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STAINING
In theory, an unexposed coating of the sensitive emulsion should completely dissolve during development, leaving a region with the same colour as the virgin paper.
This is generally not followed by most papers coated with emulsions made from various pigments, and a more or less evident residue is shown by the unexposed portions, which resists current development procedures.
This residue can be partially removed by a light action on the surface of the paper, either by a brush or by a water jet, but still something remains which resists also after long immersion in water.
It is not difficult to imagine why this happens.
The pigment in suspension, as soon as it becomes in contact with the paper fibers, is attracted by the active binding sites on the surface, but also sucked in by capillarity in the many crevices of the cellulose fibers.
Both mechanisms contribute to the onset of the so called 'staining', well known since the beginning of gum printing, as evident from the strong recommendations to use a pigment concentration below the one producing a stain, lower for automatic development, and a bit more for the forced one.
To test the behaviour of the paper with a given pigment/gum/water coating I tried the methods described below.
Drop test to evaluate staining
Materials: A solution of 10% pigment in 33% gum in water, in a eyes drop vial.
An eyes dropper vial full of water
An eyes dropper for liquid transfer
Two plastic egg packaging
A strip of the paper to test 4x21 cm.
With a pencil draw a line across the strip so defining 11 rectangular zones.
Mark 0,1,2.. 10 in the lower parts of the rectangles.
Put two drops, one above and one below, of the 10% pigment soliution in the square marked 0.
Put 4 drops of the 10% pigment in the first cavity of the egg package.
Put 4 drops of water in the first cavity, thus diluting the pigment.
Stir the mixture with the transfer eyes dropper, suck all andd put two drops in the rectangle marked 1
With the same eyes dropper put 4 drops in the next cavity of the egg package.
Add 4 drops of water so diluting again the pigment and put two drops in rectangle marked 2.
Continue so up to rectangle 11.
Leave the strip to dry completely (several hours, especially for the richest in gum.
Develop in water for 1 hour.
With a small brush force the development of the lower drop on each rectangle.
Observe and possibly measure the RGB of each drop.
The upper corresponds to the staining in free automativ developmen, the lower the one left by forced development, corresponding to the deep fiber staining.
By this way the concentration of the pigment is halved each time, from 10% to 0,02%.
This method contempletes several variants.
Variant 1
On each rectangle of the strip are drawn two squares of 1x1 cm.
With a soft tiny brush the drop is spread to cover the square.
Variant 2
From a thin aluminum foil, or a plastic one, or even a piece of paper under test cut 22 squares 1x1,5 cm and fold a part of them as a handle of a 1x1 cm square.
With a tweezer put delicately one square on each drop to spreaad it on the square area.
Let everything dry,
The added squares will split during development.
The aluminum or plastic squares can be used again.
Some round pins can also be used.
Variant 3
With a pencil draw a line across the strip so defining 11 rectangular zones.
Mark 0,1,2.. 10 in the lower parts of the rectangles.
Put 4 drops of water in each cavity.
Put two drops, one above and one below, of the 10% pigment soliution in the square marked 0.
Put 4 drops of the 10% pigment in the first cavity of the egg package.
With the transfer eye dropper mix thoroughly, charge the dropper and add 4 drops in the following cavity.
Continue until the end of cavities.
With the transfer eye dropper pick up 2 drops from the last cavity.
Put 2 drops in the last rectangle marked 10.
Discharge the dropper in the last cavity.
Pick up the solution in the near cavity and put two drops in the rectangle marked 9.
Continue until first cavity and rectangle 1.
Variant 4
Working as in the case of variant 3, when all the cavities are filled, in each of them put a 1x4 strip of paper and leave in for 1 hour.
Extract the strip and allow to dry placing it properly not to disturb the coated extremity.
Develop in water for 1 hour.
TEST OF STAINING BY DRYNG DROP VARIANT 1
To evaluate the extent and the rate of onset of staining, one test was made with Variant 1. The drop were allowed to dry completely, then developed 1 hour in water.
The strip shows a marked presence of stain after the first dilution (with pigment at 5%, as common in sensitized emulsion, where same volumes of pigment suspension and dichromate solution are mixed.), then a more smooth increase of stains up to the last dilution at a concentration of the pigment equal to 10/1024% = 0,97%.
The brushed spots appear somehow lighter than the undisturbed spots.
TEST OF STAINING BY DRYING DROP VARIANT 3
The former experiment was repeated without spreading the drop.
Here also there is a big increase of staining at the first dilution, then a smooth increase but with a peak at the 4th dilution, followed by a smooth decrease. here also the brushed spots are lighter, but at the highest dilutions the difference vanishes.
The spots show also some difference among each other. The unbrushed ones in dilutions 1 to 6 show a darker core, which becomes lighter in dilutions 8 to 10. The brushed ones show an almost uniform colour but a lighter core (like the unbrushed) from dilutions 8 to 10, like the unbrushed. (and almost identical to them in grey intensity.)
TEST OF STAINING TO EVALUATE THE RATE OF STAINING.
1 drop of 10% pigment was added to 4 ml of water and thoroughly mixed. 7 small piece of paper (2x3cm) were marked 1,2,4,8,16,32,64.
On each piece two drops of the mix are deposed. Each piece is then immersed in water after waiting a number of minutes equal to the numbers marked.
After one hour of water development no trace of stain is visible on the paper with the exception of the last, which was left in air for an hour before developing.
COMMENTS ON THE ABOVE EXPERIENCES
After having seen the strong stain left by even very diluted pigment, I thought that at least after an hour I could get enough penetration of the pigment in the paper fibers, but it is not so, and only a light shadow is left on paper in correspondence of the drops.
Now the staining at different dilutions, quite high, I think is due to the fact that during drying, even the most dilute drop become concentrated because of water evaporation, and in the last moment a very concentrated solution penetrates in the fibers. At this moment I do not know if the brushable part is also deposited at the end of evaporation or sometime before. Also the different shapes of the stain left by the drops is for the moment unexplained.
To know if a longer permanence in a pigment solution could force some more pigment in the paper fibers, a strip of paper was immersed in a dilute( 1 drop 10% pigment in gum to 32 drops water. At this dilutions the drops leave a strong stain after evaporation)pigment solution and left for a time ranging from 1 hour to 16 hours.
After measuring the RGB of the stain left a slight increase with time of immersion is observed, but the absolute level of stain is still very low.
A similar result is obtained when a strip is left 24 hours in the concentrated mother solution of pigment (10% in 33% gum)
Test to evaluate the effect of evaporation rate on staining.
An author mantains that paper dried under high humidity show a lower stain. At high humidity the drying takes more time, therefore the effect of rate of drying was investigated. Four pieces of paper were coated with an emulsion made with 3 drops of pigment mother solution and 3 drops of pure water, similar to the concentration used in reaal work, with equal parts of mother solution and water saturated with dichromate . Two of them were left to dry naturally, one was placed over a wet paper and the last was dried with a phon. the pieces were then developed in water. The two pieces dried normally show a similar staining, not too different from the one dried with the phon, while the piece dried slowly show a little more stain.The test will be repeated an also tried on a sized paper.
After having observed the difficulty in leaving a consistent stain from the pigment in solution, while a drying drop leaves a strong spot, the mechanism of such a behaviour is the main reason for continuing the experimentation.
PROBABILITA'
INDICE VIDEO DELCAMP D01D02D03
STAMPA AI PIGMENTI
LARGE NEGATIVES FROM DIGITAL IMAGES
