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The Dufaycolor Process Product booklet
Product booklet for The Dufaycolor Process. 71 pages in English. Read the original PDF, download or print a copy without registration.
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- The Dufaycolor Process
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- English
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- 71
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By courtesy of Tunbridge.
Rellected light from half.watt Blocks by John Swain & Son,lamps totalling 2,5oo watts and Ltd., Screen negatives rnade
two 5oo watt spots. Exposure direct on to Ilford Rapid Process
3 seconds atFf4.5 on Dufaycolor Panchrornatic Plates through
Flat Filrn through Dr/z filter. Ilford Tricolour Fihers. rio
line screen.View original page 3Page 4 · Read text
)DufaycoX.on PnocessThe
SECTION I
INTRODUCTNON
Colour photogruphy is by no means a new invention, but
its use has been gteutly restricted, partly on the ground of expensg
but primarily b""*" of the necessity for long exPosures and
somewhat difficult manipulation.
The new Dufaycolor process, which is remarkably easy
to manipulate, will undoubtedly provide a stirnulus to colour
photogriphy which will lead it to ito rightful place in the
photographic world.
Whenever it is desired to record an event or reproduce a
scene or person in colour, the correct startilg point is a_photo'
in colour, and now with the Dufaycolol Process this canSaph and with very little moreL i"oduced with ordinary lpparatus*kiil than is required to *o[e a black and white photograph.
From this colour photograph further reproductions can be made
in a variely of ways.
What it i.s and how it wor[<s
The Dufaycolor-film process gives transparencies on a flexiblenon,inflammable urra repioduces robiects in their full
brilliance of colour. The transparency is viewed by holding it
in front of any convenient difiused light source or by projection
in a suitable apparatus.
The film consists of a transparent non.inflammabl.e base
on which is printed a very fine transparent colour patte-rn- con'
sisting of allernating blue and green s_quares and red lines. "patternThis is callei the "r6seau" and in this the complete
colour pattern (of three colours) is reproduced twenty times
fineness,per millimetre or 5oo times per inch. Its extreme
*-ill, therefore, be readily appreciated.
The method of preparation of the reseau is a natter of
much interest and also of lte"y considerable practical difrculty;
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The DUFAYCOLOR Process
the main stelx in the process are shown in the illustration on the
colour plate opposite, and the following is a brief outline of the
procedure:-
The base is first coated with a very thinlayer ofcollodion
dyed blue; afterwards a set of greasy ink lines is printed at an
uoglu to the length of the roll, there being twenty lines to the
millimetre and the spaces between being equal in width to the
lines. The method of printing is comparable with that of
letterpress printing and it will be appreciated that the machinery
required to produce such a result is a triu-ph of engineering
enterprise and skill. I,l
In the next operation the dye between the ink lines is .l
bleached and the second dye bath results in the clear spaces I
being coloured green. The ink lines are now removed and a cta
second set printed at right angles to the first. The lines this
time are broader than in the first instance and the spaces between
are narrower so that there are the same number of lines per
millimetre as before.
The second bleaching bath now removes the green and
blue dyes where there is no protective cover of ink. In the final
bath these clear spaces are dyed red, and lastly the ink lines are
removed. The reason for making the red lines narrower is to
approximately equalise the areas of red, blue and green.
On top of the r6seau to protect it, there is a very thin
layer of varnish and then a coating of a special very hlghly
sensitive panchromatic emulsion (see Fig. r).
.ADHESIVE TAPE EXPOSE ]TIIS SIDE NON FLAI.I SASE
t
RESEAU I Y
THIN VPRNIsH I.AYEA
PROTECIIVE
PAPER. COVER
FIG.I. CROSS SECT|ON THROUGH PIECE OF DUFAYCOLOR FILM
The film is exposed through the base so that the light
passes through the colour r6seau before reaching the emulsion.
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PHOTOMICROGRAPHS OF DUFAYCOLOR RESEAU.
Magnification Xroo Linear.
(t Base dyed blue, then (") BIue bleached
over,printed with greasy
resist in lines.
*4
{'
(:) Dyed grecn. (+) First resist washed off
and reprinted in lines at
right angles to the first set.
(6) Dyed red and resist re-
moved to give final reseau.View original page 7Page 8 · Read text
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Process The DUFAYCOLOR
In this way the image is broken up into three sets of minute
areas which represent respectively the red, green and blue
components of the subject photographed.
Consider now a red object photographed on Dufaycolor
Film; the red light passes through the red areas of the
r6seau but is stopped by the green and blue areas. On
development there rvill be a black silver deposit behind each red
BLUE' GREEN RED
''ln' LIGHT LIGHTI LIGHTI Y Y {
ilmR,-FrR,sT
DEVEIOMAENT
AFTB.
IIEACHINO
ATT€R
Rl-DEytlort|a|l
I I I I
WHITE BLUE GREEN
FIG.2. REPRODUCTION gtVER Bf,OMIDE EMUISION OF COLOURS BY
DEYEI.OPED ATVER IMAGE DUFAYCOLOR
GELANN AFTERDigitallyDN: cn=MikeREMOVALsignedButkus,by Mikeo=ButkusOFButkusSIIVERcamera manuals,IMAGEou=butkus.org,
email=mike@butkus.org, c=USMike Butkus Date: 2023.03.21 20:13:24 -04'00'
filter element, the silver bromide behind the blue and green
elements being unafiected. The next step in the processing of
the film is the bleaching operation, which dissolves away the
developed silver without affecting the unexposed areas. An even
exposure to white light is then grven to make these unexposed
areas responsive to a final treatment with developer, which
converts all the remaining silver bromide to silver. Thus
it will be seen that there is now a black silver deposit
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Process The DUFAYCOX,OR
behind both the blue and green areas. When the film
red is illuminated the light only Pass through .h1 "uo- accurate rePresentation of the filter elements and thus an
original red object is obtained. Similar reaJoning ?PPltT to the
of any_other_ colour and the three colour elements
""p"rodo"tion are such-that all colours can be reproduced. See Fig. z. eriployed
' The fineness of the r6seau pattern avoids any coarseness or
graininess in the image when viewed in the hand or when pro'
j"ected on the screen, the magnification is not excessive' frovided
eye_ by since the final colour picture is formed in the
adding together the light transmitted by all the seParate colour
i'f th" r6seau] this process is known as additive colour "I"*"it photography.
the principles of this for* of were first -photoflophy laid down in 1869, while the first colour ltlates were lrld'rrll'
factured in 1895, and throughout this period uplo the present
day many i*iiov"orents hive been made. Dufaycolor i1 the
lut"rt of ihit fong line of processes and is the outcome of con'
tinued and concJntrated ,lsearch, which has been carried on
with the sole purpose of obtaining accurate colour rendering
combined with-uo method of manipulation. "uty Dufaycolor takes its name from M. Louis- Dufay, who
manufactot"d the Dufay Diopticolor and Dioptichrome Screen
Plates in r9o8 and has ,irr"u b"9" e,ngaged_ in tlie- developing of a
colour mafrix on film sufrciently fine io be applicable to colour
cinematography. such a prgcgss could ogt reach perfection
without iL" df lutgu sums of money and much "*p""diture research, both of which have been made co,op€rative scieltific
ulruiiuble by the foresight of Spicers Ltd.-arrd Ilford Limited,
who have c-o,operated ii the development of this Process and have
brought it to its present_stage of perfection. Intensive experimental
*o"f is proceed^iog uod improvements are being m'ade' "Jortait
The Advantages of D'affaYcotror
Dufaycolor is the most perfect syste_m of additive.colour
photography yet devised in tlat it combines the following
improvements over all previous Processes:
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The DUFAYCOLOR Process
(r) Considerable increase in speed, permitting snaphot
elposures with ordinary inexpensive cameras under a wide range
of conditions.
(z) Greater latitude in elposure
screen ft) Finer and very much less obtrusive colour
elements.
(4) Absence of frilling and blistering.
G) Absence of parallax.
(6) Ease with which three,colonr separation negatives
can be made for preparing colour photographs on paper or
colour blocks for machine printing by any process.
(7) Ability to make duplicate transparencies without
serious loss of colour.
Tlpes of Dufaycolon lFilm
There are two types of Dufaycolor Film:
Tyae D.r . . Film Packs and Films for Leica and *r,Ttl"T*l,.
Type D.a for Flat Film and sub,standard Cinematogrephy.
Note:-Yariations of this method of packing cannot be undertaken, tfius
Typ" D.z material will Not be packed as Roll Film, etc.
Typ" D.r Dufaycolor Film is suitable for exposure in day,
light without any fiher and is therefore particularly suitable for
the amateur user. A filter is necessary for ltgha other than
daylight.
Typ" D.z Dufaycolor Film requires a fiIter under allci,rcumstances (see page ro). It has been decided to market this
type of material for Dufaycolor Flat Film because the highest
possible speed and colour correction is obtainable in this way,
with the various light sources which may be used in practice.
Flat Film therefore grves the perfection required for technical
work. The necessity. for using a filter does not lead to Typ"
D.z material with filter being slower than the Typ" D.r
without filter in daylight.
A further difference is that the Flat Film is on a thicker
non,inflamnable base to make it suitable for use in large sizes.
7View original page 10Page 11 · Read text
SECTION II
\MORKNNG NNSTRN.JCTIOI{S
The development technique and subse_quent tre_atment
recommended in the following sections of this booklet are
applicable to both the Type D.r and Typ" D.z materials.
Arrangemenrs have been made by Ilford Limited for the
processing of Roll Films and sub,standard cin6 Films, so that
materials.|rocessin[ instructions are not issued with these
FUt a"tuits are, however, gven in each carton for loading the
camera, etc. In the case of nilm Packs and Flat Films full
processing instructions are enclosed in each box but the
iollowing pages give further information which will enable
even the-novice to carry through the neoessary operations.
Flat Film-Loading the Dar[< Slide
Owing to the extreme speed and colour sensitiveness of
the emulsion on Dufaycolor Film, it is essential to carry out all
loading operations in complete darhness. A little experience yill
soon this to be done. If the bench is clear and the worker "iu6l"stands still there is little difficulty in doing everything by touch.
The film is packed six sheets in a box, all facing the same direction,
and each piu." of film has hinged to it at one end a sheet of black
paper of exactly the same size which covers and. protects the
beioi.rlrioo.coated side (see Fig. r). This PaPr should notremoved until immediately before development, when it should
be opened our and a gentle sideways pull will cause the
adhes^ive strip to peel eveoly off the film without tearing (see
Fig. :).
As previously mentioned, Dufaycolor Film has to be
'uxposed through the base and therefore special care must be
tut"o in loadingi the base side can be recognised readily j" .1"
dark because it is smooth to the touch and is not protected by the
paper cover. Care should be taken, however, not to impress
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Process The DUFAYCOLOR
finger prints on the base, since thel' -1y _b-e recorded on the
during exposure. Where film holders or sheaths are"-itl.idoavailable, these may be used successfully, but it is importa:rt
to insert first that end of the film which is attached to the
backing paper so that the emulsion is adequately protected from
scratches and the paper cannot crease.
Fig. 3.
For use in plate dark slides of the single metal type, _the
afiln may be placed with its paper protector in contact with
sheet of thin fiat glass and then the edges joined to th9 glass with
narrow strilx of ihin adhesive tape. If it is preferred to use stiff
4cardboard in place of glass it is desirable to use best quality
cardboard, otlierwise t[.e emulsion may be fogged if stored for
long in this way.
With some double dark slides it may be better to Put the
film behind a sheet of glass, that is to say, the glass is_inserted
first, then the film with its emulsion side away from the glass.
This prevents any difficulty due to curlin_g of the film, but neces,
sitatei allowance being made for the thickness of the glass when
focussing, unless the exposure is to be made at a very small
aperture. The thickness of the glass can be comPensated for
ajutomatically by reversing the ground glass of the focussing
screen so that the plain side of the glass is towards the lens.
/Fitrters Since different light sources vary very greatly in their
use spectral composition it is necessary in colour work to
J difi"r"ot filter for each type of light, and in order to obtain
the best results on Dufay"olor Flat Film under all conditions,
special highly efficient fiIters have been m,ade for use with
each type of light. (See also page T.)
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The DUF'AYCOLOR Process
The following are the filters which are available for
Dufaycolor Type D.z.
Code Number.
Dr/t for Daylight.
Dt/z for Half,watt Light (including high eficiency or Pro,
jector type lamps) and Pointolite.
D"/S for Photoflood and similar types of lamp.
D"/+ for Sashalite and similar types of bulbs.
Dr/S . . for Arc Light (white,flame carbons).
Filters can also be supplied for other special purposes on
request, but full details must be given when ordering.
It will be noticed that flashlight is not mentioned above,
because different makes of flash powders differ greatly in the
colour of the emitted light. Flash bulbs of the Sashalite type
are preferable and more reliable.
For Dufaycolor (Typu D.t) Roll Films and Film Packs
are(see page 7), filters issued under the D.r code number
required as follows:-
Code Number.
No filter. . for Daylight.
Dr/z for Half,watt Light (including high efficiency or Pro,
i::Tilli-i"Hm):*ff:ill1r;f ramps. D./t Dt/+ for Sashalite and similar typ& of bulbs.
IMpORTANT NOTE. In order to prevent errors ir is
desirable always to refer to filters by their code numbers.
Filters made for orthochromatic or panchromatic materials are
of no value for Dufaycolor.
Filters consisting of gelatine film are ideal for experi,
mental work, as they are cheap and reliable wheu properly
handled. It is essential that they should not be buckled or
have finger marks, as they may then produce double images and
other defects. They should be handled only by the edges and
stored between paper and they should not be stuck 1o any
zupport. The most satisfactory way of employing a gelatine
filter is to place it between the lens components.
- For regular use cemented glass filters should be employed.O"ly the B quality filters are recommended, as the cheaper A
qualiqy are not suitable for high,class colour work.
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The DUFAVCCLOR Process
Glass filters preferably should be placed in front of the ^but Iens, as they will nofthen affect the focus, if placed berween
the lens and the film it will be necessary to focus on the ground
glass as the focussing scale will be inaccurate. Dufaycolor Filters
can be supplied in three different types of filter mounts.
It should be pointed out that a Dufaycolor transparency
will look correct in colour only if viewed in daylight, no matter
what light was used for exposing, provided the correct taking
filter was employed. If it is desired to view in any other light, a
suitable compensating viewing filter should be used. (So
Page 33).
Focussioug
Focussing is very important since transparencies which
are not quite sharp are usually very ineffective. As mentioned
previously the emulsion is on the back of the film during expos,
ure. With the ordinary type of camera with a lens aperture
have no effect, but when lenses of veryf /+.5 or smaller this will
large aperture and short focal length are employed special care
will be needed to ensure correct focussing. It is also very import,
ant that the film should be held flat and in the correct plane. If
results are obtained which are not sharp the method of holding
the film should be examined.
Exposure
To obtain first,class colour transparencies, considerable
accuracy in exposure is essential, even though Dufaycolor Film
has greater latitude in this respect than any other colour,screen
process. Variations in exposure of at least 8 to r will not cause
serious divergencies in the colour positives if they are processed
individually in accordance with the methods given here. If all
exposures are developed for a fixed time, the latitude is of the
order of .4 to r, but can be increased by subsequent reduction
or intensification. Nevertheless, transparencies of maximum.
brilliance will only be obtained by correct exposure.
Hence, we wish to stress the importance of accurately
estimating all exposures. Success in this direction may depend
very largely on personal skill and experience, but the fol,
lowing notes will enable this experience to be gained very
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Process The DUF'AYCOLOR
rapidly. The table given below will be found to be a reliable
guide for all outdoor exposures on Dufaycolor, but it will be
noticed that special categories are not given for different types
of subjects since in colour photography the correct exposure re,
mains the same whether the subject is relatively near to or far
from the camera and whether or not the subject includes
shadows; this is because exposure should be correct for the high,
lights. Hence, subjects including both very dense shadows and
strong high,lights should be avoided as far as possible. The
times grven below are for the film in conjunction urith its day,
light filter in the case of the Type D.z material, or without
filter in the case of Roll Films, etc., Type D.r.
f+ fs.c fs fn f'6 Beach and sky soenes r/zoo tftoo r/5o t/rs rfto
All outdoor subjects in rfroo t/so t/>S tfrc t/S
sunlight except the above
AII outdoor subjects on t/so t/r5 lrc t/S t/" bright days with sun
obscured
AII outdoor subjects ;;
dull days lrc t/S t/* r '/rS
These exposures are correct for the middle hours of the day
during the summer months. For other times and seasons multiply
by the following factors:-
April, May, November,
Time of day June, July, March, December,
(not summer time). August. September. January.
a.m. P.m. roto2 I , , I
8-ro or r,-^4 I , I
6-8 or ,4-6 , I
I As further useful evidence of the speed of Dufaycolor to daylight
I it *uy be said that it requires Lpproximately lwice the ex'
I ooror" of Selo Roll Film or four timis that for Selochrome Roll
L i it*, when the exposure^Again,for the latter materials yields well
f explsed negatives. Dufaycolor is slightly _slower than tt Ilford Special Rapid Panchromatic Plate to daylight andI "
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Process The DUFAYCOLOR
only upproximately one,third the speed of the latter to half,
watt light when it is used with its appropriate filter and the
plate without a filter.
Naturally, the use of a reliable etlPosure meter will give
greater accuracy than is possible with either of the above
methods. Photo,electric exposure meters are particularly
applicable to colour photography because thel record primarily
t[e light from the brighter parts of the subject. Hence the
reading grven by the instrument is usually correct without
multiplying by any factor intended to compensate for nearness
CELL WINOOW
RED SCAL€
BLACK SCALE
GREEN SCALE NEEDTE
BLACK ARROW
r/nur,reens
RE D ARROW
GREEN ARROTT/
IXED
CENTRE EOSS
MILLED
OUTER DISC
of subject, depth of shadow, etc. The Ilford Photo'Electric
and yet most- reliableExposure Meter (FiS. 4) it of the simplest
and can be strongty recommended for use with_Dufaycolor;
"yp"experience shows that w"ith an exposure,-"t9t of this type the
peicentage of perfect exposures is very high. This- exPosure P"F" is not ooly rrrituble for outdoor work but is a reliable gui{e for
indoor subjects taken by daylight when the exposures _do_qot
exceed one second; also if the mete" readings are multiplied by
be found to be a useful g"id" to studio exposures with5 it will
13View original page 16Page 17 · Read text
rI
t
r
f
1 The DUFAYCOLOR Process
IT lanps. When Photoflood Iamps are em, r $lflrlat_t (gasfilled) ployed_ the factor by which the meter readings uhoua be
F details for the use of th" Ilford multiplied is also 5. Full
Photo,Electric Meter are supplied with the instrument.
For further notes on Photoflood lighting see p. 27.
Points to Renaem.ber ' In colour photography it is well to remember the fol,
lowing points:-
It is not necessa,ry when using Dufaycolm rnaterial
to _seeh for lytghtly coloured obiects tb ph,otograph. Dufay,
colorshould be employed in the normal wuy,pho?ogruphing
the ordinary colours met with in Natur", wheo ihd-ori
pleasing efects will be obtained.
The best transparencies are usually those taken
ytth the lens at a wide aperture; therefore, do not stop I
down much, unless it is esJential to do so to obtain deptl of focus. I
- Strong contrast in the lighting is not required, as
the colour provides the contrast.
Brilliant sunshine grves brilliant colours and is,
therefore, desirable, but heavy shadows may result which
are not always pleasing.
Large masses of colour are more efiective than
small scattered patches.
Colour pictures should, if possible, be taken with
the light_behind the camera: photographing against the
light is often disappointing.
Under exposure leads to dense results and hard
contrasts, while over exposure grro light colours
and lower contrasts, unlesJ the in exposure are
subsequently_ by the development"rtors technique _corrected
recommended here.
Processing
It is important to follow closely the instructions given_here, since ooly^ in this way will the 6est results be obtalned.
Dufaycolor Film possesses the valuable property of withstanding
t4
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The DUFAYCOTOR Process
continual treatment in a variety of baths under all normal con,
ditions without frilling, blistering, or other defects; rherefore, it
will be found quite easy to carry out the processing operations.
The so,called reversal method of processing must be ernployed to -piece produce_ a colour transparency (positivQ from the of
Dufaycolor Film which is actually exposed in the cum&a. On -obtained developing, a negative image is first but, instead of
fixing at_ this stage, the black silver negative image is dissolved
out in a bleaching bath, the remaining silver bromide is exposed
to light during the second exposure and then redeveloped to give
the final positive. This procedure presents no diffi"ul?y in piac,
tice and can be carried out by any amateur.
The use of most ordinary developers on Dufaycolor Film
willlead to failure, and itistherefore desiiable to usethedeveloper
formula recommended and to follow the instructions closely.
For the convenience of different rypes of workers, two
elternative development techniques are given. Where the work
is mainly repetitive or the conditions of lighting are srandar&sed,
the exposures should be accurate and a standard developmentt time will give the best result. Should errors in exposure occur
the resulting transparencies will be too thin or too dense and
can be partially corrected by reduction or intensification, as
explained on page 2,r.
For the amateur or other outdoor worker who cannot
control his exposures so accurately, a method of Factorial
Development is of great vaJue, since small errors in exposure are
autornatically compensated for and larger errors are made to
yield the best results obtainable under the circumstances without
the necessity for any after treatment. But if Constant First
Development is preferred, the after,treatment can be made to
yield as high a percenrage of good resuks as would be obtained
by Factorial Development.
t whichever method is ernployed great care must be taken
I to avoid fogging, as the speed of the emulsion is exceptionally
high. fogging, as from an unsafe darkroom ^lu*p, is -Unifnrm not readily detected, as it does not lead to veiling in rhe ordiiary
sense, but causes a general flattening of gradaiion and colour,
together with a loss oU U"^trt;,View original page 18Page 19 · Read text
\
Process The DUFAYCOn-OR
CONSTANT FIRST DEVELOPMENT METHOD. ((G"Work in total darkness or with an Ilford Safelight shielded
so that the direct light cannot fall on the film.
Immerse in one of the following developers for three
minutes at 65oF. (two,and,a,half minutes at TooF: or two
minutes 7fn.), vigorously rocking the dish the wholetime; "tincreases thJbrilliance of the transparencies. Formula Bagitation
gi.r". slightly more contrasty results than A and has the advan'
i"ge thui the thiocyanate is a more constant and stable chemical
than ammonia. When using the formula A it is important to
ensure that the ammonia is up to its labelled strength.
DE\IELOPER A
Metol r3o grs. 6'5 g-t.
Sodium Sulphite (cryst.) I ozs. roo ,,
Hydroquinone 40 grs. 2'o t,
,.75 ,, Potassium Bromide 50 ,,
Ammonia (Sp. G. 88o) zro minimc rr c.ct.
Water uP to 1o ozs. rooo ,,
DEVELOPER B
Metol ..r30 grs. 6.5 gms.
OZS. roo ,) Sodium Sulphite (cryst.). . I
Hydroquinone 1o grs. 2'O ,,
roo ,, Sodium Carbonate (cryst.) I ozE. or( Potassium Bromide 50 grl. 2'75 ,,
Potassium Thiocyanate (sulphocyanide)
(p*") . . 16o gru. 9', o7,8. rooo c.cs. Vy''ater up to 1o
theIt is important that the chemicals should be dissolved in
order gr*o. Alternative am.monia strengths are Ammonia
Sp. G.-9ro:z9o minilns or 15 c.cs.; Solution of Arnnonia
c.cs.B.P.: r oz. r7o minims or 3.4
These developers do not keep indefinitelyi if a stock solu,
tion is to be kept, tlh"o formula B is preferable, but it is not
advisable to use this if more than two or three weeks old. If
old developer is employed rather weak and flat transparencies
will be obtained.
It is preferable to use fresh developer for each film
unless several^films are developed at the same time; the used
developer malt however, be imployed again for the second
development process (see page zo).
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The DUF'AYCCtOR Process
I When it is desired to develop u number of films together
it is essential that thgl should be held on some ngd supporr so
that they do not one anorher and to ensureihut thi edges -overlap do not curl out of the solution. A sheet of Ilford Filmflat mounled
o_n gfass can be used, the film being squeegeed into contact with
this before commencing development, when it will adhere firmly.
Yh"l pr_ocessing_is _completed the film is srripped off and the Filmflat thoroughly dried, when it is ready for re,use.
FACTORIAL FIRST DEVELOPMENT METHOD. FoT
Factorial Development to be employed, it is essential to desensit,
ise the errulsion in order that the progress of development can be
watched. While there are a number of desensitisers available,
.lZ are not necessarily all suitable for use with the special
developers containing silver solvents employed for the reietsal
development of colour films.
For Desensitising immerse the film in total darkness in a
r : 2ooo solution of Pinacryptol Yellow for two minutes and
then rinse for one minute and truttsfer to the developer.
After desensitising, the Ilford Iso Safelight (red) should
be employed, and, rsing_this safelighr in a darliroom lamp con,
a z5.w_att bulb, the film should not be exposed to its lightlainjng
for long periods, nor at a shorter distance than tlree feet.
The Development Technique recommended is as fol,
lows:-Take two dishes and in the hrst pur one part of either of
the de'eloprs given above and three puti. of waier, and into the
second dish pour full strenEh developer. After the two minutes
desensitising and oneminute rinsing, tiansfer the film to the weak
developer and note the time required in seconds for the first
appearance of the image. The brightest high.lights, such as
sky, etc., can be ignored. Immediate-ly this tiire h-as been noted
transfer the film to the full strength developer and continue
dcvelopment without further inspection for i period equal to
three tines the period in the fiist bath. the ?ish should be
rocked continuously throughout the whole period of develop,
ment.
Itis not satisfactory to give less than two minutes or more
than five minutes in the full Jtrengthdeveloper at65"F., asthis
would indicate that the film is so fai ov€r, or irnder,exposed as to
17View original page 20Page 21 · Read text
Process The DUFAYCOLCR
be beyond correction by this method; at higher temperafirres
development for rather less than two minutes may be required for
a fully exposed film.
BLEACHING BATH. After development rinse thefilmfor
^abouttwo minutes or place immediately in a one per cent solution'of acetic acid to stop divelopment. Then transfer^to the followilg
bath for five minutes until all the negative silver image is dissolved.
.| 2 gms. Potassium Pennanganate. . *o 9".''.
Sulphuric Acid (conc.) . .2oo mlnms ot IO C.CS. I
I rooo c.cll. Water up to .. 1o ozs,
It is important to agitate the film during the bleaching
operation.
In making up this solution it should be remembered that
the perrnanganate may take some hours to dissolve completely
and the sulphuric acid should be added a few drgps at a time
and not suddenly. It is desirable to use water free from any
appreciable quantity of chlorides or else a trace of free silver
nilrate (ro grains or o'j gm.) should be added, as this will
prevent ih" det""ioration of the solution during storage. The
solution should not be kept after use.
White light may be turned on in the darkroom when the
filn has been in the bleaching bath for about one minute, and
all subsequent operations carried out in the white light.
CLEARING BATH. After bleaching, the film is
rinsed and placed in the following bath the brown stain 'rntil
has disappeared:-
gms. Potassiurn Metabisulphite I OZ. I r 25 torl Water up to .. 1O ozs. J ( rooo c.cll.
ALTERNATI\IE BLEACHING BATH. Some workers
prefer the bichromate bleach because it is more stable and no
clearing bath is required after it, thus obviating one o_Peration.
If the Fi"hro*ute Fleach is used the resulting Dufaycolor trans,
parencies will be definitely softer and more de_graded in colour.
Fo" some purposes these relatively degraded efiects may be
considered more pleasing and harmonious.
18View original page 21Page 22 · Read text
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The DUF'AYCOtOR Process
Bleaching requires three to five ninutes in a bath prepared
by diluting r part ofthe following solution with r o parts of water.
Potassium Bichromate 2 o7,s. I r 50 gms. roo c.cs. Sulphuric Acid conc. I ,, I o" .{ Waterupto.. .. qo ,, J L rooo ,,
Dissolve the bichromate in the water fust and then add
the sulphuric acid.
REDUCTION OF DENSE TRAI{SPARENCIES. When
the film is viewed in white light after bleaching it will be
obvious whether or not the exposure has been satisfactory. A
correctly exposed film will show the brightest portions of the
highJights as almost dead black when the film lies face up in the
dish: this is because the image in these portions should be
almost free from silver bromide. If on the other hand the high,
lights are covered with a white veil it follows that the exposure
has been too short, or if Factorial Development has been
employed the correction has either been insuffi.cient or the
exposure so short that adequate compensation is impossible. On
redevelopment this white veil will yield a silver deposit, so that
the transparency will either be flat or unduly dense. This can
be avoided and the transparency consequently improved by
carefully regulated reduction before the second development,
but if the reduction has to be carried far the picture will tend to
be unduly soft in contrast. This treatment is of great value in
practice and was employed as long ago as rgog when it was
included in the instructions for the Thames Colour Plate but
more recently it has been less employed than its obvious merits
would warrant.
The best reducing agent is a very dilute solution
(r : rooo) of potassium cyanide; a stronger solution acts too
rapidly. This substance is extremely poisonous and must
therefore be handled with care; those who prefer not to use
cyanide can employ r per cent solution of hypo or a 2 per ^cent solution of ammonium thiocyanate (sulphocyanide), but
these give less satisfactory results. The film is immersed in the
reducing solution and gently rocked until the high,lights are
almost clear, but are still just covered with a slight milky
deposit; the action is stopp"d by transferring to water. After
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