dosch_photographic developer storage life

8/13/2019 Dosch_Photographic Developer Storage Life

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08/01/14 Photographic Developer Storage-Life

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Storage life of photographic developers: Phenidone-ascorbic acid,

and metol-hydroquinone types

What are the practical time limits for the individual darkroom?

Michael P. Dosch

Rev. Nov 2000

Abstract

It is well-known that photographic chemicals change their potency over time. The decline

in activity is accelerated when these chemicals are mixed in water solution (1, 2). An

experiment was designed to describe the decrease in activity of phenidone powder,

phenidone in aqueous solution, and a phenidone-ascorbic acid film developer, when these

are kept for various lengths of time. A second part tested various scratch-mixed metol-

hydroquinone (also called MQ or D76-type) developers. Exhaustion was arbitrarily defined

as a 0.2 log density unit decrease in density of the most-dense sect ion of a step tablet,

as compared to the density of the same step when developed at the initial time interval.

The main outcome was measured by densitometer.

Phenidone developers: Mixing one phenidone developer ("PC") from dry phenidone powder

produced inconsistent results. A stock solution of phenidone was exhausted within 4weeks. Storage life of more than 6 months was obtained through a different method of

mixing PC, from PX, and from XtolTM(Eastman Kodak, Rochester NY). The last three

named were also more consistent than the first two methods.

D76-type developers: The longest lasting was a metol-Kodalk variant without

hydroquinone (called DK76b here) at 22 weeks. The rest were all exhausted in 6-12

weeks, except D76TM(Eastman Kodak, Rochester NY) which lasted more than 6 months

and was the most consistent performer. However, with the commercial D76, attempts

made to exclude air in an accordian-type plastic bottle reduced both the storage life and

the c onsistency considerably.

It was concluded that the commercial products D76 and Xtol are consistent performers

whose storage life is underestimated by their product literature (at least, when mixed with

distilled water and stored under the conditions of this experiment). For scratch mixers,

DK76b and two varieties of a phenidone-ascorbic acid developer warrant consideration

since their performance is consistent and they last nearly 6 months before exhaustion.

Key words

Phenidone (1-phenyl-3-pyrazolidone), phenidone A, film development, phenidone-ascorbic

acid film developer, XtolTM, Mytol, photographic solutions-storage, black and white

photography, D76, MQ, metol, hydroquinone.

Introduction

Part 1- Phenidone-Ascorbic acid developers

Phenidone (1-phenyl-3-pyrazolidone, CAS No. 92-43-3), also called phenidone A, is a

developing agent useful for black and white film or paper development. Its formula weight is

162.2 (3). It is said to lose its potency after about six months (2), when kept as the dry

chemical. This claim is difficult to evaluate, since no studies are cited in the sources.

Perhaps even more importantly, storage conditions will vary from darkroom to darkroom.

Unless these are specified, it is difficult or impossible to apply safe-storage figures to one's

own photographic processes. DimezoneTM(Phenidone B; 4-hydroxymethyl-4-methyl-1-

phenyl-3-pyrazolidone) is thought to resist oxidation in solution better than phenidone A ( 4),

but it is not as readily available. Phenidone A can be substituted for phenidone B on a

weight- for-weight basis (4).

Like most developing agents, phenidone should lose potency at a faster rate due to

oxidation once it is mixed in water solution, particularly if stored in partially-filled bottles.

One suggestion for keeping phenidone, which also aids in measuring small quantities, is to

mix a stock solution (Table 1). This stock solution was chosen for testing. It has been

modified from the formula given in the source by the substitution of sodium metabisulfite for

sodium bisulfite, and a change in the total amount of phenidone from 2 to 1.5 gm/L, so that


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each 100 mL contains 0.15 gm of phenidone.. No claims about the keeping properties of this

solution were stated in the original source (5). "The preservative will have no appreciable

effect on any developing formula" (4).

Table 1 Phenidone

stock solution

Distilled water (65.5 C/150 F) 750mL

Sodium metabisulfite 6 gm

Phenidone1.5

gm

Water to make 1.0 L

Phenidone has been combined with ascorbic acid derivatives into a new developer, XtolTM. It

is thought to produce better image quality (less granularity, increased sharpness) and faster

film speed, store longer once mixed, be more environmentally friendly, and present less risk

of allergic reactions than developers based on metol or hydroquinone (2, 6). It also mixes atroom temperature. Because of these advantages, it was chosen for test, as well as a a

modified version, suitable for compounding by anyone with access to the raw materials ("PC"

developer, see Table 2). It was modified from the Paul Lewis Mytol formula (see 2, 6) by the

substitution of ascorbic acid (formula weight 176.1) for sodium ascorbate (formula weight

198.1). Scales are generally only accurate to the nearest 0.1 gm. To prepare phenidone

0.15 gm, dissolve 0.3 gm Phenidone in 200 mL distilled water (65.5 C /150 F), then use

100 mL.

Table 2 Phenidone-Ascorbic

acid developer ("PC")

Distilled water

750

mL

(27 C / 80 F)

Sodium sulfite anhyd. 60 gm

Sodium metaborate 4 gm

Ascorbic acid10.7

gm

Phenidone 0.15gm

Sodium metabisulfite 3 gm

Distilled water to make 1.0 L

Another phenidone-ascorbic acid developer ("PX") was compounded for testing. This was a

formula modified by the author from the patent documents for XtolTM(6) by omitting the

chelating agent EDTA, the use of phenidone A instead of B, and the use of ascorbic acid

instead of sodium isoascorbate (Table 3).

Table 3 Phenidone-ascorbic acid

developer ("PX")

Distilled water (65.5 C /150 F)200

mL


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Sodium sulfite anhyd.10

gm

Sodium metaborate 4 gm

Phenidone0.2

gm

Distilled water (27 C / 80 F) to

make

750

mL

Sodium sulfite anhyd.75

gm

Sodium metabisulfite3.5

gm

Ascorbic acid10.7

gm

Distilled water to make 1.0 L

Part 2- Metol-hydroquinone (D76- or MQ-type) developers

Once the methods were established, it was decided to study several MQ developers. These

are listed in Table 4. D76, as produced commercially by Eastman Kodak, was chosen as a

control. After all, the reason for compounding chemical solutions is to ensure optimum

freshness and consistent activity. If I can't do better than purchased D76, it is time to re-

examine my assumptions. The published formula for D76 is given (5). The commercial

product D76 probably contains proprietary additives in addition to the ingredients shown.

D76H is advocated by Anchell and Troop (2) as consistent. As compared to D76, the formula

omits hydroquinone, and increases metol to 2.5 gm. My customary developer is DK76a,

which is D76 with the substitution of sodium metaborate (Kodalk) for borax. I also created

and tested DK76b, which is similar to D76H, but with a similar substitution of sodium

metaborate for borax. Finally, I studied what I call MC here. This formula is a metol-ascorbic

acid developer (7) which is thought to promote surface development, which is associated

with higher definition and less graininess. Also, I was interested in it as an example of an

ascorbic acid developer.

Table 4 Metol-

hydroquinone

(D76- or MQ-

type)

developersD76 D76HDK76a DK76b MC

Water mL 125

F/52 C750 750 750 750 750

Metol gm 2 2.5 2 2.5 2.5

Sodium sulfite

anhyd. gm100 100 100 100

Hydroquinone

gm 5 5

Ascorbic acid gm 10

Borax

decadhydrate gm2 2


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Sodium

metaborate gm2 2 35

Potassium

bromide gm1

Water to make1.0

L1.0 L 1.0 L 1.0 L

1.0

L

Thepurposeof the first part of the study was to describe the loss of activity over time of:

1. Dry phenidone powder

2. Phenidone stock solution

3. PC developer solution

4. PX developer solution

5. XtolTMsolution

The purpose of the second part of the study was to describe the loss of activity over time

of the MQ-type devlopers.

A secondary purposeof the study was to use the various developers, become familiar with

them, and create a family of curves that would provide useful information to anyone desiring

to compound them. The importance of this study is not only economic ("How can I getsolutions to last for the maximum amount of time and save money?") but the removal of

another barrier to consistency in black and white processing ("How can I be sure I am using

chemicals and solutions as long as they are effective, and no longer?").

HypothesesIt was expected that dry phenidone powder would lose activity after six

months, and phenidone stock solution, PC developer, PX developer, or XtolTMafter one to

two months. It was expected that most of the MQ deveopers would lose act ivity after 1-2

months, and that D76H would last longer.

AssumptionsIt was assumed that loss of activity could be measured as the difference

between the maximum density recorded from a step tablet contact printed on film initially,

compared to the density of the same step on a similar piece of film, exposed and developed

under the same conditions, but at a later time, with "aged" chemicals or solutions. It wasassumed that if these test procedures, time intervals, and storage conditions were

specified, other photographers could benefit from the information.

Materials and methods

Design

This prospective, controlled study was designed and performed over a twelve month period

beginning April, 1999.

Instruments and materials

A Gossen LunaPro Digital light meter and Beseler Audible/Repeating Enlarger Timer (CatalogNo. 8177) were used to ensure consistency of film exposure. The same step tablet was used

throughout. An XRite 810 densitometer was calibrated before each use, and used to read all

films. An Eastman Kodak Process Thermometer Type III was used to establish a temperature

of 20 degrees C for all developing solutions, and for the tempering water bath used to keep

solution temperatures constant during the course of development, stop bath, and fixer. A

GraLab 300 timer was used to time processing steps. An AccuLab V-400 electronic scale,

readable to 0.1 gm, was used to weigh chemicals. It was allowed to warm up for 30

minutes, and calibrated against a supplied 200 gm weight, before each use. The materials

used are specified in Table 5 ("Formulary" is Photographers Formulary, Condon Montana,

800-922-5255, http://www.photoformulary.com).

Table 5 Stock

numbersSodium

metabisulfiteFormulary

10-

1280

10-
http://www.photoformulary.com/


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Phenidone Formulary 0870

Sodium sulfite

anhyd.Formulary

10-

1340

Sodium metaborate Formulary10-

1285

Ascorbic acid Formulary 10-0241

Acetic acid 28% Kodak146

2829

Universal Rapid

FixerIlford

741

850

PhotoFlo 200 Kodak146

4502

Xtol Kodak 8888182

Metol Formulary10-

0770

Hydroquinone Formulary10-

0670

Borax decahydrate Formulary10-

0260

Potassium bromide Formulary 10-0930

Procedures- Film testing

Only distilled water was used to prepare developers and the phenidone stock solution. All

developers were diluted to working strength with distilled water. Whether diluted or not, all

developers were used one-shot and then discarded. Tap water was used in all other

solutions unless specified below.

Tmax 400TM4 x 5 inch sheet film from two 100-sheet boxes (expiration March 2001) was

used for all tests. Over the course of the experiment it was stored in the freezer, in its

original container or a 25 sheet sized box, wrapped in a sealed plastic bag to excludemoisture. The freezer temperature was not monitored, but no power failures were

encountered during the experiment. The film package was removed from the freezer at least

three hours (small box) or six hours (large box) prior to exposure. Development began within

an hour after exposure (usually immediately). For each test in Part 1 (phenidone-ascorbic

acid developers), one sheet of f ilm was exposed in contac t with the step tablet, and

developed for 11 minutes. In Part 2 (D76 types), standard development time was 8 minutes.

The density of the most-dense area of the step tablet was determined initially, and then at

each time interval. Testing of each solution ceased when exhaustion occurred, defined as a

0.2 log D unit density loss, as compared to the maximum initial density.

Testing procedures were based on methods specified by Phil Davis (8). Consistent film

exposure was ensured by using the same light source (Beseler 45M with condenser head),

the same time and f/stop setting (0.4 sec and approximately f/11), and the same step

tablet throughout. The enlarger head height or f stop of the enlarger lens (but not the time)

was adjusted for each exposure, which produced incident light meter readings of EV +0.5 on

the easel. Temperature of the developer, and the tempering water bath, were measured and

brought to 20 degrees C (68 degrees F) prior to beginning development. Film was processed

in BTZS tubes (Darkroom Innovations, Fountain Hills, AZ), with constant manual rotary

agitation.
http://www.darkroom-innovations.com/


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For selected developers, a family of curves was generated in Plotter (9) in the first week of

the experiment to describe their characteristics. Testing intervals planned for phenidone

developers are shown in Table 6. Because it was anticipated that MQ-type developers

would exhaust more quickly, it was planned to test them every one to two weeks initially,

then every month after 8 weeks had elapsed.

Table 6 Testing schedule for phenidone developers

Storage

life of...Solution Dilution

Testing

intervalPreparation

1

Powder

PC from

powder1 + 1

Initial,

then

monthly

Mix PC fresh

each month

from dry

phenidone

2 Stock

solution

PC from

stock1 + 0

Initial;

each

week x4; every

two

weeks x

4; then

monthly

Mix PC at each

interval from

phenidone0.15% stock

solution

(prepared at

initial time

interval)

3 PCPC from

stock1 + 0

Initial;

each

week x

8; every

two

weeks x

2; then

monthly

Use PC (mixed

from stock at

initial

interval)

4 PX PX 1 + 1

Initial;

eachweek x

8; every

two

weeks x

2; then

monthly

Use PX (mixed

at initial

interval)

5 XtolTM XtolTM 1 + 1

Initial;

eachweek x

8; every

two

Use XtolTM

(mixed at

initial


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weeks x

2; then

monthly

interval)

Post-development procedures

At the end of development, film was placed in SB1 stopbath (48 mL acetic acid 28%, in 1L

tap water) for one minute, then fixed in a tray of Ilford Universal Rapid Fixer (1+3 in tap

water) with constant rocking agitation. Fixing time was set at 5 minutes (normal clearing

time in this lab with this combination is approximately 1.5 to 2 minutes). Film was rinsed in

tap water for 3 to 5 minutes, then rocked in a tray of sodium sulfite 2% for 5 minutes as a

wash aid. Washing in an archival 4 x 5 washer (Red Village Products, Geneva NY) proceeded

for 25 minutes. Film was rinsed in PhotoFlo 200 (1 mL in 200 mL distilled water) for one

minute, then hung to dry at room temperature.

Storage Conditions

Chemicals were kept in tightly closed, amber glass bottles. After mixing, all developers and

the phenidone stock solution were stored in narrow mouth 500 mL size amber glass bottles,

partially filled (unless noted otherwise). These were placed in a basement darkroom, in a

closed cabinet, and a weekly log of temperatures was kept. The temperature of this room

ranged from 18.2 to 23.8 degrees C (64.7 to 74.8 degrees F) during the course of datacollection (April 1999 to January 2000).

Data analysis

Densitometer readings were recorded for each time interval in Microsoft Excel. Graphs were

prepared in Excel or Phil Davis' Plotter software. Missing values were interpolated for the

graphs so they would be clearer, but tables of original data with missing values were also

prepared. Curve shapes were plotted for the initial family of curves for each developer

solution at varying times of development, and for the initial and final film test for selected

developers.

Results

Developer storage life

Log density readings for the phenidone developers are found in Table 7. The results of one

film test are shown in each cell of the table (i.e. these are not averages of two or three film

readings). Coefficient of variation is the sample standard deviation, divided by the sample

mean. It, as well as standard deviation and range, are ways to show how much variation

there was in density, over the time course of the study.

The initial density for Xtol is much higher than the other developers. The three developers

with the least variation are PC from stock, PX, and Xtol. Useful life, defined as the time

before a change of 0.2 units from the initial density, was longest in these three developers

as well.

Table 7 Phenidone developers

WeekPC from

powder

Stock

solution

PC from

orig stockPX

Xtol

1 0.9 1.18 1.18 0.99 1.78

2 1.24 1.19 1.02 1.93

3 1.15 1.17 0.98 1.89

4 1.11 1.15 1.02 1.81

5 0.68 1.05 1.16 0.95 1.9

6 1.23 1.04 1.97

7 0.95 1.13 0.98 1.83


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8 1.14 0.94 1.9

9 0.6 0.56 1.11 0.84 1.86

11 0.06 1.0 0.88 1.67

13 0.41 1.09 0.88 1.82

17 0.85 1.06 0.88 1.75

21 0.57 1.02 0.85 1.94

25

28 1.04 0.96 0.79 1.89

Mean 0.72 0.91 1.11 0.93 1.85

Std.

deviation0.22 0.40 0.08 0.08 0.08

Coeff. of

variation 0.30 0.44 0.07 0.08 0.04

Range 0.6 1.2 0.3 0.3 0.3

Exhaustion

(weeks)4 6 27 27 40

Figure 1shows the same results for the phenidone-ascorbic acid developers graphically.

The initial densities are shown as zero, and changes over time are shown as deviations

above or below the zero horizontal line. Values were interpolated for intervals which were

not tested to produce a more readable graph.

Table 8shows similar data and summary statistics for the MQ-type developers. The initial


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density for MC is much higher than the other developers. The developer with the leastvariation is clearly Kodak D76, although the variability is also low in D76 (no air), D76H, and

DK76b. Useful life (time to change of 0.2 units from the initial density) was longest in D76,

followed closely by DK76b. All the rest lasted less than 14 weeks.

Table 8 MQ-Type developers

Week D76D76 (no

air)D76H DK76a DK76b MC

1 1.44 1.29 1.20 1.56 1.35 2.243 1.43 1.40 1.23 1.63 1.40 2.16

5 1.42 1.33 1.14 1.55 1.41 2.15

7 1.39 1.52 1.16 1.64 1.38 2.15

11 1.40 1.52 1.10 1.57 1.30 1.99

13 1.36 1.43 0.96 1.70 1.25 1.89

15 1.46 1.53 0.92 1.81 1.29 1.82

21 1.46 1.59 1.24

23 1.48 1.80 1.16

25 1.56 1.94 1.11

Mean 1.44 1.44 1.11 1.67 1.29 2.06

Std. deviation 0.05 0.10 0.11 0.13 0.10 0.15

Coeff. of

variation0.04 0.07 0.10 0.08 0.07 0.07

Range 0.20 0.24 0.31 0.39 0.30 0.42

Exhaustion

(weeks)

>

246 12 14 22 8

Figure 2shows the same results for the MQ-type developers graphically. The initial

densities are shown as zero, and changes over time are shown as deviations above or below

the zero horizontal line.


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Gradation characteristics of developers at exhaustion

Curve shapes at exhaustion did not differ greatly from that found initially. They gave the

appearence of a curve with shorter development time. Figure 3shows PC developer curves

initially and at 28 weeks.

Figure 3 PC curves at 0 & 28 weeks.

Figure 4shows the curves for D76H developer initially and at 14 weeks.


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Figure 4 D76H at 0 & 14 weeks.

Comparability of scratch-mixed versions to the commercial products

The curve families for the phenidone-ascorbic acid developers were similar in appearence.

Thus PC or PX could be expected to produce generally similar results to Xtol in gradation,

while they differed in development time needed and effective film speed.

Figure 5 Xtol


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Figure 6 PC developer

Figure 7 PX developer

The curve families for the MQ-type developers were also similar in appearence. DK76b

produced identical results to Kodak D76 in terms of effective film speed, gradation, and

development time.

Figure 8 D76 (Kodak)


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Figure 9 DK76a

Figure 10 DK76b

Discussion

Individual developers

Phenidone-ascorbic acid developers

Xtol was the most-active of these developers, producing a maximum density of 1.8, as

compared to 1.0-1.2 for all other phenidone developers tested, with a standard developing

time of 11 minutes.

Phenidone was hard to measure accurately. This may have been the cause of the wide

swings in activity I saw when testing PC developer made from the dry phenidone at each

time interval. This is due in some part to the fact that any scale made to measure to the

nearest 0.1 gm will read "0.2 gm" when masses of 0.151 to 0.249 gm (0.098 gm range) are

on the balance. This range is an significant fraction- nearly half- of the mass you are

weighing. Compare this to the same errror with a larger mass: when you are weighing 10.0

grams, the scale displays "10.0 gm" with weights from 9.951 to 10.049 gm (still a 0.098 gm

range, but this time only around 1% of the weight on the pan).

Because of the inherent difficulty of weighing small quantities of phenidone, I had high hopes

for the stock solution mixed at the initial interval, and used to measure phenidone in makingPC developer at each test ing interval. But the act ivity of this stock solution was

significantly poorer than the keeping qualities of PC, PX, or Xtol developer themselves. I

wouldn't recommend using it.

I didn't make a formal test of the gradation of PC or PX compared to Xtol. But the shapes of

the curve families are reasonably similar, so they would be worth exploring further. They did


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not produce the same effective film speeds as Xtol. They required longer developing times

to produce the same densities on film. But either lasted six months or so before exhaustion.

If you are interested in scratch-mixing your own Xtol, either formula would be worth testing.

Finally, I was surprised by how hard phenidone is to dissolve. I tried dissolving it in a small

quantity of isopropyl alcohol, which was not particularly effective. Heat was less effective

than I would have guessed before beginning. I had to stir phenidone for 7-10 minutes before

it would dissolve, even with distilled water at 150-170 degrees F.

D76-type deve lopers

There's certainly nothing wrong with Kodak D76. Its useful life exceeded 6 months in this

experiment. Its variability (standard deviation) week to week was half that of any other

developer tested.

Strangely, attempts to prolong its useful life by storing it in an accordian-type plastic bottle

and excluding air resulted in lesstime before exhaustion occurred! I can only guess that the

plastic container was not impervious to oxygen in the room air. Otherwise I cannot account

for this- excluding air from the bottle "should" have worked, according to everything I have

read.

D76H has been advocated by Anchell and Troop (2) as a developer which is much more

stable than D76. I found that it was reasonably stable (exhaustion 12 weeks).

Two of the other developers in this experiment are modifications of D76H. DK76b is adeveloper with increased metol and no hydroquinone (like D76H), but with sodium

metaborate substituted for borax. MC is a developer with increased metol, but with ascorbic

acid instead of hydroquinone, and a large amount of sodium metaborate instead of sodium

sulfite.

DK76b turned out to be the developer of choice. Its gradation, film speed, and developing

times mirror those of D76 or my former standard developer, DK76a. And it was quite stable

week to week, with a very long shelf-life (22 weeks) besides. Relying on metol alone rather

than the MQ superadditive combination may mean that this developer might be more

compensating (less tendency to block highlights), like D23 is said to perform.

MC still interests me, in spite of its modest shelf-life of 8 weeks, because of the large

quantity of ascorbic acid I now have on hand, and because it has the potential for

compensating development (no hydroquinone), room temperature compounding (nohydroquinone), and sharpness (no sodium sulfite). I will be examining the curve family and

pictorial qualities of this developer in the future.

Storage life

Why are my results so much more liberal than published recommendations? Kodak suggests

that the shelf life of D76 is 1-2 months. I found excellent activity remained at 24 weeks (6

months). I would speculate that glass bottles were important and would recommend their

use instead of plastic- they are impervious to oxygen. The difference in shelf-life between a

single batch of D76, split between a glass bottle and a Falcon plastic ac cordion-type bottle

was notable. Secondly, consistent pH and lack of any contaminants makes distilled water a

clear choice if you want maximum storage life and consistent results. Again, this is

speculation, because my data only relate to storage life, I did not test what factorspromote it in any given developer.

When a manufacturer makes their recommendations, it is on the basis of multiple tests, with

a variety of storage temperatures, water samples, and conditions of storage such as varying

temperatures. My data show a way, certainly not the only way, in which to produce

prolonged shelf-life in your own darkroom.

Limitations

This method doesn't formally evaluate curve shape, or the gradation of photographs taken

with progressively aging developer. Nor does it evaluate any corrective actions (could one

simply develop longer and get good results with an "exhausted" 9 week old batch?). The

results can be applied to other photographer's process controls only to the degree their

storage conditions match mine. Finally, it is unknown how long the phenidone I received in

April 1999 had been on the shelf prior to shipping.

The formal language I use masks one technical flaw. Because of budget and time

constraints, I tested each developer with one film sheet at each time. Had I tested two or

three sheets of film, I would have had a better idea of the test to test variability, and been


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able to apply statistical techniques such as life table analysis or analysis of variance.

Without these techniques, I cannot state with confidence what part of the week to week

variability is due simply to random variations in the methods (such as the small variations

between one sheet of film and another), and what part is due to change in developer

activity over time.

Conclusion

It was concluded that the commercial products D76 and Xtol are consistent performers

whose storage life is underestimated by their product literature (at least, when mixed with

distilled water and stored under the conditions of this experiment). For scratch mixers,DK76b and two varieties of a phenidone-ascorbic acid developer (called PC and PX here)

warrant consideration since their performance is consistent and they last nearly 6 months

before exhaustion.

References

1. Davis P. Photography 5th ed. Dubuque Iowa: Wm C Brown. 1986: 126.

2. Anchell S, Troop B. The Film Developing Cookbook. Boston: Focal Press. 1998.

3. Sigma Chemical Company. Biochemicals and Reagents for Life Science Research 1998.

www.sigma.sial.com

4. Chapman R. Photochemistry. PHOTO Techniques. 1997 March/April; 13.

5. Anchell S. The Darkroom Cookbook. Boston: Focal Press. 1994: 24.

6. Opitz RJ, Zawadzki S. Weakly alkaline ascorbic ac id developing composition,processing kit and method using same. US Patent No. 5,756,271; May 26, 1998.

7. Zakia R, Stroebel L. The Focal Encyclopedia of Photography 3rd ed. Stoneham,

Massachusetts:Butterworth-Heinemann: 1993;774.

8. Davis P. Beyond the Zone System 4th ed. Boston: Focal Press. 1999.

9. Davis P. Plotter version 4.9a, 1993; Matcher version 4.44, 1991.

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Author's Disclosure Statement

The author has no financial or paid consulting relationship with the manufacturers of any

device or material mentioned here. The recommendations made here are the author'spersonal opinions based on his own darkroom practice, consultation with published

materials and colleagues, and his own research.

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