Mike Ware describes the version ‘S’ of his chrysotype process.
Always be careful when handling chemicals. Read the health and safety instructions.
This is a novel and chemically sophisticated version of Herschel’s original gold-printing process, whose difficulties have previously prevented its adoption into the photographic repertoire. New chrysotype has qualities and permanence like platino-palladiotype, but the added feature of beautifully muted, non-literal colours, controlled by the parameters of the process.
The new chrysotype sensitizer is unstable and must be prepared by mixing carefully-measured volumes of three stock solutions a few minutes prior to coating the paper. For brevity, the three solutions required will be designated as: A. Ligand, B. Gold, and C. Iron. The Version ‘S’ (for the ‘Sodium’ method) described here is completely ‘user-friendly’, employs simple chemistry (a
mixture), and is quite safe to work. It is recommended for your first
experience of gold printing. There are other Versions of the process,
involving more variables, and some hazards; some of them are
described in ‘The Chrysotype Manual’.
In the instructions that follow, you will find a number of options
offered for making up the solutions, which are completely equivalent
in their outcome, but may be adopted according to your local
availability and price of the chemicals, especially the gold salt.
Chemicals Required for the Sensitizer
A. 3,3′-Thiodipropanoic acid (aka 3-3′-Thiodipropionic acid)
B. either B1. Sodium tetrachloroaurate(III) dihydrate
or B2. Hydrogen tetrachloroaurate(III)trihydrate
C. Ammonium iron(III) oxalatetrihydrate
D. either D1. Sodium carbonate(anhydrous)
or D2. Sodium hydrogen carbonate
E. Tween 20
Chemicals Required for Processing
Developing agents;one or more of the following:
Disodium Edta(1,2-Diaminoethanetetraacetic acid, disodium salt)
Tetrasodium Edta (1,2-Diaminoethanetetraacetic acid, tetrasodium salt)
Sodium sulphite orsodium metabisulphiteorKodak Hypo Clearing Agent
Apparatus Required for Making up the Sensitizer
One small Pyrex or Corning glass beaker (100-200 cc)
One graduated glass measuring cylinder (50-100 cc)
One large beaker or plastic measuring cylinder (ca. 500 cc)
One small conical filter funnel (ca. 5-6 cm diameter)
Filter papers: Whatman Grade #1, (ca. 8-10 cm diameter)
Two glass stirring rods, one long and one short.
Dropping pipette (2-3 cc capacity) or small graduated syringe
Balance or scales accurate to ± 0.1 g
Electric hotplate/magnetic stirrer (or, simply a basin of hot water)
Spatula or small plastic spoon
Washbottle filled with pure water
Three brown glass bottles with plastic screw-caps, ca. 50 cc capacity
Self-adhesive labels and indelible pen
Preparing Stock Solutions for Version ‘S’ Sensitizer
A. Ligand solution (1.4 molar)
- Weigh out 12.5 g of 3,3´-thiodipropanoic acid and transfer it into a beaker or measuring cylinder of ca. 500 cc capacity (This vessel is necessarily large in order to contain the foam generated in the reaction that follows).
- Weigh out separately 7.4 g of sodium carbonate (anhydrous powder), or 11.8 g of sodium hydrogen carbonate.
- Add ca. 30 cc of pure water to the thiodipropanoic acid and stir it into a suspension with a glass rod.
- Slowly add the solid sodium (hydrogen) carbonate powder, in small portions of ca. 1 g, to the suspension of thiodipropanoic acid. There will be effervescence as carbon dioxide gas is evolved – this is harmless. The suspension will foam up alarmingly, but it should be contained by the tall vessel. Stir well and allow the foam to subside between additions. The solution becomes cold, so may be slightly warmed in a bath of hot water to hasten the reaction. Continue until there is no further effervescence, leaving a colourless solution. A few residual solid particles do not matter.
- Pour the solution into a small measuring cylinder (50-100 cc capacity) and make it up to a final volume of 50 cc with pure water from the wash bottle and mix well.
- Filter the solution through a #1 filter paper directly into the stock bottle (this is a slow process because the solution is somewhat syrupy.) This solution should keep indefinitely, if correctly made up. Label it with the date and the following:
New Chrysotype: Version ‘S’
Solution A: Ligand
Disodium thiodipropanoate 1.4 molar
B. Gold solution (0.35 molar)
- Carefully transfer or weigh out 5.0 g of sodium tetrachloroaurate(III) into a Pyrex glass beaker of 100 cc capacity. Use a plastic spatula or spoon – this gold salt will attack stainless steel and nickel.
- Add ca. 25 cc of pure water at room temperature to the gold salt in the beaker. You should use some of this water to wash out and transfer any residual crystals of gold salt from your weighing bottle or vial, by means of a dropping pipette. With gentle agitation, all the solid will easily dissolve to a yellow solution at room temperature.
- Filter the solution carefully into a measuring cylinder (50-100 cc capacity) using a small conical filter funnel and a Whatman #1 filter paper, or equivalent.
- Add pure water from the washbottle, a little at a time, to the filter-paper, allowing it to pass through to wash out most of the residual yellow solution, and to make up the final volume to exactly 36 cc in the measuring cylinder. Mix thoroughly.
- Transfer the solution carefully from the measuring cylinder to the stock bottle. This solution is stable indefinitely. Label it with the date and the following:
New Chrysotype: Version ‘S’
Solution B: Gold
Sodium tetrachloroaurate 0.35 molar
If hydrogen tetrachloroaurate(III) is cheaper or more readily available than the sodium salt, then you can make your gold solution from it, using the following procedure for neutralizing this acid with anhydrous sodium carbonate or sodium hydrogen carbonate:
- Carefully transfer or weigh out 5 g of hydrogen tetrachloroaurate(III) into a tall measuring cylinder (100 cc capacity). Use a plastic spatula or spoon – this gold salt attacks steel and nickel.
- Add ca. 30 cc of pure water at room temperature to the measuring cylinder. You should use some of this water to wash out and transfer any residual crystals of gold salt from your weighing bottle or vial, by means of a dropping pipette. With gentle agitation, all the solid will easily dissolve to a yellow solution at room temperature.
- Weigh out (precisely, if possible) 0.673 g of sodium carbonate (anhydrous powder), or 1.07 g of sodium hydrogen carbonate, using a chemical balance. If your balance is not this precise, the figures may be rounded up to 0.7 g or 1.1 g respectively.
- Slowly add the sodium (hydrogen) carbonate in small portions (tip of a spatula), to the solution of chloroauric acid. There will be vigorous effervescence as carbon dioxide gas is evolved. The spray should be contained by the tall measuring cylinder. Swirl the solution, and continue adding the solid to completion and no further effervescence.
- Add pure water from the washbottle to make up to a final volume of exactly 36 cc in the measuring cylinder with thorough mixing.
- Filter the solution carefully into the stock bottle through a small #1 filter paper to minimise the residual loss of solution. Label it:
New Chrysotype: Version ‘S’
Solution B: Gold
Sodium tetrachloroaurate 0.35 molar
C. Iron solution (1.4 molar)
The following procedure must be carried out under subdued indoor lighting, not in daylight (a 60 watt incandescent tungsten bulb at least 1.5 meters away).
- Weigh 30 g of ammonium iron(III) oxalate into a small Pyrex glass beaker of ca. 100 cc capacity.
- Add exactly 33 cc of pure water (from a measuring cylinder) at room temperature to the salt in the beaker. Stir well with a small glass rod to dissolve the bulk of the solid. The solution becomes cold, so gently warm the beaker in a bath of hot water to assist dissolution.
- Within 5 minutes the solid will have dissolved to form an emerald-green solution (any slight cloudiness from a few remaining fine crystals of impurity should be ignored). The solution does not need to be ‘made up’, because its total volume should already be correct = 50 cc.
- Filter the solution through a Whatman #1 paper in a conical funnel, directly into a clean, dry brown bottle.
- Store this bottle in the dark at room temperature. If, after a few days, a few white needle-like crystals (probably of ammonium oxalate) have appeared, re-filter the solution to remove them. This solution is close to saturation, and if it is allowed to cool much below 20 °C (68 °F), some emerald-green crystals may slowly grow at the bottom of the bottle. Warm gently and swirl to re-dissolve these. Stored carefully at room temperature in the absence of light, this solution will keep indefinitely. Label it with the date and the following
New Chrysotype: all Versions
Solution C: Iron
Ammonium iron(III) oxalate 1.4 molar
Mixing the Sensitizer Solution for Version ‘S’
The primary control of contrast in this process is provided by the molar ratio of ligand: gold, which can be varied between 2 and 6. A ‘standard’ value of 4 is recommended for your first trials. The molar ratio of gold: iron is always 1. Given the concentrations of the solutions made up as above (the ligand and iron solutions have the same concentration, 1.4 molar, which is 4 times the concentration of the gold solution, 0.35 molar), this determines that the volumeratios for these three solutions, A: B: C to mix a ‘standard’ sensitizer must be:
Volume Ratios of Ligand: Gold: Iron
A: B: C = 4: 4: 1
However, these volume ratios can be varied between the limits:
A: B: C = 2: 4: 1 and 6: 4: 1
in order to vary the contrast.(Note that the first of these ratio numbers also tells you, conveniently, the molar ratio of ligand to gold). The values you prefer will be decided by your personal taste and the density ranges of your negatives, but here are some guidelines:
Lower ligand: gold ratios than the ‘standard’ value of 4 give less stable sensitizers, which will print faster, with a longer tonal range and higher maximum density, but they have a greater tendency to fog in the highlights and to decompose before coating.
Higher ligand: gold ratios than the ‘standard’ value of 4 give more stable sensitizers, which will print less rapidly, showing higher contrast and slightly lower maximum densities, with clean highlights.
There is no benefit in significantly increasing the proportion of iron (C) in the ratios, because this tends to cause ‘blocking up’ of tonal gradation in the shadows, ‘bleeding’ of gold pigment from overexposed regions, and lower maximum densities. However, a small excess of iron (C) will do no harm.
The actual volumes of the solutions you need to measure out will, of course, depend on the total area to be coated, i.e. number and size of your coatings, the nature of your paper, and the ligand: gold molar ratio you have chosen. As a guide, typical figures are set out in Table 1 for making up 10 cc of mixed sensitizer.
Table 1. Component solution volumes to make 10 cc of chrysotype sensitizer.
|Volume of A
|Volume of B
|Volume of C
|2||2.86 cc||5.71 cc||1.43 cc|
This is sufficient to coat about eight 10.25" x 8.25" (260 x 210 mm) areas of paper, on the assumption that each sheet requires ca. 1.25 cc of sensitizer solution (which is true for Fabriano 5, HP 210 gsm paper). For total volumes other than the 10 cc used in this illustration, you should scale these numbers accordingly, selecting the row of volume figures, A, B, C that corresponds with your chosen ligand: gold molar ratio. For example, if you find that your chosen paper requires 1.6 cc for each coating, and you wish to coat 8 sheets, then you will require 8 x 1.6 = 12.8 cc of sensitizer, so each figure in the appropriate row should be multiplied by 1.28 to give the volumes to be measured out. These small volumes are conveniently and accurately measured out with the small graduated syringes, of capacity 1, 2, and 5 cc. It is good practice to dedicate a particular syringe to each solution, with an identifying mark, to avoid cross-contamination.
The solutions must be mixed in the following sequence:
- Deliver the volume A of ligand solution into the mixing vessel using a graduated syringe.
- Add the volume B of gold solution, drop by drop, from another syringe, with stirring or, preferably, careful swirling of the vessel (which avoids introducing a stirring rod). Allow time between each drop (1-2 seconds) for the gold solution to decolorise.
- When step 2 is complete, add the volume C of iron solution, from a third syringe, to give a pale yellowish-green sensitizer.
- Mix thoroughly and, using a fourth syringe, coat the paper sheets in the way described in the Preparations article.
Although these volume figures have been quoted rather precisely, do not be daunted if you find it difficult to achieve this degree of accuracy in measurement: even with a calibrated syringe, you may have to guess the second decimal place. The process is quite forgiving over a range of values and you will obtain a result, albeit less predictable, with almost any reasonable combination of volumes.
The stability of the made-up sensitizer solution depends on the ligand: gold molar ratio and the temperature, but all solutions should last for at least 30 minutes at 20 °C without showing any perceptible decomposition, which is sufficient time to coat a batch of sheets. The mixed solution cannot be stored; although the ‘standard’ solution should last at least 2 hours. Decomposition is indicated by the deposition of metallic gold, which plates the container.
Coating and Drying
The method of coating the paper has been described fully in Preparations. If difficulty is encountered in achieving an even coating, a little Tween 20 may be added to the sensitizer to improve the absorption of the sensitizer by the paper fibres: 0.1-0.3 cc of a 20% v/v solution, per 10 cc of sensitizer solution, is appropriate (i.e. a final Tween concentration in the sensitizer of 0.2 to 0.6%). Tween may also assist in ‘smoothing out’ the tones if a grainy or fibrous image is obtained, but tends to be incompatible with gelatin-sized papers.
Immediately following the coating, allow the sheet to rest horizontally for a few minutes so that the sensitizer can soak into the surface fibres, as indicated by the disappearance of the reflective sheen. It may then be dried in a uniform stream of warm (40 °C) air for about 10 minutes. After heat-drying, the coated paper may be stored in a light-tight desiccator at a Relative Humidity (RH) of 10% or less. The lifetime of prepared paper in storage is variable, depending on the parameters of the sensitizer. For preference, to avoid introducing this uncertainty, the paper should be exposed and processed on the same day that it is coated. If you adopt this as your method of working, then heat-drying the coating is not essential (unless you desire the result of a low RH): just leave it in the dark, e.g. in a drawer with plenty of air circulation to the environment, at room temperature for an hour or so, which suffices to reach equilibrium with the ambient RH.
Regulating the Humidity of the Paper
To attain a predictable result for the colour of the image, the water content of the sensitized paper must be controlled before exposure. Varying degrees of hydration will produce different colours in the print and different extents of printout: high RH generally yields a nearly complete printout in fairly neutral grey tones, or with a hint of blue or green. The lowest RH values give almost no printout: only the shadow tones are faintly visible, and the image is generated almost entirely by development, with pinkish-brown or dull red shadows and greyish-blue high values. The intermediate values of RH can provide strongly split-toned scales, with purple and magenta shadows graduating into blue highlights. This results from partial printout coupled with a some degree of development. Hydration can be controlled in two different ways, which will now be described. The first is simpler, but requires more attention because the process must be timed.
Water hydration chamber (time-dependent method)
A heat-dried paper may be humidified over pure water, at 100% RH, for a timed period of up to 30 minutes, as described in Preparations. If the ambient temperature is much different from 20 °C, then some correction to the time is necessary for consistent results.
Salt hydration chambers (equilibrium method)
These are chambers of known, constant RH, provided by saturated solutions of specified salts. Details of the construction and use of such enclosures has been described in Preparations. It is unnecessary to have a full range of RH chambers, however, and initially I recommend that you prepare just three, using the substances in Table 2, which will convey some idea of the range of colour behaviour. Following that, the use of other RH values will be a matter of exploration and personal taste.
Table 2. Salts for the control of Relative Humidity atmospheres.
|H1 Calcium chloride anhydrous||CaCl2||9|
|H4 Calcium chloride sat. aqueous||CaCl2||45|
|H7 Sodium chloride sat. aqueous||NaCl||75|
|H8 Ammonium chloride sat.aqueous||NH4Cl||80|
The RH box H1 can also be used for the storage of coated paper. Paper should be heat-dried before placing in this box, so as not to exhaust the desiccant too quickly. The RH box H4 can be made up using the spent calcium chloride from an H1 box, when it becomes too moist. A saturated solution is prepared, in contact with excess solid, in each case, as described in Preparations. Hydration in these chambers requires a minimum of half an hour, but papers may be left longer. The paper may tend to fog in the higher RH baths if it is left for much more than an hour, but if you intend to equilibrate at high RH, H7 or H8, the initial drying in hot air is unnecessary.
Exposure times vary somewhat with the ligand: gold molar ratio, but are generally as short as for palladium sensitizers, for example. Using even modest UV light sources, adequate exposures may only be one or two minutes. The extent of printout is nearly total at high RH values, leaving less than 1 stop of development, but the printout decreases in the dried sensitizers, leaving 4 to 6 stops of development at the lower RH values. Test strips or step tablet tests are a useful guide in the latter case. The negative density range appropriate for a full tonal range in the print is indicated in Table 3 below, under ‘controlling the contrast’. However, ‘softer’ negatives can still yield effective prints, especially if a high ligand: gold ratio is used.
Making up the Processing Solutions
Disodium Edta, ca. 1% w/v solution: dissolve ca. 10 g (one rounded 5 cc teaspoonful) in 1 litre of tap water at room temperature. Use this for a few prints only, in one session. Do not store it. When it acquires the colour of colloidal gold it is liable to stain subsequent prints, and should be replaced. Do not use tetrasodium Edta, which is alkaline, and will cause iron stains.
Any of the following can be made up and used as 1-2% w/v solutions: tartaric acid; citric acid; oxalic acid. Because they are acids, overlong treatment may tend to block up the shadow tones, due to the coagulation of the gold, and better results may be obtained by using their sodium, potassium or ammonium salts.
These ‘developers’ should ideally all be employed as ‘one-shot’ process baths, or nearly so. Practically speaking, a 1 litre bath should be sufficient for 2 or 3 prints 10" x 8" or the equivalent area of smaller prints. It is risky to re-use these ‘first bath’ solutions, because they acccumulate most of the excess iron and gold salts, and soon acquire the ruby-red colour of colloidal gold which may cause staining of subsequent prints. Do not store these solutions: make them up, ca. 1-2% in strength, as needed, by dissolving a rounded teaspoonful (8 cc) of the solid in one litre of water at 20 °C.
Clearing Baths #I and #III
Tetrasodium Edta, ca. 5% w/v solution: dissolve 50 g in 1 litre of tap water at room temperature. Two such clearing baths are required. They may be stored, and will have a capacity of around 50 10" x 8" prints per litre. It is economical to use ‘2-bath fixing procedure’, by replacing the #I bath by the #III bath, when it is exhausted.
Clearing Bath #II
Sodium sulphite, sodium metabisulphite, or Kodak Hypo Clearing Agent: ca. 2.5% w/v: dissolve ca 25 g (1 level 15 cc tablespoonful of solid) in 1 litre of tap water at room temperature. This bath should not be stored, but made up fresh from solid for one day’s printing.
Processing of the Exposed Print
This is an optional step in which the print is humidified, after exposure but before wet processing, by placing it in a water hydration chamber for a period of 2 to 15 minutes. The print is held parallel to and above, but out of contact with, the surface of water (larger than the print in extent). If the water is warmed to ca. 40 °C the processing time can be shortened to 1-2 minutes, and the degree of development will be very complete. (A photographic dish-warmer serves well for this purpose. The arrangement is essentially the same as that for humidifier enclosures; for details see Preparations.) If you desire the lowest contrast, with the longest possible tonal scale, and the most delicate high values, then this procedure is strongly recommended. It allows the image to develop almost completely before it is immersed in the wet baths, which immediately start to wash out the chemicals. The hues are generally fairly monochromatic, i.e. without strong ‘split tones’, because of the lack of development. Post-hydration also yields very smooth tones. However, prolonged post-hydration can lead to chemical fogging of the image. Uniformity in exposure to the water vapour is important, and the humidifying tank should be kept in a draught-free location.
One of the following ‘developers’ may be chosen, although ‘development’ is something of a misnomer here for the higher RH values, because the chief action of these baths is as ‘clearing’ agents. However, each does have a slightly different effect in ‘fine-tuning’ the resultant colour of the gold image. Vigorous agitation is important for the first couple of minutes. Prints may remain in this bath for up to 10 minutes.
Water: this is the least ‘energetic’ option, and gives the greatest apparent contrast, because the high values may be truncated by lack of any developing agent.
Disodium Edta; 5-10 minutes gives a clean result with fairly neutral tones.
Tartaric acid; yields redder tones, but still fairly subdued.
Citric acid; from 1 to 6 minutes can produce a long tonal range of rose-pink hues crossing over into blue highlight tones if the sensitizer was exposed at low RH values. The intensity of the red colour in the shadows increases progressively with time and intensifies in the clearing baths. Overlong treatment may cause ‘blocking up’ of the shadows and a loss of tonal separation when the print dries down.
Oxalic acid; causes the most intense and striking red/blue colour splits and the longest tonal range.
These are not the only possibilities – the list of reagents could be very long – and opportunities for experiment are endless. The main criteria for selecting a chemical for the ‘first bath’ are: it should be a non-alkaline, non-reducing, chelating or complexing agent for ferric iron.
To avoid carry-over of chemicals to the next bath, the print should be briefly washed for half a minute in gently running water.
- Clear #I.
10 minutes in a bath of 5% w/v tetrasodium Edta, with only occasional agitation needed. Red tones will tend to intensify in this bath.
briefly for a few seconds.
- Clear #II.
10 minutes in a freshly-made bath of sodium sulphite or metabisulphite or Kodak Hypoclearing Agent of concentration ca. 2.5% w/v. Very little is agitation needed.
briefly for a few seconds.
- Clear #III.
10 minutes in a second bath of 5% tetrasodium Edta.
30-60 minutes in gently running water.
Drain, face outwards, on a near-vertical sheet of glass, or Perspex (Plexiglass in the USA), and then peg up on a line to air-dry, or place on a horizontal drying screen. Alternatively, if cockling of the paper sheet is a problem, blot or roller it, and allow it to dry between sheets of high quality blotting paper, such as Multisorb, preferably under some pressure.
Summary of the Steps in the New Chrysotype Process
- Pre-hydrate paper [only if RH < 50%]
- Mark up for coating with template, and number the paper
- Dust down paper and clip or tape up for coating
- Mix sensitizer [A:B:C = 4:4:1 standard]
- Coat paper [5 passes standard] & blot off excess
- Rest paper horizontally till sheen goes
- Dry in warm air stream [40 ºC] 10 mins
- Store in CaCl2 [9% RH] if not immediately required
- Hydrate @100% RH @ 20 ºC for timed period or use constant humidity box
- Register with negative in print frame
- Expose to UVA light source 1 to 10 mins
- Post-Hydrate @100% RH @ 20 ºC, 5 to 15 mins, or @ 45 °C 1-2 mins
- Develop in disodium Edta [1%] or other,10 mins
- Rinse in running water
- Clear #I in tetrasodium Edta [5%] 10 mins
- Clear #II in sodium sulphite or KHCA, 10 mins
- Clear #III in tetrasodium Edta [5%] 10 mins
- Wash in running water 30 mins
- Drain on vertical Plexiglass sheet 15 mins
- Dry in air at room temperature and RH
Controlling the Contrast
The contrast of the sensitizer is defined by its printing exposure range, which is conveniently expressed as the negative density range, D, to produce tones from print highlight to deepest shadow. Control of this is achieved, as indicated earlier, by varying the molar ratio of ligand to gold, through the volumes and concentrations of the solutions A: B. A minimum value of molar ratio 2 gives the longest range of tones; however this ratio also corresponds to a very ‘fast’ and unstable sensitizer which tends to suffer fogging of the highlights. As the molar ratio is raised, the contrast increases as indicated approximately in Table 3 below; the performance also becomes more predictable; the clearing of the highlights is cleaner and the exposure times will require lengthening.
Table 3. Dependence of sensitizer contrast on ligand: gold molar ratio.
|Ligand:Gold molar ratio:||2||2.5||3||4||5||6|
|Print exposure range, D:||2.8||2.4||2.2||2.0||1.8||1.6|
Controlling the Colour
The colour of a new chrysotype print depends on four main factors: the Version of the chemistry; the humidity of the coating during exposure; the sizing agent in the paper; and the developer used in the wet-processing procedure. In addition, the result also depends on the acidity of the sensitizer: a more acidic sensitizer produces darker and duller reds with less printout.
Hydration of the coating prior to exposure
The primary control of colour is the RH of the coated paper’s environment before exposure. At low values (ca. 10%), the colours are predominantly pink and reddish-brown. As the RH is raised, the dominant colour shifts through magenta, via purple (45%), to bluish-black (80%). At high RH a fine range of grey/black tones is obtained, which compares very favourably with the best that the platinotype process can achieve.
Effect of the sizing agent
To obtain the best red and pink colours, choose a paper that has been tub-sized with gelatine, e.g. Fabriano 5, Arches Aquarelle, T.H. Saunders ‘Waterford’, or Ruscombe Mill’s ‘Talbot’ paper. Alternatively you can surface-size papers yourself with gelatin as described in Preparations. Gelatin tends to stabilise and ‘protect’ the smallest particles of colloidal gold, which appear red. For more neutral blue-black tones and dull magentas, the papers internally sized only with Aquapel may be used – especially Ruscombe Mill’s ‘Buxton’ paper.
Choice of developing agent
The choice of ‘developer’ also influences the final colour, as outlined above: ranging from water through to oxalic acid. These developers act differently in conjunction with the RH. At low RH, citric acid produces pinks, and disodium Edta browns, however at high RH citric acid produces bluish, and disodium Edta greenish, results.
Full instructions on all aspects of chrysotype printing are contained in Mike Ware’s 211 page book The Chrysotype Manual below.
The Chrysotype Manual
The Science and Practice of Photographic Printing in Gold
by Mike Ware
Well researched encyclopedia.
Gold in Photography
The History and Art of Chrysotype
by Mike Ware