Experiment: Tonal Scale, Contrast and Image Colour in Palladium

Purpose

To measure the characteristic curve of the palladium process as you actually run it, and then to move one variable at a time until you can say which of them is worth using as a control.

The hypothesis. The exposure scale of a palladium print is longer than that of the silver gelatin paper measured in Part XIII, and of the four variables available to a printer — metal load, humidity, developer and an oxidiser dose — humidity moves the curve furthest.

The control. One coated sheet, at the coating volume and drop count established in the coating lab, dried at 55 per cent relative humidity, exposed at the dose established in the printing lab, developed in 28 per cent potassium oxalate at room temperature, cleared through the three-bath sequence and dried flat. Every other strip is compared against it and against nothing else.

The one variable that changes — in each condition, one only, from this list: metal per unit area, relative humidity at drying and exposure, developer identity, developer temperature, oxidiser dose. A fifth condition, print-out at high humidity, is a condition rather than a variable and is treated separately for the reason given under Preparation.

And a constraint the other experiments in this course do not have: every strip costs money. A one-variable-at-a-time design with a fresh sheet per condition would use eight sheets of palladium. The design below uses three, by borrowing Ware’s own laboratory trick.

!Two of these variables interact, and the design says so before the results

A one-variable-at-a-time design is only honest where the variables are separable, and two of these are not.

Humidity and developer are coupled through print-out. Ware’s own table records “Development”, in log exposure units, as a property of the humidity: for palladium it is 0.4 log units at 32 per cent relative humidity, 0.2 at 55 per cent and 0 at 80 per cent — that last meaning total print-out, with nothing left for a developer to do. So at a high enough humidity the developer stops being a variable at all, and comparing three developers at 80 per cent would be comparing three clearing baths.

Humidity and image colour are coupled to exposure length. Ware’s instruction for obtaining near-neutral tones at high humidity carries a condition attached: “provided that exposures are not too short in duration”. Colour therefore moves with two things at once in the humid conditions.

What this design does about it. The developer and oxidiser conditions are all run at the control humidity, so that the developer comparison is a developer comparison. The humidity conditions are all run in the control developer. Nothing is run at both extremes at once, and the interaction itself is tested by one deliberate cross — the print-out condition — rather than pretended away. Say this in your report before you say what you found, because a reader who does not know the variables interact will over-read your humidity result.

Learning objectives

By the end of this session you will be able to:

• Plot a measured characteristic curve for the palladium process as you run it, with the measurement method and its precision stated.
• State your own exposure scale and maximum density and compare them with the paper you measured in Part XIII, as a ratio rather than as an impression.
• Rank the four controls by how far each moves the curve, from your own data.
• Say what a variable does to the curve’s shape — whole-curve shift, slope change, shoulder only, or colour only — rather than reporting that it “changed the print”.
• Record image colour reproducibly by the course’s own stated method, and state what that method cannot do.
• Read a published platinum curve against your own palladium measurement without confusing the two.
• Cost an experiment before running it, in metal and, where the price file allows, in money.

Prerequisites

• Exposing, developing and clearing a palladium print, and one successfully cleared print from it. Do not measure a process you have not yet made work.
• Cyanotype exposure scale and contrast, which is the same experiment on a process that costs pence. If you have not run it, run it first.
• Experimental design for the darkroom, for control, replication and the discipline of recording a prediction before the result.
• The characteristic curve and film and developer characterisation, because this page measures both and re-teaches neither.

Safety classification

Level B, inherited entire from the two labs it depends on, with one addition and one clear statement about duration.

The hazards are those pages’: a concentrated oxalate developer, palladium salt solutions that are skin sensitisers and must never dry into dust, clearing baths, and an enclosed ultraviolet source. Their safety classification sections carry the reasoning and are not restated.

What is new here is exposure time rather than exposure kind. This session coats, exposes and processes more pieces than either lab, over a longer bench session, which means longer contact with every one of those hazards and more opportunities to put a wet hand somewhere. The controls that follow from that are procedural rather than chemical: gloves and goggles stay on for the whole session rather than being removed between prints, the bench stays covered, every waste container is set out and labelled before the first sheet is coated, and the developer tray is covered when it is not in use.

The one addition is hydrogen peroxide at 3 per cent, the pharmacy-strength solution, added to the developer by the millilitre. At that dilution it is the substance sold over a counter, and the operation is measuring 2 mL of it with a syringe. It is included because it is the only published contrast agent for this process that this course can run, and the argument for that is under Preparation.

⚠Declared raised step: the developer temperature series

The step. Warming the potassium oxalate developer to a maximum of 38 °C for the temperature series in condition 6.

Its level and its controls. Level B, and the controls are exactly those declared in the printing lab: the tray warmed by standing in a larger tray of hot water, never on a hotplate and never over a flame, carried at not more than half depth, handled with tongs behind goggles and an apron, with the hot-water tray filled before the developer tray goes into it.

The ceiling is the same and the reason is the same. The kits work this bath at 32 to 38 °C; Bostick and Sullivan say it may be used up to boiling and Mougin allows 7 to 100 °C for palladium; the historic trade ran it at 60 to 77 °C, and Willis’s own 1892 announcement identifies the problem that practice caused — workers scalding their fingers. No source read for this course establishes a safe handling temperature or a scald control for a domestic tray of concentrated oxalate solution, so the course stops at the highest figure a modern maker publishes for routine use. A temperature series that stops at 38 °C is a smaller result than one that runs to 70 °C, and it is a result the course can stand behind.

What is not a hazard here, and why. There is no chromium in this session and no chlorate, because both of the traditional contrast agents are excluded — the first under the chromium policy, the second at Level D on the availability of a better route — and so the substances that make the historical version of this experiment hazardous are simply not on the bench. There is no strong acid either: the clearing sequence is a chelating one. And the peroxide, at 3 per cent, is not the concentrated oxidiser its name suggests; a 30 per cent solution would be a different substance to handle and is not what this page uses.

Hazards

Hazard	Where it arises	Control
Systemic oxalate poisoning; skin and eye irritation	The 28 per cent developer, in three trays across the session	Gloves and sealed goggles for the whole session; tongs; nothing eaten at the bench; trays covered between conditions
Scald	The warmed developer in condition 6	The declared raised step above
Skin sensitisation, H317	Wet coated strips, from coating to the first clearing bath, on eight pieces rather than one	Gloves on throughout; nothing touched with a bare hand until it is out of the wash
Severe skin burns and eye damage, H314	Sodium sulfite when weighed and mixed	Gloves, goggles, weigh in a still room
Oxidiser	3 per cent hydrogen peroxide	Measured with a dedicated syringe; kept away from the paper stock and from any organic waste; not stored beside the developer
Ultraviolet, skin and eye	Repeated exposures, so a longer aggregate	Enclosed and interlocked, used closed; do not defeat the interlock to peek at a print-out condition — take the frame out
Deliquescent salt solutions	The saturated-salt humidity boxes	Excess solid left in contact with its solution; boxes lidded and labelled; not stored where a spill would reach paper stock
Cross-contamination	Eight strips through three developers on one bench	One tray per developer, labelled; one set of tongs per tray; strips identified in pencil on the back before they are wet

Required PPE

Nitrile gloves and sealed chemical splash goggles, on for the whole session and not between prints. That is the specific control this page adds, and it is the one most likely to be neglected: a three-hour session with eight pieces produces a moment when it seems reasonable to move one strip with a bare hand. See the glove ruling.

An apron, for the warmed developer.

Dedicated tongs per tray, and none of them shared with a silver bench: oxalate and silver must not meet, on the incompatibility recorded on oxalic acid’s page.

No respiratory protection is specified, because no operation in this session generates a dust or a characterised vapour. If you mix the developer or the sulfite from the solid rather than from stock, that is a weighing operation and it needs the mask the price file names as a gap.

Ventilation

A ventilated room, and the developer trays covered between conditions. Nothing here is classified as producing a hazardous vapour and no source read gives an airborne figure for any of these baths. What the sources give is Sarah Van Keuren’s account of her own reused citrate developer producing fumes that gave her headaches, and her conclusion that the tray belongs under a fume hood or outdoors — a working printer’s observation of a bath carrying many prints’ worth of dissolved iron, not a measurement and not a classification.

This session runs three developer trays for longer than either lab does, which is a reason to take her report seriously enough to keep the room’s air moving and the trays covered. It is not a reason to specify extraction, because nobody has said what would be extracted.

Materials

Material	Quantity	Note
Coated sheets from the coating lab	3, at 12 × 10 in	The whole point of the design: three sheets carry eight conditions
Calibrated transmission step wedge, 21 steps	1, plus a second if you have it	The price file names this as one of its gaps; a 21-step wedge in 0.15 increments spans 3.0, which comfortably exceeds this process’s scale
The standard negative	1	For one pictorial condition alongside the wedges
Masking material, Rubylith or black polythene	enough for eight margins	Every strip gets a masked margin, because that is the clearing test
Saturated salt solutions for the humidity boxes	see Preparation	Sodium chloride and calcium nitrate tetrahydrate; excess solid in each
Distilled water	about 1 L
Drying screen	1

Chemicals

Chemical	Quantity this session	Form
Ferric oxalate	about 3 mL total	Bought solution, as in the coating lab
Sodium tetrachloropalladate(II)	about 3 mL total, of which about 1 mL goes on the double-coated sheet	Bought solution
Potassium oxalate monohydrate	1 L of 28 per cent w/v, reused	Solid or bought developer
Ammonium citrate	500 mL of Mougin’s bath, or a bought pre-measured powder	Dibasic salt; the formulary carries the strength and its single-source provenance
Sodium citrate with citric acid	500 mL at 20 per cent with 2.2 per cent	Willis’s own palladiotype developer
EDTA disodium dihydrate	about 95 g across baths 1 and 3	Clearing sequence
Sodium sulfite	25 g	Clearing bath 2, fresh
Sodium hydroxide	9.6 g, only if bath 3 is made from the disodium salt
Hydrogen peroxide	about 5 mL of 3 per cent	The oxidiser condition
Sodium chloride and calcium nitrate	enough for saturated solutions	The humidity boxes: 76 per cent and 55 per cent relative humidity at 20 °C

Equipment

Everything from the two labs, plus:

• Two lidded humidity boxes — Ware specifies cat litter trays with close-fitting lids, because photographic dishes are too shallow and their lip makes sealing difficult. Strips are held on the underside of the lid, face down, over but not touching the solution; two strips of self-adhesive magnetic tape hold them without marking the paper.
• A hygrometer with a remote probe, so a box can be read without opening it.
• The densitometer of Part XV, in reflection mode.
• A scalpel and rule for cutting sheets into strips before they are coated.
• A dated physical colour reference — see Preparation.

Estimated cost

Band ££££, and this page asks you to work the figure out before you start rather than after. That is part of the discipline of the process rather than a formality: an experiment on an expensive material that is designed after the metal is bought is an experiment that will be cut short in the middle.

The capital is two lidded boxes, two salts and a syringe. Everything else is already on the bench from the two labs.

Estimated consumables cost

Every figure with a number comes from the planner’s dated UK price file, and every quantity from the Materials and Chemicals sections above.

Consumed	This session	Sourced price	Cost this session
Nitrile gloves	3 pairs	£6.64 to £14.99 per box of 50 to 100	£0.40 to £0.90
Sodium sulfite, anhydrous	25 g	£13.68 to £19.98 per kg	£0.34 to £0.50
Unbuffered cotton paper, 12 × 10 in	3 sheets	Not priced. Hot-pressed cotton paper is among the gaps src/data/prices.json names, and not in these sizes	—
Sodium tetrachloropalladate(II) solution	about 3 mL	Not priced. It is a gap the price file names, and it dominates this table	—
Ferric oxalate solution	about 3 mL	Not priced. A gap the file names as well, and for a reason of its own: the bottle is sold at three strengths under one name	—
Potassium oxalate; ammonium citrate; sodium citrate and citric acid	1 L plus 500 mL plus 500 mL	Citric acid alone is priced, at £10.00 per 250 g of the monohydrate; the other three are named gaps, and this row’s quantities are bath volumes rather than weights	—
EDTA disodium dihydrate; sodium hydroxide	about 95 g; 9.6 g	Not priced; cleaning alkali is a named gap	—
Hydrogen peroxide, 3 per cent	about 5 mL of a pharmacy bottle	Not priced	—
Sodium chloride; calcium nitrate tetrahydrate	enough to saturate two small volumes	Not priced	—
Step wedge	not consumed, but named here because the file cannot price it	Not priced; a 21-step transmission step wedge is among the named gaps	—

The priced subtotal is about £0.74 to £1.40, and it is a floor rather than a total by a very wide margin — because the row that dominates the real cost of this session is unpriced, and the course cannot yet price it.

What can be costed exactly is the metal, and that is what the design section below does. Three 12 × 10 in sheets at about 2.3 mL of mixed sensitiser each is about 7 mL of sensitiser; at 0.25 mmol of palladium per millilitre of mixed sensitiser, that is about 1.7 mmol, or about 190 mg of palladium metal, before the double coating adds a further sheet’s worth to one of them. Report that figure in your lab report whether or not you can attach a price to it: a metal quantity is a fact, and a price is a date-stamped observation the course does not currently hold.

Waste streams

As the printing lab, in larger volume, plus two that are specific to this page.

• Noble-metal-bearing solutions: three developers now rather than one, plus clearing baths 1 and 3, plus every tray rinse. All into the one labelled container.
• The sulfite bath, discarded at the end of the session.
• The peroxide-dosed developer, which is the one tray that should not go back into the stock bottle. An oxidiser has been added to it; it is a different bath from the one it started as, and returning it to the stock would carry the oxidiser into every subsequent print.
• The saturated salt solutions, which are kept rather than discarded — they are the humidity boxes and they last indefinitely as long as excess solid remains.

Label everything through the labelling SOP, follow the general chemical waste procedure, and check your local regulations; they govern. See the disposal ruling.

Alternative route

No ultraviolet unit. The whole eight-condition series can be run in sunlight only if it is run in one session under an unchanging sky, because the comparison between strips is what matters and the absolute dose does not. Record the conditions rather than a dose, and say so in the analysis. What you lose is the ability to compare today’s family of curves with a family measured next month.

No densitometer. Then this is not this experiment. What you can still do is the visual version: print the same wedge under each condition, lay the strips side by side, and record for each the step at which a tone first lifts from paper white and the step at which two adjacent steps stop being distinguishable. That is a coarse exposure scale in wedge steps, it is reportable, and its precision is one step. Mark it in the atlas as a step count rather than as a density range, because it is not the same measurement.

No palladium, or not enough for three sheets. Two routes, and both are honest.

Run the identical design on a kallitype at a tenth of the cost. Every variable except the metal identity transfers: humidity, developer, oxidiser dose and coating weight are all the same experiment on the same iron chemistry, and the curves you get are real curves for a process the atlas needs a row for anyway.

Or run the reduced design: the control, one humidity condition and one developer condition, at 4 × 5 in rather than 8 × 10. Three quarter-sheets is about a fifth of the metal of the full design. You will not be able to rank all four controls, and you should say so rather than implying a ranking from three points.

One thing has no alternative. There is no way to measure the exposure scale of this process without printing it. The published figures in this page are Ware’s measurements of his own sensitiser on his own paper under his own lamp, and they are the comparison rather than the answer.

Preparation

The design, and why it is three sheets rather than eight

Ware’s own test target is the trick this experiment is built on: eight identical step tablets on one sheet, mounted together, “allowing subsequent division of the sheet, differential processing and semi-quantitative comparisons”. One coating, one exposure, eight strips that were identical until you started treating them differently — which is a better control than eight separately coated sheets could ever be, as well as an eighth of the cost.

Three sheets, eight conditions

1. 1Sheet A — the control sheet, four wedge exposures, cut into four strips after exposureA1 is the control: 28 per cent potassium oxalate at room temperature. A2, A3 and A4 take the developer and temperature conditions. Because all four were coated and exposed as one piece, any difference between them is the developer and nothing else
2. 2Sheet B — the humidity sheet, cut into strips BEFORE dryingCoated as one sheet, then cut wet-edge-free into three and dried in three different humidities: the desiccated box at about 20 per cent, ambient at about 55 per cent, and the sodium chloride box at 76 per cent. All three developed identically in the control developer
3. 3Sheet C — the metal-load and oxidiser sheetHalf of it double-coated and half single, exposed together, then cut so that one single-coated strip takes the peroxide dose and one takes none. This is the only sheet that costs more metal than the others
4. 4Plus one pictorial print, from the standard negative, at the winning conditionRun last, once the family of curves has told you which condition you actually want. This is the sheet that goes into the comparison atlas

Three coated sheets and one pictorial print. The saving comes from cutting after exposure rather than coating separately, which is Ware's own method for his platinum test targets.

The eight conditions

#	Sheet	Variable	Setting	What it tests
1	A1	—	Control. 55 per cent RH, single coat, 28 per cent potassium oxalate, room temperature, no oxidiser	The base curve, and the answer to the hypothesis
2	A2	Developer	Ammonium citrate, 15 to 20 °C	Three independent claims that it is cooler and more neutral
3	A3	Developer	Sodium citrate 20 per cent with 2.2 per cent citric acid, 4 to 5 minutes	Ware: the citrate bath leaves clear high values where oxalate fogs them
4	A4	Developer temperature	28 per cent potassium oxalate at 38 °C	Whether warming changes contrast, colour or both
5	B1	Humidity	Dried in a desiccant box, about 20 per cent RH	Speed, colour, and the reversal risk
6	B3	Humidity	Dried over saturated sodium chloride, 76 per cent RH	The other end of Ware’s own range
7	C1	Metal per unit area	Double coating	Maximum density, curve shape and colour
8	C3	Oxidiser dose	2 mL of 3 per cent hydrogen peroxide per 100 mL of developer	The only contrast agent this course permits

B2 is the ambient-humidity strip and doubles as a replicate of the control; C2 is the single-coated, no-oxidiser strip on sheet C and does the same job. Two replicates of the control across two sheets is the cheapest insurance in the design, and if they disagree, you have learned that your coating or your exposure is not repeatable and every other comparison here is suspect.

The contrast agent question, settled before the bench

Three of the four traditional contrast agents for this process are excluded, and each for a different reason. State them in your report, because “the course does not permit it” is not a scientific reason and the actual reasons are.

Four published contrast agents, and what happens to each

from excluded to the one this page runs

1. Potassium chlorate in the sensitiser — excluded, Level DThe classical drop system's third bottle. It re-oxidises part of the iron(II) photoproduct so that less survives to reduce the palladium. Its own encyclopaedia entry puts it at Level D on the rubric's second criterion — a better route to the same result exists — and the deciding operation is weighing the dry salt on a bench carrying paper and dust. Ware's objection is separate and aesthetic: the effect "is not to uniformly contract the tonal scale, but simply to truncate the high values", with increased graininess and a "false sparkle" of lost gradation, and the Getty records the same defect analytically as a visually detectable patchiness that leaves no residue to detectLevel D
2. A dichromate in the developer — excluded, chromium policyWillis and Clements recommended a very small amount of potassium dichromate in the oxalate developer for "brilliant prints" by palladiotype, and Paul Strand and Ned Scott used it. It leaves this course by a route that has nothing to do with this page: /safety/chromium/ rules chromium(VI) out at any level anywhere. Ware adds that it is not even stable in the bath, being reduced by oxalic acid to oxalato-chromium(III) complexeschromium policy
3. Sodium hexachloroplatinate(IV), the "Na2" of the modern kits — excluded, because it is platinumWare records it as the nineteenth-century alternative oxidant, still sold at a very high price and said to cause less image deterioration than chlorate. Its trick is kinetic: platinum(IV) is thermodynamically reducible at +0.68 V but far too inert to yield an image in the few minutes available, so it can act as an oxidant without becoming image metal. It is still a halogeno-platinum compound, and the 4 September 2026 ruling closes itplatinum
4. Hydrogen peroxide in the developer — the one this page runsJean-Claude Mougin publishes a dose table: 0 cc for a negative of density range 1.8, 0.5 cc for 1.35, 1.0 cc for 1.20 and 2.0 cc for 1.05, of a 3 per cent solution per 100 cc of ammonium citrate developer, with twice those doses to the potassium oxalate bath. It does the same chemistry as the chlorate — re-oxidising iron(II) before it can reduce palladium — in the tray rather than in the coating, from a pharmacy bottle, with no solid to weighpermitted

Every exclusion is a standing course policy rather than a judgement made on this page, and each names the policy it rests on.

⁇How much weight the peroxide dose can carry

It is one printer’s published method and he says so himself. Mougin marks the oxidiser section of his own text with his personal symbol for an interpretation of his own rather than received practice. The ammonium citrate developer entry records that the same table prints two oxidisers under one column — a 3 per cent hydrogen peroxide solution and a 4 per cent potassium dichromate solution — without distinguishing their doses, so only the peroxide doses are published by this course and the chromium half is governed by the chromium policy.

What that means for your report. This is not a well-established control with a literature behind it; it is one documented method from a printer Ware calls the leading contemporary French exponent of platinum-palladium printing. Treat your own result as evidence about it rather than as a confirmation of it, and note that a dose table indexed by negative density range is telling you what to do about a negative you already have rather than describing what the reagent does to the curve.

And there is no balanced equation for it here, for the same reason there is none for the chlorate on the drop system page: no source read gives a stoichiometry for the re-oxidation of an iron(II) oxalato complex by either oxidant under these conditions. The direction of the reaction is established; its arithmetic is not, and an equation assembled from first principles would be an invention with a citation attached.

The humidity boxes

Two lidded trays, each holding a saturated solution with excess solid left in contact with it — that is the condition that makes the humidity constant. Ware’s own three are:

Saturated solution	Relative humidity at 20 °C
Ammonium chloride	80 per cent
Sodium chloride	76 per cent
Calcium nitrate tetrahydrate	55 per cent

with lithium chloride at the dry end at 15 per cent in the equivalent table for his gold process. For the dry condition this page uses a desiccant box — silica gel or anhydrous calcium chloride — which Ware’s own laboratory practice runs at 9 to 27 per cent.

Allow at least half an hour in the box for evenness; the upper limit is not critical and can be a few hours. Read the box with the remote probe before you take a strip out, and record the figure.

The colour reference

Record colour, do not describe it, and use the course’s own method rather than inventing one.

🔬How the course records image colour without a spectrophotometer

Part XX established this and this page adopts it unchanged, because a second method would make the atlas incomparable with itself.

The eye is the comparator. Two strips edge to edge under one light show a hue difference far below anything you could name in isolation. So every strip is laid against the control strip, never judged from memory, and the record is a comparison rather than an absolute.

The vocabulary is the conservation literature’s. The AIC gives platinum as a matte, deep, rich, yet warm black, and palladium as warmer, yellower and browner; Ware’s own table gives palladium as sepia, Van Dyke brown and warm black at 32, 55 and 80 per cent relative humidity. A hue outside those terms is described by naming the two it sits between. Do not invent a scale.

The number comes from your own densitometer, and it is a signal rather than a measurement. Part XV’s reflection head carries two matched green LEDs, so every density in this course is a green-channel density. Fit a matched pair of blue LEDs as well, read one mid-tone patch in green and again in blue, and the difference between the two channels moves when the hue moves. It is reproducible, it is defined by your instrument, and it is not a colour measurement in any colorimetric sense. Report it as “green-minus-blue density difference on my Part XV head”, with the instrument’s own precision, and it will be comparable with your other strips and with nobody else’s.

And keep a dated physical reference. Cut one control strip in half at the end of the session, mount one half in the dark with the date and the full condition record on the back, and keep it. Every future comparison is against a physical object rather than a memory, and in five years it is also a fading test you did not have to design.

A spectrophotometer would settle all of this in CIELAB coordinates and a colour difference. The course specifies none and the planner prices none, and this page will not pretend a phone camera is one.

And write the prediction down

Before you coat anything: which single variable do you think will move the curve most, and in which direction? Seal it, or at least date it. The comparison at the end is worth more than the result.

Procedure

Stage 1 — Coat (60 minutes)

1. Cut three sheets to 12 × 10 in and mark four wedge areas on sheet A, three strip areas on sheet B and three on sheet C, in pencil, with the cutting lines marked too. Cut nothing yet.
2. Coat sheet A in one pass, at your established volume scaled to the whole coated area. Rest ten minutes, dry an hour at ambient.
3. Coat sheet B the same way. Cut it into three strips as soon as the surface has dulled, before drying, so the three can go into three humidities.
4. Coat sheet C, then coat the left half of it a second time once the first coat has dried, at the same volume per unit area. Record both volumes. Dry an hour at ambient.
5. Put B1 into the desiccant box, B2 on the bench at ambient, B3 into the sodium chloride box. Read all three hygrometers after an hour and record them.

✕The double coating is the condition most easily botched

Struss, writing in 1913, describes multiple platinum printing as superimposing image on image by repeated sensitising, printing and developing — a different and far more laborious thing from what this condition does. What this condition tests is a second coat of sensitiser before any exposure, which is the practice Ware attributes to the platinum salt’s solubility: the sensitiser could not carry enough metal, so printers put on more of it.

Two failure modes. If the first coat is not fully dry the second lifts it and you have one uneven coat rather than two even ones. And if the second coat is spread with more rod pressure than the first, the comparison is confounded by technique. Dry fully, use the same volume, the same number of passes and the same hand — and record both volumes so that the metal load is a number rather than “double”.

Struss’s own observation is worth keeping as a prediction to test: repeated printings add to the blacks, and yet “the print seems actually to lighten, due no doubt to the lengthening scale of tones or intermediate gradations, and the resulting increased contrast”. That is a claim about curve shape, and your densitometer can settle it.

Stage 2 — Expose (45 minutes)

6. Run the unit up five minutes.
7. Expose sheet A as one piece, with four identical wedge exposures at the dose established in the printing lab. Then cut it into A1 to A4.
8. Expose B1, B2 and B3 at the same dose, taking each from its box immediately before exposure and returning the others to theirs. Record the box humidity at the moment each strip left it.
9. Expose sheet C as one piece and cut it into C1 (double-coated), C2 and C3.
10. Record the print-out appearance of every strip as it comes out of the frame. On the humid strips there will be more of it, and how much more is one of this experiment’s results.

Stage 3 — Develop and clear (90 minutes)

11. Develop each strip in its assigned bath and record the time to full appearance for each.
12. Clear every strip identically through the three-bath sequence, ten minutes each with half-minute rinses, then wash together. The clearing must not become a hidden variable.
13. Dry all strips flat, together, on the same screen.

Stage 4 — The pictorial print (30 minutes)

14. Read the wet strips, decide which condition you actually want, and print the standard negative at it. That print is the one that goes into the comparison atlas.

Stage 5 — Measure (30 minutes, next day)

15. Twenty-four hours after drying, read every step of every strip on the densitometer, in green.
16. Read one mid-tone patch of every strip in blue as well, for the colour signal.
17. Lay every strip against the control under one light and record the colour comparison in words.

Expected observations

The print-out image is bigger on the humid strips and almost absent on the dry one. Ware’s account of the ferric oxalate sensitiser is that under dry conditions there is very little print-out at all, because the photoproduct is insoluble, and that the customary 2 per cent oxalic acid in the bottle converts about a tenth of the sensitiser to the soluble ferrioxalate anion, which produces a little more. On B3 at 76 per cent you should see appreciably more brown in the frame than on B1.

The developers should differ in the high values before they differ anywhere else. Ware’s comparative tests find an oxalate developer reducing some palladium(II) on its own, fogging the high values a brownish grey — and he is explicit that this is “quite distinct from the yellow stain of iron”, which is the diagnostic that lets you tell a developer fault from a clearing fault. The citrate bath, he reports, leaves clear highlights.

The warmed developer should develop faster and warmer, and may streak. Photographers’ Formulary warn that with a heated bath development occurs almost immediately, so the sheet must be submerged quickly and evenly. If A4 streaks and A1 does not, that is a real observation about working hot rather than a spoiled strip.

The double-coated strip should be denser and its curve should change shape, if Ware’s explanation for why printers double-coated is right.

And the dry strip is the one to watch for reversal. If the heaviest steps of B1 come back lighter than the steps next to them, you have reproduced the phenomenon Ware associates with a coating dried below about 30 per cent relative humidity — and you have done it deliberately, which is worth more than avoiding it.

What is happening chemically

Four different mechanisms are being separated, and naming them is what turns a set of prints into an experiment.

Metal load changes how much there is to reduce. More palladium per unit area means more metal available at every exposure level, so the whole curve can go higher before it runs out. Ware attributes the historical practice of double-coating directly to this: the platinum salt’s solubility limited how much metal a single coating could carry, so maximum density was limited by the bottle rather than by the chemistry.

Humidity changes how far the iron(II) can travel. At about 70 per cent relative humidity a cellulose paper carries roughly 8 per cent water by weight, which Ware calculates as about ten water molecules per sensitiser ion — enough for short-range ion diffusion. That single variable therefore changes the speed, the amount of print-out, the colour by way of the particle size, and the reversal risk, all at once. It is the most powerful lever on this page and the least visible.

The developer changes what dissolves and how fast. All three baths do the same job — mobilising the iron(II) — by different ligands at different pH.

FeC₂O₄ + C₂O₄²⁻ → [Fe(C₂O₄)₂]²⁻

The oxalate bath: free oxalate takes the photoproduct into solution

The citrate baths chelate the same iron with a different ligand, and Mougin’s insistence that his citrate developer be held at pH 5.5 to 6 is not fastidiousness: his own rule is that for platinum as for palladium “it is imperative to have an acid pH; with a basal pH, the clearing of the paper will indeed be impossible”, which is the same hydrolysis argument the clearing sequence rests on. And the reason the oxalate bath fogs palladium highlights while the citrate baths do not is a redox difference between the two iron complexes rather than anything about the palladium.

The oxidiser removes iron(II) before the palladium can use it. This is subtraction, not redistribution, and the shape of the effect follows from that: an oxidiser spread evenly through the bath consumes iron(II) wherever it finds it, so a shadow with plenty left is barely touched while a highlight with little left loses all of it. The high values are removed rather than the scale being compressed. Watch your own curves for that signature — a toe that has been cut off, with the rest of the curve unmoved — rather than for a general steepening.

Data to record

Field	Why it is here
The prediction, dated, written before coating	The comparison that makes this an experiment
Everything from the coating and printing records, per sheet	Bottle strengths, drop volume, coating volume, paper, dose
Coated area and volume for every sheet, and both volumes for the double-coated half	The metal load, as a number
Total palladium used in the session, in mmol and mg	The cost figure the course can actually compute
Box humidity at the moment each strip left it, and room humidity at each exposure	The variable with the widest effect and the shortest memory
Developer identity, strength, temperature, age and prints-through, per strip
Peroxide dose, as mL of 3 per cent per 100 mL of bath	Stated as the source states it, not as a percentage of the whole
Time to full appearance in the developer, per strip	The one timing that carries information
Print-out appearance out of the frame, per strip, in words
Clearing: identical for all strips, confirmed	A hidden variable if it is not
Reflection density of every step of every strip, in green, 24 hours after drying	The measurement
Green and blue density of one mid-tone patch per strip	The colour signal
Colour comparison against the control, in the conservation literature’s vocabulary
Densitometer precision, from your own repeat readings	A curve without it is a drawing
Masked-margin inspection of every strip under a bluish light	The clearing check, on eight pieces

Analysis

1. The base curve and the hypothesis

Plot density against log relative exposure for A1. Read off:

• Maximum density, the plateau. Ware’s own figure for print-out platinum-palladium is about 1.45.
• Exposure scale, which Ware defines for his own tables as running from fog plus 0.04 to 0.9 of maximum density. His figures are 2.4 for palladium and 1.9 for platinum.
• Mid-tone slope, which he gives as about 0.78 for palladium and 0.96 for platinum.

Then answer the hypothesis with a ratio:

R = ΔlogH(palladium) ÷ ΔlogH(silver gelatin paper)

How much longer the scale is

using your own Part XIII figure for the paper you measured. Report the ratio, not the impression. If your palladium scale is shorter than Ware’s, the likely causes in order are: a coating volume below his, a humidity outside his range, an under-exposure that never reached maximum density, and a densitometer reading a different spectral band. Say which you checked.

2. Which variable moved what

For each condition, classify the change rather than describing it. There are only four things a variable can do to a curve and knowing which one you are looking at is most of the interpretation.

Four kinds of change, and what each one means

1. The whole curve shifts sidewaysA speed change, and nothing else. The process is doing the same thing, faster or slower. Expect humidity to do this: Ware's relative speed for palladium runs from 0.5 at 32 per cent to 2.5 at 80 per cent, which is a factor of five, or more than two stopsspeed
2. The slope changesA contrast change in the proper sense — every tone is redistributed. This is what a paper grade does in Part XIX, and it is what the sensitiser controls of this process mostly do not docontrast
3. The toe is cut off and the rest is unmovedTruncation rather than compression, and it is the signature of an oxidiser. Ware's objection to the chlorate is precisely that this is not what "more contrast" usually means: the weakest exposures are cancelled outright, giving separation in the mid-tones bought with empty paper in the high valuestruncation
4. The plateau movesA maximum-density change, and on this material it is almost always about how much metal is on the sheet. This is the double-coating condition's expected signatureDmax

Every result in this experiment should be classified into one of these four before it is explained. A change that is described as 'more contrast' without saying which of these it was has not been analysed.

Then rank the four controls by the size of the change they produced, and compare that ranking with your prediction.

3. Reading a platinum curve against your palladium measurement

The course does not print platinum, so the comparison has to be made against published data — and the difference between a measured result and a read one must stay visible in every sentence.

Ware's published platinum and palladium curves, and where your own control should sit

• Ware, palladium — ΔlogH about 2.4, slope about 0.78
• Ware, platinum — ΔlogH about 1.9, slope about 0.96
• Where your own control would fall if it matched

Show the numbers behind this plot

Three curves on one pair of axes. Ware's palladium curve rises from base density through a long non-linear toe, straightens at a shallow slope of about 0.78 through the middle values, and reaches a maximum density near 1.45 after about 2.4 log units. Ware's platinum curve rises from a shorter toe at a steeper slope of about 0.96 and reaches a similar maximum after about 1.9 log units, so it sits above and to the left of the palladium curve throughout the middle values. A third, dotted curve marks where a reader's own control strip would fall if it matched Ware's palladium figures exactly; a real measurement will differ, and the gap between the dotted line and the reader's own plot is the thing to explain rather than to ignore. Both published curves are drawn to their stated exposure scales, slopes and maximum density and were not measured by the course.

Series	log relative exposure	Reflection density
Ware, palladium — ΔlogH about 2.4, slope about 0.78	0.00	0.06
Ware, palladium — ΔlogH about 2.4, slope about 0.78	0.30	0.09
Ware, palladium — ΔlogH about 2.4, slope about 0.78	0.60	0.16
Ware, palladium — ΔlogH about 2.4, slope about 0.78	0.90	0.29
Ware, palladium — ΔlogH about 2.4, slope about 0.78	1.20	0.48
Ware, palladium — ΔlogH about 2.4, slope about 0.78	1.50	0.72
Ware, palladium — ΔlogH about 2.4, slope about 0.78	1.80	0.98
Ware, palladium — ΔlogH about 2.4, slope about 0.78	2.10	1.24
Ware, palladium — ΔlogH about 2.4, slope about 0.78	2.40	1.42
Ware, palladium — ΔlogH about 2.4, slope about 0.78	2.70	1.45
Ware, platinum — ΔlogH about 1.9, slope about 0.96	0.00	0.06
Ware, platinum — ΔlogH about 1.9, slope about 0.96	0.30	0.13
Ware, platinum — ΔlogH about 1.9, slope about 0.96	0.60	0.29
Ware, platinum — ΔlogH about 1.9, slope about 0.96	0.90	0.55
Ware, platinum — ΔlogH about 1.9, slope about 0.96	1.20	0.86
Ware, platinum — ΔlogH about 1.9, slope about 0.96	1.50	1.15
Ware, platinum — ΔlogH about 1.9, slope about 0.96	1.80	1.38
Ware, platinum — ΔlogH about 1.9, slope about 0.96	2.10	1.45
Ware, platinum — ΔlogH about 1.9, slope about 0.96	2.40	1.46
Ware, platinum — ΔlogH about 1.9, slope about 0.96	2.70	1.46
Where your own control would fall if it matched	0.30	0.09
Where your own control would fall if it matched	0.90	0.29
Where your own control would fall if it matched	1.50	0.72
Where your own control would fall if it matched	2.10	1.24
Where your own control would fall if it matched	2.40	1.42

The shapes are drawn to teach and were not measured from prints. The exposure scales, mid-tone slopes and maximum density are Ware's own densitometry of his print-out sensitisers on an X-Rite 312 in reflectance mode; the curve between those anchor points is the course's drawing. Your own plot replaces the dotted series. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Three cautions, and they are the point of the exercise rather than hedges around it.

Ware’s curves are of his print-out sensitiser, not of the develop-out one you ran. The metal is the same and the iron salt is not, so a difference between his figures and yours is expected and is not an error in either. His figure for the traditional develop-out sensitiser’s exposure scale, given separately, is about 2.0.

Every published colour statement is somebody’s judgement of somebody else’s print. The AIC’s “matte, deep, rich, yet warm, black” for platinum and “warmer, yellower brown” for palladium are identification vocabulary from conservators looking at collections, and they are the right words to borrow and the wrong thing to treat as a measurement.

Label every claim in your report. Ware’s exposure scale of 1.9 for platinum is published; your 2.2 for palladium is measured; the statement that platinum would therefore need a shorter-scale negative than yours is inferred. Three sentences, three classes of evidence, and the atlas assignment will ask for exactly that discipline.

4. The print-out condition

This is the fifth condition and it is not a variable in the same series, for a reason worth stating plainly.

Full print-out needs a different iron salt. Ware’s print-out palladiotype uses ammonium iron(III) oxalate, whose photoproduct is soluble; the ferric oxalate you coated has an insoluble photoproduct, which is why the traditional process needs a developer at all. So a ferric oxalate sheet humidified to 76 per cent gives partial print-out and still needs development — which is itself the measurement. Ware’s own figures for his ammonium system put development at 0.4 log units at 32 per cent relative humidity, 0.2 at 55 per cent and 0 at 80: the amount of work left for the developer falls to nothing. Your B3 strip measures how far the ferric oxalate route gets along that road, and the honest report is a comparison against his figures for a different sensitiser rather than a claim to have printed out.

If you hold ammonium iron(III) oxalate as well, the full condition is available and the formulary carries the composition: equal volumes of a 60 per cent w/v iron solution and a 19 per cent w/v ammonium tetrachloropalladate(II) solution, humidified to 80 per cent, exposed until the highlight detail resolves, and processed by clearing alone.

5. The interaction, revisited

Look at whether the developer differences you measured on sheet A would survive at sheet B’s humidities. If B3 shows substantially more print-out than B2, then part of the image on B3 was made before any developer touched it, and a developer comparison run at that humidity would have been measuring less than it appeared to. Say so. A design’s limitations reported in the results section are a finding; the same limitations discovered by a reader are a fault.

Troubleshooting

“My two control replicates disagree.” Then stop and find out why before interpreting anything else. The usual causes, in order: the coating volume differed between sheets; the two sheets dried at different humidities because one was nearer a radiator; the exposure unit had not run up; the densitometer drifted. Two replicates that disagree have done their job.

“Nothing changed between the three developers.” Check the developer temperatures were actually different from each other only where intended, and check the strips were cleared identically. Then consider that this may be a real result: the AIC’s own summary is that contrast in this family is achieved mostly by exposure rather than by the developer, so a small developer effect on the curve alongside a visible effect on colour is what the literature would predict.

“The dry strip is much slower than I expected.” Expected in direction, and Ware’s numbers say how far: palladium’s relative speed at 32 per cent relative humidity is 0.5 against 1.3 at 55 per cent — most of a stop and a half, from drying alone.

“The double-coated strip is not denser.” Three checks: was the first coat fully dry, was the second volume actually equal to the first, and did the exposure reach maximum density on either strip? A comparison of two under-exposed strips shows nothing about maximum density.

“The peroxide strip lost its highlights entirely.” That is the mechanism working, and it is the effect Ware objects to. Note that Mougin’s own table gives 2 mL per 100 mL as the dose for a flat negative of density range 1.05, and a step wedge is not a flat negative — its range exceeds this process’s scale. On a wedge you should expect the strongest dose to cut the toe off severely. Report it as the truncation it is.

“Everything has a yellow cast.” A clearing failure across the whole session rather than a result. Check the masked margins under a bluish light, and check whether clearing bath 1 has passed its capacity of about 50 to 60 prints of 10 × 8 in per litre — eight strips plus a print is roughly one sheet’s area, so capacity is unlikely to be the cause on one session unless the bath was already old.

Clean-up

1. Return the two unmodified developers to their bottles, filtering if a sludge has formed. The peroxide-dosed developer does not go back; it is waste.
2. Keep clearing baths 1 and 3, discard bath 2.
3. Rinse every tray into the noble-metal container.
4. Lid the humidity boxes and label them with the salt and the nominal humidity; they keep indefinitely with excess solid present.
5. Wipe the bench wet, wash the tongs by tray, and wash hands before removing goggles.

Storage

The strips are the data and they should be stored as such. Mount the control replicate as the dated physical colour reference described under Preparation, with the full condition record on the back, in the dark. Keep the rest flat, interleaved with materials that pass the Photographic Activity Test, on the AIC’s own storage terms: about 20 °C with not more than 2 degrees of drift, and 50 per cent relative humidity varying by not more than 5 per cent over 24 hours.

Re-read three of them in a year. A dated set of strips with a recorded density is an ageing experiment you have already paid for, and the Clarke and Hemmenway result — that stain appears on ageing in proportion to residual iron, on prints that looked acceptable on the day — is the reason it is worth the drawer space.

The baths store as the printing lab sets out: developers in the dark because a used bath is still photosensitive, EDTA baths between sessions, sulfite never.

Disposal considerations

As the printing lab, in larger volume, with two additions.

The peroxide-dosed developer is a separate decision. It carries oxalate, iron, palladium and an oxidiser. It goes into the collected noble-metal waste rather than back into stock, and it should not be mixed with anything organic on its way there.

The saturated salt solutions are not waste while excess solid remains — they are apparatus. When you do finally discard them, calcium nitrate tetrahydrate is an oxidiser — the encyclopaedia has no entry for it yet, so this page names it without a link — and sodium chloride is not, and they should not be poured into the same bottle as the noble-metal waste, which would dilute a valuable stream with a large volume of salt for no reason.

Local regulation governs all of it, and this course cannot tell you what it says where you are. See the disposal ruling.

Questions

1. Your control strip gives an exposure scale of 2.1 and your Part XIII paper gave 1.05. State the ratio, and say what negative density range this process therefore wants.
2. One strip’s curve has the same slope and maximum density as the control but starts a third of a log unit further right. Which variable did you change, and which did you certainly not change?
3. Why can the developer not be compared meaningfully at 80 per cent relative humidity?
4. Mougin’s dose table is indexed by negative density range rather than by a desired contrast change. What does that tell you about how he intends the reagent to be used, and why does it make a step wedge an awkward test subject?
5. You measure a green-minus-blue density difference of 0.11 on the control and 0.19 on the dry strip. What may you claim from that, and what may you not?
6. Struss claimed in 1913 that repeated printings add to the blacks and yet the print seems to lighten. Express that as a claim about a curve, and say which of your measurements tests it.

Further experiments

Extend the humidity series to five points and fit it. Lithium chloride at 15 per cent, calcium nitrate at 55, sodium chloride at 76 and ammonium chloride at 80, plus the desiccant box. Plot relative speed and maximum density against relative humidity and compare the shape with Ware’s Table 7.1 — remembering that his figures are for a different iron salt, so a difference in shape is a finding rather than an error.

Test the “false sparkle” claim directly. Print one wedge with the peroxide dose and one on a higher-contrast negative made by correction curve, matched so that both give the same mid-tone separation. Then compare the high values. Ware’s objection is that the oxidiser truncates where a negative correction redistributes, and this is the experiment that would show it on paper you made.

Run the reversal condition on purpose and map its boundary. Ware names six factors; you can vary two cheaply — humidity below 30 per cent, and the amount of print-out — on a sensitiser you already have. Find the humidity at which reversal first appears on your paper. Nobody in this course has that number.

Cross the developer and humidity variables deliberately, three developers at three humidities, nine strips, on kallitype rather than palladium so the cost is bearable. That is the design this page declined to run on palladium, and the iron chemistry is the same. If the interaction the design section warned about is real, it will show up as a difference between developers that changes size with humidity.

Read the strips again at three months, six months and a year, under the same light, on the same instrument, against the dated reference. The measurement costs nothing and the course has no data of its own on the ageing of a palladium print.

Recap

You measured the curve of the palladium process as you run it, and then moved four controls one at a time against a control strip cut from the same coating. The design was three sheets rather than eight because the strips were cut after exposure rather than coated separately, which is Ware’s own laboratory method and is also a better control.

Two of the variables interact — humidity changes how much of the image is made before the developer is involved at all — and the design says so before the results rather than after. Of the four traditional contrast agents, three are excluded by standing course policies and the fourth, a peroxide dose in the tray, is one printer’s published method rather than an established control. And the comparison against platinum is a comparison against published curves: a measured result and a read one, kept visibly apart in every sentence of the report.

Check your understanding

Question 1. One strip's curve has the same slope and the same maximum density as the control, but sits a third of a log unit to the right. Which variable did you most likely change?

• The oxidiser dose, which shifts the whole curve
• The humidity, which changes speed — Ware's relative speed for palladium runs from 0.5 at 32 per cent to 2.5 at 80 per cent, and a pure sideways shift with slope and maximum density unchanged is the signature of a speed change and of nothing else
• The metal load, since more metal takes longer to reduce
• The developer temperature, which shifts the curve without changing its shape

Show the answer and why

Answer: The humidity, which changes speed — Ware's relative speed for palladium runs from 0.5 at 32 per cent to 2.5 at 80 per cent, and a pure sideways shift with slope and maximum density unchanged is the signature of a speed change and of nothing else

Classifying the change before explaining it is most of the analysis. A sideways shift alone is a speed change; a slope change is a contrast change; a cut-off toe with the rest unmoved is truncation by an oxidiser; a moved plateau is a maximum-density change and on this material that is almost always metal load. Option one is the one to reject on mechanism: an oxidiser consumes iron(II) wherever it finds it, so it takes the weakest exposures out entirely while barely touching the strongest — truncation, not translation.

Question 2. Why is a three-developer comparison meaningless at 80 per cent relative humidity?

• Because the developers decompose faster in humid air
• Because at that humidity Ware's own table gives "Development" as 0 log units for palladium — the image has printed out during the exposure — so the three baths would be functioning as clearing baths and the comparison would be measuring nothing about development
• Because the paper is too wet to absorb developer evenly
• Because the exposure scale is longer at high humidity, so the wedge no longer fits

Show the answer and why

Answer: Because at that humidity Ware's own table gives "Development" as 0 log units for palladium — the image has printed out during the exposure — so the three baths would be functioning as clearing baths and the comparison would be measuring nothing about development

This is the interaction the design section declares in advance. Ware's table records development, in log exposure units, as a property of the humidity: 0.4 at 32 per cent, 0.2 at 55 and 0 at 80 for palladium. A variable that vanishes at one setting of another variable cannot be studied at that setting, and a design that ran the comparison there would produce three plausible-looking curves that meant something else entirely. Option four states a true fact — the exposure range does lengthen to 2.4 at 80 per cent — that is not the reason.

Question 3. Your control gives an exposure scale of 2.1; the enlarging paper you measured in Part XIII gave 1.05. What follows for your negative?

• The ratio is 2.0, and the negative for palladium needs about twice the density range — around 2.1 in ultraviolet — so a negative made for that paper will print flat here
• The ratio is 2.0, so the palladium print will have twice the contrast of the silver print from the same negative
• Nothing follows; exposure scale and negative density range are unrelated quantities
• The negative needs half the density range, because a longer scale accommodates more with less

Show the answer and why

Answer: The ratio is 2.0, and the negative for palladium needs about twice the density range — around 2.1 in ultraviolet — so a negative made for that paper will print flat here

Exposure scale is what the process can accommodate, so a longer scale asks for a longer negative, not a shorter one — which inverts option four and is the single most common error here. Ware asks for about 2.4 in the ultraviolet for palladium and the suppliers ask for 1.35 to 1.50 for a kit that includes a contrast agent this course does not use, and the gap between those two numbers is exactly what the chlorate bottle was for. Option two has the direction of the contrast relationship backwards as well: a longer scale from the same negative gives a flatter print.

Question 4. You read a green-minus-blue reflection density difference of 0.11 on the control strip and 0.19 on the strip dried in the desiccant box. What may you claim?

• That the dry strip is 0.08 units warmer in CIELAB terms
• That the two strips differ measurably in hue on your own instrument, in the direction the sources predict for a drier coating, and nothing about the magnitude in any colorimetric sense — because a two-channel density difference is a signal defined by your densitometer rather than a colour measurement
• Nothing, because a densitometer cannot detect colour at all
• That the dry strip is sepia and the control is Van Dyke brown

Show the answer and why

Answer: That the two strips differ measurably in hue on your own instrument, in the direction the sources predict for a drier coating, and nothing about the magnitude in any colorimetric sense — because a two-channel density difference is a signal defined by your densitometer rather than a colour measurement

Part XX established the method and its limits together, and both halves have to be reported. The number is reproducible on your instrument and comparable between your own strips; it is not comparable with anybody else's instrument and it is not a colorimetric quantity, because a density is defined only by its geometric and spectral conditions and yours are a pair of LEDs. Option four over-claims in the other direction: those are the conservation literature's words for particular appearances, and assigning them from a number rather than from a side-by-side comparison is exactly the invented scale the method exists to avoid.

Question 5. Of the four published contrast agents for this process, why does the course run only the hydrogen peroxide dose?

• Because peroxide is the most effective of the four
• Because each of the other three is closed by a standing policy that has nothing to do with this page — chlorate at Level D on the availability of a better route, dichromate under the chromium policy, and sodium hexachloroplatinate(IV) because it is a halogeno-platinum compound — while the peroxide dose is published, needs no solid to be weighed and comes from a pharmacy bottle
• Because peroxide is the only one that raises contrast rather than lowering it
• Because the other three are not documented well enough to reproduce

Show the answer and why

Answer: Because each of the other three is closed by a standing policy that has nothing to do with this page — chlorate at Level D on the availability of a better route, dichromate under the chromium policy, and sodium hexachloroplatinate(IV) because it is a halogeno-platinum compound — while the peroxide dose is published, needs no solid to be weighed and comes from a pharmacy bottle

Three different policies, three different arguments, and none of them is about efficacy — which is why option one and option three are both wrong in kind rather than in detail. Option four is wrong on the facts: the chlorate system is the best-documented contrast control in the whole part, with published drop tables from two suppliers and a provenance running back to 1882. What the page must add, and does, is that the peroxide dose is Mougin's own marked personal interpretation rather than established practice, so a result from it is evidence about the method rather than a confirmation of it.

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