Chiba Type -A Non-Toxic Alternative to Gum Dichromate

106

J. Soc. Photogr. Sci. Technol.Japan.(2007) Vol.70 No.2: 106-112

一般論文

千葉方式-重クロム酸塩ゴム印画法に代わる毒性のない写真法

Halvor BJORNGARD*and Hiroyuki KOBAYASHI*

Halvor BJORNGARD*・小林裕幸*

Abstract The historical photographic printing system gum-dichromate or" gum printing", is used worldwide by artists and in art based photography education. It utilises the highly dangerous chemicals dichromate and often formaldehyde. Here we present two alternative methods for making "gum prints" with gelatine or gum arabicum. These are based on free radical polymerization with ammonium ferric citrate as sensitizer. The gelatine-based process is simple, using hydrogen peroxide as developer, the gum-based process includes ammonium persulphate in the photosensitive layer and has therefore problems with a" dark reaction". Our process also avoids hardening of the substrate, by a modified work path.

要旨歴史的な写真印画法であるゴム印画法は芸術家や写真教育の場で広く用いられている.このシステムでは非常に危険な化合物,重クロム酸塩が用いられている.また,硬膜剤としてホルムアルデヒドが用いられることも多い.本研究は,これらの危険な薬品を使わなくてすむ,ゼラチンあるいはアラビアゴムを用いたゴム印画法の代替法を紹介する.これらの方法は,クエン酸鉄(III)アンモニウムを感光化剤としたフリーラジカル重合に基づく.ゼラチンを用いたプロセスは簡単で,感光により生成したFe(II)と反応してフリーラジカルを発生させるのは現像液中に加えられた過酸化水素水である. 一方 ,アラビアゴムを用いたプロセスでは,フリーラジカル発生剤として感光層に過硫酸アンモニウムが加えられる.したがって,後者のプロセスでは暗反応が問題となる.これらのプロセスは硬膜剤を用いなくてすむよう工夫されている.

Key words: Non-toxic gum printing, gelatine polymerization, gum arabicum, ammonium ferric citrate, hydrogen peroxide キーワード:毒性のないゴム印画法,ゼラチンの重合,アラビアゴム,クエン酸鉄(III)アンモニウム,過酸化水素水

1. Introduction strong paper (about 200-300 gsm) is first shrunk by soaking in water and dried, it is then given an extra size with gelatine and We have previously made a non-toxic version of Carbon print- hardened, using either formaldehyde, alum or glyoxal. This pro- ing using ferric-based polymerization of gelatine 1). A related and cedure prevents staining from the pigment and strengthens the perhaps more popular printing method is gum dichromate 2) paper as it goes through several cycles of wetting and drying and which utilises the same chemistry as Carbon printing and is will shrink further, which impairs registration of the negative. therefore equally dangerous to the user. The hardening may also contribute as a preservative. The paper The target here is to emulate gum-dichromate in a non-toxic is then coated with a solution of gum arabic, dichromate and a manner. Gum printing is mainly performed with gum arabicum pigment. It is exposed and the unexposed parts are removed in (acacia gum) but also with any material that can be cross-linked water. The obtainable density and exposure range from one layer with dichromate, and that will hold the pigment. is limited; therefore, a full image is built up from repeated print- Attempts were done with gelatine, gum arabic, poly vinyl ings. The process can be varied and manipulated almost infi- alcohol, albumin, starch and dextrin, using ammonium ferric nitely by employing the various binders, pigments and multiple citrate as sensitizer (hereafter AFC). While image-forming coatings. activity was observed for all these materials, we have focused on The two points we avoid with our method are dichromate and gelatine and gum arabicum. The way to use these two materials hardeners. differs due to the easy water solubility of gum arabic. The primary interest here is the concept of the process and 2. Experimental the behaviour of the sensitizer. 1.1 Gum Printing Sizing and solutions are made with distilled water, develop- Gum printing can briefly be described as follows; a suitable ment and clearing steps with tap water. Working environment

平成18年12月25日受付平成19年1月31日受理 Received 25th, December 2006, Accepted 31st, January 2007 *千葉大学自然科学研究科〒263 -8522千葉市稲毛区弥生町1-33

Graduate School of Science and Tbchnology, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan H. BJ0RNGÅRD and H. KOBAYASHI Chiba Type 107

was lit by 20 watt tungsten bulbs and kept at 40-60% RH. remove eventual stain. Therefore it must also be applied anew While these printing methods can be performed on any mate- in-between each coating. As this size dissolves, a higher pig- rial compatible with the ferric, and which will hold the binder, ment concentration than what has been usual might be possible. the usual substrate is art papers. We have tried the following From about the 3 printing, two layers of sizing might be neces- brands: Arches; Platine and Aquarelle, Fabriano Artistico, and sary between each coating, depending on the paper. some hard sized Japanese papers, all of them gave good results. Because this approach depends on dissolving the sizing, the The important point here is that the paper is not pH buffered 3). gelatine process can be sized with either gelatine or gum arabic, 2.1 Materials as both these will dissolve in warm water. The gum process uses We used Nitta P-3201, acid processed pigskin gelatine for all only cold water therefore gum has to be used as a sizing agent as samples with the gelatine-based method. We have also tried the gelatine will not dissolve. lime-extracted bone gelatine (ossein), but found it more difficult This treatment avoids the use of hardeners. If a preservative to coat due to the higher viscosity4). Ammonium ferric citrate is necessarily, we suggest thymol.

(AFC) is the green variety from Wako with an iron content of Humidity of the paper affects sensitivity therefore this factor 14.5-16%. Gum Arabicum powder and all other chemicals men- should be kept constant. tioned are from Wako. Pigment is Holbein's HWC Lamp Black, a

Japanese produced watercolour pigment which is very reliable 3. Results and Discussion when measured in very small samples. All tests shown are made on Arches Platine paper. We present two methods, first results with gelatine then for

2.2 Sensitometry gum arabic.

The samples shown are primarily from one layer, which is a 3.1 Gelatine as Binder small part of the characteristic curve for the final image. When The paper is prepared and brush coated (sized) with 3% clear analysing the samples we have used the values at about 10% and gelatine, then dried. It is important that the paper is as dry as

90% of the Density for gamma, and for maximum Density possible before coating the photosensitive solution, since a values. Speed comparison is done at about 10% of Density. The humid paper surface may lead to staining. Staining has been presented data is as reliable as we could make it, however coat- observed with working environment above 60% RH. ing skills is a very influential factor here, also fluctuations in The solution is coated using three brushes, one to spread the humidity (the weather) affects absorbency of the paper and solution, one to dry it, and one to smooth it, an alternative sensitivity. method here is to use a foam roller 6). The brushes should be

2.3 Equipment able to absorb excess solution. We have used 1ml of this solu-

pH was measured with a Horiba pH meter M-81. Printing tests tion (Table 1) to coat about 50cm2. It should be noted that tem- were exposed with a 2 KW metal halide bulb at 1 meter distance, perature of the work environment is an important factor here. with a vacuum frame, using a Stouffer T2115 step tablet. Based on our results we suggest this recipe as a starting point

2.4 Processes for optimisation.

Using gelatine as a binder we could utilise a process more or The coated paper is dried without heat, exposed and devel- less identical to the ferric based Carbon method, however gum oped in a 0.3% solution of hydrogen peroxide. We have not found arabicum dissolves in cold water where gelatine needs water a minimum time in the peroxide bath, but one second is suffi- above •`40•Ž to melt. This means that while the gelatine based cient. We suggest 5 seconds to ensure an even development. process is developed in a hydrogen peroxide solution at room What is important is that the paper is smoothly and fully temperature, the gum arabic coating dissolves in this solution immersed in the solution. If it breaks the surface and come in before it can be polymerised. Inclusion of ammonium peroxodis- direct contact with air, uneven development results, presumably ulphate in the photosensitive solution is an alternative approach because of oxygen inhibition 7). for gum arabic as binder. For clearing, we can simply pour 40•Ž water over the paper,

2.4.1 Registration and sizing the unexposed parts are washed off in about one minute.

To reduce shrinkage we have used the following procedure: To ensure removal of leftover iron, a final bath with a solution

The paper is coated on the backside with gelatine then glued of EDTA (ethylenediaminetetraacetetic acid, disodium salt) can onto a sheet of glass. The edges of the paper are taped to the be used 8) However, there is usually no visible traces of iron left sheet with gaffa tape. This tape may also aid registration of the negative. The gelatine glue prevents buckling of the paper and Table 1 While every user will have to reach a preferred recipe because the tape limit shrinkage and prevents water from reaching the factors as paper type and humidity of environment will vary, backside of the paper. This gelatine can be removed later with we suggest this as a starting point for optimisation. The 3% warm water. This is an adaptation from a method using dry gelatine solution gives a longer exposure range and the 0.5 ratio AFC to gelatine performs well regardless of pH of mounting of the paper 5). solution. Pigment concentrations up to 0.8g pr 10ml has been The actual wet time in our processes is reduced to a total of a used without staining. few minutes per layer, compared with the 30 minutes to one A suggested recipe is: hour usual with gum dichromate. This reduces the strain on the 0.3g gelatine paper and may allow a larger selection of papers to be used than what has been normal in gum printing. 0.1•`0.8 g Lamp Black 0.15 g AFC The mounted paper is coated (sized) with a solution of Water to 10 g gelatine or gum arabic on the surface. This prevents staining. This coating will dissolve in the wet procedure and therefore Adjust to pH4 with HCl, this is optional, but increases speed. 108 J. Soc. Photogr. Sci. Technol.Japan Vol.70 No.2 (2007)

Fig. 1 Effect on the characteristic curve in one layer when increasing weight ratio of AFC to gelatine content, pH of solution adjusted to 4 with HCl, pigment 0.25g in 10ml 5% gelatine solution. Exposure time 5 minutes. The 0.5 ratio gives best combination of exposure range and speed. Speed and contrast increases with concentration.

Fig.2 Effect on the characteristic curve with one layer by increasing Fig.3 Overview of influence of varying AFC concentration on; weight ratio of AFC to gelatine content, pH of solution 6. gamma, exposure range and sensitivity for two pH values. At Pigment 0.25g in 10ml 5% gelatine solution. Exposure time pH4 higher concentrations gives higher sensitivity, has a lim- 5 minutes. A minimum acceptable ratio is 0.5. ited influence on exposure range and gives a slightly higher gamma. For pH 6 there is a large improvement in sensitivity and exposure range at a ratio of 0.5, but gamma increases with after the clearing. concentrations. 3.2.1 Ammonium Ferric Citrate The performance of ferric as a sensitizer is strongly related to pH8). On exposure the ferric is reduced to ferrous, the ferrous photoproduct is easily oxidized back to ferric again, which reduces efficiency of the system. Optimum performance is at about pH4. Depending on gelatine type the suggested recipe (Table 1) has a pH of 6, Figure 1 and 2 shows the different behaviour of one coating with 5% w/v gelatine and 0.25 gram pigment to 10ml solution while varying the ferric concentration towards minimum values. Fig.4 The Influence of increased gelatine concentration on the At pH6 the minimum concentration is a 0.5 weight ratio AFC characteristic curve for one layer. Ratio AFC to gelatine 0.5 to gelatine. When the solution is adjusted to pH4 with HCl a (increasing with gelatine content), pigment constant with weight ratio of 0.1 AFC to gelatine still gives acceptable results 0.25g per 10ml, pH of solution 6. Increasing gelatine concentration increases contrast and density. [Fig.1, 2], because a higher amount of ferrous is present. The most important aspect of the performance is a long exposure range. The sensitivity is not so important here and should sitivity 9) brief attempts on varying gelatine types did not show not be to high as this complicates manipulations of the tone scale any significant differences. With higher viscosity, the coating is by reducing exposure time for the consecutive layers. Density more difficult. can be increased by pigment type, concentrations and the multi- Figure 4 shows how increasing gelatine concentration and ple layers. A long exposure range reduces the necessary num- therefore amount of coated materials affects the characteristic bers of layers for an acceptable print and is of interest for low curve for one layer. Using a higher concentration of gelatine with Density prints. The minimum suggested weight ratio of 0.5 AFC a static ratio AFC to gelatine also increases the amount of ferric to gelatine does at pH6 give an exposure range of 0.75 [Fig.3], coated, which affects the gamma [Fig.3]. The 3% solution gives higher concentrations gives no improvement and lower concen- a longer exposure range and is preferred. trations is reducing log E. At pH4 an optimum exposure range is Increasing gelatine concentration raises gamma, increases also found at the 0.5 ratio, with 0.9 log E. For both pH values the Density, reduces exposure range [Fig.5] and reduces speed [Fig. 0.5 ratio gives best results. 4]. Increasing gelatine concentration means increasing ratio 3.2.2 Gelatine concentration gelatine to pigment [Fig.7, 8], the coating is therefore able to The exact recommended gelatine concentration depends on hold more pigment (less pigment is washed out in the wet proce- the gelatine type and its viscosity. Any type of gelatine should dure), this gives a higher Density affecting gamma and apparent work, while the molecular weight of the gelatine affects the sen- speed. H. BJ0RNGÅRD and H. KOBAYASHI Chiba Type 109

Fig.5 Effect of increasing gelatine concentration on gamma, density Fig.7 Increasing pigment concentration gives a higher gamma, a and exposure range. 0.25g pigment in 10ml, 0.5 ratio AFC to reduction of speed and a higher Density. The reduction of gelatine with increasing gelatine concentration in one layer. A speed is linear for the chosen samples. Increase in Density is higher concentration of gelatine decreases exposure range and limited by the gelatines capacity to hold the pigment, while increases density and gamma, because it holds more pigment. gamma increases as exposure range decreases (Fig.6).

Fig.6 The influence of increased pigment concentration on the characteristic curve for one layer. 3 samples with increasing amounts of pigment in 10ml of 3% gelatine solution, AFC to gelatine ratio 0.5. pH of solution 6, exposure 5 minutes. Lower Fig.8 Example of characteristic curve built up from three layers by concentration gives longest exposure range. increasing pigment concentration. AFC Ratio to gelatine 0.5, gelatine concentration 5%, Pigment concentration at 10ml solution for; Layer 1-0.2g, Layer 2-0.4g and Layer 3- Figure 6 shows the influence on gamma, Density and expo- 0.8 g. Exposure for each layer constant at 5 minutes. Solution sure range from increasing gelatine concentration, using 0.25g pH6. The increased pigment concentration reduces sensitivity and increases density for each layer; building the curve. pigment in a 10ml solution and a 0.5 ratio AFC to gelatine. We prefer a 3% gelatine solution as again, it gives the longest exposure range and because with higher viscosity a smooth coat- sure range as possible and a higher density per layer. With one ing becomes more difficult. Factors such as; relative humidity, layer, the possible obtainable values are limited compared with temperature of the work environment and paper type will have what is normal in printing. Density is a value that can be com- an influence here. pensated for through repeated printings, therefore exposure A second, opposing, consideration is that a higher ratio binder range has initially the priority because it will limit the necessary to pigment is less prone to staining. With Holbein's Lamp Black number of layers. Figure 6 shows the effect of increasing pig- we have not had problems at a 3% gelatine solution with our siz- ment concentration for one layer, using a 3% gelatine solution, a ing method, but again other paper types or pigments may give 0.5 ratio AFC and no pH adjustment. Strictly, there is no one diferent results which may favour a higher concentration of correct pigment concentration, but for this experiment values binder. above 0.4 gram in the 10ml solution significantly increases 3.2.3 Pigment Concentration gamma [Fig.7], this is useful when adding deep shadow details. Ideally, what one want is the combination of an as long expo- Increasing pigment concentrations for the 3% gelatine solu- 110 J. Soc. Photogr. Sci. Technol.Japan Vol.70 No.2 (2007)

Table 2 Two parts of the gum solution is mixed prior to coating with one part each of the sensitizer solutions. The mixed solution is not stable and has to be processed immediately. The ferric concentration is a minimum value that gives acceptable results, because higher concentrations will speed up the "dark reaction". The method gives less sharp images and tends to have a higher contrast than the gelatine based method.

A suggested recipe is: Gum stock solution 1.75 g Gum Arabic 5 g water

～0 .59 Lamp Black Sensitizer solution A: 0.05 g AFC 2.5 g water Fig.9 Example of characteristic curve built from four layers by reduc- Sensitizer solution B: ing exposure time. AFC Ratio to gelatine 0.5, Pigment 0.4g at 10 ml with a 5% gelatine solution. Exposure time for Layer 1 is 1g Am. Persulphate 10 min, Layer 2 is 7 min, Layer 3 is 5 min and Layer 4 is 2.5 g water 2.5 min. The reduction of exposure is 0.15 log H per layer with a constant pigment concentration. prints. However successful printing requires time control from mixing the components to the clearing-step. tion reduces sensitivity [Fig.7] because it blocks the exposure. Comparing prints made immediately after mixing the solution Gamma and Density increases with pigment. This is utilised and prints made 30 minutes after mixing the solution shows a when building a full tone scale where the initial layers has a speed increase of 0.6 Log H with identical characteristic curves. priority of exposure range for the highlights and the later layers After one hour, the speed-increase is about 0.75 Log H, but prioritise Density for the shadows [Fig.8]. with loss of tone rendition. Similar results were obtained using 3.2.4 Multiple Layers ammonium ferric oxalate (which have a higher purity) instead of A full tone scale is built up by any number of layers, normally ammonium ferric citrate. between 3 to 6 layers are used, but up to 30 has been claimed. In These results were about similar for both coating and drying the available literature, we have seen two approaches: the tests and then waiting 30 minutes, or mixing the compo- 1: One approach is to keep the exposure time constant and nents and waiting 30 minutes before coating and drying, i.e. then on repeated coatings increase the pigment concentration 10), humidity is not an important factor. this reduces sensitivity and increases density [Fig.7]. Figure 8 It is a possible that this "dark reaction" is caused by im- shows one curve built up by 3 layers using this method. purities in the sensitizer reacting with the persulphate as poly- 2: Another approach is to use the same pigment concentra- merization can be initiated from very low concentrations of tion, but reduce the exposure time 11) [Fig.9]. The reduction of catalysts 14). Additionally the gum arabicum may decompose the exposure in figure 9 is 0.15 Log H for each layer. Probably this persulphate initiating polymerization. Temperature of the envi- method gives best control as it only manipulates one factor (Log ronment is probably a factor to consider. Although, we have H). detected the reaction, we have not yet fully studied its prop- Variations and combinations of these two approaches are erties. possible often a combination is used. Use of several kinds of 3.3.2 Process for gum arabic pigment and colours will also affect the behaviour. The paper is prepared with a sizing of clear gum arabicum. 3.3 Gum arabicum as Binder The standard 17.5% w/v solution is suitable. Addition of ammonium peroxodisulphate to the photosensitive Before coating, mix two parts of the stock solution with one solution makes it possible to use gum arabic. In our gelatine part of each sensitizer solution (Table 2). The stock and the fer- solution, an image cannot be obtained when including persul- ric solution can be stored but the persulphate solution should be phate. mixed for each session. This approach is similar to gum dichro- The basic reaction is well known; on exposure, the ferric is mate. reduced to ferrous. The ferrous is then oxidised back to ferric The concentration of gum arabicum, and lamp black is from a (Eq.1), the oxidiser produces a free radical that initiates poly- generic gum-printing recipe 10). The pigment concentration merization 12): should be varied to manipulate the tone scale! An approach giving consistent results is to mix the compo- S2O82-+ Fe2+ → SO42- +SO4- ・+Fe3+ (1) nents, coat the paper, dry it with a table fan for 10 minutes exactly and proceed with exposure and clearing. Because sensi- 3.3.1 "Dark Reaction" tivity is continuously increasing, a consistent work pattern is One problem with dichromate systems is that gum or gelatine necessary. can reduce the dichromate leading to fogging. This is known as Exact concentration of AFC should be reached after personal the "dark reaction" 13). We have found a similar problem here. experimentation. It is possible to use very small amounts of When the components are mixed, a hardening reaction starts. sensitizer. Increasing AFC concentrations simply increases This reaction is relatively slow and gives an opportunity to make speed [Fig.10]. For weaker light sources, a higher sensitivity H. BJ0RNGÅRD and H. KOBAYASHI Chiba Type 111

Fig.11 Comparison of characteristic curves of one layer; gum sensi- tised with ammonium dichromate and cleared for 20 minutes, and the ferric-based systems for gum and gelatine. AFC 0.05g to 10ml gum, and AFC 0.05g to 10ml 3% gelatine solution at pH4. The ferric-gelatine method gives a result Fig.10 Influence on characteristic curve when varying AFC addition closest in characteristics to the gum-dichromate method. from 0.05g to 0.2g in 10ml standard gum solution with one layer. 5 minutes exposure time. Increased AFC concentration increases speed only, probably partly due to the "dark reac- dissolve the print. With gum dichromate, long development tion". times is utilised to achieve a longer exposure range [Fig.11]. It is possible to force the clearing by spraying water on the might be preferable. Because time affects the sensitivity, image to achieve better definition in the shadow areas, however building up the image by reducing exposure time as shown this also tends to reduce sharpness. Multiple layers with with Figure 6, will not give reliable results unless the reduced reduced pigment concentration will achieve a normal tone scale. time of the exposure is compensated for by waiting the remain- ing time before clearing the image. Increasing the pigment 5. Conclusion concentration might be a simpler way to reduce sensitivity when building up the image as shown with Figure 5. Between these methods, the gelatine-based process is easi-

Our recommended amount of 0.05g AFC is correct for a est, and most reliable. It also gives a sharper result, which with drying time of 10 minutes. With 5 minutes drying time it is this particular genre of photographic printing is not necessarily insufficient, but a 0.1g addition of AFC gives results. the usual positive quality. Gum arabicum (and dichromate) is

Increasing or decreasing the persulphate concentration gives capable of a very subtle quality of sharpness, which may make it either reduced or increased speed in a similar manner to adjust- worth to pursue this particular binder. ing the AFC concentration. The gelatine-based method is functional and can be used as

The dark effect does also influence sharpness and a gum print described. The gum-based method should be seen as a starting made in this manner has a lower sharpness and a higher contrast point for further experimentation. than a gum dichromate print or the gelatine based method sug- As we see it, this is not so much a question of whether ferric gested here. or dichromate gives the best results, often "best" is a direct We have cleared the samples for 2 minutes in water at 15•Ž. result of the printer's skills and experience, but whether one is

Because the sizing dissolves, one should wash water over the willing to or have facilities to, work with toxic chemistry. Basic print during this step to avoid staining. After 20 minutes in laboratory safety is seldom taught at art schools. water, the image completely dissolves. There remains to establish the mechanism of the dark reaction in the gum-ferric method, it is also possible that other per 4. Comparison compounds or other ferric salts could give better results. 5.1 The Name Figure 11 shows a simple comparison of gum-dichromate and Chiba is the place this process was made. In Japanese Chiba is the two ferric-based methods. The two gum curves are from the spelt with two kanji;千葉which literal meaning is "thousand same stock solution. The short clearing time for the gum-ferric leaves". This is not a bad name for a non-toxic printing method, system (2 minutes) gives a higher density and contrast, partly leaves after allbeing light sensitive.(^o^) because less pigment is washed out. It is obvious that a lower pigment concentration would be suitable for this method. References The gelatine curve is more similar to the gum-dichromate result. In the gelatine process, the actual gelatine concentration 1) H. Bjorngard, H. Kobayashi," On Ammonium Ferric Citrate as Sensi- affects density (Section 3.2.2) in addition to pigment concentra- tizer for Carbon Printing", Imaging. Sci. J., in press. tion. For this gelatine curve [Fig. 11] the amount of pigment in 2) W. Crawford, The Keepers of Light. Morgan & Morgan, 1979, the coating solution is half of the gum curves. pp. 199-212. The gum-ferric curve [Fig. 11] shows a speed increase corn- 3) M. Ware, The Chrysotype manual, fotofilm publishing, 2006, pp.47- 49. pared to the gelatine curve. Since the speed increase is observed 4) A. G. Ward, A. Courts, The Science and Technology of Gelatine, with an identical amount of sensitizer, it is presumably primarily Academic Press, 1977, p.113. the result of the dark reaction (Section 3.3.1). 5) http://www.bostick-sullivan.com/Technical_papers/substrat.htm After clearing, the gelatine layer is relatively strong and con- 6) http://www.alternativephotography.com/process _temperaprint.html sistent. The polymerized gum is less strong than the gelatine, 7) G. Odian, Principles of Polymerization, 4th Ed., John Wiley & Sons, leaving the gum image for extended periods in water will Inc., 2004, p.261. 112 J. Soc. Photogr. Sci. Technol.Japan Vol.70 No.2 (2007)

8) M. Ware, "An investigation of Platinum and Palladium Printing", J. 11) B. Andersson, Handbok i Adelforfarande, Studio F, 1987, pp. 197- Photogr. Sci., 34, 19 (1986). 215. 9) H. B. Abrahamson, A. B. Rezvani, J. G. Brushmiller, "Photochemical 12) J. Kosar, Light Sensitive Systems, John Wiley & Sons, Inc., 1965, and spectroscopic studies of complexes of iron (III) with citric acid p. 173. and other carboxylic acids", Inorg. Chim. Acta, 226, 117 (1994). 13) J. Kosar, Light Sensitive Systems, John Wiley & Sons, Inc., 1965, 10) E. J. Wall, F. I. Jordan, J. S. Carroll, ed., Photographic Facts and p. 82. Formulas, Prentice-Hall and Amphoto, 1975, pp. 313-314. 14) US Patent, 3, 101-270 (1963).