4. Is the loss steady, or fast at first and then slower? What would each shape imply about how much unexposed halide is left?
2. Compare A and B on day 7. State, in two sentences, what a fixing bath would have had to remove from B to make it behave like a finished print, and what it would have cost the image.
3. Read the exposure series. Between which two bands is the difference largest? A material whose tone changes most between 2 and 4 minutes and least between 16 and 32 has a shape to its response, and that shape is the characteristic curve Part XIII measures properly.
4. Does doubling the time double the effect? The reciprocity law, named after Bunsen and Roscoe from their photochemical investigations of the 1850s, says that equal products of intensity and time give equal blackening. Schwarzschild measured its failure on gelatine plates in 1900, and found he needed roughly twice the total light at a hundredth of the intensity. Ware notes the same failure at the slow end of print-out processes: a very long exposure never quite compensates for a weak light. Your series doubles the time at constant intensity, which does not test the law directly - but if you repeat sheet C on a bright overcast day and compare, you have the beginning of a test. Say what you would have to control to make it a real one.
5. Interpret the filter result. Explain the ordering in terms of photon energy, then state two reasons why you cannot turn it into a spectral sensitivity curve.
6. Draw the safelight conclusion. Your red strip is the reason darkroom safelights are red or amber: Kodak’s table specifies a light-amber filter for contact and enlarging papers with a 15 W bulb no closer than 1.2 m, and ILFORD limits direct safelight illumination of Multigrade papers to four minutes at 1.2 m or more. Now read Kodak’s warning alongside it: the colour sensitivity of most emulsions does not end abruptly at a wavelength, most materials keep some sensitivity to the very colours a recommended filter transmits, and safelight exposure should therefore always be minimised. Write one sentence explaining why “the paper cannot see red” and “a red light is unconditionally safe for any paper” are different claims, and why only the first is defensible.

Troubleshooting

What you see	Most likely cause	What to do
The whole sheet is uniformly dark, including under the objects	The paper was exposed before the objects were laid on it - the room was brighter than you thought, or the packet was open too long	Redo in a darker room. Open the packet, remove one sheet, close it immediately. Test the room by leaving a sheet out for five minutes with a coin on it.
No tone at all after the longest exposure	Emulsion side down, or the sky is much weaker than you judged	Check the curl and the gloss; the emulsion side curls inwards. Re-expose with a four times longer bracket.
Tone appears but the shapes are soft and doubled	The objects were not in contact - the glass was not weighted, or the objects are thick	Weight the glass at the corners. Thick objects give a penumbra; that is physics, not a fault, and is worth recording.
Dark fingerprints on the finished sheet	The image surface was touched	Handle by the edges with freshly washed hands. Note it in the log rather than hiding it: it is a real result about what the emulsion responds to.
Sheet B has changed as much as sheet A	The envelope leaks, or it was opened more than the log says	Test the envelope with a fresh offcut and a coin for a day. Re-run the twins; the control is the experiment.
The bands of the exposure series all look the same	The bracket is entirely above or entirely below the useful range	Move the whole bracket by a factor of four in the direction indicated and repeat.
The green strip is darker than the blue strip	The gels are not what their names suggest, or the blue gel is much denser	Record it and say so. This is a result about your gels. Compare each gel against the open control rather than against each other.

Clean-up

Bring the glass in, untape it and store it flat. Collect the gels, the mask and the objects. Sweep up any offcuts of paper. Wash your hands - not because of a hazard from the paper, but because sunscreen, dust and grease on your fingers are the enemy of the next sheet. Put the paper packet back in its black bag, inside its box, in a cool dark place.

Storage

The two photograms. Sheet A lives in room light, because that is the experiment. Sheet B lives in the sealed opaque envelope, dated and labelled, and this course asks you to keep both for the rest of Level 1. They are the exhibits Part XI opens with.

Label them as unfixed. An unfixed print looks like a print. Write UNFIXED on the back of each in pencil and store them apart from any finished work, so that neither you nor anyone else mounts one in a frame and wonders in a year why it has gone.

The paper. ILFORD’s technical sheet is the authority for the product you bought; the general rule is cool, dry, dark, flat and in the original light-tight bag. Heat and damp fog paper slowly even in the dark, and a fogged sheet will spoil the next run of this experiment.

Disposal considerations

There is no liquid waste and no fixer on this page, which is a large part of why it is Level A.

The solid waste is dry paper offcuts and any spoiled sheets, carrying a small quantity of silver halide in gelatin on a polyethylene-coated base. ILFORD’s published guidance for domestic users in the United Kingdom states that small quantities of scrap film and paper should be treated as normal household waste, and that trade users should route them through a licensed waste operator, because silver recovery from film and paper is part of the treatment process. That is the manufacturer’s statement for one jurisdiction, not a universal rule: local regulation governs, and it differs between countries, regions and even water companies. Check your local regulations. Where a collection route for photographic material exists, use it, and keep the offcuts dry and separate so that it can.

Nothing on this page goes down a drain, because nothing on this page is a liquid.

Questions

1. Wedgwood and Davy wrote in 1802 that the image “may, indeed, be examined in the shade, but in this case the exposure should be only for a few minutes”, and that candlelight did not affect it perceptibly. Using your own log, say what they were describing and why candlelight was the safe choice, in terms of the spectrum of a candle flame and the response of a silver halide.
2. Before you exposed sheet D, you predicted an order for red, green, blue and the open control. Compare your prediction with your result and account for any difference. Which part of the difference is about the paper and which is about the gels?
3. Your window-glass strip darkened almost as fast as the open control. What does that tell you about how much of the paper’s response lies in the ultraviolet, and what would you need to measure to turn “almost as fast” into a number?
4. A friend proposes to keep the unfixed photogram permanently by sealing it in an airtight box with a sheet of dark card. Predict what will happen over a year and state the reasoning. What has been removed from the sheet, and what has not?
5. In one paragraph: what could Wedgwood have concluded in 1802 from the same observation you have just made, and what could he not have concluded, given that neither the mechanism of photolysis nor any solvent for silver halide was available to him?
6. Sheet C shows tone appearing between one exposure step and the next. Explain why “the paper needs four minutes” is not a statement about the paper alone, and list three things it is also a statement about.

Further experiments

• Solargraphy. An unfixed print-out image inside a pinhole camera, exposed for weeks or months, is the basis of solargraphy: the sun’s daily arcs record themselves on paper that is never developed, and the result is scanned rather than fixed because fixing would destroy it. Part VII returns to it, with the pinhole camera built in Part VI.
• The lumen print. The same material, the same absence of chemistry, but with fresh plant material laid on the paper and left for hours. Practitioners report colours the dry photogram does not give, and moisture from the plant is the usual explanation. The course has not tested this and states it as practice, not chemistry; it is a good candidate for your own controlled comparison, dry leaf against fresh leaf, one variable.
• A better tone scale. Repeat sheet C with a purchased calibrated step wedge in place of the sliding mask, so that every step is a known ratio rather than a time you slid a card at. Compare the two and say which errors the wedge removes.
• The fingerprint. Deliberately press a clean, dry thumb on one corner of an offcut, and a slightly damp one on another, expose alongside a clean control, and record the result. Then decide what you would need to do to find out why.
• The one you must not do yet. Do not substitute an ultraviolet lamp for the sun to speed this up. Ultraviolet A is invisible and damages skin and eyes with no sensation at the time. The course builds a UV exposure unit in Part XVI, inside an enclosure, with its own hazard assessment, and that is where lamps enter.